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Sediment gravity flows transport large volumes of sand and clay minerals into submarine systems, which store some of the world's major reserves of oil and gas. However, knowledge about grain-coating clay mineral formation and its role in preserving reservoir quality in deep marine settings is poorly documented. Here we present a case study on the Agat Formation, a deep marine deposit interpreted as a series of turbidites, using a multimethod approach including petrographical, petrophysical and sedimentological data. This study investigates the occurrence and origin of chlorite coating and demonstrates how extensive chlorite coating substantially affects reservoir quality. The presence of green marine clay pellets suggests an initial shallow marine origin and sedimentological evidence reveals that the sediments were later remobilized by gravity flows and deposited at their present location. We suggest that the precursor clay coating was emplaced prior to sediment remobilization because of the presence of clay coating on grain contacts and all detrital components, the continuous nature of coating and the lack of clay bridges between the grains. Therefore, the origin of chlorite coating in deep marine environments may be recognized using the characteristic properties of inherited precursor clay coating. Chlorite coating thickness varies between an upper and lower sand unit, with an average of ca. 4.5 µm and ca. 24 µm, respectively. Permeability is significantly reduced in the interval with exceedingly thick chlorite coating but shows only a subtle decrease in helium porosity. This study enlightens the importance of crucially evaluating porosity in sandstones with thick chlorite coating using a multimethod approach. The results from this study can be useful in future exploration endeavours in the area and in other deep marine systems with a similar setting worldwide.

Lower Cambrian quartz arenite deposits have a world-wide occurrence and are also present on Baltica. However, the processes influencing the deposits from source to sink have not been accordingly investigated. The provenance of these deposits is crucial for the understanding of the extent of chemical weathering in the cratonic drainage area and reworking at the broad shallow shelves of Baltica during early Cambrian time. Provenance analysis and study of weathering effects was done for lower Cambrian sandstone from southern Scandinavia, including southern Norway, southern Sweden, and Bornholm (Denmark). For the quartz-arenite sandstone of the Ringsaker Member and the Hardeberga Formation, predominantly moderately weathered felsic–intermediate plutonic and meta-igneous source terranes are suggested from negative Eu anomalies, high LaN/YbN, and low to moderate Ti/Nb ratios, as well as trace amounts of plutonic lithoclasts. Similarly, a felsic–intermediate igneous and metamorphic signature is indicated in the heavy-mineral assemblage and a dominance of dark cathodoluminescence of quartz in a special study of samples from Bornholm, thus suggesting a mixed provenance of local granitoid–orthogneissic Mesoproterozoic basement and distant sources from the Transscandinavian Igneous Belt for the Hardeberga Formation on Bornholm. High percentages of mostly rounded ultra-stable heavy-mineral grains, quartz, and extremely high SiO2/Al2O3 ratios indicate a compositionally and texturally mature sand that was subjected to extensive to moderate weathering following removal of detritus from the source area. The main controls on the alteration of framework composition and the heavy-mineral assemblage are interpreted as surface weathering, based on moderate Chemical Index of Alteration values and meteoric flushing, and reworking by waves, based on an estimated low amount of feldspar before burial. Thus, the maturation of lower Cambrian quartz arenite on southwestern Baltica can be ascribed to a combination of processes including weathering in the source-rock area, extensive reworking, and early diagenesis. This study also highlights the importance of understanding alteration processes affecting the deposits from source to sink, and to be careful to describe quartz-rich rocks as simply recycled deposits.

[Abstract A general shift towards higher mineralogical and textural maturity changes the reservoir character across the Triassic–Jurassic transition in the southwestern Barents Sea basin (SWBSB), largely affecting the hydrocarbon prospectivity in the region. Petrographic and geochronological provenance data presented in this paper suggest that the shift from mineralogically immature to mature sandstones initiated during the deposition of the Norian–Rhaetian Fruholmen Formation, and varies with basin location. Strong contrasts between the Fruholmen Formation and underlying formations are associated with proximity to the rejuvenated Caledonian and Fennoscandian hinterlands and are mainly restricted to the southern basin margins. In the basin interior, subtle petrographic variations between the Fruholmen Formation and older Triassic sandstones reflect a distal position relative to the southern hinterland. The long‐lived misconception of a regional compositional contrast in the Arctic at the turn of the Norian can be attributed to higher sampling frequency associated with hydrocarbon exploration activity along the southern basin margins, and masking by increased annual precipitation and subsequent reworking during the Jurassic. Geothermal signatures and rearrangement of ferric clay material across the Carnian–Norian transition support a recycled origin for the Fruholmen Formation in the basin interior. As the closest tectonically active region at the time, the Novaya Zemlya fold‐and‐thrust belt represents the best provenance candidate for polycyclic components in Norian–Rhaetian strata. In addition to recycling in the hinterland during the Late Triassic, local erosion of exposed intrabasinal highs and platforms at the Triassic–Jurassic transition represents a second process where thermodynamically unstable mineral components originally sourced from the Uralides may be removed. Textural and mineralogical modification may also have occurred in marginal‐marine depositional environments during periods with elevated sea level. Mature sediment supply from the rejuvenated hinterland in the south, multiple cycles of reworking and gradual accumulation of polycyclic grains have likely led to the extreme compositional maturity registered in the Tubaen, Nordmela and Sto formations in the SWBSB. It is likely that increased annual precipitation since the latest Carnian had an amplifying effect on sandstone maturation across the Triassic–Jurassic boundary, but we consider the effect to be inferior compared to provenance shifts and reworking. Findings from this study are important for understanding compositional and textural maturity enhancement processes in siliciclastic sedimentary basins., Structural reorganization of the western Barents Sea basin and surrounding hinterland terrains resulted in a shift from compositionally immature to mature sandstones during the Late Triassic. ]

Recently, the petroleum industry has focused on deeply buried reservoir discoveries and exploring potential CO2 storage sites close to existing infrastructure to increase the life span of already operating installations to save time and cost. It is therefore essential for the petroleum industry to find an innovative approach that exploits the existing core- and well log data to be successful in their endeavor of effectively characterizing and predicting reservoir quality. Continuous data sources (e.g. wireline logs) have a huge potential compared with expensive, time inefficient and sporadic data from cores in determining reservoir quality for use in a regional context. However, whereas core analysis offers in-depth knowledge about rock properties and diagenetic processes, continuous data sources can be difficult to interpret without a formation-specific framework. Here, we demonstrated how the pre-existing core data could be effectively used by integrating petrographic- and facies data with a pure predictive machine learning (ML) based porosity predictor. The inclusion of detailed core analysis is important for determining which reservoir parameter(s) that should be modeled and for the interpretation of model outputs. By applying this methodology, a framework for deducing lithological and diagenetic attributes can be established to aid reservoir quality delineation from wireline logs that can be used in frontier areas. With the ML porosity model, a Random Forest Regressor, the square of the correlation was 0.84 between predicted- and helium porosity test data over a large dataset consisting of 38 wells within the Stø Formation across the SW Barents Sea. By integrating the continuous ML porosity logs and core data, it was possible to differentiate three distinct bed types on wireline log responses within the Stø Formation. Particularly, the relationship between Gamma ray (GR) and porosity was effective in separating high porosity clean sand-, low porosity cemented clean sand and more clay and silt rich intervals. Additionally, in the P-wave velocity (VP) - density domain, separation of high porosity clean sand- and heavily cemented low porosity clean sand intervals were possible. The results also show that the ML derived porosity curves coincide with previously published and independent facies data from a selection of the wells included in the study. This demonstrates the applicability of the model in the region, because the Stø Formation has been described to exhibit similar lithological- and mineralogical properties over large parts of the Western Barents Sea area. Even though, continuous porosity data could be estimated from other sources like VP, neutron or density logs, this would generally require matrix and fluid information. This study demonstrated the effectiveness of the ML model in generating continuous porosity logs that are useful for characterizing and predicting reservoir properties in new wells. This methodology offers a workflow for exploiting already acquired core and well log data for frontier exploration that can be adapted to other formations and exploration scenarios worldwide.

