Subtle reservoirs and implications for hydrocarbon exploration in terrestrial lacustrine fan-delta deposits: Insights from the Triassic Baikouquan Formation, Mahu Sag, Junggar Basin, western China

Abstract

Petroleum reserves are abundant in the large-scale lacustrine fan delta reservoirs containing mixed oil in the Baikouquan Formation (MA131 Block, Junggar Basin). However, detecting such subtle reservoirs in the lacustrine foreland basins is challenging. High-resolution sequence stratigraphy, sedimentary basin evolution, and reservoir models are established by integrating the results obtained via wireline log data, cores, thin sections, and seismic data. A fifth-order sequence stratigraphic framework for the Baikouquan Formation is established, which enables the reconstruction of the sedimentary basin evolution and further detailing of the reservoir model, such that the correlation to multistage progradation and retrogradation of the fan delta complex can be determined. The study of core samples led to the identification of eight microfacies, of which sandy debris flows and distributary channels dominate approximately 80% of the depositional system. Stratal slices enable the characterization of the northern Xiazijie and eastern Madong fan-delta systems. The northern Xiazijie fan delta is widely distributed throughout sequences SSQ1–SSQ6, whereas the Xiazijie fan delta underwent, in general, gradual retrogradation during the formation of sequences SSQ6 and SSQ7. The eastern Madong gravity flow fan-delta system only developed during the formation of sequences SSQ1–SSQ5 at the location of an abrupt slope break. There are nine categories of reservoir models in the study area based on the source rocks, faults, and reservoirs. Geomorphology and climate are crucial in controlling the sedimentary infill. Deep faults and an unconformity between the Permian and the Triassic of the Mahu Sag played a decisive role in controlling the hydrocarbon migration pathways. The faults, source rocks, and microfacies together exert considerable control on hydrocarbon accumulation. Within the nearshore fan-delta front, variable reservoir quality can be observed. Crucial factors in improving reservoir quality are fine-grain size and relatively low clay content. This study may facilitate subtle reservoir characterization in other basins.