The origins of carbonate minerals of a source-controlled lacustrine carbonate succession in the Shulu sag, Bohai Bay Basin: Implications for porosity development and paleoenvironment

Abstract Lacustrine carbonates are widely distributed in the third member of the Eocene Shahejie Formation (Es3L) of the Shulu sag and other rift basins in the Bohai Bay Basin, which can act as reservoirs for hydrocarbon and provide insights into paleoenvironment. However, the origins of lacustrine carbonate deposition and their implications for porosity development and paleoenvironment are not well understood. Based on micropetrological characteristics, X-ray diffraction (XRD), stable carbon and oxygen isotope analyses, and organic geochemistry data, four types of calcite (calcite debris; calcareous biological detritus; micrite in fine-grained marlstone; fibrous sparry calcite) and two types of dolomite (dolomite debris; rhombic euhedral dolomite) with distinctly different characteristics and origins are identified. The δ13C and δ18O values of calcite and dolomite debris are consistent with that of the Cambrian-Ordovician carbonate rock in the provenance area, suggesting that the carbonate debris was sourced from the surrounding Cambrian-Ordovician carbonate bedrock. Microcrystalline calcites are commonly developed in organic-rich marlstones with a large number of algae. The δ13C values of the microcrystalline calcites are higher than that of carbonate debris and the positive excursion of the δ13C value is along with the increase of TOC content. These indicate that origin of the microcrystalline calcite was authigenic calcite and was formed by biochemical process. Bedding-parallel fibrous sparry calcite occurs in intervals with high TOC contents and within the thermally mature zone of hydrocarbon generation, indicating that oil generation should be responsible for the sparry calcite formation. Rhombic euhedral dolomite commonly occurs as isolated micro-crystals with euhedral to subhedral rhombs shape. Micro-sized spherule-like dolomite in the early stage of crystal growth were also present. Rhombic euhedral dolomite are commonly associated with organic matter and developed in organic-matter rich intervals with plenty of fossilized bacterial filaments and framboidal pyrite. These suggest that rhombic euhedral dolomite were probably microbially mediated dolomite. Mineral components can affect reservoirs quality. The porosities of samples show a positive correlation with dolomite content and the porosities of fibrous sparry calcite-bearing intervals are higher than non-parry calcite-bearing intervals. The increase in authigenic microcrystalline calcite content reflects a rise in lake level and an increase in water depth and primitive productivity.