Reservoir Quality Of Sandstones Research Articles

Diagenetic history of the Miocene Surma Group sandstones from the Eastern Fold Belt of the Bengal Basin

The Eastern Fold Belt (EFB) of the Bengal Basin remains an under-explored petroleum province in Bangladesh. Interbedded sandstones and shales of the Miocene Surma Group are thought to host hydrocarbon accumulations in EFB. The diagenetic history and their controls on sandstone reservoir quality and distribution are largely unknown in this region. Therefore, this paper aims to improve the understanding of diagenetic processes and their impacts on reservoir quality in the Surma Group sandstones. Thin section petrography, Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis have all been used to this end. The results of this study show that the sandstones are very fine- to medium- grained, moderately to moderately well-sorted, matrix- rich and dominantly sublitharenites. The principle diagenetic minerals/cements are calcite, authigenic chlorite, illite/illite–smectite, minor quartz cement, and minor kaolinite. Poikilotopic calcite cement drastically reduces the porosity and permeability down to 0%. Authigenic chlorite rims along quartz grain prevent quartz overgrowth, which acts to preserve porosity. The primary porosity (0–22%, average 9%) is by far the most dominant porosity in the Miocene Surma Group reservoirs, with burial enhanced secondary porosity caused by dissolution. Reservoir quality is mainly controlled by mechanical and ductile grain compaction and calcite cement.

Grain-Size Analysis of Ancient Deep-Marine Sediments Using Laser Diffraction

Grain-size analysis of siliciclastic sedimentary rocks provides critical information for interpreting flow dynamics and depositional environments in sedimentary systems and for analysing reservoir quality of sandstone. Methods such as sieving and thin-section analysis are time consuming and unsuited for large sample numbers. Laser diffraction particle analysis is quick and reliable for analysing 100s of samples, assuming successful disaggregation. Here, we evaluate this method utilizing samples from three siliciclastic formations in Northern Italy: the Miocene Castagnola and Marnoso-Arenacea Formations, and the Cretaceous to Palaeocene Gottero Formation, which vary in degree of lithification. We focus on: 1) methods of whole-rock disaggregation; 2) methods of subsampling sediment for laser diffraction analysis; and 3) comparison of thin-section analysis with laser-diffraction particle size analysis. Using an ultrasonic bath and a SELFRAG (high voltage selective fragmentation) as disaggregation tools, this study evaluates separation of whole, undamaged grains subsequently measured by laser diffraction analysis. We show that it is possible to disaggregate ancient, well cemented rocks using an ultrasonic bath. When disaggregating samples with the SELFRAG method, grain-size measurements become less accurate and less precise with increasing sample lithification and increased presence of cement. This is likely a combination of incomplete grain disaggregation in the SELFRAG and heterogeneity within samples. Following disaggregation, we compare sub-sampling methods using a stirrer plate versus a pipette. Both produce accurate analyses, but the stirrer method is the most reliable and replicable. A comparative small subsample method, run as one whole sample with no need for subdivision into aliquots, is found to be reliable and replicable but is more susceptible to heterogeneity within field samples. When comparing laser diffraction results to grain-size volume methods estimated from thin-section analysis, thin-section sand grains are overestimated, and clay/silt grains are inaccurate. These results provide a framework for understanding potential biases introduced through various sample preparation and measurement methods.

Diagenetic evolution of the lower Yaojia Formation of Songliao Basin, China: Impact on reservoir quality