Chlorite-coats on quartz surfaces are ubiquitous in various sedimentary environments. Chlorite-coats shield the surface of quartz from quartz cement overgrowths, thus preserving anomalously high porosity in deeply buried sandstone reservoirs. The inhibition of the quartz cement implies that the chlorite-coats on the surface of quartz grains can significantly influence the physicochemical behavior of the quartz grains. Therefore, failure to notice the initial thin microscale coatings forming during deposition can have serious consequences for modeling several geochemical reactions occurring at liquid-solid interfaces. Despite this huge implication, the fundamental mechanisms involved in chlorite-coat formation is not well understood. Here we present an experimental study to determine the parameters that control chlorite-coat formation on the surface of quartz grains. The batch experiments were conducted in a concoction of quartz and chlorite under different conditions of ionic strength, pH, and presence of humic acid (HA), iron- (Fe) and aluminum (Al) oxides). HA, Fe- and Al oxides are suggested to aid the emplacement of chlorite-coat precursors. At pH 7, the quartz–chlorite and quartz–chlorite–Fe/Al-oxides mixing experiments performed in saline and non-saline solution result in equal chlorite-coat coverages, suggesting neither salinity nor Fe and Al-oxides explain the mechanisms of chlorite-coat formation. At pH 5 and 9, however the chlorite-coat coverage was superior only in saline solution, indicating differences in coat coverage may be caused by variable electrokinetic charge distribution due to the distribution and transport of dissolved salt. The chlorite-coat barely formed in experiments that contain HA in quartz–chlorite mixtures regardless of ionic strength and pH. Against a long-standing notion, the presence of organic matter cannot necessarily be prerequisites for binding chlorite on the surface of quartz grains. The dynamic interactions between solution chemistry and surface chemistry of solid phases (quartz, chlorite, HA, Fe and Al oxides) can result in changing the electrokinetic properties in a region near the solid phases and at mineral-solution interfaces. We therefore propose that the electrokinetic response that arises in heterogeneous systems may explain the mechanisms of chlorite-coat formation.

Evaluating top seal integrity is crucial for successful subsurface CO2 storage. Caprock shale geomechanical properties are complex and influenced by various parameters and processes. It is challenging to understand the role of various factors affecting the geomechanical properties; therefore, an integrated approach is required to evaluate top seal shales. In this study, we investigated the caprock properties of the Early Jurassic shaly Drake Formation overlying the reservoir sandstones of Early Jurassic Cook and Johansen formations. The study area is the potential CO2 storage site Aurora (the Longship CCS project), located in the Horda Platform area, offshore Norway. Based on lithological variations, the Drake Formation is subdivided into upper and lower Drake units. Variations of the geomechanical properties are investigated using wireline logs from 50 exploration wells, two 3D seismic cubes, and several 2D seismic lines. Elastic property-based brittleness indices of the Drake Formation caprock shales are evaluated to identify possible top seal quality. Moreover, seismic attributes and gas leakage scenarios are investigated qualitatively to assess the possibility of injected CO2 escaping from the reservoir. Low brittleness indices value of the Drake Formation shale near the Aurora injection site indicated that the seal rock might diffuse the injection-related stress change and act as an effective top seal. Based on the integrated qualitative assessment, it is likely that the Drake caprock shale will be acted as an effective top seal in and around the Aurora storage site. However, due to the complex nature of caprock shales, we recommend field-scale numerical simulation to evaluate the injection-induced stress-strain effect.

Relationship between different geomechanical and acoustic properties measured from seven laboratory tested unconsolidated natural sands with different mineralogical compositions and textures were presented. The samples were compacted in the uniaxial strain configuration from 0.5 up to 30 MPa effective stress. Each sand sample was subjected to three loading – unloading cycles to study the influence of stress reduction. Geomechanical, elastic, and acoustic parameters are different between normal compaction and overconsolidation (unloaded and reloaded). Stress path (K0) data differs between normal consolidated and overconsolidated sediments. The K0 value of approximately 0.5 is founded for most of the normal consolidated sands, but varies during unloading depending on mineral compositions and textural differences. The K0 and Overconsolidation Ratio (OCR) relation can be further simplified and can be influenced by the material compositions. K0 can be used to estimate horizontal stress for drilling applications. The relationship between acoustic velocity and geomechanical is also found to be different between loading and unloading conditions. The static moduli of the overconsolidated sands are much higher than normal consolidated sands as the deformation is small (small strain). The correlation between dynamic and static elastic moduli is stronger for an overconsolidation stage than for a normal consolidation stage. The results of this study can contribute to geomechanical and acoustic dataset which can be applied for many seismic‐geomechanics applications in shallow sands where mechanical compaction is the dominant mechanism. Effects of stress reduction on geomechanical and acoustic relationship of overconsolidated sands

Low salinity water (LSW) flooding has yielded promising results in improving the enhanced oil recovery (EOR) techniques in sandstone and carbonate reservoirs. Despite various studies and advances, the main mechanism behind the oil displacement in the rock-brine-oil system is unclear. To get a better understanding of the governing mechanisms, we used the molecular dynamic (MD) method to simulate a system including calcite-brine-oil under two different scenarios. At T=300 K and T=360 K each scenario is completed during 40 ns using a canonical ensemble. The results show that all mono/divalent ions are hydrated initially. Furthermore, Na+, Cl− and H2O molecules reside close to the calcite surface and their positions show little change at different time steps. However, the position of divalent ions is dynamic depending on the simulation time steps. Ca2+, which is initially detected at 2.35 Å above the solid surface at T=300 K and T=360 K, migrates away by time. On the other hand, Mg2+ and SO42− which are invisible up to t = 16 ns even at the radius of 10 Å, move gradually toward the surface with time and consolidate their position after oil displacement. Namely, prior to oil displacement, a shift in Na+, Cl− and Ca2+ locations is noticed, and the trend of double electric layer (EDL) is extended, which might be the cause of oil displacement. The findings of this study are applicable in the improvement of EOR technology, water resource treatment and contaminant transfer from geological systems, dealing with solid solutions such as ocean-soil systems.

Rock physics analyses of data from a wildcat well 7117/9-1 drilled in the Senja Ridge area, located in the Norwegian Barents Sea, reveal changes in stiffness within the fine-grained Paleogene Sotbakken Group sediments, caused by the transformation of opal-A to opal-CT, and opal-CT to quartz. These changes manifest as flat spots on 2D seismic profiles. These flat spots were mistaken as hydrocarbon–water contacts, which led to the drilling of well 7117/9-1. Rock physics analyses on this well combined with a second well (7117/9-2) drilled further NW and updip on the Senja Ridge indicate overpressure within the opal-CT-rich zones overlying the opal-CT to quartz transformation zones in the two wells. The absence of opal-A–opal-CT and opal-CT–quartz flat spots on seismic in the second well is attributed to differences in the temperature and timing of uplift. Amplitude v. angle (AVA) modelling indicates both the opal-A–opal-CT and opal-CT–quartz interface points plot on the wet trend, whereas modelled gas–brine, oil–brine and gas–oil contacts fall within quadrant-I. These findings will be useful in understanding the nature of compaction of biogenic silica-rich sediments where flat spots could be misinterpreted as hydrocarbon-related contacts in oil and gas exploration.

Oil accumulation, being difficult and complicated, is an important issue in petroleum exploration researches. Authigenic minerals, such as carbonate cements and chlorite, can in certain reservoirs alter the wettability of some pore spaces from water-wet to oil-wet. Generally, these oil-wet pore spaces are favorable for oil accumulation. The alteration of reservoir wettability induced by authigenic minerals and the impacts on oil accumulation in tight sandstone reservoirs were investigated using a suite of mineralogical and geochemical characterization techniques, including thin section observation, SEM-EDS, XRD, QEMSEM, CL, quantitative grain fluorescence (QGF), fluorescence spectral analysis, contact angle measurement and sealed coring oil saturation testing on the fourth member of the Lower Cretaceous Quantou Formation (K1q4) in the southern Songliao Basin, China. The study shows that the tight sandstone reservoirs are compositionally immature with detrital grains distributed homogeneously. Quartz, carbonates and clay minerals that show heterogeneous distribution characteristics are the major authigenic minerals in some parts of the intergranular pores. The detrital mineral assemblage suggests that the reservoir rocks at deposition and before diagenesis had characterized by strong water-wet properties. With the development of authigenic minerals, carbonate cements and authigenic chlorite tend to alter the wettability of some parts of the existing pore spaces from water-wet to oil-wet. In the K1q4 sandstone reservoirs, oil prefers to accumulate in the cemented residual pore spaces around carbonate cements and chlorite. Reservoirs containing about 4–5% carbonate cements are suggested to be more preferable to oil accumulation. These reservoirs are mainly located between sandstone-mudstone interfaces and central parts of the sand bodies. Chlorites have mainly two effects: on one hand, chlorite alters the wettability of existing pore spaces and provides preferential accumulation sites for oil in the tight sandstone reservoirs, on the other hand it can reduce the adhesion of oil through forming “clay-oil flocs” and promote further migration. Consequently, the reservoirs with moderate amount of carbonate cements or chlorite always show relative high oil saturation in the K1q4 tight sandstones.

