Grain-coating Clays Research Articles

Simulating Compaction and Cementation of Clay Grain Coated Sands in a Modern Marginal Marine Sedimentary System

Reservoir quality prediction in deeply buried reservoirs represents a complex challenge to geoscientists. In sandstones, reservoir quality is determined by the extent of compaction and cementation during burial. During compaction, porosity is lost through the rearrangement and fracture of rigid grains and the deformation of ductile grains. During cementation, porosity is predominantly lost through the growth of quartz cement, although carbonate and clay mineral growth can be locally important. The degree of quartz cementation is influenced by the surface area of quartz available for overgrowth nucleation and thermal history. Clay grain coats can significantly reduce the surface area of quartz available for overgrowth nucleation, preventing extensive cementation. Using a coupled-effect compaction and cementation model, we have forward-modelled porosity evolution of surface sediments from the modern Ravenglass Estuary under different maximum burial conditions, between 2000 and 5000 m depth, to aid the understanding of reservoir quality distribution in a marginal marine setting. Seven sand-dominated sub-depositional environments were subject to five burial models to assess porosity-preservation in sedimentary facies. Under relatively shallow burial conditions (<3000 m), modelled porosity is highest (34 to 36%) in medium to coarse-grained outer-estuary sediments due to moderate sorting and minimal fine-grained matrix material. Fine-grained tidal flat sediments (mixed flats) experience a higher degree of porosity loss due to elevated matrix volumes (20 to 31%). Sediments subjected to deep burial (>4000 m) experience a significant reduction in porosity due to extensive quartz cementation. Porosity is reduced to 1% in outer estuary sediments that lack grain-coating clays. However, in tidal flat sediments with continuous clay grain coats, porosity values of up to 30% are maintained due to quartz cement inhibition. The modelling approach powerfully emphasises the value of collecting quantitative data from modern analogue sedimentary environments to reveal how optimum reservoir quality is not always in the coarsest or cleanest clastic sediments.

Depositional and diagenetic controls on the reservoir quality of marginal marine sandstones: An example from the Early Devonian subbat member, jauf formation, northwest Saudi Arabia

The reservoir quality of marginal marine sandstones can vary significantly across different depositional lithofacies. This variation is primarily due to differences in depositional processes and subsequent diagenetic alterations occurring during various stages of burial. To address this issue, this study focuses on the Early Devonian Subbat sandstones of the Jauf Formation, which serve as an important analogue for subsurface gas reservoirs and have potential for CO2 sequestration. An integrated approach was employed, including field observations, petrography, geochemical analyses, and petrophysical analyses, to investigate the impact of depositional and diagenetic processes on reservoir quality. The findings indicate that the sandstones range from poorly to well sorted, very fine to medium-grained arkosic and quartz arenites that were deposited in a range of environments including shoreface-to-offshore transition zones, wave-dominated deltas, fluvial and tidal estuarine channels. The sandstones experienced both shallow and deep burial diagenesis, while key diagenetic events affecting reservoir quality include compaction, carbonate cementation, the formation of authigenic clays (kaolinite, illite, and chlorite), quartz overgrowth, and the dissolution of unstable feldspar and mica grains. Authigenic pore-filling cements and grain-coating clays significantly influence reservoir quality. The effect of quartz cementation was minimal where illite and/or chlorite coatings are well-developed. Mechanical compaction is more pronounced in the shoreface-to-offshore transition compared to the estuarine channels likely due to the higher detrital matrix content (21.7–35%, avg. 26.7%). The well-sorted fluvial to estuarine channel sandstones exhibit the best reservoir quality, with porosity ranging from 15.9% to 25.5% (average 20.4%) and permeability between 433.8 mD and 1261.2 mD (average 720 mD). This high reservoir quality is attributed to the low detrital matrix content (0–1%, average 0.2%) and limited pore-filling cements. In comparison, the moderately sorted, wave-dominated delta sandstones have lower porosity (5.3–11.2%, average 8.2%) and permeability (17.8–80 mD, average 42.8 mD), despite a relatively higher detrital matrix content (0–11.5%, average 2.8%). The poorly sorted shoreface-to-offshore transition sandstones exhibit the lowest reservoir quality, with porosity ranging from 1% to 2% (average 1.5%) and permeability between 0.3 and 1.1 mD (average 0.7 mD). These variations in reservoir quality among different depositional facies are primarily linked to depositional factors (sorting and detrital clay matrix) and shallow to deep burial diagenetic processes (compaction and cementation). A comprehensive understanding of these processes can enhance reservoir quality prediction in the Jauf Formation and similar reservoirs elsewhere.

