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

Abstract

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 300°C, the temperature was increased to 650°C at a heating rate of 30°C/h. The products were tested every 50°C starting at 300°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 450°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 550°C to 650°C, and the parameters Ln(C1/C2) and Ln(C1/C3), 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 ΣC21−/Σ22+, Ln(C1/C2), and Ln(C1/C3), as well as the dry coefficients, decreased within the main temperature range. During secondary oil cracking (550°C to 600°C), the Ph/nC18 and Pr/nC17 decreased. High pressure influenced the evolution of the biomarkers Ts and Tm, C31 homohopane, C29 sterane, and their related maturity parameters to different extents during oil cracking under different temperature ranges.