Genetic mechanisms of high-quality tight siliciclastic reservoirs: A case study from the Upper Triassic Xujiahe Formation in the Yuanba area, Sichuan Basin, China

Abstract

This study analyzed the petrological characteristics, diagenesis, pore types, and physical properties of the tight coarse-grained siliciclastic sequences in the third member of the Upper Triassic Xujiahe Formation (also referred to as the Xu-3 Member) in the western Yuanba area in the northeastern Sichuan Basin, China, based on the results of 242.61-m-long core description, 292 thin-section observations, scanning electron microscopy (SEM), and 292 physical property tests. The types and genetic mechanisms of high-quality tight coarse-grained siliciclastic reservoirs in this member was determined thereafter. The research objective is to guide the exploration and development of the tight coarse-grained siliciclastic sequences in the Xu-3 Member. The results of this study are as follows. Two types of high-quality reservoirs are developed in the coarse-grained siliciclastic sequences of the Xu-3 Member, namely the fractured fine-grained sandy conglomerate type and porous medium-grained calcarenaceous sandstone type. Hydrodynamic energy in the sedimentary environment is the key factor controlling the formation of high-quality reservoirs. These high-quality reservoirs are developed mainly in the transitional zone with moderately high hydrodynamic energy between delta-plain braided channels and delta-front subaqueous distributary channels. The dolomitic debris (gravel) content is the main factor affecting the reservoirs’ physical properties. The micritic algal debris and sandy debris in the dolomitic debris (or gravels) tend to recrystallize during burial, forming intercrystalline pores within. In the medium-grained calcarenaceous sandstones, intercrystalline pores in the dolomitic debris are formed at the early diagenetic stage, and a pore system consisting of structural fractures connecting intergranular pores, intergranular dissolution pores, and kaolinite intergranular micropores is developed at the late stage of diagenesis. The formation of intercrystalline pores in dolomite gravels and gravel-edge fractures, a pore system connected by gravel-edge and tectonic fractures, is closely related to the dolomite gravels in the sandy fine-grained conglomerates.