The oil-bearing layer in the first member of the Upper Cretaceous Yaojia Formation (K2y1) in the middle of the Changling Sag of the southern Songliao Basin, northeast China, has low-permeability and is deemed to be an unconventional reservoir. Therefore, better evaluation of reservoir quality and related controlling factors are crucial for the reduction in exploration risk and sustainable development. The current study integrates diagenesis, sequence stratigraphy, and sedimentary facies to decipher their influence on reservoir quality using core and thin section observations, X-ray diffraction, scanning electron microscopy, fluid inclusion, and carbon and oxygen isotope analyses. The K2y1 is composed of shallow delta sediments of transgressive system (TST) and lowstand system (LST) tracts. The depositional microfacies of the sandstones include TST deltaic front underwater distributary channels and sheet sands, and LST deltaic plain distributary channels, distributary channel intersections, and crevasse splays. The sandstones are dominantly fine-grained moderately sorted feldspathic litharenites and lithic arkoses with silt. Early diagenetic events included compaction, development of smectite, and alteration of feldspar and kaolinite whereas mesogenetic alteration included feldspar dissolution, precipitation of quartz and carbonate cements, and clay mineral alteration. The reservoir quality of the K2y1 sandstones is generally poor, exhibiting variations among different deltaic depositional facies within the LST and TST. The (underwater) distributary channel (intersection) sandstones, particularly those of the LST, have relatively higher reservoir quality owing to their larger grain size, better sorting, lower carbonate cement content, mixed-layer I/S and illite, and greater occurrence of chlorite.

Effect of petrography and diagenesis on the sandstone reservoir quality: a case study of the Middle Miocene Kareem Formation in the North Geisum oil field, Gulf of Suez, Egypt

Kareem Formation is one of the most important hydrocarbon reservoirs in the Gulf of Suez. It contributes significantly to the total Egyptian oil production; therefore, the current work is to emphasise the influence of siliclastic facies types and their diagenetic processes on the reservoir quality of the Kareem Formation. A comprehensive log and core analyses were used to determine both the petrophysical and petrographical characteristics. Three classic facies have been identified: two sandstone facies with reservoir potential and one shale facies. Facies I is coarse-grained sandstone with high-angle cross-bedding, distributary/braided channel deposits on the active part of a mid-submarine fan lobe and is represented by beds of (Shagar Member). Facies II is fine-grained sandy beds represented by low-angle crudely, cross-bedded sandstone encountered in the Rahmi Member. Facies III is mainly shale and recorded in the Rahmi clastic zone. Petrographically, the quartz arenite microfacies are dominant in the Shagar sand zone and show the highest potential zones in the Kareem Formation. On the other hand, quartz lithic arenite and calcareous lithic arenite constitute the major microfacies in the Rahmi clastic zone and show the lowest reservoir quality. The main diagenetic processes that enhance the sandstone reservoir intervals of the Kareem Formation are the microfracturing of mineral grains and the dissolution of both feldspar and carbonate cement. In contrast, the amount of cementing materials, particularly authigenic carbonate and clay cements, led to diminishing of the reservoir quality of the studied Kareem Formation.

Experimental Simulation of Burial Diagenesis and Subsequent 2D-3D Characterization of Sandstone Reservoir Quality

Characterization of deeply buried sandstones and their reservoir quality is of paramount importance for exploring, developing, and subsurface storage of energy resources. High reservoir quality in deeply buried sandstones is commonly correlated with the occurrence of grain coatings that inhibit quartz cementation. The development of reliable models that can predict reservoir quality relies on incorporating quantitative understanding of these diagenetic processes. Hydrothermal experiments simulating burial diagenesis were integrated with multi-scale X-ray tomography to quantify the 3-dimensional evolution of grain coating volume and porosity with increasing temperature; while microscopic and automated quantitative mineralogy analysis were used to track the associated mineralogical alterations. To simulate reservoir evolution, sandstone samples from the Lower Jurassic Cook Formation (Oseberg Field, 30/6-17R, Norway) were exposed to a silica supersaturated Na2CO3 (0.1 M) solution for up to 360 h at temperatures of 100–250°C. The experimental results show the main porosity and permeability reduction window is associated with pore-filling kaolinite, and lies between 150 and 200°C, above which little change occurs. Volumetric increases in grain coating start to occur at ∼150°C through precipitation of authigenic chlorite, and continue to 250°C, irrespective of the experimental duration. Together with preexisting siderite coatings, the newly precipitated chlorite prevents the loss of reservoir quality by inhibiting quartz overgrowth development. Pore flow simulations based on the observed temperature-dependent 3-dimensional pore networks allow us to characterize pore-throat and permeability evolution and gain quantitative understanding of the impact of diagenetic overprinting on deeply buried sandstone reservoirs.

