Characteristics and genetic mechanism of carbonate cements in sandstones near the overpressure top surface: A case study of the Niuzhuang Depression in Bohai Bay Basin

Abstract

Carbonate cements are important authigenic minerals in the sandstone reservoir near the overpressure top surface in the Niuzhuang Depression. However, their genetic mechanism has not been studied in detail. This study uses observations of casting thin sections and cathodoluminescence and micro-area electron probe compositional data to determine that the diagenetic sequence of these carbonate cements is penecontemporaneous dolomite → calcite filling primary pores → calcite/iron-bearing calcite filling secondary dissolution pores → ankerite. The genetic mechanisms of these carbonate cements are analyzed based on the fluid inclusion temperature at different phases of carbonate cementation, carbon and oxygen isotope composition, simulation of the thermal evolution of the hydrocarbon source rock, and elemental chemical composition. Results show that the carbon isotopic compositions of the carbonate cements are affected not only by organic acids released during the maturation of organic matter but also by the dissolution of lacustrine carbonate rock in the fourth member of the Shahejie Formation. Dissolution–reprecipitation of the lacustrine carbonate rocks is responsible for an obvious positive drift in the δ13CPDB values of carbonate cements near the overpressure top surface. There are four phases of carbonate cementation near the top surface of the overpressured zone, with phase IV carbonate cements mainly occurring near the overpressure top surface. Phase I carbonate cements are mainly dolomite, and their formation is related to the decomposition of organic matter by methanogenic bacteria. Phase II carbonate cements are related to Ca2+ and CO32− supersaturation during pore fluid concentration. Phase III carbonate cements result from overpressured fluid that is rich in Ca2+ and Mg2+ and feldspar dissolution caused by overpressure fluid intrusion. Phase IV carbonate cements are derived from CO32− formed by organic acid pyrolysis in late diagenesis, CO32− formed by early carbonate cement dissolution, and Mg2+ and Fe2+ plasma formed by the dissolution of metamorphic rock debris into pore fluid. The formation of phase III and IV carbonate cements is most affected by overpressured hydrocarbon fluids.