Geochemical evolution during the cracking of crude oil into gas under different pressure systems

Two comparative simulation experiments (a normal atmospheric-pressure opening system and a 20 MPa closed system) were conducted to study the geochemical evolution of n-alkane, sterane, and terpane biomarkers in the process of oil cracking into gas under different pressures. With an initial experimental temperature set at 300A degrees C, the temperature was increased to 650A degrees C at a heating rate of 30A degrees C/h. The products were tested every 50A degrees C starting at 300A degrees C, and a pressure of 20 MPa was achieved using a water column. The low-maturity crude oil sample was from the Paleogene system in the Dongying sag in eastern China. The threshold temperature obtained for the primary oil cracking process in both pressure systems was 450A degrees C. Before the oil was cracked into gas, some components, including macromolecular n-alkanes, were cracked into medium- or small-sized n-alkanes. The secondary oil cracking of heavy hydrocarbon gases of C2-5 to methane mainly occurred between 550A degrees C to 650A degrees C, and the parameters Ln(C-1/C-2) and Ln(C-1/C-3), as well as the dry coefficients, increased. Overpressure inhibited the oil cracking process. In the 20 MPa system, the oil conversion rate decreased, the temperature threshold for gas generation rose, and oil cracking was inhibited. Compared with the normal pressure system, high-carbon n-alkanes and other compounds in the 20 MPa pressure system were reserved. Furthermore, the parameters I C-21 pound (-)/I (22) pound (+), Ln(C-1/C-2), and Ln(C-1/C-3), as well as the dry coefficients, decreased within the main temperature range. During secondary oil cracking (550A degrees C to 600A degrees C), the Ph/nC(18) and Pr/nC(17) decreased. High pressure influenced the evolution of the biomarkers Ts and Tm, C-31 homohopane, C-29 sterane, and their related maturity parameters to different extents during oil cracking under different temperature ranges.