Enhanced effects of large-scale CO 2 transportation on oil accumulation in oil-gas-bearing basins — Implications from supercritical CO 2 extraction of source rocks and a typical case study

Abstract

Abstract Large-scale mantle-derived CO2 emplacement and co-accumulation of CO2 and crude oil have been discovered in oil-gas bearing basins in eastern China and many other areas worldwide. To investigate the effect of the natural supercritical CO2 on the migration of crude oil as well as on the formation of oil reservoirs, experiments on supercritical CO2 extraction (SCE) and traditional chloroform extraction (CE) of hydrocarbon source rocks were carried out and the geochemical characteristics of the Huangqiao CO2-oil reservoir in the Subei Basin, eastern China were investigated. The SCE yielded a large proportion of the hydrocarbons in the Permian mudstone source rocks at 110 °C under 30 MPa. Compared with the traditional chloroform extraction (CE), the SCE-retrieved organic compounds are dominated by saturated hydrocarbons and contain a relatively higher concentration of small molecular mass compounds. Many wells in the Huangqiao reservoir yielded large amounts of CO2 from several formations, such as the Triassic Qinglong Formation (T1q) and the Permian Longtang Formation (P2l). Accompanying the CO2 flow, a certain amount of oil is commonly recovered. The oil is mainly light oil or condensate oil with a relative density of 0.7933–0.8308, and it contains 90.06–97.37% saturated hydrocarbons. Compared with hydrocarbons in the Permian source rocks, the oil accompanying the CO2 in the Huangqiao reservoir contains more C20- hydrocarbons and less C21+ hydrocarbons. The oil accompanying the CO2 has a similar composition to the SCE extracts from the source rocks, namely, both contain a relatively high concentration of low molecular mass hydrocarbons. Therefore, it can be concluded that, in the Huangqiao area, during movement upward along deep faults and fractures or flow through source rocks, the deep supercritical CO2 naturally extracted hydrocarbons, especially small molecular hydrocarbons, from the source rocks, and then brought them to the shallow strata to form the CO2-oil coupling reservoirs.