Lithofacies Types Research Articles

Occurrence characteristics and factors that influence shale oil in the Jurassic Lianggaoshan Formation, northeastern Sichuan Basin

Well Ping'an 1, located in the northeastern Sichuan Basin, tested 112.8 m3/d of oil from the Lianggaoshan Formation, marking an important breakthrough in shale oil production. However, due to the influence of complex lithofacies, high thermal maturity, and limited exploration, the occurrence and factors that influence shale oil across the different lithofacies of the Lianggaoshan Formation remain unclear, thereby severely impeding exploration and development. This study systematically examines the occurrence and factors that influence shale oil (adsorbed oil and free oil) in the Lianggaoshan Formation through an integrated approach involving petrology, geochemistry, scanning electron microscopy, nuclear magnetic resonance spectroscopy, multi-temperature pyrolysis, and gas chromatography. Results demonstrate the following: (1) Residual shale oil (adsorbed oil) in the study area predominantly exists as oil films, precipitated oil, and residues (asphalt). Oil films appear on the edges of organic matter (OM), clay minerals, pyrite, and felsic minerals. Crude oil adheres as thin layers to mineral edges, with part of the oil resulting from the lipophilic properties of OM. Precipitated oil tends to accumulate around organic pores, and at higher maturity stages, heavy oil components adhere to mineral surfaces as residues. The hydrocarbon loss of the sample is serious, and free oil is difficult to observe due to experimental processing. (2) Based on the experimental results of preserved cored samples, the shale oil in the study area is mainly free oil (0.86–4.46 mg/g, average 1.99 mg/g), followed by adsorbed oil (0.0007–1.36 mg/g, average 0.43 mg/g). The presence of higher concentrations of clay minerals and total organic carbon (TOC) correlates with increased quantities of free and adsorbed oil in the Lianggaoshan Formation. (3) The two-dimensional nuclear magnetic resonance method combined with the “e” index method was employed to quantify the free oil volumes in shale reservoirs of varying maturity within the study area. The recovery coefficient initially increases rapidly and then gradually with increasing Ro (vitrinite reflectance) values. Compared with other lithofacies types, OM-rich laminated argillaceous shale and OM-poor laminated siliceous shale have higher free oil content, which makes them easier to produce, in other words, these two lithofacies should be preferred as favorable exploration and development targets.

The constraints of sedimentary environment on the evolution of bauxite reservoir characteristics within the benxi formation in the Linxing area, Ordos Basin, China

AbstractThe breakthrough of bauxite natural gas in the Taiyuan Formation in the Longdong area of the Ordos Basin makes bauxite natural gas gradually become a new exploration direction. However, the complex sedimentary conditions and diverse developmental strata of the Linxing region present difficulties for the exploration of bauxite gas. In this research, samples of bauxite cores from significant drilling sections in the Linxing area were obtained for comprehensive geochemical, mineralogical, and pore structure analysis. This study investigates the impact of sedimentary environments on the physical properties of bauxite reservoirs within the Benxi Formation, focusing on lithofacies categorization, mineral composition, and pore structure. This study established a lithofacies categorization system that iron minerals, aluminum minerals + titanium minerals, clay minerals as three end‐member components. The study identified four lithofacies types: pure bauxite, clayey bauxite, mixed bauxite, and bauxite mudstone. According to geochemical analysis, the bauxite rock in the study area is deposited in a semi‐enclosed bay with terrestrial freshwater input. The physical properties of bauxite reservoir in the study area are relatively poor, among which the physical properties of bauxite reservoir are the best. Authigenic minerals like pyrite are often filled in dissolution pores. The constraint of sedimentary environment on bauxite reservoir is mainly manifested as the influence on the formation of diaspore mineral framework, and its acidity and alkalinity are very important for the formation of diaspore. Hence, diaspore commonly forms in the shallow lake environments characterized by pure bauxite rock, exhibiting favorable reservoir conditions. This research examines the impact of mineral composition on bauxite reservoirs through the lens of lithofacies and sedimentary environments. The variations in bauxite rock reservoirs across distinct sedimentary environments have been elucidated. It provides a new guidance for the exploration of natural gas in Benxi Formation bauxite in Ordos Basin.