Role of depositional environment on clay coat distribution in deeply buried turbidite sandstones: Insights from the Agat field, Norwegian North Sea

The preservation of reservoir quality in deeply buried sandstones largely depends on the presence of grain-coating clays that can inhibit quartz overgrowth and cementation. These grain-coating clays are the focus of today's hydrocarbon exploration as they enable new oil and gas discoveries in sedimentary basins. However, the origin and distribution of these clays in turbidites are poorly documented in scientific literature. Our study addresses this knowledge gap by investigating the mechanisms and parameters governing the clay coat development in turbidite sandstones, with a focus on the Agat Formation (Fm) in the Norwegian North Sea. Petrographic observations, scanning electron microscopy, and X-ray diffraction analyses were used to identify the origin of clay minerals and their distribution, as well as to understand the diagenetic evolution of the sandstone and its impact on reservoir quality. Based on the sedimentary facies description, the Agat Fm comprises four principal members deposited in various depositional environments, ranging from distal lobe fringe to amalgamated proximal lobes and weakly confined channels. Chlorite and kaolinite are the main authigenic clay minerals in these sandstones, associated with a variable amount of inherited detrital clays consisting in Fe-bearing kaolinite and mixed-layer minerals composed of illite and hydroxyl-interlayered clay minerals formed in acidic soil environments. Well-developed chlorite coats are associated with the highest reservoir quality (φ > 10%), while discontinuous chlorite coats are linked to extensive quartz cementation and poor reservoir quality (φ < 5%). Samples with abundant pore-filling chlorite also exhibit poor reservoir quality (φ < 7%). Proximal lobe deposits and weakly confined channels show well-developed chlorite coats, whereas discontinuous chlorite coats are mainly found in proximal lobe deposits associated with high velocity escaping fluids during dewatering. Pore-filling chlorites are typically present in distal lobe fringes and levee deposits. Chlorite in the Agat Fm is mainly derived from the replacement of inherited detrital clays via an intermediate berthierine precursor forming during early diagenesis. The dissolution of Fe-rich grains such as biotite and ferric detrital clays under reducing conditions governed the kinetics of berthierine/chlorite growth. The late diagenesis is characterized by extensive feldspar dissolution in a closed diagenetic system, leading to the precipitation of kaolinite and quartz cement. The presence of chlorite coatings around detrital grains preserved the porosity from extensive quartz cementation during deep burial, highlighting the role of chlorite in maintaining the quality of deeply buried turbidite reservoirs.

The importance of facies, grain size and clay content in controlling fluvial reservoir quality – an example from the Triassic Skagerrak Formation, Central North Sea, UK

Clay-coated grains play an important role in preserving reservoir quality in high-pressure, high-temperature (HPHT) sandstone reservoirs. Previous studies have shown that the completeness of coverage of clay coats effectively inhibits quartz cementation. However, the main factors controlling the extent of coverage remain controversial. This research sheds light on the influence of different depositional processes and hydrodynamics on clay-coat coverage and reservoir quality evolution. Detailed petrographic analysis of core samples from the Triassic fluvial Skagerrak Formation, Central North Sea, identified that channel facies offer the best reservoir quality; however, this varies as a function of depositional energy, grain size and clay content. Due to their coarser grain size and lower clay content, high-energy channel sandstones have higher permeabilities (100–1150 mD) than low-energy channel sandstones (&lt;100 mD). Porosity is preserved due to grain-coating clays, with clay-coat coverage correlating with grain size, clay-coat volume and quartz cement. Higher coverage (70–98%) occurs in finer-grained, low-energy channel sandstones. In contrast, lower coverage (&lt;50%) occurs in coarser-grained, high-energy channel sandstones. Quartz cement modelling showed a clear correlation between available quartz surface area and quartz cement volume. Although high-energy channel sandstones have better reservoir quality, they present moderate quartz overgrowths due to lesser coat coverage, and are thus prone to allowing further quartz cementation and porosity loss in ultra-deep HPHT settings. Conversely, low-energy channel sandstones containing moderate amounts of clay occurring as clay coats are more likely to preserve porosity in ultra-deep HPHT settings and form viable reservoirs for exploration. Supplementary material: of data and technique used in this study are available at https://doi.org/10.6084/m9.figshare.c.6438450

