Cracking conditions of oils existing in different modes of occurrence and forward and backward inference of gas source rock kitchen of oil cracking type

Abstract：The gas source rock kitchen of oil cracking type is a kind of special gas source rock kitchen derived from high-quality hydrocarbon source rocks. The oils generated from source rocks mainly have three modes of occurrence： disperse liquid hydrocarbon inside source rocks, disperse liquid hydrocarbon outside source rocks and concentrated liquid hydrocarbon outside source rocks. The quantities and proportions of the three modes of occurrence of hydrocarbon are controlled by various factors, including internal and external ones. As to the oil expulsion efficiency, the organic carbon contents in mudstone, limestone and oil shale are 0.67%, 0.62% and 10.6% respectively and their maximum oil expulsion efficiencies are 45%, 55% and 80% respectively. The kinetic experiments of cracked gas in different media indicate that the activation energy of methane changes according to the medium conditions. Carbonate rocks have the greatest influence on oil cracking and can significantly reduce the activation energy of cracked gas and the temperatures of oil cracking. Mudstone takes the second place and sandstone has the smallest influence. The corresponding Ro values of main gas-generating periods in different media are as follows： 1.5%-3.8% for pure crude oil, 1.2%-3.2% for disperse oil in carbonate rocks, 1.3%-3.4% for disperse oil in mudstone and 1.4%-3.6% for disperse oil in sandstone. The gas source rock kitchen of oil cracking type is an intermediate and what are visible are the primary gas source kitchen and gas accumulation formed by it. However, the modes of occurrence, distribution scope and amount and resources of gas generated for the gas source rock kitchen of oil cracking type can be only determined and proved through forward and backward inference. Take the Middle-Lower Cambrian in the Tarim basin as an example of forward inference, the original oil-generating quantity is 2 232.24×108 t and the residual oil quantity and oil cracked gas quantity are 806.21×108 t and 106.95×1012 m3 respectively. Take the cracked gas in dolomites of the Feixianguan Formation in the northeastern Sichuan basin as an example of backward inference, the area of the delineated paleo-oil accumulation is approximately 735 km2, the quantity of crude oil is 45×108t, and the quantity and resources of oil cracked gas are 2.72×1012 m3 and 1.36×1012 m3 respectively.