Geochemical characteristics and organic matter enrichment of the lower cretaceous dolomite-rich interval in the Bayindulan sag, Erlian basin, northern China

A dolomite-rich interval was found in the Lower Cretaceous, Bayindulan Sag. Based on organic and inorganic geochemical data, the hydrocarbon source rock potential of this dolomite-rich interval and the factors that control organic matter enrichment were investigated. This dolomite-rich interval has TOC values ranging from 0.54% to 9.64% (average = 3.51%) and HI values varying from 263 to 974 mg HC/g TOC, suggesting good to excellent source rocks whose organic matter is dominated by type I/II1 kerogens. The dominance of short-chain and medium-chain n-alkanes (n-C16 to n-C23) in the n-alkane distribution of the extracted organic matters in the dolomite-rich interval implies that the planktonic organic matters acted as the dominant inputs for the dolomiterich interval, with a minor contribution from higher plants. The low Pr/Ph values suggest a strongly reducing environment favoring the preservation of organic matters. The high amount of gammacerane indicates water stratification during deposition time, which also helps to maintain the strongly reducing conditions in the bottom water. Meanwhile, REE and trace elements indicate that the dolomite-rich interval is closely related to the exhalative hydrothermal events, and the upwelling hydrothermal fluids provided abundant nutrients into lake water, resulting in the bloom of aquatic organisms. Overall, the strongly reducing depositional environments, the strong stratified-waterbody and the hydrothermal fluids jointly control the organic matter enrichment in the dolomitic interval.