Lower Cambrian quartz arenitic deposits have a worldwide occurrence. In this study, petrographic and mineralogical analyses were carried out on samples from the quartz‐rich Ringsaker Member of the Vangsas Formation from southern Norway and the corresponding Hardeberga Formation from southern Sweden and on the Danish island of Bornholm. The quartz arenite is almost completely quartz cemented and has an average intergranular volume of 30%. The quartz cement is the dominating cause for porosity loss. Dissolution along stylolites and microstylolites is suggested to be the primary and secondary source for the quartz cement respectively. The quartzose sandstone from southern Norway was severely cemented prior to the Caledonian Orogeny, thus limiting the tectonic influence on diagenesis during thrusting. For most samples, authigenic clay minerals and detrital phyllosilicates represent ca. 5% of the present‐day composition. This, together with a low feldspar content, of on average 4%, indicates that the sediment was extremely quartz‐rich already during deposition. The low amount of feldspar prior to burial and the formation of early diagenetic kaolinite point to weathering, sediment reworking and early diagenesis act as important controls on sediment maturity. The large variation in clay‐mineral and feldspar content between the localities, as well as within the sandstone successions, can be explained by different palaeogeography on the shelf during deposition and subsequently dissimilar subjection to reworking and early diagenetic processes. Weathering in the provenance area, reworking in the depositional shallow‐marine environment and meteoric flushing during the burial stage are suggested to explain the high mineralogical maturity of the lower Cambrian sandstone from southwestern Baltica. These processes may generally account for similar quartz‐rich shallow‐marine sandstone units, deposited as a result of intensive continental denudation and during temperate to subtropical and moderately humid conditions.

This study investigates the diagenesis and reservoir quality of Upper Jurassic Sandstones of Ula Formation from the Central Graben. Petrophysical and Petrographical studies have been done on cored interval from well 2/1-6 of Ula Field. Precipitation of quartz cement is the main porosity destroying process in deeply buried, quartz-rich sandstone reservoirs of the North Sea. Quartz cement precipitates in the form of syntaxial overgrowth over detrital grain of quartz. Grain coatings, like micro-quartz and illite, are the main reasons for preservation of porosity in sandstones as they cover the grain and inhibit the quartz overgrowth. Petrographical data in this study clearly indicates that grain coatings are present in the studied samples. Micro-quartz grain coating is the most common grain coat in the Upper Jurassic Sandstones of Ula, which is generated from the transformation of siliceous sponge spicules known as Rhaxella Perforata. Clay grain coats like illite and chlorite are also present. Relation between Intergranular Volume (IGV) versus Matrix and Quartz Cementation versus Porosity have also been studied. IGV is strongly affected by mechanical compaction, grain size, grain shape, quartz, and carbonate cements. Sandstones with high amount of matrix and fine grains have high IGV as compared to coarse grains because coarse grains are compacted more when they are subjected to mechanical compaction. Grain shape also has a pronounced effect on porosity. Angular grains tend to lose porosity easier as they are subjected to stress.

Sedimentary basins containing igneous intrusions within sedimentary reservoir units represent an important risk in petroleum exploration. The Upper Triassic to Lower Jurassic sediments at Wilhelmøya (Svalbard) contains reservoir heterogeneity as a result of sill emplacement and represent a unique case study to better understand the effect of magmatic intrusions on the general burial diagenesis of siliciclastic sediments. Sills develop contact metamorphic aureoles by conduction as presented in many earlier studies. However, there is significant impact of localized hydrothermal circulation systems affecting reservoir sediments at considerable distance from the sill intrusions. Dolerite sill intrusions in the studied area are of limited vertical extent (∼12 m thick), but created localized hydrothermal convection cells affecting sediments at considerable distance (more than five times the thickness of the sill) from the intrusions. We present evidence that the sedimentary sequence can be divided into two units: (1) the bulk poorly lithified sediment with a maximum burial temperature much lower than 60–70 °C, and (2) thinner intervals outside the contact zone that have experienced hydrothermal temperatures (around 140 °C). The main diagenetic alteration associated with normal burial diagenesis is minor mechanical plastic deformation of ductile grains such as mica. Mineral grain contacts show no evidence of pressure dissolution and the vitrinite reflectance suggests a maximum temperature of ∼40 °C. Contrary to this, part of the sediment, preferentially along calcite cemented flow baffles, show evidence of hydrothermal alteration. These hydrothermally altered sediment sections are characterized by recrystallized carbonate cemented intervals. Further, the hydrothermal solutions have resulted in localized sericitization (illitization) of feldspars, albitization of both K-feldspar and plagioclase and the formation of fibrous illite nucleated on kaolinite. These observations suggest hydrothermal alteration at T > 120–140 °C at distances considerably further away than expected from sill heat dissipation by conduction only, which commonly affect sediments about twice the thickness of the sill intrusion. We propose that carbonate-cemented sections acted as flow baffles already during the hydrothermal fluid mobility and controlled the migration pathways of the buoyant hot fluids. Significant hydrothermally induced diagenetic alterations affecting the porosity and hence reservoir quality was not noted in the noncarbonate-cemented reservoir intervals. Keywords: Diagenesis, Sill intrusions, Hydrothermal convection cells, Carbonate cement, Sericitization of feldspars

Mixed deposits are sediments consisting of external clastic (epiclastic or terrigenous), intrabasinal components and pyroclastic components. The mixture, comprising variable amounts of the three components, is defined as “mixed sedimentary rocks”. The Permian Lucaogou Formation (P2l) in the Jimusar Sag of the Junggar Basin is a promising tight oil target in western China, the fine-grained mixed sedimentary rocks of which are rich in organic matter (OM) and two sweet spot intervals with relatively high porosity. However, the sediment composition, provenance and deposition environmental settings have not been studied in detail. In this study coupled chemostratigraphic-petrographic analysis were used to reconstruct their depositional environments. The results show that the fine-grained sedimentary rocks have three major sediment sources, external clastic input (terrigenous clastics), intrabasinal autochthonous to parautochthonous components (carbonates, siliceous skeletal debris and OM) and pyroclastic input. Main lithofacies include siltstone/fine sandstone, mudstone, dolomite and tuffite. The silt/sandstones were mainly sourced from rocks with calc-alkaline composition, while the tuffaceous sedimentary rocks were sourced from high-K calc-alkaline rocks. Elemental proxies suggest that the carbonate rocks were generally deposited under warm and arid conditions, whereas the fine-grained clastic sediments were deposited under relatively humid conditions. The muddy or silty tuffaceous mixed rocks were deposited under relatively reducing conditions compared with carbonates and sandstones. Variations of lithofacies and OM accumulation of different intervals reflect changing deposition environmental settings, and the frequently altered high TOC content rocks and good reservoirs are benefit for tight oil formation. The work may provide some useful insights and serve as a reference for studying other mixed fine-grained sedimentary rocks and tight oil plays in similar lacustrine basins elsewhere.

Part one of this paper reported results from experimental compaction measurements of unconsolidated natural sand samples with different mineralogical compositions and textures. The experimental setup was designed with several cycles of stress loading and unloading applied to the samples. The setup was aimed to simulate a stress condition where sediments underwent episodes of compaction, uplift and erosion. P‐wave and S‐wave velocities and corresponding petrophysical (porosity and density) properties were reported. In this second part of the paper, rock physics modelling utilizing existing rock physics models to evaluate the model validity for measured data from part one were presented. The results show that a friable sand model, which was established for normally compacted sediments is also capable of describing overconsolidated sediments. The velocity–porosity data plotted along the friable sand lines not only describe sorting deterioration, as has been traditionally explained by other studies, but also variations in pre‐consolidation stress or degree of stress release. The deviation of the overconsolidated sands away from the normal compaction trend on the VP/VS and acoustic impedance space shows that various stress paths can be predicted on this domain when utilizing rock physics templates. Fluid saturation sensitivity is found to be lower in overconsolidated sands compared to normally consolidated sands. The sensitivity decreases with increasing pre‐consolidation stress. This means detectability for four‐dimensional fluid saturation changes can be affected if sediments were pre‐stressed and unloaded. Well log data from the Barents Sea show similar patterns to the experimental sand data. The findings allow the development of better rock physics diagnostics of unloaded sediments, and the understanding of expected 4D seismic response during time‐lapse seismic monitoring of uplifted basins. The studied outcomes also reveal an insight into the friable sand model that its diagnostic value is not only for describing sorting microtextures, but also pre‐consolidation stress history. The outcome extends the model application for pre‐consolidation stress estimation, for any unconsolidated sands experiencing similar unloading stress conditions to this study.

Reservoir quality assessment was conducted from petrophysical analysis and rock physics diagnostics on 15 wells penetrating Middle Jurassic sandstone reservoir formations in different regions of the eastern Central North Sea. Seismic interpretation on available 3D and 2D seismic reflection data was utilized to map thickness variations and to draw broad correlations to structural features such as salt structures and faults. In the central Egersund Basin, the Sandnes Formation shows good reservoir properties (gross thickness = 107–147 m, N/G = 33–53%) while the Bryne Formation exhibits poorer reservoir quality (N/G

The relationship between diagenetic evolution and reservoir quality in large-scale Middle – Late Triassic aged channel systems (up to 20 km wide) in the southwestern Barents Sea is investigated through core plug data, XRD, SEM- and modal analyses. The studied channel systems are likely sourced from the southeastern Uralide mountain range and are characterized by chemically unstable clastic sediment and well-developed, porosity-preserving chlorite coatings. Chlorite coatings occupy potential quartz nucleation sites on the framework grain surfaces and likely prevent significant chemical compaction in deeply buried sandstones. Porosity-reduction is believed to follow mechanical compaction trends of similar sandstone compositions. Modelling and prediction of porosity preservation in Middle - Late Triassic channel sandstones in the study area is therefore possible, if temperature histories and sandstone compositions are well constrained. The tidally influenced channel and fluvial-dominated channels in this study show significant variation in reservoir quality. These differences are found to be linked to amount of allogenic matrix and grain size, which significantly reduces the permeability in the tidally influenced channel. If seismic distinction between different channel types is impossible, the distribution of permeability is considered unpredictable. Chlorite coatings in the investigated channels are interpreted to be diagenetic overprints of a precursor clay phase, which appears to have a strong link to the Uralian provenance. Coating precursor emplacement likely occurs prior to significant burial, but the exact physical conditions enabling this process remain elusive without systematic laboratory and analogue studies.

