Origin and accumulation of petroleum in deep precambrian reservoir in Baxian Sag, Bohai Bay Basin, China

Abstract The Baxian Sag is a mature exploration area. The exploitation of deep sedimentary formation is becoming increasingly important as the further development potential of shallow plays faces growing challenges. The discovery of the Niudong gas-condensate field from deep Precambrian reservoirs indicates that potentially significant hydrocarbon resources remain to be discovered in deeper stratigraphic units. This study was undertaken to better understand the generation and accumulation of hydrocarbons in deep reservoirs through integrated petroleum geochemistry, fluid inclusion analysis and source rock analysis. Geochemical parameters of condensate and gas suggest that condensate and gas in Precambrian reservoir have similar thermal maturity, and are directly derived from gas-prone source rock at high maturity levels. The regional structure section and hydrocarbon distribution indicate that gas-condensate in the ND1 well is more likely derived from lacustrine source rock in the Paleogene Shahejie Formation. Refined source kitchen analysis indicates that lacustrine source rocks in the Shahejie Formation display a high degree of heterogeneity, including organo-facies C, D/E and F. Differences in the nature and composition of the expelled petroleum exist between organo-facies due to variations in kerogen composition. The orderly spatial distributions of gas/oil ratio (GOR), API gravity and gas carbon isotopes denote that the various fronts expelled hydrocarbons at different maturity levels from different organo-facies. This reflects the dynamic scenario of petroleum migration and accumulation from kitchen to shallow reservoirs. The Precambrian reservoir in ND1 well has experienced multiple-charging with hydrocarbon expulsion at different maturity levels. While hydrocarbons of lower maturity have arrived into the shallower reservoirs, condensate and gas expelled at high maturation levels have efficiently accumulated in deeper prospects.