Cementation periods of deep marine carbonate reservoirs in the Sichuan Basin, China: a case study from the Yuanba area

The research on cementation periods of deep marine carbonate reservoirs has significance to understand multiphase fluid activities and pore evolution history. Taking the Changxing Formation of the Yuanba area in northeast Sichuan Basin as a case, the formation and distribution characteristics of the reservoir cements were investigated by microscopic observation and fluid inclusions. The results show that the reservoir cements in the Changxing Formation are mainly distributed in intragranular pores, intergranular pores, and fractures in allochemical dolomites and dolomitic biolithite limestones. The cementation of reservoirs can be divided into four periods. In the first period, the intragranular calcite cements and a few non-transformed intergranular calcite cements occurred, which give out dark light or no light under cathodoluminescence, and they had low contents of Fe, Mn, and Ba, and formed in the shallow diagenetic environment. In the second period, the dolomite cements in intergranular pores and fractures occurred, which mainly give out bright light under cathodoluminescence, and they had relatively high contents of Fe, Mn, and S, and relatively low contents of K + Na, Sr, and Ba, and high homogenization temperatures of fluid inclusion, indicating that the cements formed in the middle burial diagenetic environment in early Middle Jurassic. In the third period, the intergranular calcite cements and fracture calcite cements occurred, which mainly give out dark light or no light under cathodoluminescence, and they had high contents of K + Na, Ba, and S, and wide homogenization temperature of fluid inclusion, indicating that the cements formed in the middle-deep burial diagenetic environment in middle-late Middle Jurassic. In the fourth period, the pyrite, quartz, rutile, and strontium minerals occurred, with symbiotic relationship, and they give out no light under cathodoluminescence, and formed in the deep burial environment, perhaps, in the hydrothermal environment.