Dissolution and its impacts on reservoir formation in moderately to deeply buried strata of mixed siliciclastic–carbonate sediments, northwestern Qaidam Basin, northwest China

Abstract

Petroleum exploration is increasingly extending from shallowly to moderately to deeply buried strata, with reservoir quality being a key. The formation mechanism of conventional clastic and carbonate reservoirs under such conditions has been widely investigated. However, the mechanism of mixed siliciclastic–carbonate reservoirs has not been well studied. In this paper, we address this issue using a case study in the northwestern Qaidam Basin, northwest China (N1–E1+2 age, 2500–4500 m). Dissolution pores and fractures dominate the reservoirs. However, only the fractures were the focus of previous studies. Thus, here we investigate dissolution and its impacts on reservoir formation, providing a complementary understanding of the reservoir formation in the basin. Using core observations and examining thin section, it was discovered that dissolution was both random and followed bedding. The dissolved components primarily included calcareous and gypsum minerals, with fingerprints that are characteristic of burial dissolution. Further electron probe analysis on authigenic minerals revealed that the dissolution fluid might have originated from acidic formation fluids associated with hydrocarbon generation. The fluids passed through faults and fractures. Dissolution pores were an important component of the reservoir, providing approximately 60% of the porosity. In addition, segments of high porosity generally above 5% are associated with dissolution. Based on these observations, a schematic model was established to explain the impacts of dissolution on reservoir formation. Specifically, organic acidic formation fluids enter tectonic fractures, resulting in dissolution as they pass through them. This dissolution enhances the size of the pore space and the reservoir properties of the rocks, eventually developing a fracture-dissolution reservoir.