Role of diagenetic alterations on porosity evolution in the cretaceous (Albian-Aptian) Bima Sandstone, a case study from the Northern Benue Trough, NE Nigeria

The Cretaceous Bima Sandstone is a potential hydrocarbon reservoir that is currently being explored in the Northern Benue Trough, NE Nigeria. The sedimentological characteristics and tectonic evolution of the formation have been extensively studied by previous researchers. However, the impact of diagenetic alterations on porosity evolution of the sandstone is poorly understood. This study employed a multi-technique approach, including thin-section petrography, SEM, XRD, and cathodoluminescence analyses to investigate the role of framework composition, primary depositional characteristics, and diagenetic alterations on porosity evolution of the sandstones. The results of the study indicate that the Bima Sandstone has a present-day intergranular porosity ranging from 0 to 25.9%, averaging 5.8%. Diagenetic alterations, including mechanical and chemical compaction and quartz cementation, have significantly modified the porosity of the sandstones more than the primary depositional features (grain size and sorting). Porosity reduction was mainly due to mechanical compaction (av. 32.1%) than cementation (av. 7.4%). Quartz overgrowths (up to 15%) are the most abundant cement in the sandstones, formed primarily due to lack of grain-coating clays. The main eogenetic alterations related to shallow burial include dissolution and kaolinitization of feldspars and, to some extent, mechanical infiltration of smectitic clays, which formed largely discontinuous clay coatings and pore-filling clays. Mesogenetic alterations related to deep-burial include the formation of quartz overgrowths, transformation of kaolinite and smectite into illite, and pervasive albitization of both plagioclase and K-feldspar, which is exclusively associated with the deepest, Lower Bima Sandstone Member, and has significantly reduced the secondary porosity in the sandstones. Telogenetic alterations related to uplift, mainly include dissolution and kaolinitization of unstable feldspars, which has significantly increased the present-day, secondary porosity of the sandstones to up to 15%. Pore-filling clays (chlorite, illite, and kaolinite) have had a small impact on the intergranular porosity of the sandstones, mainly because of their low volume. The study provides insights into how diagenesis can significantly affect the reservoir porosity of the sandstones and could also assist in reducing the risk of exploration of the fluvial reservoir in the basin and elsewhere.

Impact of Grain-Coating Clays on Porosity Preservation in Paleocene Turbidite Channel Sandstones: Nelson Oil Field, UK Central North Sea

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 &gt;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 (&gt;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.

Role played by clay content in controlling reservoir quality of submarine fan system, Forties Sandstone Member, Central Graben, North Sea

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.

Diagenetic alterations related to sedimentary architecture of deltaic distributary channels in red beds of the Cretaceous Yaojia Formation, Songliao Basin

Fine-scale diagenetic alterations linked to architectural patterns of the sandstones were studied in meter-scale deltaic distributary channels of the Upper Cretaceous Yaojia Formation, Songliao Basin. The deltaic reservoirs of the Yaojia Formation are mostly developed in three types of distributary channels: major distributary channels, crevasse channels, and terminal distributary channels. The three types of distributary channels have different sedimentary architecture and lithofacies. The distribution of reservoir intervals varies greatly in the upward fining distributary channel depositional cycles (meter-scale). From the diagenetic alterations in the distributary channels successions, compaction, calcareous cementation, grain-coating clays with hematite in the red bed sandstones, and dissolution of feldspar or rock fragments are main control factors of high-quality reservoirs. More porosity loss is generated by strong compaction near the top and bottom part of the distributary channel succession. This could be caused by the high clay content and mud clasts which contribute much to the porosity loss in the sandstones. Calcareous cementation is mainly developed near the clay-rich intercalations. Dissolution pores in the sandstones without hematite cement and grain-coating clays are well developed in the middle position of the successions. The cementation is restricted in sandstones with unique hematite cement and grain-coating clays in the reddish crevasse distributary channels. The distribution pattern of high-quality reservoirs within the three different types of distributary channels in red beds is proposed by linking the diagenetic processes to depositional environments and architecture. High-quality hydrocarbon reservoirs are mostly preserved in the massive and cross-bedded sandstones from the middle part of the major distributary channel successions.

