Carbonate Cementation Patterns, Potential Mass Transfer and Implications for Reservoir Heterogeneity in Eocene Tight-Oil Sandstones, Dongying Depression, Bohai Bay Basin, China: Evidence From Petrology, Geochemistry and Numerical Modeling

Abstract

This study investigates transport mechanisms involving carbonate cementation in tight-oil sandstones in the Eocene Es4s interval, Dongying Depression, Bohai Bay Basin, China, to determine potential mass transfer between adjacent mudstones and sandstones, and its implications for reservoir quality. Evidence from petrology and geochemistry recognizes two generations of carbonate cements: early calcite and dolomite with relatively low ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7078 to 0.7084 and later ferroan calcite and ankerite with more radiogenic ⁸⁷Sr/⁸⁶Sr ratios from 0.7092 to 0.7116.

The numerical model show that different distribution patterns of carbonate cements reflect episodic concentration gradients leading to diffusive transport of bicarbonate species during progressive burial of interbedded sandstone-mudstone units. Extensive precipitation of early calcite or dolomite at or near mudstone–sandstone contacts is a result of initial concentration gradients generated from different compositions in the primary mineral assemblages of sandstones and adjacent mudstones. Later ferroan calcite and ankerite cements are more concentrated at distances of greater than 2 m from sandstone-mudstone contacts. The distribution patterns of ferroan carbonates are related to introduction of CO₂ (aq) from adjacent mudstones into sandstones resulting in dissolution of early-formed calcite and dolomite cements that led to diffusive transport of dissolved bicarbonate species. These dissolved bicarbonate species were incorporated with Fe⁺² and subsequently sequestered and re-precipitated as more stable, ferroan carbonate minerals in-situ or locally within the sandstone system.

Therefore, short-distance diffusive transport is inferred to have been the predominant transport mechanism of dissolved ions associated with carbonate cementation and large-scale mass transfer between sandstones and adjacent mudstones occurred in a relatively open geochemical system on a very local scale (a few meters). Numerical modeling results show that low porosity zones correspond with high abundances of carbonate cements and, therefore, the tightly carbonate cemented beds or concretions in the Eocene Es4s sandstones commonly served as fluid-flow barriers or seals for oil. The results of this study have improved the understanding of transport mechanisms associated with fluid-rock interactions, in general, and have enhanced the predictive capacity of diagenetic models in relation to reservoir quality.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018