Hydrocarbon generation potential and model of the deep lacustrine source rocks in the Dongying Depression, Bohai Bay Basin

Abstract

The deep mudstone in the Palaeogene of the Dongying Depression has been relatively deeply buried and is a useful example of a deep source rock. We use TOC, rock pyrolysis, gas chromatography-mass spectrometry (GC-MS) and vitrinite reflectance (VRo) to evaluate organic matter types, the sedimentary environment, thermal maturity, and hydrocarbon generation potential. Thermal pyrolysis experiments and PVT phase simulation were also used to study the evolution of hydrocarbon generation in these deep source rocks. Two sets of dark mudstone are formed in a reductive salt lake environment, and the organic matter types are mainly I and II. TOC data and pyrolysis experiments show considerable hydrocarbon generation potential. The mudstone samples present low pristane/phytane, high gammacerane/C30hopane, and gammacerane/C31hopane (22R) values, reflecting a strong reduction environment with development of gypsum and salt rocks for the source rocks. A significant effect of maturity on the carbon isotope was indicated, and the carbon isotope values of aromatics are unusually heavier than those of saturated hydrocarbons and resins. The maturity parameters VRo and pyrolysis peak temperature (Tmax) are somewhat reduced owing to the influence of multiple units of gypsum-salt rocks. The VRo values of the two sets of mudstone are in the range 0.74%–1.0% and 0.75%–1.95%, respectively, indicating that these mudstones are mainly moderately mature and high to over mature. The results of thermal simulation experiments of a hypersaline mudstone show that the increase in organic matter maturity is uneven at different stages of thermal evolution, and the increasing rate is relatively fast in the oil generation window. The wide maturity range of the source rocks contributes to the formation of various types of reservoirs, including normal oil, volatile light oil, condensate gas and single dry gas in the deep part of depression; all reservoir types are distributed regularly with burial depth. A detailed hydrocarbon generation model for salt lake facies is proposed to show the evolutionary stages of biogas, bitumen, low mature oil, normal oil, wet gas, volatile oil, condensate gas, and dry gas with increasing thermal maturity. Reservoirs of volatile oil, condensate gas, and dry gas develop at depths of >4000 m. At depths of >4500 m, condensate gas and single dry gas reservoirs mainly exist; these are the main targets for deep petroleum exploration in the area.