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The Forties Sandstone Member is an important deep-water reservoir in the Central North Sea. The role of depositional characteristics, grain-coating clays, and diagenesis in controlling the reservoir quality of the sandstones is poorly understood. The main aim of the study is to understand the role of depositional characteristics, grain-coating and pore-filling clays, and diagenesis in controlling the reservoir quality evolution of turbidite-channel sandstones. The study employed a multi-disciplinary technique involving thin section petrography and scanning electron microscopy (SEM) to investigate the impact of grain size, clay matrix content, mode of occurrence of grain-coating chlorite and illite, and their impact in arresting quartz cementation and overall reservoir quality in the sandstones. Results of our study reveal that porosity evolution in the sandstones has been influenced by both primary depositional characteristics and diagenesis. Sandstones with coarser grain size and lower pore-filling clay content have the best reservoir porosity (up to 28%) compared to those with finer grain size and higher pore-filling clay content. Quartz cement volume decreases with increasing clay-coating coverage. Clay coating coverage of >40% is effective in arresting quartz cementation. Total clay volume of as low as 10% could have a deleterious impact on reservoir quality. The Forties Sandstone Member could potentially be a suitable candidate for physical and mineralogical storage of CO2. However, because of its high proportion (>20%) of chemically unstable minerals (feldspar, carbonates, and clays), their dissolution due to CO2 injection and storage could potentially increase reservoir permeability by an order of magnitude, thereby affecting the geomechanical and tensile strength of the sandstones. Therefore, an experimental study investigating the amount of CO2 to be injected (and at what pressure) is required to maintain and preserve borehole integrity. The findings of our study can be applied in other reservoirs with similar depositional environments to improve their reservoir quality prediction. [ABSTRACT FROM AUTHOR]

The Butano Sandstone forms part of an Eocene submarine fan deposited in the La Honda basin in central California. The Butano submarine fan has been subdivided into three main facies associations: southern inner-fan deposits, central middle-fan deposits, and northern outer-fan deposits. In spite of homogeneous quartzofeldspathic sandstone composition, these outcrop belts have several compositional and mineralogical differences, likely caused by post-depositional events related to the complex tectonic history of the La Honda basin, and expressed in deep-burial diagenesis and possible hydrothermal alteration. Both compaction and cementation reduced the intergranular volume (lower than 19 %) and the Compactional Index (average 0.9), indicating the major role of compaction in reducing original porosity. The southern proximal-fan and northern distal-fan outcrops exhibit different relative abundance of detrital quartz (Qm 43.5 %) than the middle-fan central outcrop (Qm 50.7 %). Primarily plagioclase and subordinately K-feldspar suffered leaching and intense dissolution during burial, particularly in the central part of the basin, even showing similar and quite constant distribution of alteration degree among the three outcrop belts (dominantly Fpc, Fpd, Fka, and Fkb grain types). Plagioclase abundance decreases in the middle-fan deposits proportionally with the increase of authigenic kaolinite, even where present in moderate amount; in contrast, authigenic kaolinite of inner-fan strata probably formed after partial dissolution of K-feldspar. Diagenetic changes are most abundant in the Butano Sandstone in the central part of the La Honda basin, likely primarily due to hydrothermal effects during deep burial. • Investigating the post-depositional processes affecting the framework composition of the Butano Sandstone. • Compaction play the crucial role, rather than cementation, in reducing the original porosity. • Dissolution affects primarily plagioclase grains and subordinately k-feldspar. • Samples belonging to the central portion of the Butano sandstone exhibits a higher volume of quartz due to diagenetic evolution of composition. • Burial diagenesis and hydrothermal effects influence the detrital mode of the Butano Sandstone. [ABSTRACT FROM AUTHOR]

