Substantial advancements have been achieved in the exploration of Gulong shale oil in Songliao Basin. Pronounced disparities exist in oil-bearing capacity and productivity between the pure shale at the base of the Qingshankou Formation and the sandy interlayer at its top. However, the investigation of the microscopic characteristics underlying the oil-bearing differences in millimeter-scale assemblages of relatively high-yield lamina shale remains ambiguous, thereby fueling debates regarding the evaluation and genesis mechanisms of oil-bearing shale. This study integrates multiple analytical approaches, including conventional thin-section petrographic analysis, organic geochemical characterization, X-ray diffraction (XRD), argon ion polishing scanning electron microscopy observations, gas adsorption experiments, and thermal simulation experiments for hydrocarbon generation. The dark, organic-rich laminae and the bright, organic-poor laminae in the core of the Guye 3HC well section are analyzed to identify distinct lamina combination modes. The relationship between hydrocarbon generation, reservoir characteristics, and oil-bearing properties of these different lamina combinations during the hydrocarbon generation and storage processes is investigated. The results are obtained in four aspects. First, based on the total organic carbon (TOC) content and lamina types, the shale of the first member of Qingshankou Formation is categorized into two types: “organic-rich matter mixed” and “organic-poor matter felsic” binary lamina combination modes. Second, Thermal simulation experiments were performed on samples from both lamina combination types. During the hydrocarbon generation process, the organic-rich mixed lamina combination, which is characterized by a high TOC content, exhibited a higher oil-bearing evaluation index (S₁). This outcome of S₁ during the storage process was primarily influenced by clay minerals, specifically the illite-montmorillonite mixed layer. In contrast, the organic-poor matter felsic lamina combination mode demonstrated superior reservoir development potential. Third, the oil content reaches its peak when the vertical thickness ratio of organic-rich to organic-poor laminae in the Gulong shale is 2:1. At this ratio, the TOC content of the organic-rich laminae exceeds 2.15%, while the clay mineral component content exceeds 55%. This combination exhibits strong hydrocarbon generation potential but relatively poor reservoir capacity. Following hydrocarbon generation, the hydrocarbons migrate into the reservoir space provided by the organic-poor laminae. The “2:1 combination mode” represents the optimal source-to-reservoir ratio for developing “lamina-type” shale reservoirs. Fourth, the 2:1 combination model, defined at the millimeter scale, was identified by high gamma-ray (GR) values, TOC content exceeding 2.15%, and clay mineral content exceeding 55%. This model offers a novel approach for predicting sweet spots and efficiently developing shale oil from the Qingshankou Formation in the Gulong Sag.

The Early Eocene Climate Optimum (EECO) represents the peak of the early Paleogene greenhouse climate. However, a comprehensive understanding of the terrestrial paleoenvironmental response to the EECO and its implications for organic matter (OM) enrichment remains lacking. We integrated sedimentological, astrochronological, and geochemical data from South China Sea sediments to reconstruct the paleoenvironment and establish the OM enrichment model during the EECO. Astronomical time scales (ATS) for the Lower Wenchang Formation (Lower WC Fm.) in the Kaiping Sag, South China Sea, were established, spanning 55.4 to 43.9 Ma. During 51.5–48.7 Ma, records of astronomical signal (with overlapping cycles of 2.4 Ma, 1.2 Ma, and 405 kyr), stratigraphy (organic-rich mudstone), and paleoclimatic reconstructions (warm and humid climate) provided convincing evidence for the EECO in Kaiping Sag. This study presented the first detailed record of the terrestrial paleoenvironment response to the EECO in the South China Sea, characterized by high terrestrial input, anoxia water conditions, and elevated paleo productivity. A transient pre-warming event before the EECO exhibited a similar paleoenvironmental response, highlighting the sensitivity of terrestrial records. Post-EECO conditions showed a reversal of paleoenvironmental trends observed during the EECO. Pearson correlation analysis reveals that the EECO influenced OM enrichment by regulating paleo productivity and preservation conditions of lake. Elevated atmospheric pCO₂ levels and increased terrestrial input promoted algal blooms, thereby enhancing lake productivity. OM preservation was controlled by water column stratification and bottom water anoxia, driven by increased terrestrial input and rising lake levels. Our findings enhance the understanding of feedback mechanisms in terrestrial environments during global warming and provide insights into future climate change predictions.