The genesis of authigenic minerals and the porosity evolution of various lithologies and their implications for identifying high-quality reservoirs in the fourth member of Xujiahe Formation (Northeast-Sichuan Basin, China)

With petroleum exploration extending to deeper reservoirs, it becomes more and more difficult to find relatively high-quality reservoirs in heterogeneous sandstones. This research integrates fluid inclusion analysis, petrographic, mineralogical, and geochemical data to determine the origin of authigenic minerals, establish the porosity evolution model and discuss diagenetic effects on reservoir quality of the fourth member of the Upper Triassic Xujiahe Formation. The main rock types of studied sandstones are identified by thin-section, SEM and XRD analysis-feldspathic litharenites, litharenites, calcareous sandstone and chlorite-rich sandstone. Of the four types of sandstone, reservoir porosity of the chlorite-rich sandstone is generally the highest. Target sandstones have experienced complicated diagenetic histories including mechanical compaction, three stages of carbonate cementation, the dissolution of K-feldspar or rock fragments, three kinds of authigenic chlorite precipitation, and two stages of quartz precipitation. Two reaction pathways between K-feldspar and kaolinite were determined and limited dissolution was generated in the closed diagenetic system. Diagenetic sequence and porosity evolution suggest that the critical factors in reducing reservoir quality can be attributed to early intense compaction and widespread calcite cementation. The genesis of high-quality reservoirs among the various lithologies is different. In feldspathic litharenites and litharenites, the key factors for the development of relatively high-quality reservoirs is the acidic dissolution of K-feldspar and less cement being present in the rock. Because chlorite helps retain primary pores by inhibiting quartz overgrowths, almost all chlorite-rich sandstone are relatively high-quality reservoirs. In general, the reservoir quality of calcareous sandstone is poor unless fractures exist that can improve permeability. Determining the porosity evolution of various lithologies is an effective method to predict relatively high-quality reservoirs, which can provide a useful reference for deep petroleum exploration in other areas.

Occurrence and origin of chlorite and associated impact on tight sandstone reservoir quality: A case study of the Xujiahe Formation (NE Sichuan Basin, China)

Chlorite is beneficial to the preservation of primary pores, and its formation mechanism in marine sandstone has been widely reported. However, few studies have focused on the genetic mechanism of various types of chlorite in lacustrine deltaic sandstone. Therefore, the occurrence, diagenetic time and origin of chlorite in Xujiahe tight sandstone were investigated using microscopic images and mineralogical chemical analysis, and three stages of chlorite were identified, namely, grain-coating (Ch1), pore-lining (Ch2), and pore-filling (Ch3) chlorite, which belong to the Fe chlorite, Al chlorite and transitional types, respectively. The variations in morphology and chemical composition reflect differences in the sources and formation mechanisms. Chlorite sandstones are mainly distributed in the underwater distributary channel, which has strong hydrodynamic conditions. The formation of Ch1 is controlled by the sedimentary environment and flocculation. Salinity differences resulted in a larger amount of flocculation deposits in the estuaries during the syngenetic period. In the shallow burial stage, the clay coating underwent in situ transformation under thermal impacts and formed the Ch1. Xujiahe sandstones experienced complex diagenetic processes, which include early compaction, three stages of chlorite cementation, two stages of quartz precipitation, dissolution and two stages of carbonate cementation. Organic acid dissolution of volcanic rock fragments, phyllite, and biotite likely supplied sufficient Fe 2+ for the growth of Ch2 in a relatively closed diagenetic system. As the pore water became weakly alkaline, diagenetic fluid migration originating from the clay mineral transformation in mudstone mainly provided Fe 2+ and Mg 2+ for Ch3 growth in sandstones. Chlorite is considered to have “composite” impacts on deep tight sandstones, with grain-coating and pore-lining chlorites protecting the primary pores by restricting the silica supply and occupying potential quartz nucleation sites; however, grain-coating chlorite caused throat obstruction or even closure under strong mechanical compaction, which reduced the permeability of the reservoir. • Three different types of chlorite have been found in the Xujiahe tight sandstones. • It has been clear that the formation time of chlorite and the paragenetic sequence of authigenic minerals. • The origin and formation mechanism of authigenic chlorite in three types have been investigated. • We considerd that authigenic chlorite has a “composite” impact on deep tight sandstone: preserve primary porosity but reduce the permeability.

