Lithic Arkoses Research Articles

Terminal Fan Deposition and Diagenetic Control in the Lower Paleogene of the Shahejie Formation, Bonan Sag, Bohai Basin, China: Insights into Reservoir Quality

In the Bonan area, the lower fourth member of the Shahejie Formation (Es4x) is buried beneath a sedimentary pile ranging from 2500 to 5000 m. Understanding the impact of diagenetic alterations on these deeply buried reservoirs is crucial for effective hydrocarbon exploration and production. This study employs a terminal fan sedimentation model, encompassing depositional environments such as feeder channels, distributary channels, floodplains, and basinal zones, to provide insights into the spatial distribution of reservoir properties and their influence on the localization of optimal reservoirs within the sag. The analysis integrates diagenetic facies with well log responses, subsurface porosity trends, and permeability variations across the formation. The petrographic analysis indicates that the sandstone is composed primarily of litharenite, feldspathic litharenite, lithic arkose, and minor amounts of arkose. The dominant clay cement is illite, accompanied by mixed-layer smectite/illite, chlorite, and kaolinite. Thin section observations reveal secondary porosity formed through the dissolution of quartz grains, volcanic rock fragments, and feldspar, along with their associated cements. These sandstones exhibit relatively good sorting, with average porosity and air permeability values of 14.01% and 12.73 mD, respectively. Diagenetic alterations are categorized into three processes: porosity destruction, preservation, and generation. Key diagenetic mechanisms include compaction, cementation, replacement, and dissolution, with compaction exerting the most significant control on reservoir porosity reduction. Statistical analysis indicates that the average porosity loss due to compaction is approximately 13.3%, accounting for about 38% of the original porosity. The detrital rock cement predominantly comprises quartz (42%), feldspar (32%), clay minerals (14%), and carbonate (12%). Under the prevailing depositional conditions, porosity is enhanced by dissolution and fracturing, while late-stage diagenetic cementation by clay and carbonate minerals—excluding chlorite—adversely affects reservoir quality. Consequently, the distributary zone is identified as the primary target for exploration.

Prediction method for the porosity of tight sandstone constrained by lithofacies and logging resolution

The resource potential of the Upper Triassic Yanchang Formation in the Ordos Basin is considerably large. However, owing to the low porosity and permeability, poor connectivity, and strong heterogeneity of tight sandstone, predicting the porosity of tight sandstone in this formation poses substantial challenges. The lithology of Chang 71 sub-member is dominated by arkose and lithic arkose, wherein the porosities mainly ranged between 3 and 13% and pore types were mainly feldspar dissolution pores. Based on core sample experiments and thin section analysis, combined with conventional logging curves, we proposed a three-porosity weighted average prediction method under lithological control, and a novel method for reading logging curves controlled by logging resolution is developed. We used principal component analysis to optimise the logging curves and reduce the impact of complex coupling relationships between logging curves on lithofacies identification. The stacking algorithm, which is a combination of the random forest and extreme gradient boosting models, was applied to divide the lithofacies into five categories: homogeneous-distributary-channel fine sandstone, heterogeneous-distributary-channel fine sandstone, homogeneous-mouth-bar fine sandstone, heterogeneous-mouth-bar fine sandstone, and shallow lacustrine mudstone. Compared with the results of manual classification based on logs, the accuracy of lithofacies recognition was approximately 94.2%. Additionally, sensitivity analysis of porosity curves was conducted on four types of lithofacies (i.e., except for shallow lacustrine mudstone), and porosity models for each lithofacies were established, providing an effective and objective method for the accurate prediction of porosity. This prediction method comprehensively considers sedimentary factors and incorporates statistics that are fitted using multiple linear regression, which is highly reliable. In the Chang 71 sub-member, the fitting degree between the predicted and core porosities reached 0.912, indicating that this three-porosity weighted average method based on lithofacies constraints is reasonable, reliable, and has stronger adaptability than the those reported previously. The prediction method based on lithofacies control and weight analysis can be applied to other tight sandstones, providing reliable technical support for oil and gas exploration and development.