Origin and control of grain-coating clays on the development of quartz overgrowths: example from the lower Paleozoic Barik Formation sandstones, Huqf region, Oman

Understanding the origin of grain-coating clays and its effects on the inhibition of cementation by quartz overgrowths plays a crucial role in porosity preservation versus destruction in deeply buried sandstones. This study aims to decipher the origin of grain-coating clays and their effect on the quartz overgrowth cementation of the mouth-bar/shoreface and tidal channel sandstones of the lower Paleozoic Barik Formation in the Huqf region of Central Oman. The study is based on petrographical and mineralogical investigations using the conventional optical microscope, scanning electron microscope (SEM) equipped with a dispersed energy spectrometer (EDS), and X-ray diffraction (XRD). The results reveal that the grain-coating clays are predominantly mesogenetic-transformed illite and, to some extent, chlorite. The illitic clays occur as 2–5-μm-thick rims covering the entire detrital grain surface, as well as at grain-to-grain contacts. The illitic clays display honeycomb and cornflake textures with filamentous termination. The chloritic clays occur as 1–3-μm-thick rims, apparently covering the entire detrital grains and at grain-to-grain contact; they display honeycomb-like and platelet morphologies. Such textural morphologies demonstrate the development of illite and chlorite through a transformation process from smectitic clay precursors. The smectitic clay precursors are typically near-surface clay fractions that infiltrated mechanically into the mouth-bar/shoreface and tidal channel facies association sandstones through the tidal hydrodynamic pumps. The predominant occurrence of illite over chlorite is attributed to the availability of potassium ions that are sourced through the extensive dissolution of detrital potassium feldspar grains. The illitic and chloritic clay rims covering the entire detrital grains effectively inhibited the growth of quartz cement by limiting the nucleation sites on the detrital grains. This study may also serve as an analogue model for a similar depositional environment to better understand grain-coating clays’ origin and its role in controlling cementation by quartz overgrowths in deeply buried sandstone reservoirs globally.

Abrasion of detrital grain-coating clays during sediment transport: Implications for diagenetic clay coats

Detrital grain-coating clay minerals have been identified as precursors of diagenetic grain-coating clay minerals that inhibit quartz cementation and preserve good reservoir quality properties in deeply buried sandstone reservoirs. Between initial and final deposition, detrital grain coats are likely to be affected by sediment transport processes. To date, it has not been demonstrated that detrital grain-coating clays are able to survive sediment transport. Flume experiments simulating open-channel flow conditions were performed using sediments from the Ravenglass estuary (Cumbria, UK), where the distribution and characteristics of modern detrital grain-coating clays is well established. Scanning electron microscopy and automated mineralogy techniques were employed to study the sediment before and after transport to determine the stability of the detrital grain-coating clays. The experimental results show that detrital grain-coating clays are still present after sediment transport in turbulent flows with velocities up to 0.5 m s−1. The stability of the detrital grain-coating clays was enhanced by (1) high initial clay coat coverage, (2) turbulence characteristics of the flow, and (3) entrapment of the detrital grain-coating clays in bedforms. This work documents the first quantifiable evidence that estuary-derived, detrital clay coats are routinely preserved after sediment transport. Clay coat-rich estuarine sediments can therefore act as the source of clay coated grains found in cleaner estuarine sandstones and may even survive transport out into open marine settings.

Can carbonate cementation be inhibited in continental red bed sandstone?

The Lower Cretaceous Bashijiqike Formation in Kuqa depression belongs to a continental red bed sandstone with poor reservoir quality, and is deeply buried to 5.5–7.0 km but some successions can host porosity >10% and permeability larger than 10 mD. Carbonate cements are important pore-filling constituents, and they control the reservoir quality and heterogeneity of Bashijiqike sandstones. Red hematite-bearing grain-coating clay minerals inhibit the pore occluding quartz cements and preserve intergranular pores. Here we investigate the ability of grain-coating clays to inhibit carbonate cements. Thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), and cathode luminescence (CL) analysis were performed to investigate the petrography, pore systems, diagenesis and diagenetic minerals of the red bed sandstones. The results show the hematite-bearing grain coatings give the ‘‘red bed’’ colors of the sandstones. The sandstones are heavily mechanically compacted, and the predominant pore-filling cements include carbonates and clay minerals, while quartz cements are volumetrically not important in reducing porosity. The grain-coating clays are mainly hematite-bearing mixed-layer illite and smectite. The paragenetic sequences of various types and generations of diagenetic events were reconstructed. Sandstones containing effective mixed-layer illite and smectite coatings, in which the intergranular pores can be preserved, have high porosities. Carbonate cement is never found in sandstones with effective grain coating clays, and carbonate cement precipitates where there are no sufficiently thick and continuous grain coats. The clay coatings retard or inhibit eogenetic calcite cements and progressively restrict late-stage carbonate (dolomite) cementation during burial. The hematite-bearing grain-coating clays help the red bed sandstones retain good reservoir quality at great depths.

