Distinguishing gases derived from oil cracking and kerogen maturation: Insights from laboratory pyrolysis experiments

Kerogen separated from an immature Type I mudstone from the third section of the Tertiary Shahejie Formation (ES33) in the Dongying Depression of Bohai Bay Basin was subjected to stepwise isothermal pyrolysis. Products were extracted, fractionated into chemical groups, and re-mixed to obtain a synthetic oil with a group composition similar to reservoir oil and a pseudo-kerogen composed of 83% kerogen and 17% residual soluble organic matter (referred as s-oil and p-kerogen, respectively, to distinguish them from a produced reservoir oil and a real kerogen). The two samples were pyrolyzed in sealed gold tubes under constant pressure (50 MPa) and non-isothermal heating (300-600 °C) conditions and their generated gases were analyzed. The two gases are quite different in their chemical and isotopic composition. Compared with the gas derived from the p-kerogen, the s-oil derived gas is enriched in C2-C5 hydrocarbons in its early cracking stages and C1-C3 hydrocarbons are depleted in δ13C throughout the cracking stages. The maximum wetness (C2-5/C1-5) of the s-oil gas and the p-kerogen gas is 0.62 and 0.35, respectively, and the carbon isotopic ratio differences between the two type gases can reach 10‰ (δ13C1), 14‰ (δ13C2), and 9‰ (δ13C3). The δ13C2-δ13C3 difference of the s-oil gas is much more sensitive to thermal stress than that of the p-kerogen gas and plots of (δ13C2-δ13C3) versus δ13C1 and (δ13C2-δ13C3) versus ln(C2/C3) are effective in identifying the two gas types in some geological conditions. These results provide a guide to differentiate gases derived from oil cracking from gases derived from kerogen maturation using their chemical and carbon isotopic compositions in some simple petroleum systems.