Oil and gas geochemical exploration in the southern part of East China Sea Shelf Basin—Hydrocarbon anomalies and integrated evaluation of oil-gas potentials

Geochemical samples are collected from the southern part of East China Sea Shelf Basin and analyzed for oil and gas geochemical indices by the authors. Geochemical indices, such as acidolysis hydrocarbon gases and aromatic hydrocarbons are analyzed. From the analysis results, geochemical anomalies are extracted, and the distribution patterns of anomalous hydrocarbon geochemical indicators revealed, in addition to the genetic types of hydrocarbon gases and deep hydrocarbon attributes. Based upon the work, integrated geochemical anomaly areas are defined and their hydrocarbon-bearing capacity evaluated for the southern area of the East China Sea Shelf Basin. The indicators of the hydrocarbon geochemical anomalies include acidolysis methane and acidolysis ethane. The anomalies with total aromatics and their derivatives over 260 nm and the total polycyclic aromatic hydrocarbons above 360 nm, are mainly distributed in the eastern and western parts of the study area, corresponding to the Oujiang Sag and Minjiang Sag, respectively. Acidolysis hydrocarbon gas assemblages and methane carbon isotope compositions suggest that the anomalies of acidolysis hydrocarbon gas are mainly thermogenic type and have suffered surface oxidation. Deep hydrocarbon attributes suggest a source of dry gas to condensate oil -gas, dominated by dry gas. According to the distribution pattern of geochemical anomalies, the western comprehensive geochemical anomaly area and the eastern comprehensive geochemical anomaly areas are divided. The anomalies of acidolysis methane, acidolysis ethane, total aromatics and their derivatives over 260 nm and total polycyclic aromatics over 360 nm are evident in the western comprehensive geochemical anomaly area, while the eastern comprehensive geochemical anomaly area is dominated by acidolysis hydrocarbon gas anomalies. The total polycyclic aromatics at 360 nm are locally observed, the total aromatics and their derivatives at 260 nm are only sporadically observed in the east comprehensive geochemical anomaly area. Therefore, the western comprehensive geochemical anomaly area is obviously better than that of the eastern comprehensive geochemical anomaly area, that is, the oil and gas-bearing capacity of the Oujiang Sag is better than that of the Minjiang Sag.