Diagenetic and geochemical studies of the Buchan Formation (Upper Devonian) in the Central North Sea

The Upper Devonian Buchan Formation reservoirs in the UK Central North Sea are litharenite/sublitharenite and were deposited in fluvial–aeolian settings. The grain-coating clays in the aeolian sandstones have effectively inhibited quartz overgrowth. Hence, the reduction of reservoir quality is mainly due to mechanical compaction and early dolomite precipitation in both fluvial and aeolian sandstones; quartz overgrowth and kaolinite illitization in fluvial sandstones; and limited smectite illitization in aeolian sandstones. The carbon/oxygen stable isotopes of dolomite cements suggest a predominantly marine carbon source and precipitation temperatures between 25 and 58 °C indicating a shallow burial depth during dolomite precipitation. The temperatures and the dolomite distribution indicate that the cements originated from the overlying Upper Permian Zechstein carbonates. Extensive quartz overgrowths formed at 80 and 120 °C in the late and deep diagenetic burial history. The most probable silica source was from feldspar kaolinitization and pressure dissolution of quartz grains. Through detailed petrography and geochemical analyses, the burial–paragenesis–thermal history of the Buchan Formation has been constructed. Similar diagenetic processes are likely to have occurred in the Buchan Formation in other parts of the Central and Northern North Sea. This study may allow new petroleum plays to be considered in areas previously thought to have poor hydrocarbon potential.

Deep burial diagenesis and reservoir quality evolution of high-temperature, high-pressure sandstones: Examples from Lower Cretaceous Bashijiqike Formation in Keshen area, Kuqa depression, Tarim basin of China

The deep high-temperature, high-pressure Lower Cretaceous Bashijiqike sandstone (buried to depths as great as 6.5-7.1km) is an important natural gas reservoir in Keshen gas field, Kuqa depression of the Tarim Basin. Reservoir quality is a critical risk factor in the development of these ultradeep reservoirs. Integrated approaches incorporating routine core analyses, mineralogical, petrographic, and geochemical analyses have been used to investigate the diagenetic history of these rocks and their impact on reservoir quality with the aim to unravel the mechanisms for maintaining anomalously high porosities in sandstones that are buried to such a great depth. These sandstones are dominantly fine to medium-grained, moderately to good sorted lithic arkoses and feldspathic litharenite. Most primary pores have been lost by mechanical compaction or carbonate cementation, and the reduction of porosity by mechanical compaction was more significant than by cementation. Dissolution of framework grains contributed to the enhancement of reservoir quality. Eogenetic diagenetic alterations mainly include mechanical compaction, precipitation of calcite cements and grain-coating clays; mesogenetic diagenesis is characterized by dissolution of framework grain by organic acids and subsequent precipitation of clay minerals and quartz; infiltration of meteoric water related to teleodiagenesis would result in dissolution of the framework grains. The meteoric leaching events during teleodiagenesis are of great importance for the Bashijiqike sandstones. Grain-coating clay minerals (mixed-layer illite/smectite clays) help to preserve porosity at depth by retarding quartz cementation and pressure solution. The unique burial regime as early-stage shallow burial with late-stage rapid deep burial contributes to porosity preservation in eodiagenesis. Fluid overpressure caused by intense structural compression in the middle Himalayan movement retarded compaction and helped preserve porosity in the late rapid deep burial stage. Anomalously high porosities are mainly found in medium-grained, good sorted sandstones with grain-coating clays, but with low clay and carbonate cement content, of which the porosity is preserved primarily and enhanced secondarily. The lowest porosities are associated with sandstones that are tightly compacted or cemented with carbonates or rich in detrital matrix. Porosity-depth trends may vary significantly with lithofacies due to its differences in textural and compositional attributes. Five lithofacies are defined in terms of detrital composition and texture, type and degree of diagenesis. The reservoir quality prediction models of various lithofacies are constructed, and the results of this study provide insights into mechanisms for maintaining anomalously high porosity and permeability in high-temperature, high-pressure sandstone reservoirs, and may help explain hydrocarbon distribution.

