Oil–reservoir correlation: Unravelling the producing reservoir for the sulfur-rich Qianjiang shale oil play

The hypersaline Qianjiang shale oil play represents a promising yet challenging target for exploration due to its multiple stacked shale-carbonate layers, which complicate the identification of the most viable extraction sites. Additionally, the oil accumulated in these layers is immature to low-mature and considerably high in sulphur content. To trace the shale zones after fracturing, a newly developed oil-reservoir correlation method employing Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was utilised. This method proved effective in distinguishing between the acidic nitrogen-, sulphur-, and oxygen-containing (NSO) compounds identified by the negative mode of electrospray ionisation FT-ICR MS and the less polar NSO compounds and aromatic hydrocarbons detected by positive ion atmospheric pressure photoionisation FT-ICR MS. These compounds facilitated the deconvolution of oils produced from mixed layers in the hypersaline Qianjiang shale play. The zones of oil production identified through this method corresponded well with shale reservoirs previously characterised by anomalously high oil saturation index values (OSI >400 mg/g TOC) and a preferential enrichment of saturated hydrocarbons. A detailed analysis of C_31-35 homohopane distributions in shale extracts and crude oil corroborated these findings. The variability in the producibility of NSO compounds and aromatic hydrocarbons was examined based on functional group, degree of aromatisation, and molecular weight. During production, sulphur- and nitrogen-containing compounds predominantly migrated into the produced oil, whereas oxygen-containing compounds were less affected. Nonetheless, determining the production partitioning of aromatic hydrocarbons remains challenging; if present, it may be obscured by the commingled production effects from multiple layers.