Geochemical characteristics and gas-washing accumulation mechanism of hydrocarbon from the Xihu Sag, East China Sea Basin

For many petroleum production plays with large amounts of natural gas produced, their hydrocarbon accumulation processes have been reported to be commonly involved with gas-washing. The gas generation intensity of the Xihu Sag, East China Sea Basin is up to 6 × 106 t/km2, which is the primary driving force for gas-washing to occur. 18 oils and 20 gas samples produced from the Eocene – Oligocene sandstone reservoirs in the Xihu Sag has been geochemically characterized to delineate the controlling factor of the gas-washing and in turn better understand the hydrocarbon accumulation mechanism of the study area. The results show that the natural gas is primarily mature-highly mature coal-type gas based on their carbon isotope characteristics. The oils are believed to belong to a same oil family with a similar thermal maturity and have been gas-washed based on either their molecular fingerprints or other geochemical characteristics. The high toluene/n-heptane ratio (Tol/nC7) and high n-alkane mass depletion (Q) of the oils further indicate the occurrence of gas washing. A good correlation between either the reservoir depth or the gas generation intensity of the source kitchen and the n-alkane mass depletion of the crude oil indicate reservoir depth and gas generation intensity are the two primary controlling factors on the gas washing of the study area. In order to further delineate the effects of evaporative fractionation or migration fractionation on the hydrocarbon reservoir, we introduced a new cross plot of n-alkanes mass depletion (Q) versus adamantane/diamantine (A/D) to type the oils related to the gas washing in the study area. The meaning of this paper for the exploration of oil and gas resources will be related to the understanding of different gas washing mechanism, so as to further enhance the exploration efficiency.