Effects of gypsum-salt rock on mineral transformations in a saline lacustrine basin: Significance to reservoir development

Abstract

Gypsum-salt rock is typically developed in carbonate reservoirs, and this rock has both constructive and destructive effects on the reservoir. The ways in which gypsum rock controls reservoir development are closely related to the diagenetic conditions. In this study, a typical saline lacustrine basin, the Qaidam Basin in China, was selected to examine the influence of gypsum-salt rock on the development of the carbonate reservoir under different diagenetic conditions. Four main geological factors were assessed: formation condition (temperature), typical salt mineral (anhydrite), and fluid properties (Ca2+ and Mg2+), along with multi-group fluid–rock chemical reaction models devised using multiphase-flow solute-transport simulation technology. Mineral dissolution, precipitation, and transformation in the reservoir under various temperature, pressure, fluid, and mineral conditions were analyzed, and the change of reservoir porosity was calculated. The results showed that the concentration of Ca2+ in fluid controls the dissolution and precipitation of carbonate minerals in the reservoir; moreover, continuous and sufficient Mg2+ is a necessary condition for dolomitization. Precipitation of anhydrite decreases with increase of temperature, verifying that anhydrite precipitates more easily at low temperature. Dissolution of calcium-containing minerals in overlying gypsum-salt rock can lead to mineral precipitation in the subjacent reservoir and reduce its quality.