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

摘要

Kerogen separated from an immature Type I mudstone from the third section of the Tertiary ShahejieFormation (ES_3~3) in the Dongying Depression of Bohai Bay Basin was subjected to stepwise isothermalpyrolysis. Products were extracted, fractionated into chemical groups, and re-mixed to obtain a syntheticoil with a group composition similar to reservoir oil and a pseudo-kerogen composed of 83% kerogen and17% residual soluble organic matter (referred as s-oil and p-kerogen, respectively, to distinguish themfrom a produced reservoir oil and a real kerogen). The two samples were pyrolyzed in sealed gold tubesunder constant pressure (50 MPa) and non-isothermal heating (300-600 °C) conditions and their gener-ated 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 C2-05 hydrocar-bons in its early cracking stages and C_1-C_3hydrocarbons are depleted in δ~13Cthroughout the crackingstages. The maximum wetness (C_2-5/C_1-5) of the s-oil gas and the p-kerogen gas is 0.62 and 0.35, respec-tively, and the carbon isotopic ratio differences between the two type gases can reach 10%., (613C1), 14%(δ~13C_2), and 9‰(δ~3). The 2-3difference of the s-oil gas is much more sensitive to thermalstress than that of the p-kerogen gas and plots of (813C2-813C3) versus .513C1 and (813C2-813C3) versusIn(C2/C3) are effective in identifying the two gas types in some geological conditions. These resultsprovide a guide to differentiate gases derived from oil cracking from gases derived from kerogen matu-ration using their chemical and carbon isotopic compositions in some simple petroleum systems.