Genesis and dolomitization of “Khali” powder crystal dolomite in Triassic Jialingjiang Formation, Moxi gas field, central Sichuan Basin, SW China

Abstract

Taking the B bed of the second submember of the second member of Triassic Jialingjiang Formation in Moxi gas field as an example, the basic characteristics and controlling factors of the “Khali” powder dolomite reservoir is analyzed systematically, and the genesis and forming mechanism of the powder crystal dolomite are examined. Logging and core analysis show that the reservoir has intercrystalline pores and intercrystalline dissolved pores as the main storage space, mainly lamellar throats, good configuration between pores and throats, and medium porosity and low permeability, representing typical porous-type reservoir. Dolomitization genesis analysis indicates that dolomitization occurred in the early diagenetic stage, with obvious spatial evolution pattern, the degree of dolomitization decreases gradually from top to bottom, and decreases with the increase of distance from the source of brine horizontally; over-dolomitization occurs in the study area, the petrophysical property of dolomite close to the source of brine is worse than the dolomite formed by seepage-reflux far from the source of brine, which suggests the “Khali” powder crystal dolomite is of seepage-reflux genesis. Reservoir genesis analysis shows that the intercrystal pores formed by seepage-reflux dolomitization acted as superior seepage channels for reservoir reformation later, and syndiagenetic karstification is the root cause of the formation of “Khali” powder crystal dolomite reservoir. Key words: “Khali” powder crystal dolomite, reflux seepage dolomitization, karstification, Jialingjiang Formation, Moxi gas field, Sichuan Basin