[Display omitted] • The early Miocene Burqan Formation is a deep-marine syn -rift succession. • Facies analysis indicate deposition in proximal to distal fan settings. • Depositional parameters (grain size and detrital matrix) exert dominant control on reservoir quality. • Diagenesis plays only a minor role on reservoir quality. • Mid-fan channel deposits are interpreted as excellent quality reservoirs. Understanding the distribution of sedimentary facies, their texture and diagenetic alterations in deep-marine rift systems are critical for improved reservoir quality predictions in the subsurface. Syn-rift deep-marine sandstones of the Miocene Burqan Formation, deposited by various sediment gravity flow types in the Midyan Basin (Red Sea, Saudi Arabia) are characterized using detailed sedimentological and petrographic analyses. Six distinctive lithofacies (F1-F6) have been identified within the Burqan Formation and grouped into three broad facies associations namely proximal, medial, and distal facies of a submarine fan. The proximal fan facies consists of disorganized conglomerate (F1) and structureless sandstone (F2). The medial fan facies are represented by graded bedded and channelized sandstone (F3) and sandstone dominated heterolithics (F4). Deposits of distal fan facies consist of mudstone dominated heterolithics (F5) and structureless mudstone (F6). The sandstones are predominantly arkoses and lithic arkoses with a low degree of diagenetic modification (Eo- and Telo-diagenesis; ≤ 2 km and ≤ 70 °C temperature) and low to moderate volumes of carbonate cement (mean = 7%). The structureless sandstone (F2), sandstone dominated heterolithics (F4) and mudstone dominated heterolithics (F5) have poor reservoir quality due to the presence of authigenic cement and matrix. Graded bedded and channelized sandstone (F3) lithofacies consists of excellent reservoir sandstone due to the least proportion of cement and virtual absence of detrital clay. The observed variability in composition and textural parameters within Burqan Formation indicates that sedimentological transport processes control the depositional reservoir quality which in turn dictates diagenetic modification in deep-marine sandstones with important implications for prediction of reservoir quality in deep-marine systems. [ABSTRACT FROM AUTHOR]

Sedimentological and sedimentary petrographic investigations of the Hamitabat Formation sandstones exhibited a huge submarine fan formation in the northwest of Thrace Basin. In this study reservoir properties of Hamitabat Formation were found out according to sedimentological storage, sedimentary petrographic and petrophysical properties of the sandstone-core samples taken from 16 wells in the northwest of the Thrace Basin. According to analyses of the lithology, sedimentologic structure, texture and sedimentary petrography of the well samples the Hamitabat Formation sandstones were separated into three distinct lithofacies (A, B, and C). Lithofacies A sandstones exhibit coarse grain sizes (0.5-1.0 mm), smooth-point grain contact, subrounded-subangular grains, moderate-bad sorting, and corresponds to upper part of submarine fan formation. Lithofacies B sandstones exhibit medium grain sizes (0.25-0.5 mm), subrounded-rounded grains, concave-convex grain contact, moderate-good sorting, and corresponds to middle part of submarine fan formation. Lithofacies C sandstones exhibit very fine grain sizes (0.12 mm<) corresponds to lower part of submarine fan formation, and nonreservoir facies. Lithofacies A sandstones have medium quality reservoir property due to abundance of cross bedding, higher clay content, less pore areas formed by secondary dissolution, low permeability (0.1-10 mD) and porosity (6-12%). Lithofacies B sandstones have better reservoir properties than Lithofacies A due to fewer clay content, large pore throat, high permeability (16-104 mD) and porosity (15-24%). According to thisstudyhydrocarbons come from Hamitabat Formation Lithofacies B sandstones in the Thrace Basin. Mid part (Lithofacies B) of the Hamitabat Formation submarine fan sandstones have higher hydrocarbon potential than its upper part (Lithofacies A) [ABSTRACT FROM AUTHOR]