Detrital signatures of the Ghomaride Culm cycle (Rif Cordillera, N Morocco): New constraints for the northern Gondwana plate tectonics

The Carboniferous detrital suites of the Ghomaride Subdomain (Rif Cordillera, N Morocco) has been undertaken in order to obtain paleogeographic and paleotectonic constraints. Microconglomerate and sandstone samples from eight sections belonging to four sectors were petrographically studied. This northern Gondwana domain was part of the Paleo-Mesomediterranean Plate, located between the Iberian-French Plates and the African Paleo-Atlas-Saharian Domain, which outlines a gap of knowledge since these domains are now part of the Alpine peri-Mediterranean chains. Part of this data-lacking is related to the sandstone detrital modes that have never studied in this area, allowing to obtain information about provenance derived from growing orogens, as well as orogenic processes, and tectonic, climatic and/or erosional events. The study has revealed that the sandstones are litharenites to feldsphatic litharenites, evolving upward to lithic arkoses, and has evidenced a quartzolithic petrofacies with a transitional recycled orogen provenance. The metamorphiclastic detritus displays a low to middle-upper metamorphic rank upwardly referred to a subducted terrane. Serpentinite-like detritus indicates a metamorphosed oceanic crust being dismantled. A supply from the Rheic Ocean seems improbable since it was very far and beyond the Iberian Plate. Therefore, closer oceanic sutures should be proposed, probably separating the Paleo-Mesomediterranean Plate from the Iberian-French Plate and from the Paleo-Atlas-Saharian Domain. The sin-sedimentary volcanic activity (felsic to mafic) reinforces the idea of subductions. A correlation with other Culm successions of the western Paleotethys has allowed the performing of a paleogeographic-paleotectonic model during the Paleotethysian tectonic phase, in which the studied sandstones were deposited in a foredeep connected northward with limestone platforms and with a crystalline emerged domain (hinterland-orogenic front: Iberia-French Plate and part of the Paleo-Mesomediterranean Plate) and transitioning southward to an oceanic branch of the western Paleotethys, and finally with the Paleo-Atlas-Saharian band acting as passive margin in the foreland.

Compaction-induced porosity loss in the Cretaceous Bima Formation around Lakwaime and Dogon Dutsi, Yola Subbasin, Northern Benue Trough, NE Nigeria

The Bima Sandstone is an alluvial fan to braided river sedimentation that represents purely a siliciclastic depositional system. It is the oldest formation and the main reservoir unit in the entire Benue Trough. Porosity loss is an essential component of petroleum reservoirs that needs to be properly investigated for the purpose of hydrocarbon exploitation. Compaction and cementation processes are partly responsible for porosity loss in many known reservoirs around the world. Outcropping units of the formation around Lakwaime and Dogon Dutsi in Yola Sub-basin have been subjected to an integrated approach involving field observations, petrographic analysis and porosity measurements in order to understand the contribution of each of the processes. Modal composition of the samples shows that they are mostly lithic arkose to feldspathic litharenite, with quartz as the most common detrital mineral grain. Grain fracturing has been attributed to mechanical compaction, whereas grain contacts like sutured and concavo-convex are products of chemical compaction. The average values of compactional-porosity loss (COPL), cementational-porosity loss (CEPL), and compaction index (ICOMPACT) in the studied samples stand at 19.51%, 4.18% and 0.82 respectively. The study indicates that the role of compaction in porosity loss is greater than that of cementation as shown by the COPL-CEPL diagram and, as such considered to be generally responsible for the porosity loss in Bima Sandstone.

Provenance and Petrographic Analysis of Paleogene Sandstones in the Bukit Tigapuluh Area, Jambi Subbasin, Indonesia

The study investigates the provenance of Paleogene sandstone deposits from the Lemat Formation in the Lubuk Lawas and Lubuk Bernai sections of the Bukit Tigapuluh area, Jambi Subbasin, South Sumatra Basin, Indonesia. The research aims to elucidate the formation history and stratigraphic evolution of these sedimentary rocks through detailed petrographic analysis. Twenty-two rock samples were analyzed to classify the sandstones and determine their source rock types and tectonic settings. The results indicate that the sandstones are primarily classified as Lithic Arkose, Arkosic Subarkose, Feldspathic Litharenite, Silty Claystone, Sublitharenite, and Subarkosic Wacke. The provenance analysis suggests that these sandstones originated from a tectonic setting of a recycled orogenic zone, comprising quartzose recycled, transitional recycled, and mixed sources, indicating a combination of primary and recycled orogenic materials. Paleocurrent analysis from sedimentary structures in the Lubuk Bernai area suggests deposition directions from the southwest and southeast. This study provides significant insights into the paleogeography and tectonic evolution of the Lemat Formation within the Jambi Subbasin. Further research should focus on integrating geochemical data to refine the understanding of sedimentary provenance and tectonic implications.

