Fine-grained Sedimentary Rocks Research Articles

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.

Depositional characteristics in the Lower Congo Basin during the Cenomanian-Turonian stage: Insights from fine-grained sedimentary rocks

The Cenomanian-Turonian (C-T) stage, marked by extensive black shales and Oceanic Anoxic Event 2 (OAE 2), is pivotal for deciphering paleoceanographic conditions and the main drivers of hydrocarbon formation. Despite its significant oil and gas potential, the Lower Congo Basin (LCB) in central West Africa, particularly during the C-T stage (Likouala Formation), has been severely understudied, primarily due to limited exploration and a historical preference for other basins. Therefore, understanding the depositional characteristics of the C-T stage in the LCB is essential for elucidating paleoceanographic conditions, evaluating hydrocarbon potential and interpreting environmental changes. Due to the limited availability of published data and samples, we employed a targeted and integrated geological analysis to gain comprehensive insights, encompassing logging, lithofacies identification, thin-section observations, grain size measurements, and geochemical analyses (major, trace, and rare earth elements), to provide a cohesive and confirmatory assessment. Utilizing relative sea-level curves and geochemical indicators, and by comparing with other relevant basins, this study preliminarily identified OAE 2 stratigraphic intervals and attempted to discuss the impact of OAE 2 on organic matter enrichment in the LCB. The results indicate that, during the C-T stage, the LCB featured a restricted sea bay environment with limited terrestrial input and anoxic waters. Multiple geochemical proxies suggest that relative sea-level changes significantly affected paleoproductivity, thereby influencing organic matter enrichment. During the depositional period of Lower Likouala Formation (LLF), a relative sea-level drop resulted in weakly reducing conditions in a marginal restricted sea bay environment, leading to low primary productivity. Conversely, during the depositional period of Upper Likouala Formation (ULF), enhanced aeolian input and sea-level rise increased nutrient supply from the open ocean. This led to elevated surface marine productivity and intensified reducing conditions in the water column, forming a marginal semi-restricted sea bay environment. Enhanced reducing conditions within the lower stratigraphic intervals of the ULF are likely consequences of OAE 2. However, the marginal restricted sea bay environment and limited water exchange may have constrained the enhancement of paleoproductivity by OAE 2. These insights not only significantly enhance our understanding of the organic-rich fine-grained sedimentary rocks in the LCB during the C-T stage, but also contribute to a broader comprehension of the South Atlantic's paleoceanographic conditions and the interplay between regional geological processes and global climate events during the C-T stage.

Geochemistry of fine-grained sedimentary rocks: Implications for source area, paleoweathering, paleoclimate, and provenance of the Santa Maria Formation (Triassic, Paraná Basin, Brazil)

The integration of ICP-OES, XRD, chemical analysis, and scanning electron microscopy was used to investigate the source area, weathering, paleoclimate conditions, and provenance of the siltstones of the Santa Maria Formation (Middle to Upper Triassic, Southern Brazil). This unit is regionally subdivided, from base to top, into the Passo das Tropas Member (Late Anisian–Middle Ladinian) and the Alemoa Member (Middle Ladinian–Carnian). The Chemical Index of Alteration (CIA) was calculated for both members to highlight the weathering processes of primary minerals. The CIA results indicate moderate to extreme weathering of the source rocks for the siltstones. The extreme chemical alteration observed in the Passo das Tropas siltstones (CIA = 76.97–90.83) and in the mid-section of the Alemoa Member (CIA = 87.96–89.63) suggests weathering under more humid conditions, related to an increase in paleorainfall. Paleoclimate was interpreted as semi-arid to arid, with alternating wet and dry seasons. These results align with the geochemical behavior of uranium and thorium. Samples with higher CIA values show higher Th/U ratios, indicating a shift toward a more humid climate. X-ray diffraction analysis of the fine fraction (FF < 2 μm) of the samples identified mixed-layered illite-smectite (I/S) clay minerals with minor amounts of quartz. Regarding provenance, Th/U ratios coupled with Th vs. Sc and La-Th-Sc plots, suggest an upper crustal protolith for the southern Santa Maria siltstones, consisting of uplifted rocks from the crystalline basement of the Sul-Rio-Grandense shield and sedimentary rocks of the Camaquã Basin. The Th/Sc, Sc/Th, Co/Th, and La/Sc ratios, along with the contents of Al₂O₃, SiO₂, TiO₂, Co, V, Ni, and Sc, indicate a more felsic composition for the Passo das Tropas Member. Meanwhile, the source rocks of the Alemoa Member sediments demonstrate a more mafic composition in the mid-section, transitioning to a felsic composition in the uppermost succession.

