Mechanism of carbonate cementation and its influence on reservoir in Pinghu Formation of Xihu Sag

Carbonate cements are the most abundant authigenic mineral and impact on physical properties greatly in sandstone reservoir. In this paper, Pinghu Formation of Xihu Sag was taken as a target. Characteristics, distribution and formation of carbonate cements were investigated via optical microscopy, cathodoluminescence (CL), electron probe and in-situ carbon-oxygen isotope. The results showed that carbonate cements varied in types and shapes. Calcite/dolomite mainly present as poikilotopic cements, while ferrocalcite/ferrodolomite/ankerite generally present as pore-filling cements. Carbon isotope (δ13C) values of carbonate cements were ranging from −7.77‰ to −2.67‰, with an average of −4.52‰, while oxygen isotope (δ18O) values were ranging from −18.94‰ to −12.04‰, with an average of −14.86‰. The δ13C/δ18O indicated that the paleo-fluid of carbonate cement was mainly freshwater. Organic carbon mainly came from organic matter within mature source rocks, and inorganic carbon came from dissolution of carbonate debris and early carbonate cements. Distinctive δ13C/δ18O values manifest that carbonate cements with different types formed in different periods, which make different contributions to the reservoir properties. Calcite/dolomite formed during eodiagenesis (70–90°C) and early mesodiagenesis stage (90–120°C), and were favorable to reservoir owing to their compacted resistance and selective dissolution. Ferrocalcite/ferrodolomite/ankerite formed during middle-late mesodiagenetic stage (above 120°C), and were unfavorable to reservoir due to cementing the residual intergranular pores. Hence, in order to evaluate the reservoir characteristics, it is of significantly important to distinguish different types of carbonate cements and explore their origins.