Hydrocarbon accumulation and main controlling factors of reef-shoal gas reservoirs in Changxing Formation in the complex tectonic area, Eastern Sichuan Basin

Abstract

Reef-shoal gas reservoirs of Upper Permian Changxing Formation, which are the important targets for increasing gas reserve and production in Sichuan Basin, are highly heterogeneous in Eastern Sichuan Basin. However, because of the complex tectonic situation, the source and accumulation of natural gas are still being controversial. The lack of systematic research and relevant experimental analysis on controlling factors of hydrocarbon accumulation restrict the effective natural gas exploration. Based on petrographic observation, 3D seismic data and geochemistry, the reservoir spaces, fluid-inclusion of filling mineral, reservoir solid bitumen, and restoration of tectonic evolution are analyzed, which indicates that the reef-shoal gas reservoirs of Changxing Formation are fractured-vuggy type and formed mainly by dolomitization. The reservoir porosity system consists of pores, vugs, and fracture network. The gas reservoir boundary is mainly controlled by reef dolomite reservoirs distributed intermittently along the platform-margin. Both water and gas production wells have undergone two-stage accumulation process. The first stage is the crude oil charging of Wujiaping Formation from the Middle Jurassic to the early of the Late Jurassic; the second stage is that the crude oil of Wujiaping Formation was cracked into gas, accompanied with the formation of a large amount of reservoir bitumen from the middle-late of the Early Cretaceous to the middle of the Late Cretaceous. Tectonic-lithologic traps in both east and west side of the Kaijiang-Liangping Trough are formed in the late period of Indosinian. The faulting activity in the Yanshanian period, which gradually dropped in both east and west side of the Trough and slightly increases the tectonic-lithologic traps, is consistent with the peak period of hydrocarbon charging in the Wujiaping Formation source rock. The direct contact with the active fault above and damage to the regional caprock of the 4th member of Feixianguan Formation jointly result in the destruction of the reef-shoal gas reservoir in Changxing Formation. The Himalayan faulting activity, which further elevated the tectonic amplitude of tectonic-lithologic traps in both east and west side, forms the unique northwest–southeast strike faults in the east side of the Trough.