Organic matter enrichment mechanism of Youganwo Formation oil shale in the Maoming Basin

The Youganwo Formation oil shale located in the Maoming Basin represents a large commercially valuable lacustrine oil shale resource and a potential bio-shale gas reservoir in South China. With the aim of deepening the understanding of factors that influence organic matter enrichment, this research conducted a geochemical investigation to reconstruct the depositional paleoenvironment of bioproductivity, preservation and dilution. Youganwo Formation oil shale is mainly deposited in semi-deep to deep-lake environments with relatively warm and humid paleoclimate in the subtropical-temperate zone. The total organic carbon (TOC) content (1.46–11.85%), S(2) values (4.79–115.80 mg HC/mg rock) and HI (328–1040 mg HC/mg TOC) indicate that the oil shale has a good oil source rock potential. TOC content, (S(1) + S(2)) values and vitrinite reflectance values show that its marginally mature organic matter (OM) belongs to kerogen type I-III with good oil-generating potential. A 3rd order sequence was identified in the Yougnwo formation. Subsequently, the multiple factors including bioproductivity, preservation and dilution that control the OM enrichment of oil shale within system tracts were discussed. Moderate-quality oil shales (O(y-1)) were developed in the transgressive systems tract (TST) in an oxidizing condition with abundant detrital input. High-quality oil shales (O(y-2)) were deposited during the high-stand systems tract (HST) with increased accommodation space, improved preservation conditions, warm and humid climate, higher water bioproductivity and minimum detrital matter input. During the regressive systems tract (RST, O(y-3)), higher detrital matter input and fresher water led to lower TOC values. Among these multiple factors, dilution condition was the major one that influences OM abundance and variation on the basis of sufficient organic matter input. Thus, OM enrichment models of O(y-1), O(y-2) and O(y-3) sub-members were established. The OM enrichment and quality in oil shale were controlled by the combined effect of bioproductivity, preservation, and dilution.