Analysis of Controlling Factors of Pore Structure in Different Lithofacies Types of Continental Shale—Taking the Daqingzi Area in the Southern Songliao Basin as an Example

Due to the influence of terrigenous debris, the internal pore structure of continental shale is highly heterogeneous, and the controlling factors are complex. This paper studies the structure and controlling factors of shale reservoirs in the first member of the Qingshankou Formation in the Southern Songliao Basin using core data and various analytical test data. The results show that the original deposition and subsequent diagenesis comprehensively determine the shale reservoirs’ pore structure characteristics and evolution law. According to the severity of terrigenous debris, the shale reservoirs in the study area are divided into four categories and six subcategories of lithofacies. By comparing the characteristics of different shale lithofacies reservoirs, the results show that the lithofacies with a high brittle mineral content have more substantial anti-compaction effects, more primary pores to promote retention and a relatively high proportion of mesopores/macropores. Controlling the organic matter content when forming high-quality reservoirs leads to two possibilities. An excessive organic matter content will fill pores and reduce the pore pressure resistance. A moderate organic matter content will make the inorganic diagenesis and organic hydrocarbon generation processes interact, and the development of organic matter mainly affects the development of dissolution pores. The comprehensive results show that A3 (silty laminated felsic shale) reservoirs underwent the pore evolution process of “two drops and two rises” of compaction, cementation and pore reduction, dissolution and pore increase, and organic matter cracking and pore increase, and they are the most favourable lithofacies of the shale reservoirs in the study area.

Types of lithofacies in the Lower Cambrian marine shale of the Northern Guizhou Region and their suitability for shale gas exploration

The lithofacies and thermal maturity of the over-mature Lower Cambrian marine shale in the Northern Guizhou Region, and their impacts on reservoir properties in this shale were analyzed by combining geochemistry, mineralogy, and gas adsorption methods. Ten lithofacies were identified, and the dominant lithofacies in the studied shale are lean-total organic carbon (TOC) argillaceous-rich siliceous shale (LTAS), medium-TOC siliceous shale (MTSS), and rich-TOC siliceous shale (RTSS). Since the gas generation potential of organic matter was weak, meso- and macro-pores were compressed or filled during the thermal evolution stage with a vitrinite reflectance (RO) range of 3.0%–4.0%. The controlling factors for methane adsorption capacity in the shale samples are significantly influenced by TOC content rather than thermal maturity. Among the RTSS, MTSS, and LTAS samples, RTSS exhibits the highest favorability for preserving hydrocarbon gas, followed by MTSS. The shale types in this study play a significant role in determining the properties of shale reservoirs, serving as an effective parameter for evaluating shale gas development potential. The RTSS and MTSS with a RO range of 2.0%–3.0% stand out as the most favorable target shale types for shale gas exploration and development.

Organic matter enrichment model of fine-grained rocks in volcanic rift lacustrine basin: A case study of lower submember of second member of Lower Cretaceous Shahezi Formation in Lishu rift depression of Songliao Basin, NE China

Based on sedimentary characteristics of the fine-grained rocks of the lower submember of second member of the Lower Cretaceous Shahezi Formation (K1sh2L) in the Lishu rift depression, combined with methods of organic petrology, analysis of major and trace elements as well as biological marker compound, the enrichment conditions and enrichment model of organic matter in the fine-grained sedimentary rocks in volcanic rift lacustrine basin are investigated. The change of sedimentary paleoenvironment controls the vertical distribution of different lithofacies types in the K1sh2L and divides it into the upper and lower parts. The lower part contains massive siliceous mudstone with bioclast-bearing siliceous mudstone, whereas the upper part is mostly composed of laminated siliceous shale and laminated fine-grained mixed shale. The kerogen types of organic matter in the lower and upper parts are types II2–III and types I–II1, respectively. The organic carbon content in the upper part is higher than that in the lower part generally. The enrichment of organic matter in volcanic rift lacustrine basin is subjected to three favorable conditions. First, continuous enhancement of rifting is the direct factor increasing the paleo-water depth, and the rise of base level leads to the expansion of deep-water mudstone/shale deposition range. Second, relatively strong underwater volcanic eruption and rifting are simultaneous, and such event can provide a lot of nutrients for the lake basin, which is conducive to the bloom of algae, resulting in higher productivity of types I–II1 kerogen. Third, the relatively dry paleoclimate leads to a decrease in input of fresh water and terrestrial materials, including Type III kerogen from terrestrial higher plants, resulting in a water body with higher salinity and anoxic stratification, which is more favorable for preservation of organic matter. The organic matter enrichment model of fine-grained sedimentary rocks of volcanic rift lacustrine basin is established, which is of reference significance to the understanding of the organic matter enrichment mechanism of fine-grained sedimentary rocks of Shahezi Formation in Songliao Basin and even in the northeast China.

