Bio-precursors of organic matter, referring to formerly living precursors, can influence content and distribution of organic pores significantly. However, insufficient attention has been paid in previous studies. To research the impact of bio-precursors of organic matter on shale organic pores, we conducted palynology and thin section analysis, total organic carbon analysis, and N₂ gas absorption experiments on the Wufeng and Longmaxi Formations shales and kerogen samples from the Shuanghe outcrop section in southern Sichuan Basin, China. Generally, there are three bio-precursor assemblages being developed from bottom to top in the Wufeng and Longmaxi Formation, namely benthic algae, benthic–planktonic algae, and planktonic algae assemblages. Porosity in kerogen contributes greatly to shale porosity, accounting for 13 − 53% of total porosity. The total porosity and mesopore volume of samples (kerogen and shale) dominated by benthic algae are higher than those by planktonic algae. Pore size distributions of kerogen samples containing mainly benthic algae and planktonic algae are unimodal and multimodal type, respectively, when the pore diameter is larger than 5 nm. The different features between benthic and planktonic algae assemblages could be attributed to their different hydrocarbon generation potential and biological structure. Smaller fractal dimension of pores in kerogen samples mainly containing planktonic algae suggested that the planktonic algae are responsible for smoother pores in shales.

The sandstones interbedded with shales in the lacustrine black shale have great potential to become important targets for oil and gas exploration, but there has been a lack of systematic research regarding their types and genesis. This study focused on the investigation of the Triassic Chang 7₃ member deep lacustrine sandstone. Eleven lithofacies are identified and classified into three different types of deposits: ash fall and intra- and extra-basinal turbidite deposits. Vitric tuff, pumice-bearing shale, and ash are the main ash fall lithofacies. The presence of reverse grading and a significant concentration of plant fragments/micas suggest extra-basinal turbidite deposits. However, the collapse of deltaic intrabasinal sediments has been well sorted and does not contain plant debris and low-density materials. These three different types of sediments combined with a humid climate are the main causes for the deposition of a large number of sandstone layers in the deep lacustrine environment. Furthermore, the abundant volcanic eruptions are associated with increased organic matter accumulation and promoted the bloom of algae. In addition, the generated oil in the black shales would then migrate and accumulate in the interbedded sandstones. This research provides geological evidence for the prospective prediction of lacustrine shale oil accumulations.

Organic matter (OM), composed of various macerals, has a strong influence on the enrichment of shale gas. Nevertheless, the connection between OM-hosted pore structure and maceral type is not yet fully understood because of the difficulty to identify the maceral types by traditional scanning electron microscope (SEM). Using a combination of the reflected light microscopy, focused ion beam SEM (FIB-SEM), and Raman spectrum, three maceral types, including alginite, graptolite, and solid bitumen, are identified in the Longmaxi Shale of the Sichuan Basin. The alginate is characterized by the linear arrangement of OM-hosted pores due to the inherited biological structure of benthic algae. Pores in the structureless solid bitumen are randomly distributed with the highest abundance. The graptolite containing pore rarely is unfavorable for the pore generation but can be a good proxy for thermal maturity. Variation in thermal maturity levels accounts for the change of total pore volume in a given marcel type in the Longmaxi Shale obtained from different shale gas fields.