New insights on the retention and migration of shale oil within the hypersaline Qianjiang Formation in the Jianghan Basin, China

Abstract

Super giants of oil are present in the hypersaline Qianjiang shale of the Jianghan Basin. Although the oil in place of intersalt shale is controlled potentially by the total organic carbon (TOC) content, the extremely high values of the oil saturation index (OSI), ranging from 100 to 600 mg/g TOC, are widely revealed to be a result of lateral charging of self-sourced petroleum from deep sags. However, a comprehensive review of the literature combined with a case study of producing wells leads to new insights into the retention and migration of oil within the hypersaline Qianjiang shale. Except for the lateral migration, vertical oil migration and oil retention within sulfur-rich kerogen in situ are important replenishments, if not key factors, accounting for the enrichment of high-quality petroleum fluids in the Qianjiang shale. Intraformational migration of oil vertically from source rock layers to juxtaposed shale reservoirs is evidenced by (1) anomalously high OSI values of >400 mg/g TOC, (2) negative expulsion efficiency values, and (3) preferential expulsion of saturate hydrocarbons > aromatic hydrocarbons > polar compounds. Notably, the widely used maturity parameters are unsuitable for assessing the sulfur-rich Qianjiang shale play. Therefore, previous findings on the updip oil migration within individual cyclothem units are questionable. We believe that the present concerns regarding oil retention and migration within the hypersaline Qianjiang shale deserve the focus of much debate and suggest their re-examination.