The characteristics of volcanic lithofacies and volcanic reservoirs in the Permian Jiamuhe Formation, Southern Zhongguai Rise, Junggar Basin, NW China

The southern Zhongguai Rise has gained wide attentions due to its commercial hydrocarbon discoveries from volcanic deposits in Jinlong Oilfield. The integrated analysis of geological and geophysical data assist in determination and depiction of volcanic lithology and volcanic facie, characterization of volcanic reservoirs as well as discussion of constrains on volcanic lithology/lithofacies. The conclusions suggest that two types of volcanic lithology, including volcaniclastic rocks and silicic moltenlava, can be identified based on cores. In addition, two types of volcanic lithofacies can be delineated according to cores and wireline responses. The explosive facies are mainly developed in early depositional period while the effusive facies are developed through depositional period. Furthermore, the reservoir space predominantly consists of pores and fractures according to thin sections. The physical property varies from time to time. That is, porosity distribution varies slightly. In contrast, permeability distribution varies significantly. The volcanic reservoirs can be generally attributed to high-porosity and low-permeability reservoirs no matter in volcaniclastic rocks or silicic molten lava. The volcanic lithofacies, facies distribution, diagenism and paleogeomorphology exserts controls on physical property and reservoir performance of volcanic reservoirs together.

Diagenesis Variation in Different Distributary Channels of Shallow Water Lacustrine Delta Deposits and Implication for High-Quality Reservoir Prediction: A Case Study in the Chang 8 Member in Caijiamiao Area, Sw Ordos Basin, China

Tight oil reservoirs are considered important exploration targets in lacustrine basins. High-quality reservoir prediction is difficult as the reservoirs have complex distributions of depositional facies and diagenesis processes. Previous research has found that the diagenesis process of tight oil sandstones varies greatly in different depositional facies. However, diagenesis variation in different depositional facies is still poorly studied, especially in distributary channels of shallow water delta deposits in lacustrine basins. Based on the description of core samples, the observation of rock slices, the interpretation of well logging data, and the analysis of porosity and permeability data, the differences in the lithofacies types, diagenesis processes, and pore structures of different distributary channels have been clarified. Ultimately, a model of diagenesis and reservoir heterogeneity distribution in the shallow-water delta of Chang 8 Member of the Yanchang Formation in the Caijiamiao area of the Ordos Basin has been established. This research indicates that the main distributary channels in the study area are dominated by massive bedding sandstone lithofacies, while the secondary distributary channels are primarily characterized by cross-bedding sandstone lithofacies. There are significant differences in the compaction, dissolution, and cementation of authigenic chlorite and carbonate among different parts of the distributary channels. Plastic mineral components, such as clay and mica, are abundant in sheet sands, and are more influenced by mechanical and chemical compaction. Influenced by the infiltration of meteoric water and hydrocarbon generation, dissolution pores are relatively well-developed in the underwater distributary channel reservoirs. A large amount of carbonate cementation, such as calcite and siderite, is found within the sandstone at the interface between sand and mud. The occurrence of authigenic chlorite exhibits a clear sedimentary microfacies zonation, but there is little difference in the kaolinite and siliceous cementation among different microfacies reservoirs. Finally, a model of diagenetic differences and reservoir quality distribution within dense sand bodies has been established. This model suggests that high-quality reservoirs are primarily developed in the middle of distributary channels, providing a theoretical basis for the further fine exploration and development of oil and gas in the study area.

