Geological and geochemical characterization of lacustrine shale: A case study of the Jurassic Da'anzhai member shale in the central Sichuan Basin, southwest China

Abstract

Although numerous progresses have been achieved in characterizing marine shales, studies that are associated with lacustrine shales are limited. To study lacustrine shales, samples were collected from the Jurassic Da'anzhai Member in the central Sichuan Basin of China, and their mineralogical, reservoir, OM, and paleoenvironmental characteristics were determined, as well as the relationships between them. Analysis of trace elements reveals that the shales formed in paleoenvironments that were oxic to suboxic, dry to humid, had moderate to strong weathering, and were characterized by fresh to salt water conditions. These environments are more variable than those of marine shales. The paleoenvironmental conditions and mineralogy of the shales, particularly the oxic to suboxic paleo-redox conditions, resulted in the relatively low levels (0.11–2.18%, average 0.97%). However, based on evaluation criteria for continental source rocks, the OM is of high quality because of its high level of maturity (Ro: 0.95–1.43; Tmax: 428–500 °C) and favorable kerogen type (II2). There are well-developed intraparticle pores, interparticle pores, and microcracks. The SBET (5.42–10.69 m2/g, average 6.92 m2/g) and VBJH (1.19–4.10 mL/100 g, average 2.97 mL/100 g) values also indicate good nanoscale storage space. Terrestrial minerals (i.e., quartz and clay) and authigenic carbonate minerals (i.e., calcite) are, respectively, positively and negatively correlated with the nanoscale storage space,. Small pores (3–5 nm) dominate the nanoscale storage space. The isotherms and hysteresis loops are of Type Ⅱ and Type H3, respectively, which indicates wedge-shaped pores. However, the hysteresis loops indicate that the lacustrine shale has more dead-end pores and larger pores with more complex microstructures than other lacustrine reservoirs. In general, the Da'anzhai lacustrine shale has the potential for unconventional oil and gas exploration. The lacustrine shale's mineralogy, reservoirs, and OM are closely related to each other, and their differences are mainly caused by the paleoenvironmental conditions in which the shale formed.