Formation of fine crystalline dolomites in lacustrine carbonates of the Eocene Sikou Depression, Bohai Bay Basin, East China

Abstract

The genesis of the fine crystalline dolomites that exhibit good to excellent reservoir properties in the upper fourth member of the Eocene Shahejie Formation (Es 4 s ) around the Sikou Sag, Bohai Bay Basin, is uncertain. This paper investigates the formation mechanisms of this fine crystalline dolomite using XRD, SEM, thin section analysis and geochemical data. The stratigraphy of the Sikou lacustrine carbonate is dominated by the repetition of metre-scale, high-frequency deposition cycles, and the amount of dolomite within a cycle increases upward from the cycle bottom. These dolomite crystals are 2–30 μm in length, subhedral to anhedral in shape and typically replace both grains and matrix. They also occur as rim cement and have thin lamellae within ooid cortices. Textural relations indicate that the dolomite predates equant sparry calcite cement and coarse calcite cement. The Sr concentrations of dolomites range from 900 to 1200 ppm. Dolomite δ18O values (−11.3 to −8.2 ‰ PDB) are depleted relative to calcite mudstone (−8.3 to −5.4 ‰ PDB) that precipitated from lake water, while δ13C values (0.06–1.74 ‰ PDB) are within the normal range of calcite mudstone values (−2.13 to 1.99 ‰ PDB). High 87Sr/86Sr values (0.710210–0.710844) indicate that amounts of Ca2+ and Mg2+ have been derived from the chemical weathering of Palaeozoic carbonate bedrocks. The high strontium concentration indicates that hypersaline conditions were maintained during the formation of the dolomites and that the dolomites were formed by the replacement of precursor calcite or by direct precipitation.