Porosity and permeability are crucial indicators in the identification of high-quality reservoirs and favorable “sweet spot” zones, as well as key parameters when predicting and evaluating the development potential of fossil fuels like oil and gas. However, it is impracticable to collect enough core samples on vertical and horizontal planes for analysis due to the associated time and cost demand. Machine learning algorithms have shown remarkable capabilities in predicting the petrophysical properties by capturing non-linear relationships among logging data. In this study, to quantify the selection of logging curves and reduce the redundant logging data input, a novel and interpretable Permutation Importance-Set algorithm is proposed on the basis of logging data from the Upper Triassic Xujiahe Formation in the Sichuan Basin. The results indicate that, because of compaction, burial depth is the primary feature affecting the physical properties of tight sandstone reservoirs. Acoustic and spontaneous potential logs are critical for porosity, while density and spontaneous potential logs are pivotal for permeability, reflecting the complex diagenesis caused by the widespread sand-mud interbedding. Basin-level prediction models for porosity and permeability were developed using ten machine learning algorithms, then ablation studies confirmed the effectiveness of our feature selection and the reduced model complexity and over-fitting. This study offers a concise, interpretable prediction model with superior accuracy and interpretability for tight sandstone reservoirs.

Alkaline lacustrine shale is highly heterogeneous, and the complex relationship between the organic-inorganic porosity network and hydrocarbon occurrence restricts the effectiveness of shale oil exploration and development. Herein, we investigated the Fengcheng Formation (P₁f) in Mahu Sag. This study integrated geochemistry, Soxhlet extraction, scanning electron microscopy, gas adsorption, and nuclear magnetic resonance T₁-T₂ spectroscopy to elucidate the microscopic oil occurrence mechanisms in shales. Results indicate the presence of felsic shale, dolomitic shale, lime shale, and mixed shale within the P₁f. Matrix pores and microfractures associated with inorganic minerals are the predominant pore types in P₁f. Adsorbed oil primarily resides on the surfaces of organic matter and clay minerals, while free oil predominantly occupies inorganic pores and microfractures with larger pore sizes. Variations exist in the quantity and distribution of shale oil accumulation across different scales, where free oil and adsorbed oil are governed by dominant pores with diameters exceeding 10 nm and ineffective pores with diameters below 10 nm, respectively. Shale oil occurrence characteristics are influenced by organic matter, pore structure, and mineral composition. Felsic shale exhibits a high abundance of dominant pores, possesses the highest oil content, predominantly harbors free oil within these dominant pores, and demonstrates good mobility. Fluid occurrence in dolomitic shale and lime shale is intricate, with low oil content and a free oil to adsorbed oil ratio of 1:1. Mixed shale exhibits elevated clay mineral content and a scarcity of dominant pores. Moreover, ineffective pores contain increased bound water, resulting in medium oil content and limited mobility predominantly due to adsorption. Presently, shale oil mainly occurs in the dominant pores with a diameter larger than 10 nm in a free state. During the exploration and development of alkaline lacustrine shale oil resources, emphasis should be placed on identifying sweet spots within the felsic shale characterized by dominant pores.

