The sedimentary environment and organic matter (OM) accumulation are vital indicators for shale gas exploration. However, research on deep shale gas systems is relatively limited; moreover, the exploration of deep shale gas in the southeastern Sichuan Basin has entered a period of stagnation. In this study, systematic geochemical analysis of Wufeng (WF) and the first member of the Longmaxi (Long-1) deep shale samples from the recently drilled DY7 well in the Dingshan area of the Sichuan Basin is carried out, and the longitudinal variations in major and trace elements are revealed. The differences in the WF, lower section of the Long-1 (Long-1₁) and upper section of the Long-1 (Long-1₂) shales are studied in terms of redox conditions, paleoproductivity, terrigenous detrital input, sedimentation rate and paleoclimate, and the different main controlling factors of OM accumulation for these three layers are discussed. The WF shale has a higher TOC content (mean: 5.73%), the Long-1₁ shale has a high TOC content (mean: 2.89%), while the Long-1₂ shale has a low TOC content (mean: 1.44%). For the WF shale, due to complex geological events and large fluctuations in element contents, its TOC content is poorly correlated with these indices, redox and paleoproductivity proxies have a positive association with the Long-1₁ shale’s TOC content, but negatively correlated with terrigenous input and sedimentation rate indices. The formation of these two sets of organic-rich shales (TOC > 2%) is jointly controlled by good preservation conditions. In contrast, the TOC content of the WF shale is higher than that of the Long-1₁ shale as the result that terrigenous input and sedimentation rate of the Long-1₁ shale represent the dilution and destruction of OM, which is different from the former. During the Long-1₂ depositional period, the water column experienced weak reducing conditions and low productivity, and its high terrigenous debris input further diluted the OM, leading to a low TOC content.

Current models of sedimentary environments and organic matter (OM) enrichment for the Lower Cambrian black shales in the Yangtze Platform have not yet incorporated its northern carbonate platform margin where the related research is lacked. This study focuses on the SNZ1 well in the northern carbonate platform margin, utilizing total organic carbon (TOC) content and major and trace element data to reveal the main controlling factors of OM enrichment during the Early Cambrian. The results show that the shale stratum is tentatively ascribed to the Lower Cambrian Stage 3 and that, during its deposition, the redox transitioned from anoxic to suboxic–oxic conditions, the hydrodynamic conditions weakened initially and then strengthened, the primary productivity first increased and then decreased, the paleoclimate shifted from arid–cold to warm–humid conditions, and the terrigenous clastic input gradually diminished. Overall, the OM enrichment is primarily controlled by preservation conditions. By systematically analyzing the data from the intraplatform basin to the deep-sea basin across the Yangtze Block, a model of the sedimentary environments and OM enrichment during the Early Cambrian was suggested. Additionally, this study highlights the intrinsic link between the expansion of oxygenated surface water and the Cambrian explosion. These results provide critical insights for shale gas exploration in this region.

Substantial heterogeneity in lacustrine shale brings significant challenges to oil exploration. Therefore, a clear and effective resource evaluation standard can significantly reduce the exploration risk and cost, thus further guiding the prediction in productive areas. However, due to the lack of consideration of the thermal maturity and kerogen type, the present evaluation standards may result in the misjudgment of the resource quality of shale oil reservoirs. In this study, a method based on mass balance involving a hydrocarbon generation statistical model was proposed to calculate oil movable threshold (OMT) values. The OMT values for different types of kerogens are determined from simple and easily obtained pyrolysis parameters. Based on the OMT values, a three-dimensional resource quality evaluation model is constructed and applied to the source rocks in Member (Mbr) 1 of the Shahejie Formation (Fm) Nanpu Sag, Bohai Bay Basin, eastern China. The results show that the Mbr 1 of Shahejie Fm shale is a set of high-quality source rocks with high total organic matter (TOC) and S-1c (calibrated free hydrocarbons) content. Meanwhile, the hydrocarbon generation potential of the studied lacustrine shales are in the order of type I > type II1 > type II2 > type III, whereas the OMT values show a similar order. From type I to type III, the hydrocarbon expulsion threshold (HET) values for the four types of shales correspond to pyrolysis peak temperatures (T-max) at 438, 426, 428, and 432 & DEG;C with the maximum OMT values being 143, 128, 127, and 122 mg HC/g TOC, respectively. The movable and favorable shale oil accumulations are mainly associated with type II1 and II2 shales. Our work provides a novel method for distinguishing the resource quality and locating a favorable exploration target for lacustrine shale, improving efficiency and reducing exploration risks.

Shale gas production from the Longmaxi Formation in the Sichuan Basin, southern China, faces rapid decline and challenges in maintaining stable output. The upper part of the Long-1 Member is considered a potential replacement interval, yet detailed investigations into its reservoir heterogeneity characteristics and gas content controlling mechanisms remain lacking. This study conducted geochemical analysis, connate water content measurement, porosity testing, FE-SEM imaging, and low-pressure N₂/CO₂ adsorption experiments on core samples from the upper and lower Long-1 Member in the Jiaoshiba gas field. Multifractal characteristics were analyzed, and their influence on gas content was compared and discussed. The results show that the upper interval is jointly influenced by organic matter pores, clay mineral interlayer pores, and carbonate cementation, with water likely occurring as clay-bound water, exhibiting higher structural heterogeneity at the non-micropore scale. In contrast, the lower interval is affected by carbonate mineral dissolution, with dissolution pores and microfractures developed, and water likely occurring as water clusters within organic matter pores, showing stronger heterogeneity at the micropore scale. Furthermore, in the upper interval, higher pore uniformity and continuity correspond to higher gas content, whereas in the lower interval, higher micropore heterogeneity and more pronounced dominant pore size characteristics correspond to higher gas content. These results reveal significant differences in the heterogeneity characteristics of the upper and lower intervals of the Long-1 Member at different pore size scales and their distinct responses to shale gas enrichment, providing a reference for detailed comparison in three-dimensional shale interval development and for the evaluation of lower-quality shale intervals.

The eastern Ordos Basin holds substantial natural gas resources, yet its development performance varies considerably across different areas, and the underlying causes merit close attention. Taking the Mizhi–Qingjian Block as a case study, this paper integrated petrophysical tests, well-logging interpretation, and volumetric resource estimation to characterize the development of TSM (tight sandstone methane) and deep CBM (coalbed methane) reservoirs in the BX Fm (Benxi Formation) to H₈ Mb (He 8 Member) interval. The results indicate that the H₈ Mb effective sandstones exhibit the best reservoir development. The No. 8 coal seam is markedly thicker than the No. 5 coal seam. Water-bearing zones are extensively distributed in the Qingjian Block. Resource potential assessment shows that the TSM resources in the BX Fm–H₈ Mb interval total 1962.72 × 10⁸ m³, of which Class I + II amount to 905.47 × 10⁸ m³. The deep CBM resources of the No. 5 and No. 8 coal seams together reach 9974.31 × 10⁸ m³, with Class I + II accounting for 5503.83 × 10⁸ m³. On this basis, a synergistic co-production technology has been developed to enable efficient and coordinated extraction of both TSM and deep CBM resources. Northern Mizhi is identified as the primary development target, while the Qingjian area requires cautious planning due to high water content.