Control and prediction of bedding-parallel fractures in fine-grained sedimentary rocks: A case from the Permian Lucaogou Formation in Jimusar Sag, Junggar Basin, Western China

The fine-grained sedimentary rocks have numerous bedding-parallel fractures that are essential for the migration, enrichment, and efficient development of oil and gas. However, because of their variety and the complexity of the factors that affect them, their spatial prediction by the industrial community becomes challenging. Based on sample cores, thin sections, and well-logging and seismic data, this study employed a multi-scale data matching approach to quantitatively predict the development of bedding-parallel fractures and investigate their spatial distribution. Bedding-parallel fractures in the Lucaogou Formation in Jimusar Sag frequently occur along preexisting bedding planes and lithological interfaces. Unfilled bedding-parallel fractures inside or near source-rocks exhibit enhanced oil-bearing capacity. They were identified on micro-resistivity scanning images by the presence of regularly continuous black or nearly black sinusoidal curves. Overall, the developmental degree of bedding-parallel fractures was positively related to the brittle mineral and total organic carbon contents and negatively related to single reservoir interval thickness. The maintained porosity of the reservoir matrix contributed to a thorough response to factors affecting the development of bedding-parallel fractures. Here, an effective and objective method was proposed for predicting the development and distribution of bedding-parallel fractures in the fine-grained sedimentary rocks. The method was based on the matched reservoir interval density, reservoir interval density, and matched sweet spot density of bedding-parallel fractures. The prediction method integrated the significant advantages of high vertical resolution from logging curves and strong lateral continuity from seismic data. The average relative prediction error was 8% in the upper sweet spot in the Lucaogou Formation, indicating that the evaluation parameters for bedding-parallel fractures in fine-grained sedimentary rocks were reasonable and reliable and that the proposed prediction method has a stronger adaptability than the previously reported methods. The workflow based on multi-scale matching and stepwise progression can be applied in similar fine-grained sedimentary rocks, providing reliable technological support for the exploration and development of hydrocarbons.

Source Rock Characteristics of Loa Janan Area, Kutai Kartanegara, East Kalimantan, Indonesia

The source rock analysis was involved examining the geochemical content of hydrocarbons in fine-grained sedimentary rocks. This study was conducted on claystone in the Loa Duri area to determine the source rock characterization, which includes the level of richness, the organic matter type, and the level of maturity. The methods used are field data collection and laboratory analysis. A field study is used to collect rock sampling, while laboratory analysis consists of the analysis of Total Organic Carbon content, Rock-Eval pyrolysis, and vitrinite reflectance (Ro). The results of the analysis of Total Organic Carbon on six claystone samples showed values ranging from 0.50-8.97%. Generally, they had the potential to form hydrocarbons in the moderate to special category. Rock-Eval pyrolysis shows Hydrogen Index values from 52–149 mg HC/g Total Organic Carbon has the potential to generate gas, with Tmax values ranging from 414–432, indicating an immature to early mature level of thermal maturity. Ro analysis on three claystone samples with values ranging from 0.40–0.48 %. The hydrocarbon potential of the research area indicates the category of organic material richness is quite excellent, with kerogen including type III. The quality of the source rock based on the value of the Hydrogen Index is included in the category of gas-prone. The low maturity value (both Tmax and Ro) is thought to be caused by the suppression phenomenon in coal-dominated areas, so the values displayed by the two parameters do not correspond to actual values.

