Characteristics of reservoir-source rock and hydrocarbon accumulation model of tight sandstone gas reservoirs in the third Member of Xujiahe Formation in western Sichuan Depression

Abstract

The tight gas reservoirs of the Xujiahe Formation in western Sichuan Depression have become an important field for expanding oil and gas exploration and discovering new reserves.The development characteristics of tight gas reservoirs and their accumulation mechanism have become one of the key scientific problems that need to be solved urgently. Applied geochemistry, geophysics and sedimentology analysis methods, systematic analysis of the source and reservoir characteristics and accumulation model of the Xu-3 Member was conducted, and the high-efficiency source rocks and high-quality reservoirs were identified and predicted. The accumulation period and accumulation mode were determined.The following conclusions were drawn: ①The lacustrine source rocks in the mature and highly mature stage of the Xu-3 Member in western Sichuan Depression are dominated by type Ⅲ kerogen, and occasionally type Ⅱ2 kerogen.It is developed as a high-efficiency source rock when it is developed near the largest lake flooding surface in the lower sub-segment of Xu-3, with shale content above 65% or a single layer with a large thickness and requiring 1.50% ≤ TOC ≤ 10%.②According to the median value of sandstone grain size, the 4th stage (S1, S2, S3 and S4) high-quality reservoirs were identified, which belonged to low-porosity and ultra-low-permeability-ultra-low-porosity and ultra-low-permeability tight reservoirs. Reservoir space is dominated by secondary pores, micropores and microfractures.Most developed in the front edge of the delta and a small amount developed in the front delta.The particle size distribution range is 0.5-0.062 5 mm, the thickness is generally 5-10 m.③The hydrocarbon generation and expulsion time of the Xu-3 Member source rock in western Sichuan Depression lasted longer, and the Middle Jurassic began to generate hydrocarbons until the end of the Early Cretaceous; the Late Jurassic began to expel hydrocarbons and reached a peak at the end of the Early Cretaceous.High-quality reservoirs experience early natural gas continuous charging and late condensate gas charging.The natural gas charging time is earlier than the condensate gas charging time.The accumulation mode of tight gas reservoirs in the Xu-3 Member is "vertical fault transmission, first gathering and then dense, and later transformation and adjustment".

Keywords

• high efficiency source rock
• high quality reservoirs
• tight sandstone
• hydrocarbon accumulation model
• western sichuan depression