Depositional and diagenetic controls on anomalously high porosity within a deeply buried CO2 storage reservoir—The Cambrian Mt. Simon Sandstone, Illinois Basin, USA

Current diagenetic models predict that sandstone porosity and reservoir quality decrease with burial depth and formation age. However, the opposite trend is true for the Cambrian Mt. Simon Sandstone. The lowermost section of the Mt. Simon is highly porous—it has an average log-derived effective porosity of 17.0%. To understand the possible reason(s) for this anomaly, the amount of porosity reduction due to compaction porosity loss (COPL) and cementation porosity loss (CEPL) was analyzed. The COPL and CEPL were coupled with sedimentary characteristics in order to ascertain which factors controlled porosity and reservoir quality in the Mt. Simon. The underlying Argenta sedimentary unit disconformably underlies the Mt. Simon and is included for diagenetic and depositional comparison.Similar to expected trends from other deep basin reservoirs, the COPL generally increases with depth in the Mt. Simon. The CEPL is significantly lower in the lower half of the Mt. Simon compared to the upper half of the Mt. Simon, indicating that higher porosity is attributed to having a lower volume of intergranular cement. Early episodes of diagenesis within the arkosic lower half of the Mt. Simon, such as clay infiltration and feldspar alteration, produced diagenetic grain-coating clays, which inhibited the development of quartz cements. Thus, these grain coatings assisted in porosity preservation within the Lower Mt. Simon by reducing CEPL. The upper half of the Mt. Simon is more heavily cemented as a result of changes in the depositional setting and thus changes in the lithology. Later, largely post-compaction feldspar dissolution further increased porosity in the Lower Mt. Simon Sandstone.

Reservoir sedimentology and depositional environment studies of Alam El Bueib Formation using microfacies and nannofossils in Betty-1 well, Shoushan Basin, northern Western Desert, Egypt

The reservoir sedimentology and depositional environment of the Lower Cretaceous Alam El Bueib Formation in the Betty-1 well, Shoushan Basin, were investigated by studying lithofacies, petrography, and calcareous nannofossils. The sedimentary lithofacies indicate a fluvial to shallow-marine depositional environment. We have lithologically identified and described five lithofacies assemblages (massive-sandstone facies; cherty massive-sandstone facies; argillaceous-sandstone facies; heterolithic, laminated sandstone/shale facies; and sandy/silty–shale facies); we have petrographically identified and described seven microfacies (laminated claystone and siltstone; ferruginous quartz–arenite; feldspathic ferruginous quartz–wacke; quartz–arenite; anhydritic quartz–arenite; biomicrite; and sandy-limestone microfacies). Calcareous nannofossils were used to determine the age of the investigated deposits. The calcareous-nannofossil species led to the recognition of two nannofossil zones of the Early Cretaceous (Nannoconus bermudezi zone of the Hauterivian and Nannoconus colomi zone of the Barremian). The studied sandstone reservoirs can be classified as compositionally immature feldspathic arenite and wacke. The main diagenetic minerals of the sandstones include authigenetic clay minerals, calcite cement, quartz overgrowth, and later ferroan carbonate. Wide porosity variations in sandstones correlate with an abundance of grain-coating clays and consequent inhibition of quartz cementation. Secondary porosity has been created mainly by feldspar, rock-fragment dissolution, and clay-matrix dissolution.

Distribution of Diagenetic Alterations in Fluvial Channel and Floodplain Deposits in the Triassic Narrabeen Group, Southern Sydney Basin, Australia

Abstract In the Narrabeen Group, floodplain shale and siltstone are abundant in the Wombarra Claystone, Stanwell Park Claystone and Bald Hill Claystone whereas the Newport Formation is a fine-grained lagoonal deposit. Medium-and coarse-grained fluvial channel sandstone is common in the Coalcliff, Scarborough and Bulgo. Sandstone composition includes mostly lithic grains with minor quartz, feldspar, mica and heavy minerals; litharenites and sublitharenites are common and quartzarenites are rare. The diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement and authigenic feldspar. Quartz overgrowths are present in the channel, floodplain, lagoonal and shallow marine facies but are more common in the channel facies. Also, the channell facies are characterised by pore-filling carbonate cement, grain-coating carbonate cement and carbonate replacement of grains. Carbonate cement, pore-filling clays and grain-coating clays occur in all the channel, floodplain, lagoonal and shallow marine facies. Fluvial channel and floodplain facies contain feldspar overgrowths which disappear in the lagoonal and shallow marine facies. Dissolution of unstable grains is observed in the channel, floodplain and lagoonal facies but is more abundant in the channel facies in the Narrabeen Group. Mechanical and chemical compaction is conspicuous in the channel facies but also occurs in the floodplain and lagoonal facies.