The majority of oil and gas resources in the world are related to saline sediments, which mainly occur in sedimentary strata in the form of cap rocks or salt-associated shales. A large number of shale oil resources have been discovered in the saline shale sediments of the Cenozoic terrestrial lake basin in China. The hydrocarbon generation ability and the reservoir capacity of shale control the oil and gas generation. The reservoir capacity is mainly characterized by pore type, structure and porosity. Most of China’s shale oil and gas resources belong to salt-bearing formations. The role of gypsum-salt rocks in the formation and evolution of organic matter (OM) in such formations has received extensive attention. However, systematic understanding is lacking. Research on the pore formation and evolution in shale under the action of gypsum-salt rock sediments is especially weak. Taking the shales in the third member of the Shahejie Formation (Es₃) of the Bohai Bay Basin as an example, the influence of halite on the formation and evolution process of pores was studied in this paper. The results show that halite and gypsum minerals were associated with OM, which made them more likely to develop OM pores. The samples with a high halite mineral content (HC) are more developed regarding the pore volume and specific surface area than those with a low HC. The formation of thick salt rocks is influenced by factors of deep thermal brine upwelling, sea erosion and arid environments. The frequent alternation between humid and arid environments led to the outbreak and death of organisms and the precipitation of gypsum-salt rock, which formed the simultaneous deposition of OM and halite minerals. Finally, we have established a model of shale pore evolution under the participation of the gypsum-salt rock, and halite minerals contribute to pore development in both Stage Ⅱ and Stage Ⅳ. This study provides strong microscopic evidence for the pore system formation and evolution in salt-bearing reservoirs.

The middle Eocene climatic optimum (MECO, ca. ~42 Ma) is a key time period for understanding Cenozoic cooling of the global climate. Still, midlatitude terrestrial records of climate evolution during MECO epoch are rare. In this study, continuous high-resolution record of shale sediments in mid-Eocene Shahejie Formation (MES shales) in the Bohai Bay Basin were performed with major-element and wavelet analysis. The midlatitude paleoweathering and paleoclimatic evolution during MECO epoch were analyzed in this study. The MES shales experienced weak-moderate paleoweathering under a subtropical monsoon paleoclimate with mean annual temperature of 8.3–12.9 ℃ and mean annual precipitation of 685–1100 mm/yr. The MES shales record a mixed provenance involving intermediate igneous rocks, and low compositional maturity. The nutrient-rich environment led to enrichment in organic matter in the MES shales. Wavelet analysis revealed good periodicity about the paleoclimate and weathering during MECO epoch. In the stage Ⅰ of MES shales depositional process, the paleolake was high in nutrients, and the MES shales experienced high chemical weathering due to a relatively warmer and more humid climate. In contrast, the climate in stage Ⅱ was relatively cold and dry, and the maturity of the MES shales was relatively high during this stage, suggesting a relatively stable tectonic background. This work provides more terrestrial records of MECO epoch for midlatitude region, and is benefit for better understanding of the palaeoenvironment when MES shales formed. The implication of organic matters enrichment in this study is meaningful for the shale oil/gas exploration in Nanpu Sag.

Halite and gypsum minerals in saline shale make the retention mechanism and chemical fractionation of residual oil unique. The Dongpu Depression in North China is a typically saline lacustrine basin with developing halite and gypsum. The effect of gypsum minerals on residual oil content and chemical fractionation remains unclear. In this study, shale samples with different gypsum contents were used in organic geochemical experiments, showing that the high total organic matter (TOC) content and type Ⅱ kerogen leads to a high residual oil content, as shown by high values of volatile hydrocarbon (S₁) and extractable organic matter (EOM). XRD and FE-SEM result indicate that the existence of gypsum in saline shale contributes to an enhanced pore space and a higher residual oil content in comparison to non-gypsum shale. Additionally, the increase in the gypsum mineral content leads to an increase in the saturated hydrocarbon percentage and a decrease in polar components percentage (resins and asphaltene). Furthermore, thermal simulation experiments on low-mature saline shale show that the percentage of saturated hydrocarbons in the residual oil is high and remains stable and that the storage space is mainly mesoporous (> 20 nm) in the oil expulsion stage. However, the saturated hydrocarbons percentage decreases rapidly, and oil exists in mesopores (> 20 nm and < 5 nm) in the gas expulsion stage. In general, gypsum is conducive to the development of pore space, the adsorption of hydrocarbons and the occurrence of saturated hydrocarbon, leading to large quantities of residual oil. The data in this paper should prove to be reliable for shale oil exploration in saline lacustrine basins.