Analysis of factors affecting the reservoir quality of deepwater gravity flow tight sandstone: a case study of the Chang 6 oil reservoir of the Yanchang Formation in the Huaqing area, China

Analysis of factors affecting the reservoir quality of deepwater gravity flow tight sandstone: a case study of the Chang 6 oil reservoir of the Yanchang Formation in the Huaqing area, China

Pore structure evaluation in ultra-deep tight sandstones using NMR measurements and fractal analysis

The reservoir quality, pore spaces and pore size distributions of Bashijiqike sandstones in Kuqa depression were described by thin sections, SEM (scanning electron microscopy) analysis and nuclear magnetic resonance (NMR) tests. In order to quantitatively characterize the complexity of pore structure, NMR T2 (transverse relaxation time) spectrum was used for fractal analysis. This study unravels the relationships between petrophysical parameters, NMR parameters and fractal dimension of the ultra-deep tight sandstones. Primary intergranular pores are the main component of pore spaces, followed by secondary dissolution pores and micropores, and there contain microfractures. The NMR T2 spectra are either bi-modal or uni-modal distribution. The uni-modal T2 spectrum reflects uniform pore spaces. The coexistence of intragranular pores and intergranular pores leads to the bi-modal T2 spectrum. There is a positive correlation between fractal dimensions and T2gm (geometric mean of the T2 distribution). Consequently, four pore structure types are determined according to irreducible water content, T2gm, RQI (reservoir quality index), and the characteristics of individual pore structure are summarized. Because of the uniform pore space, Type I and Type IV have the lowest fractal dimensions. The structure of Type II and Type III are most heterogeneous due to their combination of intergranular and intragranular pores. The results help clarify the internal relationships between petrophysical parameters and microstructure, and have implications for pore structure evaluation of ultra-deep sandstones worldwide.

Sandstone reservoir quality in light of depositional and diagenetic processes of the Messinian Qawasim Formation, onshore Nile Delta, Egypt

• Sandstone in the upper part of the Qawasim Formation is the main reservoir in the El Tamad field. • Primarily, the sandstone is arenite in composition, well sorted with sub-rounded to rounded grains. • Diagenesis enhanced the reservoir quality of the sandstone with low-cement content, poor compaction and weak fracturing. • Clastic rocks of the Qawasim Formation were deposited in a shelf environment as stacked barrier-island bars. • Carbonate rocks of the Qawasim Formation were deposited in a shelf lagoon with open circulation. The Nile Delta region is the main gas-producing province in Egypt. Late Miocene sediments are the target of drilling in the onshore Nile Delta where it has the potentiality to store and produce hydrocarbons. Five wells of the Messinian Qawasim Formation at El Tamad Field were selected for the present study. Detailed petrographic investigations were carried out to reveal the depositional and diagenetic features of the Qawasim Formation rock units. Two main facies were distinguished in the studied formation: clastic and carbonate facies. The clastic rocks include conglomerate, sandstone and shale, while the carbonate rocks include limestone and dolostone. Depositional, diagenetic and petrophysical parameters of the sandstones of the Qawasim Formation reflect that this rock unit an excellent reservoir for hydrocarbon. These microfacies display depositional features such as well sorting, open packing and poor cementation in both coarse and fine-grained microfacies. The porosity of sandstone was enhanced by diagenetic processes such as dolomitization of calcite cement and low compaction that resulted in grain cracking, while the dissolution of feldspars had a subordinate impact on increasing porosity due to their low abundance and the resultant authigenic clay minerals.