Prediction of lithofacies and reservoir quality using well logs, Late Cretaceous Williams Fork Formation, Mamm Creek field, Piceance Basin, Colorado

Understanding the controls and distribution of reservoir quality is important for the economic success of tight-gas reservoirs in which diagenesis interacts with primary depositional variations in environment and texture to exert a strong control on pore networks, rock mechanical properties, and natural fractures. In the Upper Cretaceous Williams Fork Formation of the Piceance Basin, framework grain composition is a major control on compaction and the occurrence of authigenic phases. Alteration of volcanic grains in the upper Williams Fork led to grain-coating clay precipitation. Ferroan dolomite cement is found only in the deeper marine-influenced intervals in which dolostone fragments are present. This study shows that careful petrographic assessment of lithofacies heterogeneity can be upscaled by correlation with log properties to yield tools for field-scale reservoir quality prediction. Twelve lithofacies are identified based on sand-grain populations, cement types, and clay matrix content. Sandstones of the highest reservoir quality are those with grain-coating clays that inhibit quartz cementation; these sandstones can be identified based on high-density porosity log values. Sandstones with the poorest reservoir qualities are tightly cemented with carbonate and quartz cement or are rich in clay matrix. Carbonate-cemented intervals are identified by low-density porosity. Clay matrix–rich samples have high gamma-ray and low-density porosity values. The presence of abundant potassium feldspar in the upper intervals results in high gamma-ray readings even in the clean (clay matrix–free) sandstone.

&lt;b&gt;Contrast in clay mineralogy and their effect on reservoir properties in sandstone formations&lt;/b&gt;

Adigrat sandstone formation in the Blue Nile Basin is dominated by quartz arenite and subarkosic arenite, and cemented by carbonate, clay minerals and quartz overgrowths. Clay minerals in the Adigrat sandstone formation are dominated by kaolinite, illite and chlorite. Illite is the common grain-coating clay mineral. The common pore-filling minerals are kaolinite, illite and chlorite. The formation reaches a depth of 3km, which facilitated the change of kaolinite to illite. Clay minerals are the major reducer of porosity andpermeability in the Adigrat sandstone formation. The Permo-Triassic sandstones in the Ulster Basin were deposited in NE-SW trending fault-bounded asymmetrical graben. The Permo-Triassic sandstones reaches a burial depth of 1/2 km in the marginal areas and 4.5 km in the basin center. The basin underwent multiple episodes of uplift and erosion in the Upper Jurassic, and Mid-Cretaceous, and uplift and inversion in the Upper Tertiary. The difference of the clay mineralogy in the Permo-Triassic in the Ulster Basin depends upon the depositional environments, source area geology and burial history of the basin. Illite-smectite is grain coating, whereas smectite, illite-smectite, smectite-chlorite, chlorite,illite and kaolinite are the common pore-filling clay minerals in the Permo-Triassic sandstones in the Ulster Basin. Clay minerals reduced the porosity and permeability of the Permo-Triassic sandstones. KEY WORDS: Adigrat sandstone, Authigenic mineralization, Clay minerals, Diagenesis, Permo-Triassic sandstones, Reservoir characteristics Bull. Chem. Soc. Ethiop. 2008, 22(1), 41-65.