Distribution Characteristics and Hydrocarbon Significance of Deep-Water Fine-Grained Sedimentary Rocks in the Steep-Slope Zone of a Graben Lake Basin: A Case Study of Es3l sub-Member in the Jiyang Depression, Bohai Bay Basin, China

Distribution Characteristics and Hydrocarbon Significance of Deep-Water Fine-Grained Sedimentary Rocks in the Steep-Slope Zone of a Graben Lake Basin: A Case Study of Es3l sub-Member in the Jiyang Depression, Bohai Bay Basin, China

Mapping the composite nature of clay matrix in mudstones: integrated micromechanics profiling by high-throughput nanoindentation and data analysis

AbstractMudstones and shales serve as natural barrier rocks in various geoenergy applications. Although many studies have investigated their mechanical properties, characterizing these parameters at the microscale remains challenging due to their fine-grained nature and susceptibility to microstructural damage introduced during sample preparation. This study aims to investigate the micromechanical properties of clay matrix composite in mudstones by combining high-speed nanoindentation mapping and machine learning data analysis. The nanoindentation approach effectively captured the heterogeneity in high-resolution mechanical property maps. Utilizing machine learning-based k-means clustering, the mechanical characteristics of matrix clay, brittle minerals, as well as measurements on grain boundaries and structural discontinuities (e.g., cracks) were successfully distinguished. The classification results were validated through correlation with broad ion beam-scanning electron microscopy images. The resulting average reduced elastic modulus (Er) and hardness (H) values for the clay matrix were determined to be 16.2 ± 6.2 and 0.5 ± 0.5 GPa, respectively, showing consistency across different test settings and indenter tips. Furthermore, the sensitivity of indentation measurements to various factors was investigated, revealing limited sensitivity to indentation depth and tip geometry (when comparing Cube corner and Berkovich tip in a small range of indentation depth variations), but decreased stability at lower loading rates. Box counting and bootstrapping methods were applied to assess the representativeness of parameters determined for the clay matrix. A relatively small dataset (indentation number = 60) is needed to achieve representativeness, while the main challenges is to cover a representative mapping area for clay matrix characterization. Overall, this study demonstrates the feasibility of high-speed nanoindentation mapping combined with data analysis for micromechanical characterization of the clay matrix in mudstones, paving the way for efficient analysis of similar fine-grained sedimentary rocks.

Selective anion exchange adsorption of molybdenum(VI) at low concentrations from an acid leached vanadium shale solution containing aluminium and phosphorus

With a decline in high-grade molybdenum reserves, the development of other types of molybdenum resources have received increasing attention. Vanadium shale is a multi-metal shale and fine-grained sedimentary rock comprising small grains and various minerals. After leaching and extracting vanadium, the solution often contains a low concentration of molybdenum. However, because of the low molybdenum concentration, many processing plants treat it as an acidic wastewater, which wastes molybdenum resources and carries the risk of environmental pollution. The leaching process of vanadium shale mostly involves high-temperature and high-pressure operations, which greatly increase the content of impurity ions such as aluminium and phosphorus in the pregnant leach solution. These impurity ions increase the difficulty in separating and recovering molybdenum. In this study, the adsorption and separation of molybdenum at leach liquor of low molybdenum concentration were investigated. The effects of different factors such as: (i) pH of feed solution, (ii) adsorption time, and (iii) presence of impurity ions, aluminium and phosphorus, on the adsorption and separation of molybdenum using five different anion-exchange resins, D201, D296, D301, D314, and D301R, were investigated. The static adsorption and desorption test results showed a molybdenum adsorption capacity of 222 mg/g at pH = 1.5 by the D301 resin. The desorption efficiency using 20% NH₄OH was 96.1%. The adsorption efficiencies of aluminium and phosphorus were 1.31% and 3.10%, respectively. This is a better choice for separating molybdenum from complex solutions. The experimental results from spectra and theoretical calculations showed that the -NH group of D301 resin was combined with the O atoms of MoO3·3H2O, Al(SO4)2−, and H2PO4− by electrostatic attraction. The binding energies of these three species were − 311 kJ/mol, −231 kJ/mol, and − 62.0 kJ/mol respectively, indicating that D301 resin preferentially adsorbed MoO3·3H2O. Based on the above results, the D301 resin can adsorb molybdenum(VI) in complex solutions under low pH conditions, and this study is expected to promote the comprehensive recovery of valuable metals from vanadium shale.