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.

Paleoenvironmental Transition during the Rhuddanian–Aeronian and Its Implications for Lithofacies Evolution and Shale Gas Exploration: Insights from the Changning Area, Southern Sichuan Basin, South-West China

During the Rhuddanian–Aeronian interglacial period, global geological events such as glacial melting, synsedimentary volcanic activity, biological resurgence, and large-scale marine transgressions caused frequent fluctuations in paleoproductivity, climate changes, and sea level variations. These paleoenvironmental transitions directly influenced the development characteristics of shale lithofacies. This study investigates the Longmaxi Formation shale in the Changning area in the Southern Sichuan basin, focusing on 28 core samples from Well N1. Using scanning electron microscopy, QEMSCAN, TOC, XRD, and major and trace element analyses, we reconstructed the paleoenvironmental transitions of this period and explored their control over shale lithofacies types and mineral compositions. Four shale lithofacies were identified: carbonate rich lithofacies (CRF), biogenic quartz-rich lithofacies (BQRF), detrital clay-rich lithofacies (CRDF), and detrital quartz-rich lithofacies (DQRF). During the Rhuddanian period, rising global temperatures caused glacial melting and rapid marine transgressions. The low oxygen levels in bottom waters, combined with upwelling and abundant volcanic material, led to high paleoproductivity. This period primarily developed BQRF and CRF. Rich nutrients and abundant siliceous organisms, along with anoxic to anaerobic conditions, provided the material basis and preservation conditions for high biogenic quartz and organic matter content. High paleoproductivity and anoxic conditions also facilitated the precipitation of synsedimentary calcite and supplied Mg2+ and SO42− for the formation of iron-poor dolomite via sulfate reduction. From the Late Rhuddanian to the Mid-Aeronian, the Guangxi orogeny caused sea levels to fall, increasing water oxidation and reducing upwelling and volcanic activity, which lowered paleoproductivity. Rapid sedimentation rates, stepwise global temperature increases, and the intermittent intensification of weathering affected terrigenous clastic input, resulting in the alternating deposition of CRF, CRDF, and DQRF. Two favorable shale gas reservoirs were identified from the Rhuddanian–Aeronian period: Type I (BQRF) in the L1–L3 Layers, characterized by high TOC and brittleness, and Type II (DQRF) in the L4 Layer, with significant detrital quartz content. The Type I-favorable reservoir supports ongoing gas production, and the Type II-favorable reservoir offers potential as a future exploration target.

The Impact of Fractures on Shale Oil and Gas Enrichment and Mobility: A Case Study of the Qingshankou Formation in the Gulong Depression of the Songliao Basin, NE China

To study the impact of faults on the enrichment and mobility of shale oil in the Gulong area, representative rock samples were selected in this paper. Based on geochemical data and chemical kinetics methods, coupled with shale oil enrichment and mobility analysis techniques, the shale oil generation quantity and in situ oil content were evaluated from the perspectives of shale oil generation and micro migration, and the mobility of shale oil was revealed. At the same time, the hydrocarbon expulsion efficiency (HEE) of shale was qualitatively and quantitatively characterized, combined with the development of faults. The research results indicate that the study area mainly develops organic-rich felsic (ORF)/organic-containing felsic (OCF) shale, their proportion in both wells exceeds 65%, and the resource amount is the largest in this type of lithofacies. The development of a fault controls the enrichment of shale oil, and the in situ oil content and oil saturation index (OSI) of the shale in well Y58, which is close to the fault, are significantly worse than those in well S2. Well Y58 has 9.52 mg/g and 424.83 mg/g TOC respectively, while well S2 has 11.34 mg/g and 488.73 mg/g TOC respectively. The fault enhanced the migration of shale oil, increasing the efficiency of oil expulsion. As a result, the components with weak polarity or small molecules, such as saturated hydrocarbons and low carbon number n-alkanes, are prone to migration, reducing the mobility of shale oil.