Temperature impacts the quality of reservoir and source rocks, thereby representing an important aspect of petroleum prospect assessments of any basin. This case study of the Triassic basin of Edgeoya, eastern Svalbard, reevaluate earlier burial estimates that were solely based on organic matter maturation data from organic rich shales. Here, we estimate temperatures using a multi‐method approach: Rock‐Eval pyrolysis parameters, fluid inclusions in diagenetic quartz, and inorganic diagenesis signatures of sandstones. Data were collected from organic rich shales of the Botneheia and Tschermakfjellet formations and coal bearing sandstones of the De Geerdalen Formation. Rock‐Eval pyrolysis data indicate that Botneheia and Tschermakfjellet formations experienced burial temperatures of about 124–138°C while the De Geerdalen Formation experienced temperatures ≥92°C. Homogenization temperatures of the De Geerdalen Formation sandstones give similar diagenetic temperatures, from 70 to 124°C, while the kaolinite to dickite transformation implies the temperatures >90°C. Furthermore, the absence of illite formation associated with kaolinite suggest that temperature have never exceed 130°C. Integrating various methods validate spatial variations in temperature proxies and constrain the thermal history of this basin. Cretaceous intrusions have locally affected the temperatures and have obscured regional subsidence and uplift trends. The effect of igneous intrusions on inorganic matter is very limited, but intensive on organic matter. These differences between organic and inorganic thermal indices help in distinguishing of magmatic from burial heating. This study has therefore relevance in deciphering the thermal history of sandstones experiencing magmatic activity coupled with multiple burial and uplift events.

Middle to late Triassic strata exposed at Edgeoya, eastern Svalbard, represents an uplifted part of the northwestern corner of the Barents Sea. This interval is characterized by a predominantly mud-dominated deltaic depositional system where rocks with petroleum reservoir potential are expected both in delta front and channelized sandstone deposits. Recent drilling campaigns into time-equivalent rocks in the Barents Sea have however been disappointing, with porosity and permeability below expectation. This study improves the current understanding of reservoir potential within depositional systems of this kind by examining the link between different depositional facies, diagenesis and their impact on reservoir quality. Five depositional facies were mapped and correlated: channel, floodplain, shallow marine, prodelta and offshore. Our study suggests that diagenetic signatures that control the quality of reservoir rocks vary systematically with these depositional facies. Mechanical compaction was the main cause of porosity destruction in the channel, shallow marine and floodplain samples, while early carbonate cementation occludes the intergranular volume in the prodelta samples. Moreover, quartz cement reduced the porosity in the shallow marine depositional facies. The estimated maximum burial temperature of 124 °C indicates burial to several km depths. Depositional settings within a deltaic to shallow marine system do not exert a strong control on the abundance of chlorite, but it does impact whether the chlorite is pore filling or grain coating and porosity preserving. Efficient chlorite coating have aided in preserving porosities up to 32% in the very well/well sorted medium to fine grained sandstones deposited by fluvial dominated channels.

The influence exerted by the linkage and growth of fault segments on the sedimentation pattern in a lacustrine rift subbasin, the northern Bonan Sag in the Jiyang Depression in the Bohai Bay Basin, is studied by integrating drilling cores, wireline logs and 3D seismic data. The NW-trending Guxi Fault formed through the linkage of three fault segments, which display a roughly en echelon arrangement in map view and are probably related to early-stage regional transtensional and slip-strike stress. Between the vertical displacement troughs of normal faults, two narrow relay ramps, attributed to the coherent fault linkage-and-growth model, formed through the linkage of the three fault segments. A relatively wide relay ramp, attributed to the isolated fault linkage-and-growth model, developed due to the linkage of the NW-trending Guxi Fault and E-W-oriented segmented Chengnan Fault. The sedimentation pattern was strongly controlled by the geometry and evolution of the relay ramps. The sediment routing system was dominated by the relay zone, and fan-delta and sublacustrine fan depositional systems developed in the early stage of relay ramp formation. Lateral breaching of the relay ramps through extensive faulting and rifting probably caused an increase in the vertical throw and resulted in deposition of a coarse-grained nearshore subaqueous fan in front of the normal faults. The relay zone that formed from the linkage of independent faults (the Chengnan and Guxi faults) is associated with a broad drainage area and fan-delta and sublacustrine fan deposits, which contain the most effective hydrocarbon reservoirs in this deeply buried setting.

This study investigates the seismic velocity anisotropy of two organic-rich shales from the Norwegian Continental Shelf. The tested organic-rich shale samples were from the Upper Jurassic Draupne and Hekkingen formations collected from two wells (16/8-3S and 7125/1-1) drilled in the central North Sea and western Barents Sea, respectively. The two tested shales are different in organic matter richness and thermal maturation, and they have experienced different burial histories. The shale core plugs were tested in a triaxial cell under controlled pore pressure. Seismic velocities (V P VP and V S VS ) were measured along different orientations with respect to layering to identify the complete tensor of the rock elastic moduli, and to investigate the velocity anisotropy as a function of increasing effective stress. The measured velocity values exhibit strong anisotropy for the two tested organic-rich shales. The anisotropy for both shales is strongest for V S VS . Seismic velocities follow an increasing trend as the effective stress increases. The anisotropy decreases somewhat with increasing consolidation, probably due to the closing of preexisting fractures and microcracks. The reduction of anisotropy is more evident for the P-wave because it decreases from 0.32 to 0.25 for the Draupne sample and from 0.28 to 0.24 for the Hekkingen sample when the vertical effective stress increases from 26 to 50 MPa. In general, the Hekkingen sample indicates slightly higher velocity values than the Draupne sample due to more compaction and lower porosity. In spite of major differences between the two shale formations in terms of organic matter content, maturity and burial history, they indicate almost the same degree of velocity anisotropy. The outcomes of this study can contribute to better imaging of organic-rich Draupne and Hekkingen shales by constraining the rock-physics properties. The reuse of this article is subject to SEG terms of use and conditions. © 2017 Society of Exploration Geophysicists

This paper part one is set out to lay primary observations of experimental compaction measurements to form the basis for rock physics modelling in paper part two. P‐ and S‐wave velocities and corresponding petrophysical (porosity and density) properties of seven unconsolidated natural sands with different mineralogical compositions and textures are reported. The samples were compacted in a uniaxial strain configuration from 0.5 up to 30 MPa effective stresses. Each sand sample was subjected to three loading cycles to study the influence of stress reduction on acoustic velocities and rock physical properties with the key focus on simulating a complex burial history with periods of uplift. Results show significant differences in rock physical properties between normal compaction and overconsolidation (unloaded and reloaded). The differences observed for total porosity, density, and P‐ and S‐wave velocities are attributed to irrecoverable permanent deformation. Microtextural differences affect petrophysical, acoustic, elastic and mechanical properties, mostly during normal consolidation but are less significant during unloading and reloading. Different pre‐consolidation stress magnitudes, stress conditions (isotropic or uniaxial) and mineral compositions do not significantly affect the change in porosity and velocities during unloading as a similar steep velocity–porosity gradient is observed. The magnitude of change in the total porosity is low compared to the associated change in P‐ and S‐wave velocities during stress release. This can be explained by the different sensitivity of the porosity and acoustic properties (velocities) to the change in stress. Stress reduction during unloading yields maximum changes in the total porosity, P‐ and S‐wave velocities of 5%, 25%, and 50%, respectively. These proportions constitute the basis for the following empirical (approximation) correlations: Δϕ ∼ ±5 ΔVP and ΔVP ∼ ±2ΔVS. The patterns observed in the experiments are similar to well log data from the Barents Sea. Applications to rock physics modelling and reservoir monitoring are reported in a companion paper.