The dominant influence of muddy bottom currents on organic-rich fine-grained sedimentary rocks: The lower Congo Basin, west Africa

The Madingo Formation in the Lower Congo Basin (LCB) of West Africa is a fine-grained strata, which is considered to be the most promising source interval during the drift stage. Despite the extensive studies of the internal sedimentary characteristics of the Lower Mading Formation (LMF), muddy bottom currents have not been previously identified in this area. Extensive seismic (2D and 3D data), wire - line measurements and cutting samples in the LCB offer a unique opportunity to study the influence of muddy bottom currents depositional system on organic matter, and related depositional environment of the region and its depositional model of organic rich fine-grained strata. This study aims to unravel the influence of muddy bottom currents on organic matter enrichment within the LMF and build a depositional model. In combination with drilling and thin sections analysis, the M-1 member suggests the initiation and cessation of muddy bottom currents. Utilizing 3D seismic data inversion techniques, geochemical analysis, and sedimentological assessment, we studied the effect of muddy bottom currents on organic matter enrichment from the perspectives of paleoproductivity, preservation conditions, terrigenous input, and total organic carbon (TOC) distribution. Results show that muddy bottom currents have a significant impact on the redistribution of shallow water shelf sediments. They promote the transportation of nutrients and organic matter to deep water shelves, improving paleoproductivity. Most notably, terrigenous input is a key factor affecting organic matter distribution, resulting in dilution effects in deep water shelf bottom current influence areas, but conducive to the preservation of deep organic matter in low-lying areas of shallow water shelf. Muddy bottom currents have a weaker influence on the preservation environment of deep water shelves, but they have a significant effect on redox indicators (eg. U/Th, Ni/Co, V/Cr). Specifically, they reduce the enrichment in particulate elements. We established the depositional models of organic-rich fine-grained sedimentary rocks for the M-1 and M-2 members in LCB. These insights not only provide practical depositional models for marine sources rocks in the LCB, but also improves the general understanding of marine organic matter accumulation in muddy bottom currents environments. Furthermore, they serve as a reference for the study of organic-rich fine-grained sedimentary rocks in similar basins.

A Grand Challenge: Update on Improved Recovery From Tight/Shale Reservoirs

_ This is the third of a series of six articles on SPE’s Grand Challenges in Energy, formulated as the output of a 2023 workshop held by the SPE Research and Development Technical Section in Austin, Texas. Described in a JPT article last year, each of the challenges will be discussed separately in this series: geothermal energy; net-zero operations; improving recovery from tight/shale resources; carbon capture, storage, and utilization; digital transformation; and education and advocacy. _ The exploration and extraction of hydrocarbons from tight or shale formations have revolutionized the global energy landscape, unlocking vast oil and gas reserves previously considered inaccessible. According to estimates, substantial oil production has been reported for various tight plays in the US in 2024 (Fig. 1). However, the production process in tight or shale formations is not without its own set of challenges ranging from technological hurdles to environmental concerns. Therefore, understanding the current outlook is essential for stakeholders to navigate the complex terrain. To address the significant issues outlined by SPE in July 2023 (Halsey et al. 2023), this article intends to explore the complexities of hydrocarbon extraction from tight and shale reservoirs. Description of Tight/Shale Formations First, we present a succinct description of tight or shale formations. Tight or shale oil and gas are hydrocarbons found in oil-bearing mudstone, and shale gas is produced from gas shale or associated with tight oil (Boak and Kleinberg 2020). Tight unconventional formations are characterized by the presence of fine-grained sedimentary rocks that are high in organic matter, such as shale, where hydrocarbons are firmly embedded within the rock matrix. Challenges in Tight or Shale Oil Production The Existence of Complex Geometry. Tight and shale formations are known for their complex geological structures, which can make it difficult to predict the distribution and behavior of hydrocarbons within the reservoir. Additionally, shale formations can exhibit significant variability in their thickness, composition, and the occurrence of natural fractures. Hence, it is crucial to understand and navigate the intricate geology of unconventional shale formations to achieve successful shale oil production (Jiang et al. 2016). Ultralow Reservoir Permeability. The low permeability of tight formations restricts the flow of hydrocarbons, necessitating techniques such as hydraulic fracturing to create artificial pathways for extraction. However, the effective stimulation of fractures requires the use of cutting-edge drilling and extraction methods, such as multistage hydraulic fracturing (fracking) and horizontal drilling, which in turn require precise engineering and a thorough understanding of reservoir characteristics (Pokalai et al. 2015).

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.

