Contribution and significance of dispersed liquid hydrocarbons to reservoir formation

Abstract

Oil expulsion rate of different types of source rock in geologic settings are estimated by modeling hydrocarbon generation and expulsion and examining the geological sections of different source-reservoir combinations. The major gas generation peak of oil-cracked gas kitchen is determined by study on the gas generation timing of dispersed liquid hydrocarbon inside and outside source rock and ancient oil reservoirs, and the factors affecting the gas generation timing. A “five-step method” based on the origin method is set up to quantitatively evaluate the cracked gas generated by dispersed liquid hydrocarbons. The hydrocarbon generation and expulsion experiment of different types of source rock, the study on oil expulsion rate of different source-reservoir combinations, analysis of rock pyrolysis parameters of reservoir rock all show that there is a large amount of liquid hydrocarbons retained in the source rock, and the feature on the whole is the oil expulsion rate decreases with the reducing of total organic carbon (TOC). The expulsion rate of source rock with TOC of less than 2% is below 50% in general during the liquid-window stage, while that of alternating sandstone and mudstone and of thick mudstone, with TOC of 2%−4%, are 60% and 30% respectively. Affected by the ancient landform, the expelled liquid hydrocarbons accumulated in different abundance in formations, with dispersed or semi-dispersed and semi-gathered form, and they are called dispersed liquid hydrocarbons outside source rock. When buried deeper, both dispersed liquid hydrocarbons inside and outside the source can be cracked into natural gas to form conventional and unconventional gas accumulations. Exploration practices in the Sichuan and Tarim basins have proven that the cracked gas generated by dispersed, semi-dispersed and semi-accumulated, and accumulated liquid hydrocarbons is an important source of deep natural gas in China, and takes an important position in gas exploration. Key words: dispersed liquid hydrocarbon, dispersed liquid hydrocarbon inside source rock, dispersed liquid hydrocarbon outside source rock, liquid hydrocarbon cracked gas, source rock oil expulsion rate, liquid hydrocarbon cracking into gas, quantitative evaluation of cracked gas