Diagenetic controls over reservoir quality of tight sandstone in the lower Jurassic reservoir in the Lenghu area, the north margin of Qaidam basin

The Lower Jurassic Reservoir (LJR) in the Lenghu area on the northern margin of the Qaidam Basin (NMoQB) has become the most promising target for hydrocarbon exploration. The reservoir has experienced complex diagenesis; however, the porosity evolution and the influence of different diagenesis events on reservoir densification remains unclear. In this study, various analytical methods was first used to clarify the diagenetic stage and sequence, establish a porosity evolution model, quantitatively analyze the time and influence of different diagenesis events on reservoir densification, and illustrate the densification mechanism of tight sandstone reservoir in the Lenghu area. The results showed that the dominant rock types in the LJR were feldspathic litharenite, followed by litharenite, lithic arkose, and a small amount of subarkose and sublitharenite. The reservoir is a typical tight sandstone reservoir (TSR), with average porosity and permeability of 5.5% and 0.08 mD, respectively. The pore types were primarily secondary pores, followed by residual intergranular pores and occasional fractures. Diagenesis of LJR has entered the meso-diagenetic stage A, with minor progression into meso-diagenetic stage B. Quantitative calculation showed that the initial porosity of LJR is 32.2%. In eo-diagenetic stage A, compaction is the main factor for porosity reduction. In eo-diagenetic stage B, the porosity loss rates caused by compaction and cementation were 81.5% and 11.8%, respectively. In this stage, the LJR in the Lenghu area has almost been densified, mainly affected by compaction, calcite cementation and clay minerals cementation. Additionally, cementation is an important factor causing reservoir heterogeneity and is dominated by calcite and clay mineral cementation. After entering the meso-diagenetic stage A, dissolution is the main diagenesis event, which can increase porosity by 6.0%. This stage is the critical period for forming high-quality reservoir of LJR in the Lenghu area, mainly in the 4,400 m depth. In meso-diagenetic stage B, the influence of dissolution gradually decreased. Simultaneously, in this stage, the cementation of iron-bearing calcite further strengthened the densification of the LJR in the Lenghu area.

Pore Structure Characteristics of Shale Oil Reservoirs with Different Lithofacies and Their Effects on Mobility of Movable Fluids: A Case Study of the Chang 7 Member in the Ordos Basin, China

The Chang 7 member of the Triassic Yanchang Formation in the Ordos Basin is a significant continent shale oil reservoir in China. Therefore, conducting an in-depth investigation into the pore structure and fluid mobility characteristics of the Chang 7 shale oil reservoir holds immense importance for advancing shale oil exploration. This study conducts a detailed analysis of the pore structures and their impact on fluid mobility of the Chang 7 shale oil reservoir using multiple methodologies, i.e., a cast thin section, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTNA), and nuclear magnetic resonance (NMR). The results show that the sandstone in the Yanwumao area of the Chang 7 shale oil reservoir consists mainly of lithic arkose and feldspathic litharenite, which can be classified into three lithofacies (massive fine-grained sandstone (Sfm), silt-fine sandstone with graded bedding (Sfgb), and silt-fine sandstone with parallel bedding (Sfp)). Moreover, three pore structures (Type I, II, and III), and four pore spaces (nanopores, micropores, mesopores, and macropores) can be characterized. Pore structure Type I, characterized by large pores, exhibits bimodal pore diameter curves, resulting in the highest levels of movable fluid saturation (MFS) and movable fluid porosity (MFP). Pore structure Type II demonstrates unimodal pore structures, indicating robust connectivity, and higher MFS and MFP. Pore structure Type III primarily consists of dissolved and intercrystalline pores with smaller pore radii, a weaker pore configuration relationship, and the least fluid mobility. Furthermore, a correlation analysis suggests that the pore structure significantly impacts the fluid flowability in the reservoir. Favorable petrophysical properties and large pores enhance fluid flowability. Micropores and mesopores with high fractal dimensions have a greater impact on reservoir fluid mobility compared to macropores and nanopores. Mesopores mainly control MFS and MFP, while micropores govern the shift from bound fluid to movable fluid states. Among the lithofacies types, the Sfm lithofacies exhibit the highest fluid mobility due to their significant proportion of macropores and mesopores, whereas the Sfgb lithofacies have lower values because they contain an abundance of micropores. The Sfp lithofacies also dominate macropores and mesopores, resulting in medium fluid mobility levels. This study combines lithofacies types, micro-reservoir pore structure characteristics, and mobile fluid occurrence characteristics to better understand the dominant reservoir distribution characteristics of the Chang 7 shale oil reservoirs in the Ordos Basin and provide theoretical information for further optimization of production strategies.

