The effect of pressure and hydrocarbon expulsion on hydrocarbon generation during pyrolyzing of continental type-III kerogen source rocks

Abstract

In order to define the roles of water and lithostatic pressure in petroleum formation of continental type-III kerogen source rocks, semi-closed hydrous pyrolysis experiments were conducted under with 50–1200 bar water pressure and 125–2000 bar lithostatic pressure at 480 °C. Our results will be of great benefit to the exploration of shale gas and deep petroleum in China. Increasing water pressure from 50 to 325 bar did not affect the yields of gaseous hydrocarbons, oil and bitumen, but increasing water pressure leads the more oil and less gaseous hydrocarbons from continental type-III kerogen in the 325–1200 bar water pressure range under semi-closed conditions. The decreasing yields of gaseous hydrocarbons, dryness of gaseous hydrocarbons and primary hydrogen together confirm that increasing water pressure promotes the primary reaction but decreases the cracking rate of oil. The decreasing values of S2, HI and H/C in the pyrolysed samples confirm that high water pressure can increase the efficiency of hydrocarbon generation from continental type-III kerogen, while the maturation of kerogen correspondingly increases as shown by increasing values of VR and Tmax. In the 125–625 bar lithostatic pressure range, oil cracking and primary reaction of type-III kerogen may have contributed to the decreasing effect of hydrocarbon expulsion. The increasing hydrogen and gaseous hydrocarbon yields also confirm that the cracking of oil is enhanced within this pressure range. The decreasing trends of oil and gaseous hydrocarbon yields indicate that hydrocarbon generation rate was reduced by the high lithostatic pressure in the 625–2000 bar pressure range. The increasing values of S2, HI and H/C, and decreasing values of VR and Tmax of pyrolysed samples confirm the retardation effect of highlithostatic pressure on the efficiency of hydrocarbon generation and maturation. In addition, the results also indicate that expelling hydrocarbons strongly promotes further hydrocarbon generation and retards the cracking of oil from continental type-III kerogen in source rocks.