Accumulation mechanism and enrichment model of deep tight sandstone gas in second member of Upper Triassic Xujiahe Formation, Xinchang structural belt, Sichuan Basin, SW China

Abstract

Taking the second member of the Xujiahe Formation of the Upper Triassic in the Xinchang structural belt as an example, based on data such as logging, production, seismic interpretation and test, a systematic analysis was conducted on the structural characteristics and evolution, reservoir diagenesis and densification processes, and types and stages of faults/fractures, and revealing the multi-stage and multi-factor dynamic coupled enrichment mechanisms of tight gas reservoirs. (1) In the early Yanshan period, the paleo-structural traps were formed with low–medium maturity hydrocarbons accumulating in structural highs driven by buoyancy since reservoirs were not fully densified in this stage, demonstrating paleo-structure control on traps and early hydrocarbon accumulation. (2) In the middle–late Yanshan period, the source rocks became mature to generate and expel a large quantity of hydrocarbons. Grain size and type of sandstone controlled the time of reservoir densification, which restricted the scale of hydrocarbon charging, allowing for only a small-scale migration through sand bodies near the fault/fracture or less-densified matrix reservoirs. (3) During the Himalayan period, the source rocks reached overmaturity, and the residual oil cracking gas was efficiently transported along the late-stage faults/fractures. Wells with high production capacity were mainly located in Type I and II fault/fracture zones comprising the late-stage north-south trending fourth-order faults and the late-stage fractures. The productivity of the wells was controlled by the transformation of the late-stage faults/fractures. (4) The Xinchang structural belt underwent three stages of tectonic evolution, two stages of reservoir formation, and three stages of fault/fractures development. Hydrocarbons mainly accumulated in the paleo-structure highs. After reservoir densification and late fault/fracture adjustment, a complex gas-water distribution pattern was formed. Thus, it is summarized as the model of “near-source and low-abundance hydrocarbon charging in the early stage, and differential enrichment of natural gas under the joint control of fault-fold-fracture complex, high-quality reservoirs and structural highs in the late stage”. Faults/fractures with well-coupled fault-fold-fracture-pore are favorable exploration targets with high exploration effectiveness.