Zoned authigenic minerals reflect temporal changes in fluid composition and can shed light on the dynamic evolution of sedimentary systems. The chemistry of the zoned saddle dolomite-ankerite from the Bohai Bay Basin, China, changes in a cyclic way from core to rim, reflecting a dynamic system perturbated several times from thermodynamic steady-state. The distribution of the dolomite-ankerite close to faults suggests that the cyclic evolution may be related to the faulting. Oxygen isotope ratios, relatively high precipitation temperatures (80–160°C), and association with late-stage pyrite cementation, suggest that dolomite-ankerite formation may be connected to thermochemical sulphate reduction within the hydrocarbon reservoir. The chemical zones alternate between low and high Fe-content, and show evidence of corrosion before each growth stage. The complex relation between the silicate, carbonate, and sulphur/sulphide systems can be explained in terms of open-system diagenesis, where fault rupturing triggers an influx of CO2 and thermogenic H2S. Pyrite scavenges both H2S and Fe, until all H2S is exhausted, and the aqueous Fe then builds up again from slow silicate dissolution. The Fe-content of dolomite-ankerite growth after such a perturbation therefore depends on the rate of Fe-supply from the silicate dissolution, and the length of time between the chemical perturbations. If such a relation exists between time and the coupled mineral reactions, then the Fe-content of the carbonates may provide an indication of the length of time between the fault-triggered chemical perturbations of a system. This must be regarded as a working hypothesis for further detailed studies on the relation between faulting and the silicate-sulphur/sulphide‑carbonate system.

Pore-throat size is a very crucial factor controlling the reservoir quality and oiliness of tight sandstones, which primarily affects rock-properties such as permeability and drainage capillary pressure. However, the wide range of size makes it difficult to understand their distribution characteristics as well as the specific controls on reservoir quality and oiliness. In order to better understand about pore-throat size distribution, petrographic, scanning electron microscopy (SEM), pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), quantitative grain fluorescence (QGF) and environmental scanning electron microscopy (ESEM) investigations under laboratory pressure conditions were performed on a suite of tight reservoir from the fourth member of the Lower Cretaceous Quantou Formation (K 1 q 4 ) in the southern Songliao Basin, China. The sandstones in this study showed different types of pore structures: intergranular pores, dissolution pores, pores within clay aggregates and even some pores related to micro fractures. The pore-throat sizes vary from nano- to micro-scale. The PMI technique views the pore-throat size ranging from 0.001 μm to 63 μm and revealed that the pore-throats with radius larger than 1.0 μm are rare and the pore-throat size distribution curves show evident fluctuations. RMI measurements indicated that the pore size distribution characteristics of the samples with different porosity and permeability values look similar. The throat size and pore throat radius ratio distribution curves had however significant differences. The overall pore-throat size distribution of the K 1 q 4 tight sandstones was obtained with the combination of the PMI and RMI methods. The permeability is mainly contributed by a small part of larger pore-throats (less than 30%) and the ratio of the smaller pore-throats in the samples increases with decreasing permeability. Although smaller pore-throats have negligible contribution on reservoir flow potential, they are very significant for the reservoir storage capacity. The pore-throats with average radius larger than 1.0 μm mainly exist in reservoirs with permeability higher than 0.1mD. When the permeability is lower than 0.1mD, the sandstones are mainly dominated by pore-throats with average radius from 0.1 μm to 1.0 μm. The ratio of different sized pore-throats controls the permeability of the tight sandstone reservoirs in different ways. We suggest that splitting or organizing key parameters defining permeability systematically into different classes or functions can enhance the ability of formulating predictive models about permeability in tight sandstone reservoirs. The PMI combined with QGF analyses indicate that oil emplacement mainly occurred in the pore-throats with radius larger than about 0.25–0.3 μm. This result is supported by the remnant oil micro-occurrence evidence observed by SEM and ESEM.

In this study, rock physical properties and their evolution resulting from compaction processes are investigated for Mesozoic and Cenozoic mudstones and shales located in southern Viking Graben and adjacent areas within the Norwegian North Sea. The studied sediments are deposited within a progressively-subsided sedimentary basin with no major experience of exhumation events. A suite of well log data from 43 exploration wells was utilized to study the compaction behaviour of the Mesozoic and Cenozoic mudstone and shale intervals. The gamma ray log-derived shale volume (Vsh) was used to define different lithofacies and discriminate between the studied mudstones and shales. The rock properties as a function of burial depth were plotted for the identified mudstone and shale intervals. The trends could be divided into a mechanical compaction part and a chemical compaction part depending on the prevalent processes controlling the rock properties with burial depth. The transition from mechanical compaction domain to the zone of dominant chemical compaction takes place between 70 and 90 °C corresponding to a depth of 2–2.5 km. The onset of chemical compaction and cementation occurs in the same sediment found at the same depth range almost throughout the study area in spite of variable geothermal gradient indicating a lithological control on the development of chemical compaction. The degree of chemical compaction and cementation reflects the initial smectite content and the availability of potassium for the smectite to illite and quartz reaction to take place. This study contributes to the understanding of compaction processes in fine-grained siliciclastic sediments delineating the controlling factors in a region which can be regarded a natural laboratory to study compaction mechanisms due to being a subsiding basin with the extensive availability of pertrophysical data generated by hydrocarbon exploration and production activities in the area.

Development of rock physical properties in well-sorted and poorly-sorted unconsolidated mono-quartz sands and sand–clay mixtures as a function of effective stress in both dry and brine-saturated conditions is assessed in this study. The tested samples were prepared with full control on their mineralogy, grain size, grain shape, sorting, and fabric. The experiments were performed in a high-stress uniaxial oedometer up to a maximum of 30 MPa vertical effective stress. Sand–clay samples were a mixture of sand grains and clay particles (kaolinite or smectite) in different proportions. The maximum clay volume fraction used in the experiments was at most 30%. The initial bulk density of the tested sand-dominated samples was adjusted to be close to the maximum index density expected for natural sediments (sand–clay mixtures) during deposition. In pure sand samples, finer grained sand show higher initial porosity than relatively coarser grained sands.Moreover, sand–clay mixtures have lower initial porosity than pure sands. Porosity decreases as a function of increasing clay content. The poorlysorted sand samples are less compaction prone than the well-sorted sand samples. Among well-sorted sand samples, coarser grained sands are more compressible than finer grained sands. At a given effective stress level, sand–clay mixtures are more compaction prone compared with their sand component alone. Pure sands and claypoor sand–clay mixtures (either sand–kaolinite or sand–smectite) show almost the same degree of compaction when tested in both dry and brine-saturated conditions. In contrast, clay-rich sand–kaolinite and sand–smectitemixtures (clay volume >20%) are significantly more compact in brine-saturated condition. The Vp values of brinesaturated sand–kaolinite mixtures shows a positive correlation with the kaolinite content, whereas Vp starts to decrease substantially when the volume fraction of smectite exceeds 10% of the whole sand–smectite samples. Saturated bulk moduli estimated by Gassmann’s fluid substitution agree with measurements for brine-saturated clay-poor sand samples. However, the model does not yield proper predictions for sand–clay samples containing 20% clay volume and above, particularly when the clay is mainly smectite. The acoustic and physical properties derived from experimental compaction of pure sands and sand–clay mixtures show a good agreement with rock properties derived from well logs of mechanically compacted pure sands and shaly sands in progressively subsided basins such as Viking Graben in the North Sea. Thus, the outcome of this study can provide reliable constraints for rock physical properties of sands and shaly sands within the mechanical compaction domain and contribute to improved basin modelling and identification of hydrocarbon presence, overconsolidation, and/or undercompaction.

The Barents Sea is believed to have been influenced in most parts by Cenozoic uplift and erosion episodes. The rocks in the area are not currently at their maximum burial depth. The exhumation of the sedimentary rocks has had large effects on rock physical properties and hydrocarbon maturation and migration. The current study seeks to estimate exhumation from shale compaction and thermal maturity techniques and discuss its implications for hydrocarbon exploration in the uplifted Barents Sea area. This study uses well logs and thermal maturity data together with widely distributed shot gather data along long-offset seismic reflection lines. The use of shale compaction techniques to estimate exhumation was focused particularly on the regionally preserved Aptian-Albian (Kolmule Formation) and Paleogene (Torsk Formation) shales. Normal compaction reference curves were established for these units in areas currently at their maximum burial depth (e.g. Sorvestsnaget Basin and Vestbakken Volcanic Province). The results suggest widespread Cenozoic exhumation throughout the southwestern Barents Sea. The exhumation magnitudes increase towards east and northeast. The average exhumation estimates from the three data sources range from ∼800 to 1400 m within the Hammerfest Basin, ∼1150–1950 m on the Loppa High, ∼1200–1400 m on the Finmark Platform and ∼1250–2400 m on the Bjarmeland Platform. The marked differences in glacial erosion from mass balance and average erosion estimates from the current study suggest a significant pre-glacial uplift and erosion in the southwestern Barents Sea area. The observed stratigraphy and presence of significant volumes of Late Oligocene-Middle Miocene sediments in basins at the outer margin, and increased erosion rates at the same time in source areas suggest that maximum burial in the southwestern Barents Sea may have occurred sometime during the Oligocene, or even earlier in the Eocene. The results from this study are useful input for modelling of source rock maturation, generation, migration and trapping of hydrocarbons in the area. These results are also an important input for the prediction of more precise reservoir and seal rock properties in frontier areas away from the exploration wells and provide valuable knowledge for the use of interval velocities in the uplifted areas.