An experimental study of tensile stress and deformation in an anisotropic rock

Anisotropy is a very common condition in rock mass; it can be due to different factors and directly affects the failure mechanisms affecting the rock mass. For example, metamorphic rocks that are foliated, sedimentary rocks that are stratified or volcanic formations with alternating layers. Despite the existence of several studies related to anisotropy those specifically addressing the tensile strength of anisotropic rocks are quite limited. The present study is focused on the determination of the deformability, compressive and tensile strength of anisotropic rocks. It must be highlighted that the mechanical behavior of the anisotropic rock mass is dependent on the angle between the inclination of planes of weakness (e.g. foliation) and the direction of the load. Assuming a vertical load, the two extremes of tensile strength are 0°(horizontal) and 90° (vertical). A series of laboratory tests has been done in anisotropic sandstone (lithic arkose), from Burgos (Spain), including uniaxial compressive strength tests, direct tensile strength tests, and diametric compression (Brazilian tests). The tests were carried out with strain gauges that allowed estimating the elastic modulus. To determine the anisotropic direction, ultrasonic pulse wave velocity tests were also performed. The variation of strength and deformability as a function of anisotropy is analyzed, as well as the variation of elastic behavior in tensile and compressive.

Provenance of the lower Cretaceous Lumshiwal Formation, Surghar Range, northwestern Indian Plate, Pakistan: Insights from new petrographical and geochemical analysis

This paper documents the provenance and palaeoclimatic conditions of the Cretaceous Lumshiwal Formation near the western margin of the Indian plate in the Surghar Range, northwest Pakistan. The combined techniques of petrography, X-ray diffraction (XRD), and bulk rock geochemistry were utilized to reveal the source and mineral-geochemical composition and sandstone type. The petrographic analysis of the sandstones confirms the dominance of quartz, feldspar, and rock fragments, with subordinate occurrences of muscovite, magnetite, and hematite. Heavy minerals consist of tourmaline, titanite (sphene), rutile, cassiterite, monazite, and zircon. The cementing material includes ferruginous clays, jarosite, glauconite, calcite, minor dolomite, gypsum, and silica. The modal composition plot of the sandstones falls into arkose to sub-arkose, with a few lithic arkose varieties. Lithic fragments mainly include granite, with a minor occurrence of granitic gneiss, chert, phyllite, and quartz mica schist. The discriminatory provenance diagram of the sandstones suggests a transitional continental provenance. The bulk rock geochemistry of the sandstones reveals the presence of SiO2, TiO2, Al2O3, Fe2O3, FeO, MnO, CaO, MgO, Na2O, K2O and P2O5. The petrographical mineral findings were corroborated with XRD, SEM, and bulk rock geochemistry analysis (major element concentrations and their ratios), which collectively all confirm a felsic igneous source. The tectonic discrimination diagram (SiO2-log K2O/Na2O wt %) implies a dominant influx of sediment sourced from the passive continental margin of the uplifted Gondwana Indian plate. Terrigenous sediments found in the Lumshiwal Formation are interpreted to have been derived from granites and granitic gneisses of the Indian Shield. The palaeo-weathering index, including the chemical index of alteration (CIA) and the chemical index of weathering (CIW) of the Lumshiwal Formation confirms a low to moderately weathered source area. The climate discrimination plot (SiO2 versus Al2O3 + Na2O + K2O) shows that humid to semi-humid climatic conditions during the deposition of the Lumshiwal Formation.

Depositional and diagenetic controls on the reservoir quality of Early Miocene syn-rift deep-marine sandstones, NW Saudi Arabia

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.