Depositional and Diagenetic Controls on Reservoir Quality of the Oligo-Miocene Ghar Sandstone within Tectono-Sequence Stratigraphic Framework of the Persian Gulf Foreland Basin

Depositional and Diagenetic Controls on Reservoir Quality of the Oligo-Miocene Ghar Sandstone within Tectono-Sequence Stratigraphic Framework of the Persian Gulf Foreland Basin

The impact of diagenesis on the reservoir quality of the early Cretaceous Lower Goru sandstones in the Lower Indus Basin, Pakistan

AbstractThe sandstone units of the Early Cretaceous Lower Goru Formation are significant reservoir for gas, oil, and condensates in the Lower Indus Basin of Pakistan. Even though these sandstones are significant reservoir rocks for hydrocarbon exploration, the diagenetic controls on the reservoir properties of the sandstones are poorly documented. For effective exploration, production, and appraisal of a promising reservoir, the diagenesis and reservoir properties must be comprehensively analyzed first. For this study, core samples from depths of more than 3100 m from the KD-01 well within the central division of the basin have been studied. These sandstones were analyzed using petrographic, X-ray diffraction, and scanning electron microscopic analyses to unravel diagenetic impacts on reservoir properties of the sandstone. Medium to coarse-grained and well-sorted sandstone have been identified during petrographic study. The sandstone are categorized as arkose and lithic arkose. Principal diagenetic events which have resulted in changing the primary characters of the sandstones are compaction, cementation, dissolution, and mineral replacement. The observed diagenetic processes can be grouped into early, burial, and late diagenesis. Chlorite is the dominant diagenetic constituent that occurs as rims, coatings, and replacing grains. The early phase of coating of authigenic chlorite has preserved the primary porosity. The recrystallization of chlorite into chamosite has massively reduced the original pore space because of its bridging structure. The current study reveals that diagenetic processes have altered the original rock properties and reservoir characteristics of the Lower Goru sandstone. These preliminary outcomes of this study have great potential to improve the understanding of diagenetic process and their impact on reservoir properties of the Lower Goru sandstone in the Lower Indus Basin and adjoining areas.

Controls of diagenesis on the quality of shallowly buried terrestrial coarse-grained clastic reservoirs: A case study of the Eocene Shahejie Formation in the Damintun Sag, Bohai Bay Basin, Eastern China

A typical shallowly buried (800–2000 m burial depth) coarse-clastic, nonmarine, fan-deltaic reservoir is well developed in the Eocene Shahejie Formation, Damintun Sag, Bohai Bay Basin, China. In this study, diagenesis and its control on the reservoir quality of fan-deltaic sandstones and conglomerates in the Shahejie Formation were investigated in detail by petrographic and geochemical analyses, including cathodoluminescence, X-ray diffraction, scanning electron microscopy, electron probe microanalyses and X-ray fluorescence. The reservoir sandstones and conglomerates mainly consist of moderately sorted feldspathic litharenite, which has undergone various diagenetic processes such as mechanical compaction, cementation, and dissolution, and are currently in the eodiagenesis stage with porosity and permeability averaging 21.56% and 435.2 mD, respectively. Mechanical compaction and cementation caused an ∼45% reduction in the original reservoir porosity, especially the former, which exerted a more significant influence. In regard to cementation, the late calcite that formed at the mudstone-sandstone interface is more destructive and enhances reservoir heterogeneity, and its source material is believed to be primarily derived from the adjacent mudstones in conjunction with oil and gas charging. Feldspar dissolution plays a limited constructive role in reservoir properties by creating ∼10% secondary porosity and changing reservoir wettability to oil wet through the formation of authigenic kaolinite. As a result, massive primary pores (∼90% of the total pores) in the shallowly buried sandstones and conglomerates in the Eocene Shahejie Formation are still preserved. This systematic study is expected to be applicable in a general way to other shallowly buried terrestrial clastic reservoir systems.