A thick succession of fine-grained sediments in any sedimentary basin can serve as source (if organic-rich) and top seals. Both sealing and petroleum generation capacity of any fine-grained succession is dependent on compaction history (mechanical and chemical compactions) of the sediments. In the Egersund Basin, a thick succession of fine-grained sediments of Upper Jurassic-Lower Cretaceous age overly Middle Jurassic reservoir sands. The Egersund Basin, however, has experienced a complex depositional and tectonic history, particularly, late exhumation (Neogene) which complicates the overall burial, thermal and pressure history. In this study, petrophysical and acoustic properties (bulk density and P-wave velocity) of 10 exploration wells from two neighboring blocks in the Egersund Basin were utilized to investigate compaction processes in the area. In addition, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses from selected wells and available total organic carbon (TOC) data were used as guide to interpret the compaction processes and resulting rock properties. The results show that the transition from mechanical to chemical compaction is not sharp. The onset of chemical compaction imposes a measurable deviation in the burial depth trends of the petrophysical properties expected from purely mechanical compaction. The continuation of the depth related trends however, indicates that chemical compaction may occur in concurrent with continuing mechanical compaction. The observed compaction varies between the different clay dominated stratigraphic units; indicating that the major control on both mechanical and chemical compaction is the primary sediment composition.

Laboratory experiments were performed to investigate high-temperature (100–150 °C) authigenic formation of clay coatings (smectites, chlorites) on clean feldspar and quartz surfaces. Artificial formation waters were used, with Mg-concentrations in the range of North Sea brines. Experiments were run for 21–50 days in brines composed of magnesium chloride and sodium carbonate adjusted to circum-neutral pH. The experiments suggest that the silica activity is the main factor determining if grain coating smectites or chlorite form. The clay minerals form easily on both clean quartz and feldspar surfaces, but chlorite coatings were formed only on feldspar surfaces while smectite coatings were formed on both feldspar and quartz. The chlorite morphology varies between honeycomb, edge-to-face and rosette patterns, while all smectite formed with honeycomb-like textures. The clay coatings produced in this study are morphologically similar to naturally occurring diagenetic clay minerals. In natural sediments and sedimentary rocks, the formation of clay coatings is promoted by pre-existing clay drapings on mineral grains. The growth substrates used in this study were not coated with such a natural precursor materials. This suggests that the nucleation of clay coatings in nature may be possible on clean quartz and feldspar surfaces, but pre-existing clay minerals may impact the allowed levels of supersaturations and thereby the formation of the grain-coating phases.

The Lower Cretaceous Quantou Formation in the southern Songliao Basin is the typical tight oil sandstone in China. For effective exploration, appraisal and production from such a tight oil sandstone, the diagenesis and reservoir quality must be thoroughly studied first. The tight oil sandstone has been examined by a variety of methods, including core and thin section observation, XRD, SEM, CL, fluorescence, electron probing analysis, fluid inclusion and isotope testing and quantitative determination of reservoir properties. The sandstones are mostly lithic arkoses and feldspathic litharenites with fine to medium grain size and moderate to good sorting. The sandstones are dominated by feldspar, quartz, and volcanic rock fragments showing various stages of disintegration. The reservoir properties are quite poor, with low porosity (average 8.54%) and permeability (average 0.493 mD), small pore-throat radius (average 0.206 μm) and high displacement pressure (mostly higher than 1 MPa). The tight sandstone reservoirs have undergone significant diagenetic alterations such as compaction, feldspar dissolution, quartz cementation, carbonate cementation (mainly ferrocalcite and ankerite) and clay mineral alteration. As to the onset time, the oil emplacement was prior to the carbonate cementation but posterior to the quartz cementation and feldspar dissolution. The smectite to illite reaction and pressure solution at stylolites provide a most important silica sources for quartz cementation. Carbonate cements increase towards interbedded mudstones. Mechanical compaction has played a more important role than cementation in destroying the reservoir quality of the K 1 q 4 sandstone reservoirs. Mixed-layer illite/smectite and illite reduced the porosity and permeability significantly, while chlorite preserved the porosity and permeability since it tends to be oil wet so that later carbonate cementation can be inhibited to some extent. It is likely that the oil emplacement occurred later than the tight rock formation (with the porosity close to 10%). However, thicker sandstone bodies (more than 2 m) constitute potential hydrocarbon reservoirs.

The tight sandstones of the Cretaceous Quantou formation are the main exploration target for hydrocarbons in the southern Songliao basin. Authigenic quartz is a significant cementing material in these sandstones, significantly reducing porosity and permeability. For efficient predicting and extrapolating the petrophysical properties within these tight sandstones, the quartz cement and its origin need to be better understood. The tight sandstones have been examined by a variety of methods. The sandstones are mostly lithic arkoses and feldspathic litharenites, compositionally immature with an average framework composition of Q43F26L31, which are characterized by abundant volcanic rock fragments. Mixed-layer illite/smectite (I/S) ordered interstratified with R = 1 and R = 3 is the dominating clay mineral in the studied sandstone reservoirs. Two different types of quartz cementation modes, namely quartz grain overgrowth and pore-filling authigenic quartz, have been identified through petrographic observations, CL and SEM analysis. Homogenization temperatures of the aqueous fluid inclusions indicate that both quartz overgrowths and pore-filling authigenic quartz formed with a continuous process from about 70 °C to 130 °C. Sources for quartz cement produced are the conversion of volcanic fragments, smectite to illite reaction and pressure solution at micro stylolites. Potassium for the illitization of smectite has been sourced from K-feldspar dissolution and albitization. Silica sourced from K-feldspars dissolution and kaolinite to illite conversion is probably only minor amount and volumetrically insignificant. The internal supplied silica precipitate within a closed system where the transport mechanism is diffusion. The quartz cementation can destroy both porosity and permeability, but strengthen the rock framework and increase the rock brittleness effectively at the same time.

For more than a decade, a debate has been going on as to whether dawsonite can sequester CO2, and be the dominant secondary carbonate where reservoirs lack divalent metal cations. Numerical simulations have suggested large-scale formation, whereas natural occurrences in present-day or previously CO2-charged reservoirs are scarce. Natural occurrences, such as in sequences of the Hailaer and Songliao basins, may nevertheless suggest that dawsonite can form under CO2 storage conditions (

Regional assessment studies for CO2 storage plays have been carried out in the Norwegian-Danish Basin of the Central North Sea and in the Skagerrak-Kattegat area. The development of the reservoir models is a part of an ongoing interdisciplinary project with the overall goal to establish a basis for large-scale handling of CO2 in this area, including regional CO2 source and capture possibilities, transportation and infrastructure, possible storage sites as well as legal aspects relating to the whole CCS chain. The study shows that all the necessary premises for a safe, long term CO2 storage, are present in the area. Two trap types for storage have been studied more closely: 1) large gently inclined, unfaulted reservoirs in the northern Skagerrak area and 2) dome structures with four-way closures above salt pillows in the Norwegian Danish Basin. We have closely focused on the Upper Triassic-lowermost Jurassic Gassum Formation and the Middle Jurassic Haldager Sand Formation. The current study presents reservoir characteristics of the sandstones of the Gassum and Haldager formations in the Fjerritslev Trough and on the Skagerrak-Kattegat Platform.

Descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events (epigenesis), involving mineral dissolution and precipitation, which have been interpreted to have occurred during the burial history. Published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial. Near-surface diagenetic reactions may be open, involving significant changes in the sediment composition and formation of secondary porosity caused by high pore-water flow rates of meteoric water or reactions with sea water near the sea floor. Calculations show that the bulk chemical composition of the sediments below the reach of high pore-water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore-water flow. Mass transport between shales and sandstones is also limited because the pore water is, in most cases, buffered by the same minerals so that the concentration gradients are low. Recent studies show that silica released from clay-mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones. If the geochemical constraints for mass transfer during burial diagenetic reactions are accepted, the chemical reactions involved in diagenesis can be written as balanced equations. This offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance. Large-scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested, but because such changes cannot be explained chemically and physically, no predictions can be made. Burial diagenetic processes are, in most cases, not episodic but occur as slow adjustments to increased stress and temperature, driving the sediments toward increased mechanical and thermodynamic stability. As a result, the porosity of a single lithology must decrease during progressive burial, but each lithology has a different porosity curve. This article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced. This provides a theoretical basis for the modeling and prediction of reservoir quality.

This study investigates velocity anisotropy in a well-consolidated organic-rich shale drilled from the central North Sea, offshore Norway. Shales are known as anisotropic rocks showing vertical transversely isotropy. Quantification of anisotropy in shales attracts greater importance considering that shales comprise almost 70% of the buried rocks in sedimentary basins. Two core plugs were prepared perpendicular and at 45o inclination to the in-situ bedding planes in order to measure velocity variations in different directions by increasing the effective stress. The results show that anisotropy of measured shale is stronger for S-wave velocity. The anisotropy decreases with increased consolidation, but not significant, probably due to closing of pre-existing stress-induced fractures and microcracks in the tested sample. The outcomes of this study can contribute to indirect analyses of seal integrity of mudstones and shales from seismic velocity data.