The relationship between total organic carbon and bottom water redox state in North American black shales

It is clear from modern analogue studies that O2-deficient conditions favor preservation of organic matter (OM) in fine-grained sedimentary rocks (black shales). It is also clear that appreciable productivity and OM flux to the sediment are required to establish and maintain these conditions. However, debates regarding redox controls on OM accumulation in black shales have mainly focused on oxic versus anoxic conditions, and the implications of different anoxic redox states remain unexplored. Here, we present detailed multi-proxy sedimentary geochemical studies of major Paleozoic and Mesozoic North American black shale units to elucidate their depositional redox conditions. This is the first broad-scale study to use a consistent geochemical methodology and to incorporate data from Fe-speciation – presently the only redox proxy able to clearly distinguish anoxic depositional conditions as ferruginous (H2S-limited) or euxinic (H2S-replete, Fe-limited). These data are coupled with total organic carbon (TOC), programmed pyrolysis, and redox-sensitive trace element proxies, with almost all measurements analyzed using the same geochemical methodology. Consistent with expectations based on previous geochemical and paleontological/ichnological studies, these analyses demonstrate that the study units were almost exclusively deposited under anoxic bottom waters. These analyses also demonstrate that there is wide variance in the prevalence of euxinic versus ferruginous conditions, with many North American black shale units deposited under predominantly ferruginous or oscillatory conditions. TOC is significantly higher under euxinic bottom waters in analyses of both preserved (present day) TOC and reconstructed initial TOC values, although sediments deposited under both redox states do have economically viable TOC content. While this correlation does not reveal the mechanism behind higher organic enrichment in euxinic environments, which may be different in different basins, it does open new research avenues regarding resource exploration and the biogeochemistry of ancient reducing environments.

Sedimentology, depositional processes, and sequence stratigraphy of deep lacustrine fine-grained sedimentary rocks in the lower Oligocene Xiaganchaigou Formation, Qaidam Basin, Northwestern China

Lacustrine deep-water fine-grained sedimentary rocks are characterized by high heterogeneities in lithology and lithofacies stacking patterns, controlled by sediment supply and lake level fluctuation. Despite their vital roles as source and reservoir rocks of unconventional oil and gas play, depositional processes and sedimentary sequence of lacustrine fine-grained sedimentary rocks are poorly understood compared to their extensively studied deep marine counterparts. Here, we studied lithofacies, transportation and depositional processes, depositional environments, and sedimentary cycles of deep-water fine-grained deposits of the Lower Oligocene Xiaganchaigou Formation from cored wells in the western Qaidam Basin. Nine lithofacies that stack repetitively in 6–18 m thick sedimentary cycles are identified based on petrographic observation, geochemical analysis, and scanning electron microscopy. The regressive systems tract of a sedimentary cycle is composed of detrital-rich massive fine sandstone deposited by debris flows and thin-bedded fine sandstone deposited by high-density turbidity currents. The transgressive systems tract is dominated by laminated carbonate-rich mudrock facies deposited by low-density turbidity currents. A complete sedimentary cycle is interpreted to represent a transgressive-regressive cycle controlled by fifth-order lake-level variation. Coarse-grained detrital-rich debrite and high-density turbidite were deposited in the proximal sublacustrine fan and channel during lake-level regression, with sediments mainly supplied by terrigenous siliciclastic input. Carbonate-rich low-density turbidites are deposited primarily in the distal sublacustrine fan during lake-level transgression. Thin-laminated carbonate-rich mudrock facies with high organic matter abundance and well-developed laminae-induced fractures are considered both source rocks and target reservoirs of the upper Xiaganchaigou Formation.

Palynostratigraphy and Bayesian age stratigraphic model of new CA-ID-TIMS zircon ages from the Walloon Coal Measures, Surat Basin, Australia

The Surat Basin hosts significant coal and coal seam gas resources. New high-precision CA-TIMS U/Pb zircon ages from tuffs and Bayesian age stratigraphic models are combined with palynology from fine-grained sedimentary rocks and zircon trace elements to provide further chronostratigraphic and biostratigraphic constrains on the Walloon Coal Measures in the eastern margin of the Surat Basin and infer the palaeoenvironment and tectonic setting. The tuff ages range from 165.88 ± 0.11 Ma to 158.84 ± 0.05 Ma, with those from the stratigraphically lower Taroom Coal Measures ranging from 165.88 ± 0.11 to 163.05 ± 0.08 Ma and Juandah Coal Measures ranging from 159.91 ± 0.04 to 158.84 ± 0.05 Ma. This corroborates that the lower part of the Walloon Coal Measures is Callovian and the upper part is Oxfordian. The palynology results from mudstones show that all samples are dominated by microfossils of spore-pollen with conifers being the most abundant. Our samples fall within Price’s (1997) stratigraphic zonation of APJ4.2 and APJ4.3. Posterior ages for palynology samples were estimated through Bayesian age stratigraphic modelling using stratigraphic depths and U-Pb zircon ages. The palaeoenvironment in the eastern portion of the basin is inferred to be predominantly fluvial, with spores and pollen derived from fresh water or terrestrial plants. Higher concentrations of green algae in one sample suggest that at times the water was somewhat stagnant. The zircons were derived from predominantly intermediate magmas, as indicated by the generally low Ti, Ta, and Nb values. The tectonic environment that the zircons were derived from was most likely a continental subduction zone due to their high U/Yb, low Nb/Yb and relatively low Hf concentrations. These new data support previous conclusions of the Surat Basin palaeoenvironment, contribute to the ongoing discussion about the tectonic setting of the basin and add new regional age marker horizons.