A dataset of gas reservoir microscopic images in Xujiahe Formation in northeast Sichuan Basin

The gas reservoir in Xujiahe Formation in northeast Sichuan Basin holds promising potential for exploration and development. In this paper, we collected and sorted the microscopic image data of the clastic reservoir of Xujiahe Formation gas reservoir in the study area, so as to further study the petrological characteristics, diagenesis, reservoir space and physical properties of the region. The reservoir rocks are mainly lithic sandstone, lithic arkose sandstone and quartz sandstone. There are intergranular solution pores, residual intergranular solution pores, intragranular solution pores, clay mineral intergranular (solution) pores and various microfractures in the reservoir space. According to the observation of the cast thin section, the pores are relatively undeveloped, while some thin sections (micro-cracks) are relatively developed. Based on thin section identification and observation, the sandstone reservoirs of Xujiahe Formation in the study area have undergone processes such as compaction, cementation, dissolution and tectonic fracture. This dataset can provide a direct source for the comparative analysis and reference of sandstone image data. Moreover, it can serve as a benchmark for the intelligent identification of lithology.

Characterizations of Middle–Lower Triassic Reservoir Quality and Gas Accumulations in the Northeastern Margin of the Ordos Basin, China

New Middle–Lower Triassic gas reservoirs were found in the eastern margin of the Ordos Basin, which provides new possibilities for gas exploration in the Ordos Basin. However, there are few relevant studies on these gas reservoirs. In this study, we use porosity–permeability measurements, optical light microscopy, scanning electron microscopy, mercury intrusion capillary pressure experiments, core-scale low-field nuclear magnetic resonance experiments, fluid inclusion analysis, burial history reconstruction, organic geochemistry data, and wireline log data to systematically investigate the reservoir quality and accumulation pattern of Middle–Lower Triassic gas reservoirs. The Middle–Lower Triassic sandstones are mostly lithic arkose, with a small amount of feldspathic litharenite. In comparison to the upper Paleozoic tight sandstone, the Middle–Lower Triassic sandstones exhibit a superior pore structure and increased porosity and permeability. A significant portion of the Middle–Lower Triassic sandstones can be referred to as “conventional sandstones” (porosities >12% and permeabilities >1 mD). The source rocks of the Middle–Lower Triassic gas reservoirs are most likely the upper Paleozoic coal and organic-rich mudstone, and the accumulation of natural gas occurred in the Early Jurassic to Late Cretaceous. The gas accumulation in the sandstones of the Middle–Lower Triassic reservoirs was mainly driven by buoyancy. The initiation and extension of vertical faults with small displacement in the northeast margin of the Ordos Basin enabled gas to move through the faults from the deep upper Paleozoic source rocks to the Middle–Lower Triassic reservoirs. Since gas accumulation relies on buoyancy, conventional sandstone layers with good reservoir properties are more likely to reach high gas saturation and are the primary targets for exploration and development.

Facies analysis, petrography and textural characteristics of the onshore Paleogene-Neogene Lawin Basin, Perak, Peninsular Malaysia: Insights into palaeodepositional environment and provenance

Sedimentological features of the onshore Paleogene-Neogene Lawin Basin are of importance in better understanding the basin evolution and geodynamics. The N-S trending Lawin Basin, which is situated within the Western Belt of Peninsular Malaysia, is an extensional basin that formed during the sinistral movement of the Bok Bak Fault zone. This study evaluates the palaeodepositional environment, reveals the provenance, and interprets the transport dynamics of the mini-basin by integrating facies, petrographic, and grain size analyses. The basin lithologies can be broadly divided into conglomerate and sandy facies, with the former consisting of clast-supported conglomerate, matrix-supported conglomerate, cross-stratified pebble conglomerate, and bedded pebbly conglomerate, whereas the latter comprises massive pebbly sandstone, horizontally bedded sandstone, planar cross-bedded sandstone, and ripple laminated sandstone. The lithologies can be grouped into three main facies associations, including massive conglomerate, stratified conglomerate, and fluvial sandstone deposits, which reflect alluvial fan (debris/gravity flow deposits), braided gravel bar, and channel bar elements, respectively. Petrographic data indicate that the conglomerates are clast- and matrix-supported and were derived from a proximal source. The QFL range of detrital framework grains reveals that sandstones are composed of quartz (43–80 vol%), feldspar (9–54 vol%), and lithic fragments (2–16 vol%). The sandstones are coarse-grained, poorly-sorted, and sub-rounded to sub-angular in texture. They are arkose, sublitharenite, and lithic arkose in composition. QtFL/QmFLt ternary diagrams show that the sandstones are primarily derived from a provenance area comprising continental block material, including uplifted basement rocks and transitional continental deposits. This study proposes that the sandstones had a nearby granitic source (Bintang) and a debris source derived from recycled sedimentary rocks (the Baling Group sediments). Grain size parameters ranges (mean, −2.37 to 0.74 Φ; sorting, 1.22 to 2.07 Φ; skewness −0.21 to 0.91 Φ, kurtosis, 0.66 to 1.98 Φ, and average mean-sorting index of −0.24) are consistent with immature to sub-mature sedimentation deposited within a high-energy fluvial environment.