The deeply buried synrift play of the South Viking Graben is characterized by highly variable reservoir quality. An integrated approach incorporating petrophysics, petrography, and one-dimensional basin modeling methods was applied to investigate these variations. Analysis shows that average porosities below 4000 m (13,123 ft) (vertical depth below sea floor) range from approximately 5% to as much as about 25% in comparable quartz arenitic sandstones. From porosity-depth trends, three porosity categories can be recognized (normal-, low-, and high-porosity sandstones). Normal-porosity sandstones fall along the regional average porosity-depth trend. Low-porosity sandstones have been subject to extensive quartz cementation as a consequence of a higher degree of thermal maturity and plot below the regional porosity-depth trend. High-porosity sandstones plot above the regional porosity-depth trend. Here, quartz cementation has been inhibited by grain-coating microquartz, and thus porosity has been preserved. Hydrocarbon emplacement has previously been thought to have inhibited quartz cementation in the study area, but this study concludes that the reservoirs are mainly water-wet, allowing for continued quartz cementation despite the presence of hydrocarbon pore fluids. Predicting the distribution of microquartz-coated sandstones and the degree of thermal maturity is therefore fundamental for successful exploration in the deeply buried parts of the synrift play. This study presents a regional and stratigraphic framework for such predictions that may be incorporated into play models in the area.

This study presents the results of experimental compaction while measuring ultrasonic velocities of sands with different grain size, shape, sorting and mineralogy. Uniaxial mechanical compaction tests up to a maximum of 50 MPa effective stress were performed on 29 dry sand aggregates derived from eight different sands to measure the rock properties. A good agreement was found between the Gassmann saturated bulk moduli of dry and brine saturated tests of selected sands. Sand samples with poor sorting showed low initial porosity while sands with high grain angularity had high initial porosity. The sand compaction tests showed that at a given stress well-sorted, coarse-grained sands were more compressible and had higher velocities (Vp and Vs) than fine-grained sands when the mineralogy was similar. This can be attributed to grain crushing, where coarser grains lead to high compressibility and large grain-to-grain contact areas result in high velocities. At medium to high stresses the angular coarse to medium grained sands (both sorted sands and un-sorted whole sands) showed high compaction and velocities (Vp and Vs). The small grain-to-grain contact areas promote higher deformation at grain contacts, more crushing and increased porosity loss resulting in high velocities. Compaction and velocities (Vp and Vs) increased with decreasing sorting in sands. However, at the same porosity, the velocities in whole sands were slightly lower than in the well-sorted sands indicating the presence of loose smaller grains in-between the framework grains. Quartz-poor sands (containing less than 55% quartz) showed higher velocities (Vp and Vs) compared to that of quartz-rich sands. This could be the result of sintering and enlargement of grain contacts of ductile mineral grains in the quartz-poor sands increasing the effective bulk and shear stiffness. Tests both from wet measurements and Gassmann brine substitution showed a decreasing Vp/Vs ratio with increasing effective stress. The quartz-rich sands separated out towards the higher side of the Vp/Vs range. The Gassmann brine substituted Vp and Vs plotted against effective stress provide a measure of the expected velocity range to be found in these and similar sands during mechanical compaction. Deviations of actual well log data from experimental data may indicate uplift, the presence of hydrocarbon, overpressure and/or cementation. Data from this study may help to model velocity-depth trends and to improve the characterization of reservoir sands from well log data in a low temperature (

Petrographic evidence of thin sheet-like or platelet-shaped quartz cement parallel to bedding is documented in deeply buried, originally smectite-rich, Late Cretaceous mudstones from well 6505/10-1 in the Voring Basin, offshore Norway. The platelets are mainly built up of areas of patchy continuous quartz cement with various amounts of earlier-formed interlocking microquartz crystals. Cathode luminescence (CL) spectra confirm an authigenic origin for the quartz cement. The quartz platelets may originate as flakes (at c . 90–100 °C) that may evolve into well-developed near-continuous patchy quartz cement identified at 4300 m/150 °C. The quartz cement is probably sourced from silica released by the clay dissolution-precipitation processes (smectite and smectite/illite to illite and kaolinite to illite). At temperatures above about 90–100 °C, the continuous supply of silica from these clay mineral reactions results in precipitation of quartz flakes and sheet-like quartz cement. The quartz sheets may act as a mudrock stiffening agent, reinforcing and further cementing together the microquartz networks and aggregates and possibly also enhancing the schistosity and anisotropy of these mudstones during increasing burial. The quartz sheets may also act as vertical permeability barriers in the sediment possibly contributing to overpressure formation during chemical compaction.

Rock physical properties, like velocity and bulk density, change as a response to compaction processes in sedimentary basins. In this study it is shown that the velocity and density in a well defined lithology, the shallow marine Etive Formation from the northern North Sea increase with depth as a function of mechanical compaction and quartz cementation. Physical properties from well logs combined with experimental compaction and petrographic analysis of core samples shows that mechanical compaction is the dominant process at shallow depth while quartz cementation dominates as temperatures are increased during burial. At shallow depths (

Oedometric mechanical compaction tests were performed on brine-saturated synthetic samples consisting of silt-clay mixtures to study changes in microfabric and rock properties as a function of effective stress. The silt consisted of crushed quartz (∼100%) with grain size range between 4 and 40 μm, whereas the clay consisted of 81% kaolinite, 14% mica/illite and 5% microcline of grain size between 0.4 and 30 μm. Five sample pairs ranging in composition from pure silt to pure clay were compacted to 5 and 50 MPa effective stress respectively. SEM studies were carried out to investigate microfabric changes in the mechanically compacted silt-clay mixtures. The degree of alignment of the different minerals present (quartz, mica/illite and kaolinite) were computed by using an image analysis software. Experimental compaction have measured the changes in the rock properties such as porosity and velocity as a function of effective stress for different mixtures of clay and silt. Clay-rich samples showed a higher degree of mineral orientation and lower porosity compared to silt-dominated samples as a function of effective stress. Pure clay sample had 11% porosity at 50 MPa effective stress whereas the pure silt sample retained about 29% porosity at the same effective stress. The experiments showed that low porosity down to 11% is possible by mechanical compaction only. A systematic increase in strain was observed in the silt-clay mixtures with increasing clay content but the porosity values found for the 50:50 silt-clay mixture were lower than that of 25:75 silt-clay mixture. No preferential mineral orientation is expected before compaction owing to the high initial porosity suggesting that the final fabric is a direct result of the effective stress. Both P- and S-wave velocities increased in all silt-clay mixtures with increasing effective stress. The maximum P- and S-wave velocities were observed in the 25:75 silt-clay mixture whereas the minimum Vp and Vs were recorded in the pure silt mixture. At 50 MPa effective stress P- wave velocities as high as 3 km/s resulted from experimental mechanical compaction alone. The results show that fine-grained sediment porosity and velocity are dependent on microfabric, which in turn is a function of grain size distribution, particle shape, sediment composition and stress. At 5 MPa effective stress, quartz orientation increased as a function of the amount of clay indicating that clay facilitate rotation of angular quartz grains. Adding clay from 25% to 75% in the silt-clay mixtures at 50 MPa effective stress decreased the quartz alignment. The clay mineral orientation increased by increasing both the amount of clay and the effective stress, the mica/illite fabric alignment being systematically higher than that of kaolinite. Even small amount of silt (25%) added to pure clay reduced the degree of clay alignment significantly. This study demonstrates that experimental compaction of well characterized synthetic mudstones can be a useful tool to understand microfabric and rock properties of shallow natural mudstones where mechanical compaction is the dominant process.

Late Cretaceous mudstones from two wells located in the northern North Sea and the Norwegian Sea have been examined with respect to quartz cement. Two different types of quartz cement (Type 1 and Type 2) have been identified using SEM/EDS/CL-analysis of drill-bit cuttings at depths 2370–2670 m (80–85 °C). Type 1 appears as relatively large aggregates (30–100 μm) of depth/temperature related crypto- or microcrystalline to macrocrystalline irregular quartz cement formed by local re-crystallization of biogenic silica. The CL-responses of Type 1 quartz cement give a clear indication of an authigenic origin. Type 2 quartz cement represents relatively high amounts of extremely fine-grained micro-sized (1–3 μm) crystals embedded as discrete, short chains or small clusters/nests within the illitized clay matrix. The CL-responses of micro-quartz crystals indicate an authigenic origin. The micro-quartz is most probably sourced from silica released during the smectite to illite dissolution–precipitation reaction. The petrographic evidence indicates that most of the silica released by the smectite to illite reaction has not been exported out of the mudstones. The silica released produce a subtle inter-connected micro-quartz network interlocked with aggregates of micro-quartz and authigenic clay crystals. This micro-quartz cementation process causes a significant and sharp change in the mudstone stiffness at the onset of the chemical compaction regime. This is indicated by an abrupt increase in well log velocity (Vp) and change in seismic facies close to 2500 m (80/85 °C).