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.

Lacustrine shale lithofacies and depositional environment in the paleocene second member of the funing formation, subei basin, China: Insights into shale oil development prospects

Lacustrine fine-grained sedimentary rocks play a crucial role in the petroleum industry, particularly in the exploration of unconventional oil and gas resources. This study focuses on a petroliferous fine-grained succession deposited prior to the Paleocene–Eocene Thermal Maximum (PETM), specifically, the Paleocene second member of the Funing Formation. This formation represents the primary target for shale oil exploration in the Qintong Depression of the Subei Basin. Employing detailed sedimentological descriptions and utilizing multiple geochemical proxies, our investigation aimed to investigate lithofacies distribution and reconstruct the paleoenvironmental conditions prevalent in the Qintong Depression. Through the integration of lithological observation and well log data, the second member of the Funing Formation was categorized into three distinct system tracts: the early expanding system tract (EEST), the late expanding system tract (LEST), and the highstand system tract (HST). Within the established sequence stratigraphy framework, six lithofacies were identified, displaying comparable lateral and planar distributions. Results indicate that lithofacies exhibit greater thickness in the Deep Sag zone compared to the Slope and Fault Terrace. The EEST is characterized by laminated calcareous mudstone (LCM), laminated dolomitic mudstone (LDM), and massive argillaceous dolomite (MAD). Whereas the LEST predominantly comprises LCM and massive calcareous mudstone (MCM). These lithofacies were deposited under relatively arid climatic conditions, characterized by high salinity and strong terrigenous input, with a moderate abundance of organic matter consisting of type Ⅱ and type Ⅲ kerogen. In contrast, the HST is dominated by MCM with stable thickness, indicative of deposition under relatively humid climatic conditions with low salinity and terrigenous input. These conditions fostered an environment conducive to plankton and algae, resulting in a high abundance of organic matter composed of type Ⅰ kerogen. These new characterizations have significant implications for considering favorable lithofacies regarding the vertical orientation and target zones regarding the planarity of shale oil resources in the Qintong Depression of the Subei Basin.

Influences of lithofacies on fluid mobility in mixed sedimentary rocks: Insights from NMR analysis of the middle Permian Lucaogou Formation, Junggar Basin

The multi-source mixed sedimentation resulted in a unique series of mixed fine-grained sedimentary rocks evolved within the Permian Lucaogou Formation in the Jimusar Sag, located in the southeastern Junggar Basin, China. The variety of lithofacies within this series resulted in pronounced heterogeneity of pore structures, complicating the analysis of fluid occurrence space and state within reservoirs. As a result, the impact of lithofacies on fluid mobility remains ambiguous. In this study, we employed qualitative methods, such as field emission scanning electron microscopy (FE-SEM) and thin section observation, and quantitative analyses, including X-ray diffraction (XRD), total organic carbon (TOC), vitrinite reflectance (Ro), high-pressure mercury intrusion (HPMI) porosimetry, and nuclear magnetic resonance (NMR), along with linear and grey correlation analyses. This approach helped delineate the effective pore characteristics and principal factors influencing movable fluids in the fine-grained mixed rocks of the Lucaogou Formation in the Jimusar Sag, Junggar Basin. The findings indicate the development of three fundamental lithologies within the Lucaogou Formation: fine sandstone, siltstone, and mudstone. Siltstones exhibit the highest movable fluid saturation (MFS), followed by fine sandstones and mudstones sequentially. Fluid mobility is predominantly governed by the content of brittle minerals, the sorting coefficient (Sc), effective pore connectivity (EPC), and the fractal dimension (D2). High content of brittle minerals favors the preservation of intergranular pores and the generation of microcracks, thus offering more occurrence space for movable fluids. A moderate Sc indicates the presence of larger connecting throats between pores, enhancing fluid mobility. Elevated EPC suggests more interconnected pore throat spaces, facilitating fluid movement. A higher D2 implies a more intricate effective pore structure, increasing the surface area of the rough pores and thereby impeding fluid mobility. Ultimately, this study developed a conceptual model that illustrates fluid distribution patterns across different reservoirs in the Lucaogou Formation, incorporating sedimentary contexts. This model also serves as a theoretical framework for assessing fluid mobility and devising engineering strategies for hydrocarbon exploitation in mixed fine-grained sedimentary rocks.

