Influence of water on yields and isotopic fractionations of gas hydrocarbons generated from oil cracking

Abstract

Few investigations have been made to discuss the influence of water on the yields and carbon and hydrogen isotopic ratios of gas hydrocarbons generated from oil cracking although water Occurs in oil reservoirs. In this study, a pyrolysis experiment for it crude oil under hydrous and anhydrous conditions in a gold tube confined system wits performed at T = 370 degrees C, P = 2 1 MPa and times from 1 to 7 days. It is found that water has an obvious inhibition on the gas yields generated from oil cracking and a pronounced influence on the carbon and hydrogen isotopic fractionations of the cracking gas. Compared with the anhydrous pyrolysis, the hydrous pyrolysis has it smaller C1-5 alkane yields and lighter carbon and hydrogen isotopic ratios of the C1-3 hydrocarbons within maturity levels investigaled. The differences in their C,, alkane yields reach 3-12 ml/g, in their delta(CH4)-C-13, delta(C2H6)-C-13 and delta(C3H8)-C-13 values reach 1.0-2.7 parts per thousand 0.2-0.8%o and 0.2-0.3 parts per thousand, respectively, ;and in their delta D-CH4, delta DC2H6 delta D-C3H8 values reach 5-16 parts per thousand. 22-80 parts per thousand and 4-13 parts per thousand, respectively. Moreover, the hydrous pyrolysis experiment formed it greater amount of H-2 and alkenes and it smaller amount of CO2 than the anhydrous pyrolysis experiment did. The differences of H-2, ethene, propene and CO2 between the two experiment conditions are 0.44-2.12 ml/g, 0.03-0.05 ml/g, 0.20-0.34 ml/g and 0.08-0.98 ml/g, respectively. These results indicate that these differences in carbon and hydrogen isotopic ratios of generated gases between the hydrous and anhydrous experiments is possibly because water affects the free radical reactions of oil cracking and inhibits the combination between alkyl and hydrogen to form alkane to some extent, rather than due to the isotopic exchange reactions between water and some pyrolysates as usually believed for kerogen hydrous pyrolysis.