Overpressure generation is a function of the rates of sedimentation, compaction, fluid generation from kerogen and dehydration of minerals, and most importantly the lateral distribution of permeability within a basin as this controls lateral drainage. Sedimentary basins, however, are typically highly heterogeneous with respect to primary sedimentary facies, diagenesis and tectonic development. While fluid flow models based on idealised homogeneous basins may further our understanding of the processes that influence overpressure development, the results are very sensitive to the distribution of rock properties, particularly permeability. The absolute permeability of sedimentary rocks varies from more than 1 Darcy to less than 0.01 nanodarcy (nD) (10 −11 Darcy). Simple calculations, assuming vertical flow and no lateral drainage within the basin, show that overpressures approaching fracture pressure in the overburden will be reached if the effective permeability of the shale forming the seal is less than 0.1–0.01 nD (1.10 −22 –1.10 −23 m 2 ). The permeability of shales varies greatly as a function of primary textural and minnerlogical composition and it is not possible to accurately predict the effective permeability of a sequence of shales forming pressure barriers. Overpressure in uplifted basins, where there is no compaction taking place, can only be maintained over geological time if the permeabilities are much lower. Overpressure is often controlled by lateral drainage but the effective permeabilities for fluid flow across faults and the offset of permeable layers are also difficult to predict. In most cases, uncalibrated basin modelling is unable to accurately predict the magnitude and distribution of overpressures because the vertical and horizontal permeabilities in sedimentary basins cannot be determined in sufficient detail. In basins that have been extensively explored and developed, incorporation of prior geological knowledge into basin models may allow overpressures to be predicted ahead of the drill bit. However, with such a large body of information already gathered about the basin it is debatable what extra value basin modelling is providing to pressure predictions.

Basin modelling studies are carried out in order to understand the basin evolution and palaeotemperature history of sedimentary basins. The results of basin modelling are sensitive to changes in the physical properties of the rocks in the sedimentary sequences. The rate of basin subsidence depends, to a large extent, on the density of the sedimentary column, which is largely dependent on the porosity and therefore on the rate of compaction. This study has tested the sensitivity of varying porosity/depth curves and related thermal conductivities for the Cenozoic succession along a cross-section in the northern North Sea basin, offshore Norway. End-member porosity/depth curves, assuming clay with smectite and kaolinite properties, are compared with a standard compaction curve for shale normally applied to the North Sea. Using these alternate relationships, basin geometries of the Cenozoic succession may vary up to 15% from those predicted using the standard compaction curve. Isostatic subsidence along the cross- section varies 2.3-4.6% between the two end-member cases. This leads to a 3-8% difference in tectonic subsidence, with maximum values in the basin centre. Owing to this, the estimated stretching factors vary up to 7.8%, which further gives rise to a maximum difference in heat flow of more than 8.5% in the basin centre. The modelled temperatures for an Upper Jurassic source rock show a deviation of more than 20°C at present dependent on the thermal conductivity properties in the post-rift succession. This will influence the modelled hydrocarbon generation history of the basin, which is an essential output from basin modelling analysis. Results from the northern North Sea have shown that varying compaction trends in sediments with varying thermal properties are important parameters to constrain when analysing sedimentary basins.

Sandstone bodies in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, previously considered as point bars formed in meandering rivers and other types of fluvial bars, are herein interpreted as delta mouth-bar deposits. The sandstone bodies have been examined in a 2300 m long cliff section along the Argyle and Nine Mile Canyons at the southern margin of the Uinta lake basin. The sandstone bodies occur in three stratigraphic intervals, separated by lacustrine mudstone and limestone. Together these stratigraphic intervals form a regressive-transgressive sequence. Individual sandstone bodies are texturally sharp-based towards mudstone substratum. In proximal parts, the mouth-bar deposits only contain sandstone, whereas in frontal and lateral positions mudstone drapes separate mouth-bar clinothems. The clinothems pass gradually into greenish-grey lacustrine mudstone at their toes. Horizontally bedded or laminated lacustrine mudstone onlaps the convex-upward sandstone bars. The mouth-bar deposits are connected to terminal distributary channel deposits. Together, these mouth-bar/channel sandstone bodies accumulated from unidirectional jet flow during three stages of delta advance, separated by lacustrine flooding intervals. Key criteria to distinguish the mouth-bar deposits from fluvial point bar deposits are: (i) geometry; (ii) bounding contacts; (iii) internal structure; (iv) palaeocurrent orientations; and (v) the genetic association of the deposits with lacustrine mudstone and limestone.

The anisotropy of physical properties is a well-known characteristic of many clay-bearing rocks. This anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling. Preferred orientation of clay minerals is an important factor causing anisotropy in clay-bearing rocks such as shales and mudstones that are the main cap rocks of oil reservoirs. The preferred orientation of clays depends mostly on the amount of clays and the degree of compaction. To study the effect of these parameters, we prepared several samples compressing (at two effective vertical stresses) a mixture of clays (illite and kaolinite) and quartz (silt) with different clay/quartz ratios. The preferred orientation of the phases was quantified with Rietveld analysis on synchrotron hard X-ray images. Pole figures for kaolinite and illite display a preferred orientationof clay platelets perpendicular to the compaction direction, increasing in strength with clay content and compaction pressure. Quartz particles have a random orientation distribution. Aggregate elastic properties can be estimated by averaging the single-crystal properties over the orientation distribution obtained from the diffraction data analysis. Calculated P-wave velocity anisotropy ranges from 0% (pure quartz sample) to 44% (pure clay sample, highly compacted), but calculated velocities are much higher than measured velocities. This is attributed to uncertainties about single-crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in the model. In this work, we present an effective method to obtain quantitative data, helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties of clay-bearing rocks.

Vertical and lateral changes in physical properties in Cenozoic mudstones from the northern North Sea Basin reflect differences in the primary mineralogical composition and burial history, which provides information about sedimentary facies and provenance. Integration of well-log data with mineralogical information shows the effect of varying clay mineralogy on compaction curves in mudstones. The main controlling factor for the compaction of Eocene to early Miocene mudstones within the North Sea is the smectite content, which is derived from volcanic sources located northwest of the North Sea. Mudstones with high smectite content are characterized by low P-wave velocities and bulk densities compared to mudstones with other clay mineral assemblages at the same burial depths. Smectitic clays are important during mechanical compaction because they are less compressible than other types of clay minerals. A comparison between well-log data and experimental work also shows that smectite may be a controlling factor for overpressure generation in the smectite-rich Eocene and Oligocene sediments. At greater burial depths and temperatures (70–80C), the dissolution of smectite and precipitation of illite and quartz significantly increases velocities and densities. Miocene and younger mudstones from the northern North Sea have generally low smectite contents and as a result have higher velocities and densities than Eocene and Oligocene mudstones. Lateral differences in the compaction trends between the north and south for these sediments also exist, which may be related to two different source areas in the Pliocene. The log-derived petrophysical data from the northern North Sea Basin show that mudstone lithologies have very different compaction trends depending on the primary composition. Simplified compaction curves may therefore affect the outcomes from basin modeling. The amplitude-versus-offset response of hydrocarbon sands and the seismic signature on seismic sections are also dependent on the petrophysical properties of mudstones and will vary depending on the mineralogical composition.

This study determines the relationship between permeability and other petrophysical properties in synthetic mudstones as a function of vertical effective stress. Six brine-saturated clay slurries consisting of smectite and kaolinite were compacted in the laboratory under both controlled pore pressure and proper drained conditions. Porosity, permeability, bulk density, velocity (both V p and V s ) and rock mechanical properties were measured constantly under increasing vertical effective stress up to 50 MPa. The results show that smectite-rich clays compact significantly less and have lower bulk density, velocity, permeability, bulk and shear modulus but higher Poisson9s ratio compared to kaolinite-rich clays at the same effective stress. Kaolinite aggregates compacted to about 26% porosity at 10 MPa effective stress corresponding to about 1 km burial depth in a normally compacted basin, whereas a pure smectite aggregate has a porosity of about 46% at the same stress. The permeability of kaolinite aggregates varies between 0.1 mD and 0.001 mD, while that of smectite aggregates varies from 0.004 mD to 0.00006 mD (60 nD) at stresses between 1 MPa and 50 MPa. Permeabilities in clays show a logarithmic decrease with increasing effective stress, bulk density, velocity or decreasing porosity. At the same porosity or bulk density, permeabilities differ up to five orders of magnitude within the smectite–kaolinite mixtures. Applications of the Kozeny–Carman equation for calculating permeability based on porosity in mudstones will therefore produce highly erroneous results. The relationships between V p , V s , bulk and shear modulus to permeability also vary by up to four orders of magnitude depending on the clay compositions. Velocities or rock mechanical properties will therefore not be suitable to estimate permeability in mudstones unless the mineralogy and textural relationships are known. These experimental results demonstrate that smectite content may be critical for building up pore pressure in mudstones compared to kaolinite. The results help to constrain compaction and fluid flow in mudstones in shallower parts of the basins (