Reservoir Quality of Upper Jurassic Corallian Sandstones, Weald Basin, UK

The Upper Jurassic, shallow marine Corallian sandstones of the Weald Basin, UK, are significant onshore reservoirs due to their future potential for carbon capture and storage (CCS) and hydrogen storage. These reservoir rocks, buried to no deeper than 1700 m before uplift to 850 to 900 m at the present time, also provide an opportunity to study the pivotal role of shallow marine sandstone eodiagenesis. With little evidence of compaction, these rocks show low to moderate porosity for their relatively shallow burial depths. Their porosity ranges from 0.8 to 30% with an average of 12.6% and permeability range from 0.01 to 887 mD with an average of 31 mD. The Corallian sandstones of the Weald Basin are relatively poorly studied; consequently, there is a paucity of data on their reservoir quality which limits any ability to predict porosity and permeability away from wells. This study presents a potential first in the examination of diagenetic controls of reservoir quality of the Corallian sandstones, of the Weald Basin’s Palmers Wood and Bletchingley oil fields, using a combination of core analysis, sedimentary core logs, petrography, wireline analysis, SEM-EDS analysis and geochemical analysis to understand the extent of diagenetic evolution of the sandstones and its effects on reservoir quality. The analyses show a dominant quartz arenite lithology with minor feldspars, bioclasts, Fe-ooids and extra-basinal lithic grains. We conclude that little compactional porosity-loss occurred with cementation being the main process that caused porosity-loss. Early calcite cement, from neomorphism of contemporaneously deposited bioclasts, represents the majority of the early cement, which subsequently prevented mechanical compaction. Calcite cement is also interpreted to have formed during burial from decarboxylation-derived CO2 during source rock maturation. Other cements include the Fe-clay berthierine, apatite, pyrite, dolomite, siderite, quartz, illite and kaolinite. Reservoir quality in the Corallian sandstones show no significant depositional textural controls; it was reduced by dominant calcite cementation, locally preserved by berthierine grain coats that inhibited quartz cement and enhanced by detrital grain dissolution as well as cement dissolution. Reservoir quality in the Corallian sandstones can therefore be predicted by considering abundance of calcite cement from bioclasts, organically derived CO2 and Fe-clay coats.

Reservoir quality and sedimentology in shallow marine sandstones: Interplay between sand accumulation and carbonate and clay minerals

Sedimentological studies are important in understanding and predicting reservoir quality, especially in shallow buried sandstones that are dominated by eogenetic processes. Understanding the sedimentological controls on siliciclastic depositional environments will enhance the knowledge and prediction of reservoir architecture and reservoir quality, both of which are essential to resource exploitation and future CCS and hydrogen storage projects. Here, we present a study of sedimentology and depositional mineralogy, as well as controls on the deposition of siliciclastic sandstones, from the shallow-buried, Upper Jurassic Corallian marine sandstones of the Weald Basin, UK. These sandstones host small oil accumulations and, being close to large centres of population, are possible gas storage or carbon capture and storage sites. We used wireline log analysis, high resolution core logging, optical petrography, and SEM-EDS imaging to investigate reservoir architecture and the relative importance of depositional versus secondary diagenetic controls on reservoir quality. Shallow marine conditions, adjacent to a continent experiencing a warm humid climate, are interpreted based on ichnofabrics and mineralogy. Tectonic processes influenced water depth which subsequently controlled both the quantity of detrital bioclastic material (and the resulting calcite cement), and the amount of detrital clay matrix. The eustatic influence on deposition led to the development of a relatively thin intra-Corallian mudstone which compartmentalises the reservoir into discrete upper and lower sand bodies. The results of this study underline the importance of integration of detailed sedimentological and wireline log analysis in improving the prediction of reservoir quality in tectonically active environments for shallow buried, eogenetic-dominated sandstones.

