Geochemistry and accumulation of petroleum in deep lacustrine reservoirs: A case study of Dongying Depression, Bohai Bay Basin

Abstract

Numerous volatile oil reservoirs and condensate gas reservoirs were discovered in the Member 4 of Shahejie Formation in the Paleogene system, northern Dongying Depression of Bohai Bay Basin, eastern China. Deep reservoirs were examined thoroughly, including biomarker, light hydrocarbon, and diamondoid in oil; composition and carbon isotope in natural gas; fluid inclusion; and thermal history. Based on diamondoid parameters, the equivalent vitrinite reflectance (EqVRo) of the crude oil ranges from 1.55% to 1.82%, indicating a state of high to over maturity, while the low absolute concentration of methyl diamantanes indicates that the oil is still in the early cracking stage. The deep gas is a mixture of kerogen degradation gas generated from hydrocarbon source rocks and oil cracking gas produced from the reservoirs. The EqVRo of gas is mostly between 0.8% and 1.3%, which is in a moderately mature state. The oil and gas are sourced from Member 4 of the Shahejie Formation, which was formed in a saline and reductive environment with a main input of I-II algal organic matter. The kerogen degradation gas was generated in the late sedimentary period of Member 2 of the Shahejie Formation, which was the first stage of petroleum charging. The early oil and gas reservoirs underwent noticeable evaporative fractionation due to the invasion of oil cracking gas after entering the late period of the Guantao Formation in the Neogene, resulting in the formation of the present volatile and condensate oil and gas reservoirs.