Structural detachment influences the shale gas preservation in the Wufeng-Longmaxi Formation, Northern Guizhou Province

Abstract In order to reveal the restriction in shale gas enrichment of the Wufeng-Longmaxi Formation in the northern Guizhou province, the influence model of detachment layer was established through field geological investigation, core observation, logging, sample analysis, and geological background data. The response relationship between the detachment layer and the shale gas enrichment model in different structural formats was analyzed. The results show that the thickness of the Wufeng-Longmaxi Formation’s detachment layer is influenced by the conditions near the fault zones and mineralogical characteristics. The lithofacies of the detachment layer shows mainly a combination of clay-rich shale facies. This indicates that lithofacies type is one of the main factors influencing the variation in slip layer thickness. The detachment layer exhibits distinct well logging response characteristics and is influenced by nitrogen enrichment. The development of detachment fractures allows atmospheric nitrogen to infiltrate shale gas. It leads to poor gas saturation in the shale gas. In addition, the overall tectonic deformation in the northern Guizhou province was found to gradually intensify from Northwest to Southeast, and there were two tectonic models: a slot-shift tape transition belt and a spacer type deformation belt. The influence of decollements on shale gas preservation was barely found in the northern Guizhou province. It is mainly controlled by buried depth of the target layer, conditions of the cover layer, structural type, and deformation intensity.

Stratigraphic-chemostratigraphic assessment of early dolomitization and associated phosphogenesis of the Ediacaran-Cambrian Salitre Formation, Irecê Basin, Brazil

During the Precambrian − Cambrian transition, the occurrence of phosphogenesis and the rise in atmospheric oxygen levels are closely linked to the proliferation of life. However, the mechanisms associated with these phenomena and their underlying causes are not well understood. The Irecê Basin represents the remnants of intracratonic marine environments that document the Neoproterozoic sea flooding of West Gondwana. This study presents an integrated analysis of sedimentary facies and chemostratigraphy through high-resolution sampling of pristine limestones and dolomites from the Salitre Formation in east-central Brazil. The aim is to explore the relationship between environmental settings and phosphatized stromatolites by combining detailed facies descriptions with isotopic and geochemical data. A total of fifteen lithofacies types are identified and grouped into seven major facies associations. These associations reveal a depositional stacking pattern defined by flooding surfaces and transgressive–regressive cycles, allowing for the division of the Salitre Formation into two depositional sequences. The basal sequence includes sedimentary phosphorite deposits intimately associated with widespread dolomitization levels at the top. Here, we describe a combination of two major processes for the dolomitization model of the Salitre Formation, with a contribution of evaporative environment settings related to a sabkha, and the influence of a meteoric mixing zone. Furthermore, three stages of phosphogenesis have been identified, with two of them interpreted as early diagenetic deposition associated with stromatolites and early cementation, and the last one associated with a late stage of phosphogenesis. The combined analysis of REY patterns, 87Sr/86Sr ratios, and geochemical results suggests a significant continental influence on both dolomitization and phosphogenesis. The identification of depositional settings and stratigraphic patterns for these deposits is crucial for the understanding of phosphogenesis during the Ediacaran, which may have been influenced by the continentalization of the marine environments forced by the intracratonic marine setting and the tectonic evolution at the end of the Neoproterozoic Era.

The influence of different diagenesis on the elastic properties of different shale lithofacies: a case study of the upper Permian Wujiaping formation in East Sichuan Basin, China

