Effects of Crude Oil Generation and Primary Migration on Shale Oil Enrichment and Mobility: A Case Study of Biyang Depression in the Nanxiang Basin, China

Based on PY-GC, Rock-Eval, TOC, burial history and thermal history data, the oil generating quantity can be calculated by chemical kinetic methods. The more oil is generated, the more oil is discharged, reflecting the stronger shale oil migration effect. Meanwhile, the in-situ oil content of shale increases, eventually reaching the upper limit of the reservoir and in dynamic equilibrium (Figure 1). And the content of non-polar and relatively low molecular weight saturated hydrocarbons in in-situ shale oil decreases (Figure 2). Based on chloroform asphalt A, group components, and pyrolysis data, the missing light and heavy hydrocarbons in experimental value S1 can be recovered to obtain the in-situ oil content of shale (Figure 3). From this, the hydrocarbon-expulsion efficiency (HEE) can be calculated. The research results indicate that, during the migration of shale oil, heavy isotope ¹³C with strong adsorption capacity is retained and enriched due to isotope fractionation, while light isotope ¹²C is more easily migrated and discharged. In order to eliminate the influence of kerogen type on carbon isotopes, the carbon isotopes of chloroform asphalt A, saturated hydrocarbons, aromatic hydrocarbons, non-hydrocarbons, and asphaltene were subtracted from the carbon isotopes of kerogen. It was found that the difference (Δ δ¹³C) between them increased with the increase of HEE, indicating that the stronger the migration effect, the heavier the carbon isotopes of chloroform asphalt A, saturated hydrocarbons, aromatic hydrocarbons, non-hydrocarbons, and asphaltene (Figure 4). Similarly, due to the geochromatography effect during the migration process, when the migration is strong, a large amount of tricyclic terpene with relatively low molecular weight will be discharged, and pentacyclic triterpene alkane will be relatively enriched, resulting in a decrease in the ratio of tricyclic terpene to pentacyclic triterpene alkane (RTP) (Figure 5). Non-polar and relatively low molecular weight saturated hydrocarbons are prone to migration, while non-hydrocarbons and asphaltenes have high relative molecular weight and contain a large number of heteroatom components, resulting in high adsorption and difficulty in migration. This leads to a decrease in the ratio of saturated hydrocarbons to aromatic hydrocarbons (RSA), as well as the ratio of saturated hydrocarbons and aromatic hydrocarbons to non-hydrocarbons and asphaltenes, as migration increases (Figure 6). The primary migration of crude oil within source rocks leads to differential enrichment of shale oil. The increase in HEE indicates a stronger migration of shale oil, leading to a decrease in the in-situ unit organic carbon oil content of shale and a decrease in shale oil saturation index (Figure 7). The light components with weak polarity and relatively low molecular weight in shale oil decrease with increasing migration, while the heavy components with relatively high polarity are relatively enriched, leading to a decrease in shale oil mobility.