Characteristics and controlling factors of favorable reservoirs in the second member of Xujiahe Formation, Xinchang structural belt, Western Sichuan Depression

The Xujiahe Formation in Sichuan Basin has a large amount of tight gas resources and rich reserves, but its production rate is low. The development mechanism of favorable reservoirs and the enrichment and high-yield laws of hydrocarbons remain unclear, which is particularly important for the study of hydrocarbon accumulation models of deep tight gas reservoirs. Through a large number of core observation, analysis and testing, well data and other studies, the authors established an isochronous sequence stratigraphic framework, carried out the study of reservoir characteristics, clarified the formation mechanism of favorable reservoirs, summarized the patterns of hydrocarbon enrichment and high-yield, and established evolution models of the second member of Xujiahe Formation (T 3 x 2 Member) in the Xinchang structural belt of the Western Sichuan Depression. Four main understandings were obtained: (i) From bottom to top, the T 3 x 2 Member can be divided into 1 long-term cycle, 3 medium-term cycles, and 16–20 short-term cycles in total; (ii) Favorable reservoirs can be classified into two types, namely the layer-cake type with horizontal fractures and massive-bedding type with horizontal fractures; (iii) Layer-cake reservoirs are generally medium-coarse grained, quartz-rich with favorable pore structures, whereas massive-bedding-type reservoirs are mostly medium-grained, feldspar-rich with moderate pore structures; (iv) High-quality reservoirs are mainly high-energy sedimentary facies, which develop residual intergranular pores and dissolution pores, and differential densification is the key to the formation of favorable reservoirs. This study established a hydrocarbon enrichment and high-yield evolutionary model characterized by the sequence of “early reservoir formation → compaction → overpressure hydrocarbon charging → late structural reworking”, and clarified that targeting traps formed during hydrocarbon accumulation constitutes the core exploration target, while delineating sweet spots modified by late structural adjustment is the key for development deployment.