Comprehensive Comparison of Lacustrine Fine-Grained Sedimentary Rock Reservoirs, Organic Matter, and Palaeoenvironment: A Case Study of the Jurassic Ziliujing Formation and Xintiangou Formation in the Sichuan Basin

Lacustrine sedimentary formations potentially contain hydrocarbons. The lacustrine sedimentary rocks of the Ziliujung and Xintiangou Formations have been investigated for their hydrocarbon potential using low-pressure nitrogen adsorption (LP-N2A), low-field nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), total organic carbon (TOC), rock-eval pyrolysis (Rock-Eval), gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), X-ray fluorescence spectrometry (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). The results show that the normalized difference of the pore parameters between the two formations is less than 10%, and the pores are mainly slit-like mesopores with high porosity. Macropores and micropores are often developed in the quartz skeleton, while mesopores often occur among organic matter, clay minerals, carbonate minerals, and pyrite particles. The organic matter abundance of the Ziliujing Formation is relatively high. Additionally, the organic matter types of the two formations are mainly type II and type III, and the sources of the organic matter are plankton and bacteria which have reached the mature gas production stage. The palaeoenvironmental differences between the depositional periods of the two formations lie within 10% of each other. The warm and humid climate promotes the development of quartz minerals to further enhance the proportion of both micropores and macropores, and the clay minerals, carbonate minerals, and pyrite carried in the terrigenous detritus are closely associated with the total organic carbon (TOC), which promotes the development of mesopores to enhance the porosity. The reservoir, organic matter, and palaeoenvironmental characteristics of fine-grained sedimentary rocks in the two formations are similar, and both of them have good potential for development. The above results provide a basic geological theoretical basis for unconventional oil and gas exploration in the northeastern margin of the Sichuan Basin.

Study on the Differential Distribution Patterns of Fine-Grained Sedimentary Rocks in the Lower Third Member of the Shahejie Formation in Zhanhua Sag, Bohai Bay Basin

Due to the diverse types of sedimentary source supply systems, the research on the distribution patterns of fine-grained sedimentary rocks in rifted lake basins is relatively underdeveloped. The authors of this article selected the lower third member of the Shahejie Formation (Es3L) in Zhanhua Sag as a research object. Using rock core, thin section, and other data, we analyzed rock types and genesis mechanisms, identified the spatiotemporal distribution patterns of facies within the sag, and established a fine-grained depositional model for the rifted lake basin. Five lithofacies were identified, revealing the differential distribution patterns of fine-grained sedimentary rocks in the region. The northern steep slope zone and the deep depression zone are characterized by the deposition of gravity flow-formed terrestrial layered-pebbly mudstone and gravity flow-formed mixed-source pebbly mudstone, respectively. In the central lake basin zone, mixed-source laminated mudstone with vertical rhythmic interlayers has developed. In the southern gentle slope zone, biologically cemented endogenic irregularly bedded algal limestone and chemically precipitated endogenic laminated limestone have developed. The characteristics of strong northern and weak southern terrestrial inputs in the study area control the differential distribution of lithofacies, and based on this, a fine-grained rock depositional model for the rifted lake basin was established.

Logging identification of lithology in fine-grained sedimentary rocks based on the FSSA-HKELM model: a case study of the Qingshankou Formation in the Songliao Basin (NE China)

Logging identification of lithology in fine-grained sedimentary rocks based on the FSSA-HKELM model: a case study of the Qingshankou Formation in the Songliao Basin (NE China)