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 a crude oil under hydrous and anhydrous conditions in a gold tube confined system was performed at T = 370°C, P = 21 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 a smaller C_1-5 alkane yields and lighter carbon and hydrogen isotopic ratios of the C_1-3 hydrocarbons within maturity levels investigated. The differences in their C_1-5 alkane yields reach 3-12 ml/g, in their δ¹³CH₄, δ¹³C₂H₆ and δ¹³C₃H₈ values reach 1.0-2.7‰, 0.2-0.8‰ and 0.2-0.3‰, respectively, and in their δD_CH4, δD_C2H6, δD_C3H8 values reach 5-16‰, 22-80‰ and 4-13‰, respectively. Moreover, the hydrous pyrolysis experiment formed a greater amount of H₂ and alkenes and a smaller amount of CO₂ than the anhydrous pyrolysis experiment did. The differences of H₂, 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.