Diagenetic Evolution and Its Impact on Reservoir Quality of Tight Sandstones: A Case Study of the Triassic Chang-7 Member, Ordos Basin, Northwest China

The Upper Triassic Chang-7 Member of the Ordos Basin contains typical tight sandstone reservoirs. Reservoir quality is affected by diagenesis, which is a critical factor in tight oil exploration. In this study, the Chang-7 Member tight sandstones were studied by a variety of experimental methods, including thin sections, scanning electron microscopy (SEM), and X-ray diffraction (XRD), to determine the reservoir characteristics and diagenesis and discuss their influences on the reservoir quality. The Chang-7 Member sandstones are mainly lithic arkose and feldspathic litharenite with an average porosity and permeability of 7.17% and 0.13 mD, respectively. The pore type is mainly primary intergranular pores, and the secondary pores are feldspar dissolved pores, which are relatively developed, with pore radii of 2–20 µm. Diagenesis of Chang-7 Member tight reservoirs mainly includes compaction, quartz cementation, carbonate cementation, clay mineral cementation, and dissolution. The diagenetic stage develops into Mesodiagenesis A. The average porosity loss from compaction and cementation of sandstone in the Chang-7 Member is 15.93% and 18.67%, respectively. With the increase in burial depth, the porosity and permeability of the reservoir gradually decrease. In mesodiagenesis, the authigenic illite and carbonate cementation compacts the reservoir. The acid fluid carried by the two stages of oil and gas filling during diagenesis dissolved feldspar and carbonate cement, which plays a certain role in transforming the tight reservoir.

Provenance and paleoclimate of Middle Permian Tavantolgoi formation sandstones in Baruunnaran coal deposit, southern Mongolia

Baruunnaran coal deposit is located in southern Mongolia; belongs to Tavantolgoi coal zone of South Gobi Basin (Bat-Erdene, 1992). Coal seams are hosted by Middle Permian Tavantolgoi Formation which is subdivided into lower and upper member, based on lithological characteristics. Ten core samples, consisting of sandstones were collected from exploration drill holes. 
 Sandstones from Middle Permian Tavantolgoi Formation in Baruunnaran coal deposit are investigated for unravel their provenance and paleoclimate conditions andtectonic settings. Detrital modes of sandstones in lower member are characterized as slightly high amount of mineral grains (quartz and feldspar) and slightly low amount of lithic fragments (litharenite, feldspathic litharenite and lithic arkose) comparing to sandstones from upper member (litharenite and lithic arkose). Provenance study of sandstones are shown signature of recycled orogens and mixed orogenic sands. Whereas, paleoclimate condition of sandstones from lower and upper members are interpreted as they were deposited in semi humid and arid environments, respectively.

Impact of depositional and diagenetic controls on reservoir quality of syn-rift sedimentary systems: An example from Oligocene-Miocene Al Wajh Formation, northwest Saudi Arabia

Tertiary, syn-rift clastic reservoirs offer a unique opportunity to investigate the pivotal role of depositional and early diagenetic controls on reservoir quality. The Al Wajh Formation is a vital exploration target for petroleum in the Red Sea, and is characterized by early, near-surface diagenetic signatures. An integrated petrographic-mineralogical study is conducted to assess the role of depositional factors (e.g., sand grain size and clay matrix) and early diagenetic overprints on reservoir quality. The Al Wajh sandstones are primarily subarkose and lithic arkose, deposited in alluvial fan, braided fluvial, and nearshore environments. Braided fluvial sandstones of the formation have the best quality reservoirs (average porosity: 23.2 %; average permeability: 3025 mD) because of medium to coarse grain size and small proportion of clay matrix, whereas the nearshore sandstones form reservoirs of far lower quality (average porosity: 12.6 %; average permeability: 486 mD) owing to predominantly finer grain size and higher proportion of clay matrix. Clay volume between 10 and 20 % generally reduces the reservoir properties to <10 % porosity and 100 mD permeability. Compaction and grain dissolution are the major near-surface, early diagenetic processes that have strongly affected the sandstones reservoir quality. High mechanical compaction has been recorded in sandstones characterized by abundant matrix content and ductile rock fragments. Dissolution of labile grains (e.g., feldspars) has created secondary, intragranular porosity of up to 13.5 % (average 4.7 %). Kaolinite is the primary authigenic pore-filling clay in the continental units of the formation, whereas detrital and authigenic smectites are the key pore-filling clays in the nearshore units of the formation. Grain-coating smectite (up to 18.7 %; average: 7.1 %) was mainly emplaced through mechanical infiltration and has the potential for inhibiting quartz cementation should deep burial occur. The findings of the study can be utilized for modelling reservoir quality in the subsurface analogue of the Al Wajh Formation in the Red Sea or elsewhere, thereby improving reservoir quality prediction.

