Differential Enrichment of Organic Matter in Saline Lacustrine Source Rocks in a Rift Basin: A Case Study of Paleogene Source Rocks, Dongpu Depression, Bohai Bay Basin

Abstract

Organic matter enrichment in shale is one of the key factors that control shale oil resources; however, studies of the mechanism of organic matter enrichment in shale in saline lacustrine rift basins are still lacking. In this study, analyses of total organic content (TOC), stable isotopes of carbon and oxygen for carbonate, major and trace element, rare earth element, biomarker composition, and field emission-scanning electron microscopy analyses were performed on 31 core samples from the source rocks of the middle interval of the third member of the Shahejie Formation (Es3M ). The aims of these analyses were (1) to investigate the controlling effects of paleoenvironment, paleosalinity, redox conditions, dilution, sediment accumulation rate, terrestrial input, and paleoproductivity on organic matter enrichment, (2) to reconstruct the paleoenvironment, and (3) to propose a model of differential accumulation mechanism for organic matter in saline lacustrine source rocks in rift basins. Results show that obvious differences exist in the paleoenvironment of the Es3M between the southern and northern regions in the Dongpu Depression. The TOC of the northern source rocks is generally greater than that of the southern source rocks. The southern source rocks were mainly deposited in a closed, warm, and humid environment with medium chemical weathering [the chemical index of alteration (CIA) values and Mg/Ca ratios range from 57.9 to 75.8 and 0.08 to 0.61, with averages of 70.93 and 0.19], with fresh–brackish water, weak water stratification, reducing conditions, moderate paleoproductivity (the P/Ti and P/Al ratios range from 0.13 × 10−4 to 0.26 × 10−4 (mean 0.18 × 10−4) and 5.11 × 10−3 to 11.31 × 10−3 (mean 7.62 × 10−3), respectively). The northern source rocks were mainly deposited in an open, cold, and arid environment with a weak chemical weathering (CIA values and Mg/Ca ratios range from 57.9 to 75.8 and 0.08 to 0.61, with averages of 70.93 and 0.19), with hypersaline water, strong water stratification, strong reducing condition, moderate paleoproductivity [P/Ti and P/Al ratios range from 0.13 × 10−4 to 0.26 × 10−4 (mean 0.18 × 10−4) and 5.11 × 10−3 to 11.31 × 10−3 (mean 7.62 × 10−3)]. The paleosalinity, redox condition, paleoproductivity, and terrestrial input controlled the differential enrichment of organic matter in saline lacustrine source rocks. For the southern region, organic matter enrichment increased with increasing paleosalinity, reduction degree, and terrestrial input. Organic matter enrichment of the northern source rocks increased with increasing paleoproductivity, which was mainly due to high water salinity, stable deep paleowater depth, and strong reducing condition.