ILLITE/SMECTITE CLAYS PRESERVING POROSITY AT DEPTH IN LOWER PERMIAN RESERVOIRS, NORTHERN PERTH BASIN

The appraisal well Hovea–2, drilled in July 2002, was the first well in the onshore Perth Basin to discover commercial volumes of gas in sandstone reservoirs of the Artinskian (Early Permian) High Cliff Sandstone sealed beneath basal shales and siltstones of the Irwin River Coal Measures. A drill stem test in this formation, in the interval 2,370–419 mMDRT (measured depth below rotary table), flowed gas to surface at 16.5 MMcfd. Thereafter, the High Cliff Sandstone became an important play for exploration in the basin; however, no additional discoveries have been made in this reservoir, even though it has been tested in another seven wells (Jingemia–1, Eremia–1, Kunzia–1, Corybas–1, Yardarino–6, Hakia–2 and Bunjong–1).In Corybas–1, drilled in 2005, this reservoir was found to be water bearing, but an 18 m core was taken within the topmost High Cliff Sandstone. This core provided important insights into the fluvial depositional environment of these sands. Importantly, porosity and permeability are highly variable within the reservoir, with some coarser grained beds retaining anomalously high porosity and permeability due to the coating of constituent quartz grains by illite/smectite that inhibited late stage quartz overgrowth. Study of this core along with core from overlying Irwin River Coal Measures indicates that there may be some facies control on the presence of the grain-coating clays in the High Cliff Sandstone, and that the fluvial to tidal facies are more likely to be an effective petroleum reservoir than the overlying fluvial/alluvial facies. This reservoir is the primary objective in the upcoming exploration wells Beharra Springs Deep–1 and Drakea–1, to be drilled in 2007.

Distribution of diagenetic alterations in fluvial and paralic deposits within sequence stratigraphic framework: Evidence from the Petrohan Terrigenous Group and the Svidol Formation, Lower Triassic, NW Bulgaria

Elucidation of diagenetic alterations in the Petrohan Terrigenous Group (fluvial; highstand systems tract HST) sandstones and Svidol Formation (tide-dominated deltaic and tidal flat, transgressive systems tract TST and highstand systems tract HST, respectively) sandstones and calcarenite, Lower Triassic, NW Bulgaria was constrained within a sequence stratigraphic framework. Eogenetic alterations in the fluvial HST sandstones include (i) formation of grain-coating infiltrated clays as a result of percolation of mud-rich surface waters into underlying coarse-grained and permeable channel-fills and crevasse splay sandstones; (ii) formation of pseudomatrix by mechanical compaction of mud intraclasts that were incorporated into the coarse-grained channel sandstones during their lateral avulsion; and (iii) cementation by calcite (δ 18O VPDB = − 6.5‰ to − 3‰; δ 13C VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ 18O VPDB = − 6.1‰ to − 0.3‰; δ 13C VPDB = − 7.2‰ to − 5.8‰) in the crevasse splay and floodplain sediments. Mesogenetic alterations that are encountered in the fluvial HST sandstones include (i) illitization of grain-coating clays, mud intraclasts, and mica, possibly because of simultaneous albitization of feldspars; (ii) cementation by calcite (δ 18O VPDB = − 14.5‰ to − 8.4‰; δ 13C VPDB = − 7.7‰ to + 0.6‰) and dolomite (δ 18O VPDB = − 15.8‰ to − 5‰; δ 13C VPDB = − 7.9‰ to + 1.5‰); and (iii) limited amounts of quartz overgrowths in the channel sandstones owing to occurrence of thick grain-coating clays. Conversely, the tide-dominated deltaic TST sandstones and the tidal flat HST calcarenite were pervasively cemented by calcite (δ 18O VPDB = − 6.6‰ to − 3.1‰; δ 13C VPDB = − 5.1‰ to + 0.6‰) and siderite (δ 18O VPDB = − 7.2‰ to − 5.7‰; δ 13C VPDB = + 0.3‰ to + 0.9‰) particularly below marine and maximum flooding surfaces, due to the presence of abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions along these surfaces. The remaining open pores were cemented during mesodiagenesis by calcite (δ 18O VPDB = − 6.6‰ to − 3.1‰ and δ 13C VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ 18O VPDB = − 6.6‰ to − 3.1‰ and δ 13C VPDB = − 5.1‰ to + 0.6‰). This study shows that constructing a conceptual model for the distribution of diagenetic alterations is possible by integration of diagenesis with sequence stratigraphy. The model shows that tide-dominated deltaic TST sandstones and tidal flat HST calcarenite were pervasively cemented by carbonates during near-surface eodiagenesis, owing to the presence of abundant bioclasts. Conversely, fluvial LST sandstones remained poorly cemented during near-surface eodiagenesis due to the lack of bioclasts, but were cemented by mesogenetic calcite, dolomite and quartz overgrowths instead.