Diagenesis and evolution of deep tight reservoirs: A case study of the fourth member of Shahejie Formation (cg: 50.4-42 Ma) in Bozhong Sag

Abstract This study focused on the deep tight sandstone reservoir (DTSR) of the fourth member of the Shahejie Formation in the Bozhong Sag, Bohai Bay Basin, a special type of reservoir. To reveal the diagenesis and evolution of the reservoir in the study area, cores observation, thin section identification, scanning electron microscopy, grain size analysis, and petrophysical properties measurements are available to analyze the mechanics of diagenesis and densification processes. The recognition is agreed on that (1) the lithology of the fourth member is mainly composed of lithic arkose and feldspathic lithic sandstone with low compositional maturity, and grain sizes vary from middle to coarse; (2) the porosity of reservoir ranges from 4 to 11.5% (av. 6.8%), which belongs to the medium low porosity sandstone reservoir; the pore structure is complex and the type of pore spaces is mainly secondary pore, while original pores are less developed due to the deep distribution of strata; (3) based on the quantitative calculation of porosity of the DTSR, it was identified that compaction is the main reason for sandstone reservoir densification, with an average porosity reduction of 62%, followed by cementation filling intergranular pores with an average pore reduction rate of 25.1%. Dissolution plays a constructive role in improving porosity, with an average increase rate of 18.5%.

Porosity and permeability correction of laumontite-rich reservoirs in the first member of the Shaximiao Formation in the Central Sichuan Basin

The development of a large amount of laumontite cement in clastic reservoirs generate pseudo fractures at normal temperatures and pressures conditions on the ground surface, causing porosity and permeability values to be significantly higher than the actual values of buried conditions. Taking the first member of the Shaximiao Formation in the Middle Jurassic of the Central Sichuan Basin as an example, this study analyzes the material composition, reservoir porosity and permeability, diagenesis, and other characteristics of reservoirs rich in laumontite using the petrology, geochemistry, porosity and permeability characterization, and overburden pressure porosity and permeability tests and further discusses a method for correcting porosity and permeability at normal temperatures and pressures conditions on the ground surface. The reservoir rich in laumontite in the first member of the Shaximiao Formation in the Central Sichuan Basin primarily comprises lithic arkose sandstone, with porosity ranging from 2.12% to 17.48%, with an average of 10.28%, and permeability ranging from 0.004 to 120.351 mD, with an average of 2.691 mD. There are various types of diagenesis, including laumontite cementation, calcite cementation, siliceous cementation, clay mineral cementation, laumontite dissolution, feldspar dissolution, etc. Laumontite is mainly exhibited as pore filling cementation, and the extensive cementation and dissolution of laumontite are the key factors affecting porosity and permeability of reservoirs. By comparing the porosity and permeability difference between overburden pressure condition and normal temperatures and pressures condition, the influence of granular marginal pseudo fractures on the porosity and permeability of the reservoir is corrected. The corrected porosity and permeability values at normal temperatures and pressures conditions on the ground surface are linearly correlated with those before the correction. In different laumontite content intervals, the permeability of the reservoir rich in laumontite accords with the linear correction correlation. The higher the laumontite content, the smaller the correction coefficient is. Therefore, this correction guarantees accurately predict the porosity and permeability of the reservoir rich in laumontite at normal temperatures and pressures conditions on the ground surface.

