Controls of marine shale gas accumulation in the eastern periphery of the Sichuan Basin, South China

Abstract

This study examines the mechanisms that most influenced the accumulation of shale gas deposits of the Lower Cambrian Niutitang, Upper Ordovician Wufeng, and Lower Silurian Longmaxi formations of the eastern periphery of the Sichuan Basin. Abundant organic matter (OM) (2.0–5.0 wt%), appropriate thermal maturity (solid bitumen reflectance = 2.5–3.5%), and favorable kerogen types highlight the especially excellent hydrocarbon generation potential of the lower member of the Niutitang Formation and the Wufeng Formation. The development of pores hosted by OM particles and the presence of authigenic quartz account for the elevated porosity (3–5%) of these deposits and their adsorption capacity in the range of 1.5–2.5 m3/t. Strong top and base seals and a pressure coefficient (ratio of reservoir pressure to hydrostatic column pressure) greater than 1.4 of Wufeng Formation shale favor gas preservation in these deposits. However, poor associated sealing lithologies, an excessively high uplift amplitude (> 6900 m), and low pressure coefficient (< 1.0) account for diminished gas preservation in the lower member of Niutitang Formation. The low OM abundance and porosity of the upper member of Niutitang Formation and the Longmaxi Formation are more suggestive of a cap rock than a source or reservoir rock.