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Permeability and physical properties of fine-grained clastic sediments show a wide range of variations. Despite rather intensive research, the impact of grain size distribution and mineralogical composition of individual rock constituents is not thoroughly investigated. We performed mechanical compaction of brine-statured reconstituted borehole cuttings and synthetic quartz-clay mixtures to study the evolution of properties in fine-grained clastic sediments during burial. The primary objective was to examine whether the hydraulic and physical properties of fine-grained sediments could be described and constrained by binary quartz-clay mixtures. The synthetic binary mixtures were prepared by mixing quartz with non-swelling (kaolinite) and strongly-swelling (smectite) clays, which can represent the endmember properties within the clay minerals. In addition to vertical permeability, physical and seismic properties, stress-dependence of permeability, and two-phase relative permeability of brine-oil system were investigated. Experimental results show that grain size distribution and mineralogical composition control the vertical permeability. A well-constrained porosity-permeability bound is defined, where the compaction trends of pure quartz and quartz-smectite 15:85 (wt %) mixtures describe the maximum and minimum boundaries, respectively. The quartz-clay mixtures, however, fail to provide bounds to constrain the broad range of variations in physical and seismic properties of reconstituted aggregates, and consequently natural mudstones. It is crucial to incorporate microstructure into the permeability prediction models because the experiments indicated that the microscale characteristics control the macroscale fluid flow properties.

The evaluation of the interplay between kerogen maturation and total organic carbon (TOC) content within an organic-rich shale is critical to characterizing source rock shale's elastic behavior. We analysed a comprehensive database of twenty-nine well logs, two 3D seismic surveys, and geochemical data of eighteen wells within the Draupne Formation in the Norwegian North Sea. The Upper Jurassic (Kimmeridgian) Draupne Formation Shale is found throughout the Norwegian Continental Shelf and is the primary source, carrier, and seal rock in several oil and gas fields. The dataset explores a broad range of depths (2–5 km), total organic contents (0–20 wt%), and various maturation states ranging from immature to overmature. We study the relationship between rock brittleness and organic content as shale matures. We use calculated values of Young's modulus and Poisson's ratio. Academic and industry-standard cutoffs are applied to both the TOC content and maturation level. We introduce here the Organic Maturation Product (OMP) rock physics template, which incorporates a relationship between organic content and any subsequent maturation. The degree of brittleness or ductility is impacted directly by the kerogen maturation process, with lower TOC content having a dampening effect on the transition towards increasing brittleness. This template allows one to classify rock maturation in nine categories, from low to high maturation, and link these categories to rock geomechanical properties. Seismic inversion of the 3D surveys and mapping utilizing the OMP classification reveal how large-scale depositional environment and subsequent diagenetic events influence both placement and quality of source rock broadly within the basin context of the North Sea.

This study investigates petrophysical and acoustic properties of experimentally compacted reconstituted samples of seal sequences from the southwestern Barents Sea. The aggregates were collected from drill cuttings of mudstone and shale formations of two exploration wells, 7220/10-1 (Salina discovery) and 7122/7-3 (Goliat field). The washed and freeze-dried samples were characterized for grain size distributions, geochemical analyses, and mineralogical compositions. A total of 25 compaction tests (12 dry and 13 brine-saturated) were performed with a maximum effective vertical stress of 50 MPa. The laboratory measurements demonstrated that petrophysical and acoustic properties of argillaceous sediments can change within a sedimentary basin and even within a given formation. The results show that the collected aggregates from Goliat field are compacted more compared to Salina discovery. The maximum and minimum compaction are measured in samples collected from Snadd and Fuglen formations, respectively. The final porosity of brine-saturated specimens varies between 5% and 22%. The ultrasonic velocity measurements depict that samples with the same porosity values can have a broad range of velocity values. The resulting compaction trends in this study were compared to published compaction curves for synthetic mixtures of quartz and clay. All compaction trends show higher porosity reduction than the silt fraction with 100% quartz. Comparison of experimental compaction result of each mudstone and shale aggregate with its corresponding acquired well log data helps to delineate the burial history and exhumation in the study area. A net exhumation of 950 m and 800 m is estimated at Salina and Goliat wells, respectively. The outcomes of this study can provide insights for hydrocarbon prospect discovery in a pre-mature sedimentary basin in terms of exploration and production, and also for geological CO2 storage sites. The experimental results may provide information for well log and seismic interpretation, basin modeling and seal integrity of investigated horizons.

Assessment of the geomechanical properties of organic-rich shale caprocks is critical for a successful CO2 storage into a saline aquifer. In this study, we investigated the geochemical properties of the organic-rich shale caprocks of the Draupne and Heather formations, overlying the potential sandstone reservoirs of Sognefjord, Fensfjord, and Krossfjord formations in the northern North Sea, offshore Norway. The caprock's depositional variations within the sub-basins are established by analyzing the gamma-ray shape and stacking patterns. The effect due to differences in depositional environments, on the caprock compaction behavior is investigated by integrating petrographical analysis of core and cutting samples from 3 wells and by rock physical analysis of wireline log data from 27 exploration wells. Three rock physics templates are used where the wireline log data are interpreted using the published background trends. The effect of kerogen type, maturation level, and deposition environment on caprock properties within the study area are also evaluated. Moreover, the caprock property, such as brittleness, is estimated by using four mineralogy and elastic property-based, empirical relations, which is a quantitative measure of caprock property with respect to changes in stress-state. Finally, the seismic inversion method is assessed for the possibility of extracting caprock properties from surface seismic data. Regardless of compaction processes, the results indicate that the Heather Formation is mechanically stronger than the Draupne Formation. However, both formations appear to be ductile in nature. The depositional environments control the mineralogical composition and fabric of the Draupne and Heather formations, which influence the caprock properties significantly. Results also show that the effect of TOC on caprock properties is insignificant in the study area. The brittleness of the organic-rich shale caprocks in the study area follows a different trend compared to the published trends. We also observed an excellent correlation between the log-derived elastic properties and geomechanical parameters. Still, it is difficult to assess the caprock elastic properties from seismic due to the overlap of data clusters. The evaluation of caprock geomechanical behaviors is challenging as these properties are site-specific and also influenced by other factors such as exhumation, in-situ stress conditions, the existence of natural fractures, and their orientations.

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.

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.

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.