Understanding the impact of diagenesis on the elastic properties of organic-rich shale reservoirs is essential for evaluating unconventional hydrocarbon reservoirs and interpreting seismic data. Recent advancements in the exploration of the Permian Wujiaping Formation in the eastern Sichuan Basin indicate its potential to become a significant succession within the Sichuan Basin. However, the effect of different lithofacies in the Wujiaping Formation on shale elastic properties under varying diagenetic conditions remains unclear, hindering detailed reservoir interpretation. This study employs X-ray diffraction, thin section analysis, scanning electron microscopy, organic geochemistry, and dynamic elastic property tests to investigate the Wujiaping Formation shale. The results reveal three primary lithofacies types: argillaceous shale, mixed shale, and siliceous shale. Argillaceous shale, subjected to intense compaction, forms a dense rock framework of oriented clay minerals, characterized by low porosity (1.66%), low elastic wave velocity (4122.30 m/s), low elastic modulus (2174.59 m/s), and high Poisson's ratio (32.24 GPa). Mixed shale, dominated by carbonates and quartz, exhibits a rock framework formed through dissolution and cementation, with high elastic wave velocity (5196.54 m/s), relatively high elastic modulus (2975.86 m/s), and moderate Poisson's ratio (58.53 GPa). Siliceous shale, comprising biogenic quartz particles, shows strong resistance to compaction. During hydrocarbon generation, it develops abundant organic matter pores, resulting in the highest porosity (2.36%), high elastic wave velocity (5177.92 m/s), high elastic modulus (2975.86 m/s), and low Poisson's ratio (62.23 GPa). The significant differences in mineral composition and diagenetic processes across the lithofacies lead to distinct elastic properties. This study provides a rock physics framework for the detailed seismic prediction of "sweet spots" in the Wujiaping Formation shale reservoirs and offers new insights into characterizing the diagenesis of unconventional shale reservoirs using geophysical properties.

Classification of rock types of porous limestone reservoirs: case study of the A oilfield

Rock types with similar lithological components and pore structures form the basic units of porous limestone reservoirs; this influences the reservoir evaluation efficiency and water injection development. As the main oil and gas pay zone in central Iraq, the Cretaceous Khasib Formation reservoirs are influenced by deposition, dissolution, and cementation. There is strong vertical heterogeneity in the most important zone of the Kh2 layer, with diverse rock types and complex pore structures. Based on core observation and casting thin-section identification, the Kh2 layer in the study area was divided into eight lithofacies types as argillaceous bioclastic wackestone, planktic foraminiferium wackestone, lamellar bioclastic wackestone, intraclastic–bioclastic packstone, patchy green algae packstone, green algae and pelletoid packstone, benthic foraminiferium–bioclastic packstone, and intraclastic grainstone. Along with the reservoir void space types of the lithofacies, capillary pressure curves are used to quantitatively analyze the throat and pore features of the different lithofacies. From the porosity–permeability cross-plot characteristics and distribution of pore types, 14 petrophysical facies are obtained. Finally, based on the differences between the lithofacies and petrophysical facies, the Kh2 member is divided into 13 rock types with different geological origins and petrophysical characteristics. Among these, the rock type RT1-8-14 has the best and rock type RT1-1-1 has the worst physical properties among the reservoir rock types. This study provides an optimization method for carbonate reservoir evaluation and is expected to be beneficial for efficient development of similar carbonate reservoirs.

Correlation and response of astronomical forcing in lacustrine deposits of the middle jurassic, sichuan basin, southwest China

The astronomical cycle characteristics of Mesozoic lacustrine fine-grained sedimentary rocks play a crucial role in understanding the formation environment of such sediments. The fine-grained sedimentary rocks of the Middle Jurassic Lianggaoshan Formation in the Sichuan Basin, China, constitute a significant lacustrine deposit. Previous research has primarily focused on its reservoir characteristics, with limited understanding of the relationship between lithofacies changes and astronomical cycles. To fill this knowledge void, we provide a continuous lacustrine core record of approximately 260 m from the XY-4 well, spanning the Lianggaoshan Formation in the Eastern Sichuan Basin. Initially, we categorized it into eight lithofacies types and established a lithologic ranking sequence based on this classification. Through time-series analysis, we identified the 405 Kyr long eccentricity cyc lecycle, 125 Kyr short eccentricity cycle, obliquity, and precession cycle. We emphasize the significance of the 125 Kyr short eccentricity cycle as the primary controlling factor during the deposition of the Lianggaoshan Formation. Subsequently, based on the 405 Kyr long eccentricity cycle, we established an age model for astronomical tuning, creating an astronomical age scale. The calculated sedimentation interval for the Lianggaoshan Formation ranges from 170.00 Ma to 172.74 Ma, spanning a total duration of 2.74 Ma. Meanwhile, the sedimentation interval for the Lower Liang 2 Submember is determined to be from 171.54 Ma to 172.52 Ma, lasting 0.98 Ma. Further comparison of TOC content and mineral composition with the orbital cycles reveals that TOC content and clay mineral content exhibit in-phase variations with the 125 Kyr short eccentricity. This characteristic contradicts the features of Cenozoic continental basins but aligns consistently with the characteristics of the Middle Triassic in North China. In contrast, silica minerals and carbonate minerals display anti-phase variations with the 125 Kyr short eccentricity. Different lithofacies demonstrate distinct responses to orbital cycles. Lithofacies characterized by higher organic matter content generally develop during the half-period with the maximum eccentricity. In contrast, lithofacies with lower organic matter content tend to occur during the half-cycle associated with the minimum eccentricity. Finally, based on the integrated analysis of lithological characteristics and astronomical cycles, we established two sedimentary modes: “Seasonally Pronounced Type” corresponding to the maximum eccentricity and “Seasonally Obscure Type” corresponding to the minimum eccentricity.

