ABSTRACT

This paper investigates transport mechanisms involving carbonate cementation in Eocene, tight-oil sandstones in Bohai Bay Basin, China, to determine potential mass transfer between adjacent mudstones and sandstones. Evidence from petrology, geochemistry, and numerical modeling suggests two generations of carbonate cementation: (1) early nonferroan calcite (formed at 28°C–41°C) and dolomite (formed at 45°C–63°C); and (2) later ferroan calcite (formed at 105°C–124°C) and ankerite (formed at 101°C–137°C). Based on a one-dimensional model for a coupled sandstone–mudstone system under low and high temperatures, different distribution patterns of carbonate cements reflect episodic concentration gradients that led to diffusive transport of bicarbonate species during progressive burial. Firstly, extensive precipitation of early nonferroan calcite followed by dolomite at or near mudstone–sandstone contacts resulted from initial concentration gradients related to different compositions in primary mineral assemblages. Secondly, introduction of aqueous CO2 from adjacent mudstones into sandstones resulted in dissolution of early nonferroan carbonates and led to diffusive transport of bicarbonate species. These bicarbonate species were incorporated with Fe2+ and subsequently reprecipitated as ferroan carbonate minerals at distances greater than 2 m (>6.6 ft) from sandstone–mudstone contacts. Therefore, short-distance diffusive transport is inferred to have been the predominant transport mechanism associated with carbonate cementation. Large-scale mass transfer between sandstones and adjacent mudstones occurred in a relatively open geochemical system on a very local scale. Numerical model results show that low porosity zones (2.6%–5.1%) exhibit coherence with high abundances of carbonate cements (13.9%–21.2%). Tightly cemented intervals were created by different generations of carbonate cementation and resulted in destruction of sandstone reservoir porosity.

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