Integration of Post-stack Inversion and Rock Physics to Determine Sandstone Reservoir Quality: Barakan Sub-basin case

Barakan Sub-basin is assessed as potential basin for hydrocarbon reserves in the eastern region of Indonesia because it is adjacent to Masela block giant gas field. Reservoir rocks in this sub-basin are sandstones from Middle Jurassic (Lower Flamengo Formation) until Oligocene (Adi member Formation). Main sandstone reservoir rocks are knowingly studied to have good porosity in Upper Flamengo, Kopae, Ekmai and Adi member Formations. But, there is no significant study to determine sandstone reservoir distribution that have good porosity quality. Therefore, an integrated method of inversion and rock physics study are needed to determine sandstone reservoir quality. This study uses 2D marine seismic post-stack time migration and 2 wells namely Barakan-1 and Koba-1 wells. Sensitivity analysis with cross-plot of gamma ray log versus acoustic impedance values range of 20-60 API and 9000-42000 (ft/s)*(g/cc) shows a strong correlation of good porosity sandstone to low impedance in Ekmai Formation of both wells. Model based of post-stack inversion reveals sandstone distribution in Ekmai Formation of both wells. Time structure maps of top and bottom horizons in Ekmai Formation indicates Barakan-1 well within anticline height structure and Koba-1 well are deposited in a middle of sub-littoral environment.

Impact of diagenesis on the sandstone reservoir quality: a case study from the Lower Jurassic Sangonghe Formation, Junggar Basin, China

Impact of diagenesis on the sandstone reservoir quality: a case study from the Lower Jurassic Sangonghe Formation, Junggar Basin, China

Diagenetic System Control of Tight Sandstone Reservoir Quality: Case study of Xu2 Member, Xujiahe Formation, in Zhongtaishan area, Sichuan Basin, SW China

In order to discuss the control mechanism of diagenetic systems on the quality of tight sandstone reservoirs， this study analyzed the genetic mechanism of the Xu2 member tight sandstone reservoir in the Zhongtaishan area of the Sichuan Basin by combining data from cores， thin sections， cathode luminescence （CL）， X-ray diffraction （XRD）， scanning electron microscope （SEM）， inclusion analysis， pore permeability and 3-D seismic data. It was found that mainly feldspathic quartz sandstone and lithic quartz sandstone were developed in the Xu2 member of well Zhongtai1 and well Wentan1， respectively， and the reservoir space is mainly composed of feldspars and rock fragment dissolution pores， with porosity 20%. No faulting is evident in well Wentan1； calcite and quartz cementation were mainly developed at the early stage； the diagenetic fluid has a paleosalinity >20%. Siliceous and clay cementation of the dissolution byproducts were evident in both well Zhongtai1 and well Wentan1. These are generally all present and contain intra- or intergranular pores， indicating a short migration distance of the dissolution byproducts. The results show that well Zhongtai1 was in an open diagenetic system during the early stage and in a closed diagenetic system in the late stage， and that well Wentan1 was always in a closed diagenetic system. The dissolution byproducts migrated outwards through fractures to produce a net porosity increment in the open system， which effectively improved the reservoir quality. However， the dissolution byproducts did not migrate outwards in the closed system. They cemented and filled reservoir pores， causing the redistribution of the reservoir pore space. As a result， there is no net increment in reservoir porosity in the closed system， and thus no obvious improvement in reservoir quality. Therefore， it was concluded that reservoir quality in the tight sandstone was controlled by the evolution of the diagenetic system. It was mainly improved in the open system during the early stage， then preserved in the closed system during the late stage. The opening and closure of fractures was the key factor impacting the diagenetic system.