Lithofacies Characteristics of Continental Lacustrine Fine-Grained Sedimentary Rocks and Their Coupling Relationship with Sedimentary Environments: Insights from the Shahejie Formation, Dongying Sag

Lacustrine fine-grained sedimentary rocks in the Dongying Sag of the Bohai Bay Basin in China exhibit significant potential for hydrocarbon exploration. This study investigates the lithofacies types and sedimentary evolution of the Paleogene Shahejie Formation’s lower third member (Es3l) and upper fourth member (Es4u), integrating petrological and geochemical analyses to explore the relationship between lithofacies characteristics and sedimentary environments. The results show that the fine-grained sedimentary rocks in the study area can be classified into 18 lithofacies, with seven principal ones, including organic-rich laminated carbonate fine-grained mixed sedimentary rock lithofacies and organic-rich laminated limestone lithofacies. In conjunction with analyses of vertical changes in geochemical proxies such as paleoclimate (e.g., CIA, Na/Al), paleoproductivity (e.g., Ba), paleosalinity (e.g., Sr/Ba), paleo-redox conditions (e.g., V/Sc, V/V + Ni), and terrigenous detrital influx (e.g., Al, Ti), five stages are delineated from bottom to top. These stages demonstrate a general transition from an arid to humid paleoclimate, a steady increase in paleoproductivity, a gradual decrease in paleosalinity, an overall reducing water body environment, and an increasing trend of terrestrial detrital input. This study demonstrates that the abundance of organic matter is primarily influenced by paleoproductivity and paleo-redox conditions. The variations in rock components are predominantly influenced by paleoclimate, and sedimentary structures are affected by the depth of the lake basin. Special depositional events, such as storm events in Stage II, have significantly impacted the abundance of organic matter, rock components, and sedimentary structures by disturbing the water column and disrupting the reducing conditions at the lake bottom. The present study offers crucial insights into the genesis mechanisms of continental lacustrine fine-grained sedimentary rocks, facilitates the prediction of lithofacies distribution, and advances the exploration of China’s shale oil resources in lacustrine environments.

Volcanic lithofacies control the space in unconventional, rhyolitic hydrocarbon reservoirs: The Hailar Basin, NE China

