Control of Sandstone Reservoir Physical Properties by Petrographic Assemblages：A Case Study from the Upper Palaeozoic Shibox and Shanxi Formation in the Su59 Block Area

Various types of petrographic phases and petrographic assemblages are developed in the distributary channels of delta plain, and the sandstone physical properties of different petrographic phases have obvious distinction, but the control of petrographic assemblages on reservoir physical properties is not clear yet. In this paper, taking the Upper Paleozoic Shi Box and Shanxi Formations of the Su 59 Block as an exampl, we delineate the petrographic types and petrographic assemblages by means of core observation, image analysis and thin section identification, clarify the rhythmic structure of petrographic assemblages, and explore the control of petrographic assemblages on reservoir physical properties in terms of the differences in rock composition and rock-forming transformation. According to the rock grain size and sedimentary structure, the sandstones of the Su 59 Block Shibox and Shanxi Formations in the study area are derived from multiple sources in the north and have a complex and highly variable clastic composition. Five types of petrographic assemblages are developed in the Shi Box Formation and Shanxi Formation. The granularly laminated conglomeratic coarse sandstone, massive laminated conglomeratic coarse sandstone, platy interlaminated coarse sandstone and massive laminated coarse sandstone are favourable petrographic types with permeability up to 1×10⁻³ μm² or more. The petrographic assemblages are classified into composite rhyolite assemblages, positive rhyolite assemblages and anti-rhyolite assemblages according to the petrographic type and rhyolite structure. Among them, the proportion of favourable petrographic assemblages developed in the positive rhyme assemblage is the highest, which is a favourable petrographic assemblage. A comparative study of the differences in rock composition, compaction and degree of dissolution in the three types of divergent channel lithological assemblages concluded that the lower content of miscellaneous bases in the sandstones, the main diagenetic processes in Box 8 are compaction, siliceous cementation, clay mineral cementation, carbonate cementation and dissolution. The main diagenetic processes in the rock chip sandstone are compaction and clay cementation; the main diagenetic processes in the rock chip quartz sandstone are siliceous cementation and dissolution. Of these relatively weak compaction and strong dissolution in the positive rhythm assemblages are the main reasons for the better physical properties. The coarse-medium grain sandstone orthorhynchogenic assemblage is the most favourable type of petrographic assemblage due to its coarse grain size and high content of rigid clastic grains in the original material base, resulting in the preservation of primary pores and the development of more cast pores by providing access for later diagenetic fluidsThe study in this paper clarifies the control of petrographic assemblage on reservoir properties, and provides an important basis for the exploration of high-quality sandstone reservoirs in the Delta plain divergent channel phase.