Sedimentological and petrographic studies of the Haymana Formation sandstones identify a submarine fan in the Haymana Basin. This study is confined to outcrop samples from the submarine-fan sandstones (the L1, L2, and L3 sections) and focuses on documentation of the petrographic characteristics of and depositional controls on reservoir properties of the Haymana Formation sandstones. Analysis of the lithology, texture, sedimentary structure, and petrography of the outcrop samples allows separation of the Haymana Formation sandstones into three distinct lithofacies, A, B, and C. Lithofacies A sandstones exhibit medium-coarse grain sizes (0.4–0.6 mm), moderate-good sorting, and subrounded grains. Lithofacies B sandstones exhibit fine grain sizes (0.2 mm), moderate sorting, and subrounded-subangular grains. Lithofacies C sandstones exhibit very fine grain sizes (0.112 mm) and nonreservoir facies. The main aim of this study is to document the diagenetic history and reservoir quality of the Haymana Formation sandstones. Lithofacies A has the best permeability, ranging from 80 to 120 mD, which can be related to the large grain size, the presence of cross-bedding, small amount of authigenic clays or cements in the pore throats, and enlarged pore apertures. In contrast, Lithofacies B sandstones contain variable yet small pore and pore-throat sizes, which are affected by abundant pore-filling authigenic clays, small grain sizes, and no dissolution. The permeability of the Lithofacies B sandstone ranges from 16 to 35 mD, which is lower than that of the Lithofacies A sandstone. Additionally, the Lithofacies B sandstones have small median pore-throat sizes (mean = 1.52 μm) and low porosities (average 4–6%), and the Lithofacies A sandstones have large pore-throat sizes (mean = 2.06–3 μm) and high porosities (average 20–25%), with most pore throats (50% or more) ranging between 1 and 4 μm. According to these data, the Lithofacies A sandstones show good reservoir characterisation. To date, significant hydrocarbon deposits have not been discovered in the Haymana Formation sandstones, although intensive exploratory work towards this end has been carried out in the Haymana Basin. According to this study, the proximal sandstones of the submarine-fan complex of the Haymana Formation may contain high-potential reservoirs in the Haymana Basin. In the basin, hydrocarbon is sourced from deep zones and may be capped by the proximal part of the Haymana Formation sandstones. According to this study, the Lithofacies B sandstones of the Haymana Formation are not suitable as diagenetic traps for hydrocarbon exploration in the Haymana Basin. However, the Lithofacies A sandstones may be expected to have the highest exploration success. [ABSTRACT FROM AUTHOR]

Proximal to distal fan change in grain size, clay matrix content, and grain-coating clays have been identified as key contributing factors for eservoir quality evolution of submarine fan turbidite sandstones. This study evaluated the role played by grain-coating and pore-filling clays, depositional facies, and diagenesis in reservoir quality evolution of the Paleocene Forties submarine fan sandstones (Central North Sea) from proximal to distal fan settings. To help provide a comprehensive understanding of the role played by pore-filling and grain-coating clays in destroying and preserving reservoir quality, respectively, in turbidite sandstones, we have used a multi-disciplinary approach including petrography, burial history, scanning electron microscopy, and stable isotopes analysis. Results of the study showed that reservoir quality is influenced by both depositional facies and diagenesis. The proximal-fan, amalgamated sandstones facies have the best reservoir quality due to coarser grain size, lower pore-filling clays, and lower amount of ductile grains. In contrast, the distal-fan, mud-prone heterolithic facies have the poorest reservoir quality due to finer grain size, higher pore-filling clays, and higher amount of ductile grains. Pore-filling clays between 10 and 30% have a deleterious effect on reservoir quality, reducing porosities and permeabilities to generally <10% and <1 mD, respectively. Based on the relatively shallow, present-day burial depths of the studied Forties Sandstone Member (2200–3100 m TVDSS), the percentage of clay-coating coverage to significantly inhibit quartz cementation ranges from 40 to 50%. Detrital, grain-coating smectites, probably inherited from the shelf/continental environments and/or emplaced through sediment dewatering, have transformed into chlorite, illite, and illite-smectite. Calcite and siderite, where well-developed, have arrested mechanical compaction and also occluded porosity, thereby rapidly degrading reservoir quality in the sandstones; however, their dissolution by acidic pore fluids could potentially create secondary intergranular porosity, enhancing reservoir quality of the sandstones. Evidence presented demonstrates that, high quality reservoir sandstones that deviate from normal porosity-depth trends for submarine fans sandstones can be attributed to facies changes (composition and grain size) with a complex interplay of mechanical compaction, detrital clays and authigenic clay coatings inhibiting quartz cement precipitation. • Reservoir quality of submarine fan turbidites is controlled by depositional facies and diagenesis. • Down-fan fining and change in clay content impact significantly on reservoir quality of submarine fan turbidites. • Grain-coating and pore-filling clays play a significant role in reservoir quality control of submarine fan turbidite sandstones. The percentage of clay-coat coverage to inhibit quartz cementation ranges between 40 and 50%. Quartz cementation in turbidite sandstones is inhibited by grain-coating clays such as chlorite and illite/illite-smectite. • Carbonate cements, where well-developed, have affected reservoir quality of the sandstones. [ABSTRACT FROM AUTHOR]