Volcanic rocks are generally characterized by significant spatial variations of lithofacies. The characterization of volcanic lithofacies is an important tool to achieve accurate reservoir prediction during hydrocarbon exploration because of the type of lithofacies strictly controls the porosity and other physical and chemical properties of reservoir rocks. Studying the lithofacies of rhyolitic volcanic rocks is essential for uncovering the genesis and characteristics of volcanic reservoirs. The Lower Cretaceous Shangkuli Formation is located on the western boundary of the Hailar Basin in NE China. This basin hosts oil-bearing rhyolitic volcanic rocks and represents a natural laboratory for investigating the spatial distribution of different lithologies and volcanic lithofacies. Here, we present detailed field observations, TESCAN Integrated Mineral Analysis (TIMA), and fluorescence imaging to characterize seven types of rhyolitic volcanic rocks belonging to four main lithofacies associations. We examine the relationship between lithofacies associations and reservoir rocks. Field observations reveal the existence of volcanic conduit, pyroclastic, effusive, and extrusive lithofacies associations within the Hailar Basin. These lithofacies associations can be further subdivided into nine distinct subfacies. Among these, the extrusive lithofacies association is dominant and it is followed by pyroclastic, effusive, and volcanic conduit lithofacies associations. Intra-spherulite microfractures and pores, lithophysa cavities in the extrusive lithofacies associations, and inter-clast pores of the volcanic conduit lithofacies associations constitute the primary reservoir spaces of rhyolitic reservoirs. The lithofacies associations exert a control on the reservoir spaces of rhyolitic volcanic rocks by limiting the degree of supercooling (ΔT) and volatile content, which impact the formation of different textures in rhyolitic magma during syn-eruptive processes. We conclude that spherulitic and lithophysa rhyolites in the extrusive lithofacies associations, and the crypto-explosive breccias in the volcanic conduit lithofacies associations are promising targets for oil and gas exploration. The facies model we obtain from the Hailar Basin may be extended to similar rhyolitic rocks in other basins hosting unconventional reservoirs.

High-pressure mercury intrusion analysis of pore structure in typical lithofacies shale

In this paper, statistical research on the characterization of pore and fracture structures is systematically conducted on shale reservoirs in different blocks and under different lithofacies conditions. The impact of lithological characteristics on shale pore fracture structures is revealed, and the coupling mechanism of "high-pressure mercury injection, pore fracture structure, and lithofacies" is clarified. Results show that: (a). When the capillary force is equal to 0.0035 MPa, the pore throat radius is equal to 209.661 μm. (b). There exist turning pressures both in injection curve and removal curve. When the capillary pressure is lower than 100 MPa, the slope of injection curve is high, while when the capillary pressure is higher than 100 MPa, the slope of injection curve becomes lower. (c). The majority of pores is distributed in the pore size interval range from 0.063 μm to 0.004 μm. (d). There are also a certain amount of regular pores in inorganic pores. Part of the pores are secondary intergranular dissolved pores formed by later dissolution.

Pore Structure and Factors Controlling Shale Reservoir Quality: A Case Study of Chang 7 Formation in the Southern Ordos Basin, China

The lithofacies types, pore structure differences, and main controlling factors on the shale reservoirs are vital problems that need to be addressed in the process of shale oil exploration and development. This study explores the Luohe oilfield in the southern Ordos Basin, which is composed of organic-rich shale in the Chang 7 member, to clarify the reservoir properties and analyze major factors affecting the reservoir quality. The shale reservoir can be divided into five lithofacies using ternary diagrams of TOC, argillaceous minerals, and siliceous minerals: high organic-rich siliceous shale (HOSS), high organic-rich argillaceous shale (HOAS), medium organic-rich siliceous shale (MOSS), medium organic-rich argillaceous shale (MOAS), and low organic-rich shale (LOS). The type of organic matter in the studied samples is mainly Type I kerogen and Type II kerogen, predominantly Type II1 kerogen. The kerogen mostly lie within the mature zone in the study area. Various types of pores have been identified in the studied shale: intergranular pores, intragranular pores, intercrystalline pores, organic matter pores, and seams around organic matter. The pores are commonly nanoscale to micrometer in scale, with diameters ranging from 10 nm to several microns. The S1 content in shale is positively correlated with the macropore content, indicating that macropores in shale are the main effective oil storage spaces and are important for oil-bearing reservoirs. There is a good positive relationship between the macropore volume of shale and the content of organic matter. Organic matter in the shale can be beneficial for generating organic matter pores, dissolution pores, and seams at organic matter edge, resulting in better physical properties of shale reservoirs. There is a negative relationship between the quartz/feldspar content and macropores content, indicating that quartz and feldspar are detrimental for the macropore volume development. The lithofacies type is one of the important factors controlling the macropore volume. MOAS and HOAS are favorable lithofacies for the development of macropores. The findings of this study can be utilized for hydrocarbon exploration and development in the lacustrine shale formation of the Ordos Basin and other similar basins.

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.