Geochemistry and origin of the Bozhong 19-6 condensates: Implications for deep gas accumulation in the Bohai Bay Basin

Abstract

ABSTRACT The recent discovery of the large BZ19-6 gas field (condensates contained in an Archean reservoir) indicates the occurrence of significant petroleum resources in deep traps of the Bohai Bay Basin. However, the origins of this pool remain poorly understood. Here, an integrated study of oil, gas, and reservoir bitumen samples is presented to elucidate the origins of the condensates. The biomarkers and light hydrocarbon parameters of the BZ19-6 condensates reveal that they were not derived from a coaly source rock but may have originated from the first and third members of the Shahejie Formation (Es). Parameters derived from aromatic, diamondoid, and light hydrocarbons indicate that the BZ19-6 oils share a high thermal maturity with an estimated vitrinite reflectance of 1.1% to 1.3%. This was also demonstrated by the chemical and carbon isotopic compositions of the gas, thus indicating primary cracking of kerogen. The presence of high-maturity bitumen may be associated with thermal alteration by highly mature mantle-derived CO2-rich fluids. The distribution of n-alkanes reveals the loss of the charged petroleum. With increased burial depth, normal oil was cracked to light oil because of enhanced temperatures. Meanwhile, the Es source rocks in the nearby sags were rapidly matured to provide a large quantity of gas to an Archean reservoir, which may have facilitated the dissolution of oil components and the formation of condensates.

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