Impact of sedimentation and diagenesis on deeply buried sandy conglomerate reservoirs quality in nearshore sublacustrine fan: A case study of lower Member of the Eocene Shahejie Formation in Dongying Sag, Bohai Bay Basin (East China)

The sandy conglomerates of the lower member of Shahejie (Es4x) in the north of Dongying Sag, Bohai Bay Basin, were investigated to understand the formation mechanism of the good reservoir and controlling factors in deeply buried strata. This research is based on seismic, logging, and core data and comprehensively utilizes methods such as core description, petrographic analysis, cathodoluminescence image analysis, SEM-EDS analysis, and mercury intrusion. The sandy conglomerates are predominantly lithic arkose and litharenite. The reservoirs in the Es4x have experienced three diagenetic processes, compaction, dissolution, and cementation. Compaction remains the major factor affecting the reservoir quality, mainly in the progradation sequence, and cementation has a destructive effect on reservoirs, principally in the retrogradation sequence. The quantification of the compaction by the compaction index revealed that mechanical compaction is higher in the progradation sequence (0.68) than in the retrogradation sequences (0.59). Compaction is suppressed by carbonate cementation, but the extensive cementation significantly degrades the reservoir quality, especially in the retrogradation sequence. Dissolution of different rock fragments, feldspar, and the whole dissolution, in general, enhance the reservoir's quality in both intervals but more in the progradation sequence. Three types of reservoirs representing type I, type II, and Type III mercury intrusion capillary pressure curves were encountered in the Es4x. The braided channels in the progradation and retrogradation sets encompass the greatest reservoir quality and are characterized by type I and II reservoirs with good petrophysical properties. The preeminent reservoirs appear in the braided channels of the progradation set. The development of high-quality reservoirs is governed by the depositional subfacies, compaction, and dissolution in the Es4x. This study will contribute to comprehending the origin of the genesis mechanism of high-quality in deep sublacustrine sandy conglomerate and will help the exploration and pre-drill for favorable reservoirs in similar basins around the world.

RESERVOIR CHARACTERIZATION OF THE PLIOCENE RED SERIES, LAM FIELD AND SURROUNDING AREAS, OFFSHORE WESTERN TURKMENISTAN

The sedimentology, petrography and reservoir potential of Pliocene sandstones within the Upper Red Series in the offshore LAM field, Western Turkmenistan, have been examined. Depositional settings are interpreted within the framework of the Red Series palaeoenvironments across the entire Turkmen sector of the Apsheron‐Prebalkhan uplift zone, including its onshore extension to the east.Examination of 81 m of core from three separate intervals suggests that the Red Series in the LAM field is the product of a fluvial‐dominated delta system with associated floodplain deposits, periodically flooded by the saline waters of the South Caspian Lake. Relatively thick sandstones, up to around 5 m thick, are interpreted as channel and point‐bar deposits of a meandering river system, with thinner and finer‐grained sandstones and siltstones inferred to be crevasse‐splay and interdistributary floodplain deposits. Floodplain mudstones display signs of desiccation, soil formation, plant rootlets and occasional thin layers of anhydrite. Intervals with marine trace‐fossil assemblages record incursions of saline‐lake waters. Conglomeratic layers at the base of thicker mudstone intervals may be associated with abrupt transgressions of the lake. The best reservoir qualities are associated with the fluvial channel and point‐bar sandstones. Crevasse‐splay and other overbank sandstones are of poorer quality, while intercalated floodplain to lacustrine claystone/siltstone units may constitute local seals.Eighteen sandstone plug samples from the cored intervals were examined in thin‐section and by XRD and SEM to assess how mineralogy, grain size and diagenesis affect reservoir quality. The samples consist predominantly of lithic arkoses and feldspathic litharenites; higher porosities, and therefore better reservoir potential, are associated with the feldspathic litharenites. Primary controls on porosity include compaction, clay‐matrix content and calcite cementation. XRD data reveal the presence of illite, illite‐smectite and chlorite. The presence of swelling clays has been the main cause of formation damage in the field.The interpretation of meandering fluvial channels here is thought to represent the first published account of such channels within Pliocene reservoir rocks in the north of the South Caspian Basin. Previous accounts of the Red Series sandstones deposited onshore to the east have indicated deposition within braided channels of the palaeo‐Amu Darya river delta plain, and alluvial‐fan deposits sourced from uplands to the north. Deposition of the equivalent Productive Series by the palaeo‐Volga in the Azerbaijan sector to the west has also been interpreted as having taken place within braided systems, although mixed or suspended‐load fluvial channels ascribed to the contemporary Kura delta farther south may have been associated with a meandering system. Two palaeogeographic maps are presented to illustrate depositional environments for the Red Series during relative high‐ and low‐stands of the South Caspian Lake. The maps cover the area where the Productive Series of Azerbaijan passes into the Red Series of Western Turkmenistan.