Characteristics of organic pores and composition of bio-precursors in the Wufeng and Longmaxi Formation shales, Southern Sichuan Basin, China

Abstract

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 N 2 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 À 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

Origin and paleoenvironment of organic matter in the Wufeng–Longmaxi shales in the northeastern Sichuan basin

“…During the sedimentary period of the WF Formation, several paleo-uplifts occurred in the Yangtze plate due to tectonic compression, which divided the plate into distinct undercompensated marine basins. The formation of the Gondwana Glacier led to a sharp drop in global temperature and the migration of nutrient-and oxygen-rich water at high latitudes to the equator, which led to rising ocean currents and an increase in surface biological productivity and water column oxidation (Chen et al., 2011; Mu et al., 2011; Pang et al., 2018; Yan et al., 2008) (Figure 11(a)). The negative correlation between TOC and P/Ti and Ba XS in the WF Formation was due to the water redox conditions, not a decrease in primary productivity.…”

Diverse oil and gas seeps in the southern Junggar Basin, NW China (piedmont Northern Tian Shan): Origins and links to tectono‐sedimentary evolution

“…Previous studies of the southern Junggar Basin have shown that the anticlines and faults formed in multiple stages (X. W. Wang et al, ). The lacustrine source rocks reached thermal maturity after the formation of the anticline–fault systems; this meant that the traps were in place when the hydrocarbons migrated into the reservoirs (Hu, Yang, Wang, Hu, & Cao, ; Q. Pang et al, ). Late‐stage thrust faulting followed, as a result of later tectonic events and episodic basin inversion (Cai, Chen, & Jia, ; Shao, Chen, & Zhang, ; Tan, Zhang, Song, & Liu, ; X. W. Wang et al, ).…”

Special Issue: Advances of petroleum exploration and geology research in West China's petroliferous basins

“…Based on these results, they suggest that favorable reservoirs occur within high-energy shoal dolomites of the eastern and southeastern Sichuan Basin, which underwent meteoric erosion, and dolomites of the central paleo-uplift, which underwent subaerial karst formation during the Guangxi movement. Pang et al (2018) use thin section observations, total organic carbon analysis, and N 2 gas absorption experiments to characterize organic pores and determine the composition of the bioprecursor organisms in shales of the Upper Ordovician Wufeng and lower Silurian Longmaxi formations of the southern Sichuan Basin. Benthic, benthic-planktonic, and planktonic algae assemblages were observed, from bottom to top, within the formations.…”