The impact of multistage fluid and diagenetic environmental coupling on the development of deep clastic rock reservoirs: A case from the Zhuhai Formation, Wenchang A Sag, Zhujiangkou Basin

Fluid modification critically affects the quality of clastic rock reservoirs. At present, the mechanism of multistage fluid alteration in clastic rock reservoirs under deep burial conditions is still a matter of debate. To solve this problem, this study focuses on the deep clastic reservoirs of the Zhuhai Formation in the Wenchang A Sag of the Zhujiangkou Basin as an example and conducts a series of petrological and geochemical analyses. The results indicate that the fluids that primarily affect reservoir development are hydrothermal fluids and organic acids, and the hydrothermal fluid activity occurred over multiple stages, leading to a diagenetic environment with alternating acidic and alkaline conditions. In this low-permeability reservoir, hydrothermal fluids under acidic conditions were favorable for reservoir development. However, these fluids gradually became alkaline as the reaction proceeded, and under alkaline conditions, carbonate precipitation blocked the pores. In contrast, organic acids had a very positive effect on improving the reservoir. Thus, in this relatively sealed low-permeability reservoir, regions with significant hydrothermal development typically exhibit poor reservoir quality, whereas areas with substantial influxes of organic acids exhibit better porosity and permeability. Furthermore, the order of fluid injection caused substantial variations in reservoir properties. Reservoir areas that first experienced strong hydrothermal injection tend to have very poor properties. In contrast, reservoir areas that first experienced weak hydrothermal fluid influence followed by strong organic acid activity exhibit better properties. The reservoir areas initially improved by organic acids show good connectivity, and any precipitates from later hydrothermal fluids did not accumulate in reservoir pores. As a result, these reservoirs display the best reservoir characteristics. This study defines a model of reservoir development under the combined effects of multiphase fluids and diagenetic environments, which is highly important for evaluating the exploration potential of deep clastic rock reservoirs.