Abstract

The Shaximiao Formation in the Sichuan Basin possesses favorable exploration potential for unconventional oil and gas, whereas systematic studies on the genetic mechanism of its tight sandstone reservoirs remain insufficient. In this study, tight sandstones of the study area were comprehensively investigated through multiple analytical methods, including thin section observation, scanning electron microscopy, cathodoluminescence, electron probe microanalysis, fluid inclusion testing, and reservoir physical property measurement. The Shaximiao Formation belongs to typical low-permeability tight reservoirs, which are predominantly composed of lithic arkose, followed by feldspathic litharenite and arkose. A variety of authigenic minerals are widely developed in the reservoirs, including laumontite, calcite, quartz overgrowth, feldspar overgrowth, and clay minerals. The main reservoir spaces consist of primary pores, feldspar dissolution pores, and laumontite dissolution pores. The reservoirs have reached middle diagenetic stage A. A full set of diagenetic events can be identified in the study interval. These processes consist of gypsum cementation, chlorite cementation, feldspar dissolution, quartz overgrowth, kaolinite precipitation, laumontite cementation and dissolution, carbonate cementation, and pyrite cementation. Synthetic analysis of microscopic inclusion occurrences, homogenization temperature, and salinity data reveals that the Shaximiao Formation experienced three successive charging episodes of three different fluid endmembers, namely indigenous formation brine, organic acid fluid, and low-salinity surface-derived fluid. These multiphase mixed fluids sequentially altered authigenic minerals and pore spaces under variable open–closed diagenetic systems. The diagenetic system evolved progressively from an early closed environment dominated by laumontite precipitation to a middle–late semi-open-to-open environment dominated by calcite and siliceous cementation. Differential fluid migration controls diagenetic processes and the spatial distribution of cements, which fundamentally accounts for the strong heterogeneity of the reservoirs. Three types of diagenetic environments are classified in the study area, namely compaction-dominated, cementation-dominated, and dissolution-dominated environments, which jointly control the diagenetic assemblages and physical property evolution of the reservoirs. Compaction acts as the primary pore-reducing factor, causing a total porosity loss of 23.34%. Dissolution of feldspar and laumontite serves as the major pore-enhancing process, increasing the porosity by 5.26% and 4.32%, respectively. The mudstone and carbonate rock fragments in western Sichuan provide essential materials for calcite cementation, while intermediate-acid pyroclastics and plagioclase albitization collectively promote laumontite enrichment. The infiltration of meteoric freshwater and the upward migration of organic acids along faults induce feldspar dissolution, further resulting in the formation of kaolinite and quartz overgrowths. The brackish diagenetic environment under arid climatic conditions facilitates the development of early gypsum, which is finally transformed into anhydrite through burial dehydration.

IPC Classification

Keywords