Preservation and distribution of detrital clay coats in a modern estuarine heterolithic point bar in the Gironde estuary (Bordeaux, France)

ABSTRACT Estuaries provide an excellent depositional environment to study the interaction between minerals (clays, quartz sands…) and biofilms. The estuary bottom is largely covered by biofilms that impact sediment stability, the mud and clay-coat content in sands, and sedimentary-structure stability, thus influencing sandstone properties during burial. Although numerous oil, gas, and geothermal reservoirs are exploited in estuarine heterolithic point bars, many questions remain about the origin of reservoir properties and heterogeneities in these sedimentary bodies. In order to better understand the sedimentary and microbiologic processes in estuarine systems and to better predict the reservoir quality of estuarine sandstones, this study characterizes a modern heterolithic point bar located in the Garonne estuarine channel at various scales, ranging from the microscopic (thin section) to the macroscopic (core) scale. Three piston cores 4.5 to 6.8 m long were drilled in the Bordeaux North Point Bar. Three main facies were identified in these cores: 1) sandy gravel, 2) heterolithic, medium-grained sand dunes, and 3) thin heterolithic, fine-grained sand beds with mud drapes. The sands are classified as lithic arkoses to feldspathic litharenites. Detrital clay grain coats, which at deep burial depths are transformed to permeability preserving authigenic chlorite coatings, are observed from the base to the top of the point bar. These detrital clay grain coats are mainly composed of smectite, illite, kaolinite, and chlorite, intermixed with other components, such as diatoms or pyrite. Biofilms of exopolymeric substances (EPSs), mostly produced by diatoms, are believed to control the adhesion of the clay coats to the surface of sand grains. Quantification by thin section shows that on average about 30% of the sands are coated in the point bar. The proportion of clay-coated grains appears to be independent of facies. Radiocarbon age dating measured on organic matter points to significant vertical mixing, highlighting the significance of erosion and redeposition. The activities of 137Cs and 210Pb indicate a vertical sedimentation rate of ca. 0.02 m.yr–1 in the muddy chute channel. These ages, coupled to historical maps, suggest that the present-day point bar has developed over the last 300 years with a vertical sedimentation rate ranging from 0.015 to 0.036 m.y–1 and a lateral migration rate of about 1 m.y–1. The combination of sedimentary geology, thin-section petrography, and mineralogy at high spatial and temporal resolutions highlights the potential of this study area as a modern analogue for ancient tidally influenced point-bar deposits associated with clay coatings.

New method to predict porosity loss during sandstone compaction based on packing texture

The grain packing texture of underground sandstone tends to be a major controlling factor in porosity loss during compaction when the effective stress and buried depth increase. However, existing models used to predict porosity loss during sandstone compaction mostly disregard the grain packing texture of sandstone. Based on the micromechanical parameters of discrete element method (DEM) numerical simulation reported in literature, we designed one-component, binary, and ternary packing textures with different grain size distributions and subsequently performed compaction under triaxial servo simulation. We monitored the porosity loss, grain displacement, and force acting on the grain contact point during compaction. Based on the packing texture in sandstone, we propose a new method that considers varying grain sizes, grain size contents, and packing texture types to determine porosity loss during compaction without grain crushing and plastic deformation. The applicability of the method under theoretical conditions was evaluated with 5, 31, and 53 types of one-component, binary, and ternary packing textures. The correlation coefficient between the predicted and simulated values of the void ratio change (△e) was 0.999 and 0.985 for the binary and ternary packing textures, respectively. The reliability of the proposed method was verified using nine groups of physical experimental data derived from literature. The results showed a correlation coefficient of 0.88 between the measured and predicted values of △e and porosity loss (△ϕ). Errors in physical experimental data were derived mainly from grain shape and the crushing of coarse grains. Although the predicted physical experimental data △e and △ϕ were inferior to DEM simulation data, our model was deemed reliable since it showed high correlation between predicted and measured values of △e and △ϕ. Furthermore, the proposed method characterized the influence of the micromechanical process of grain rotation and grain packing texture on reservoir quality during compaction, thereby establishing its importance in predicting reservoir quality of underground sandstone.