Thermal Cracking of Oil under Water Pressure up to 900 Bar at High Thermal Maturities: 2. Insight from Light Hydrocarbon Generation and Carbon Isotope Fractionation

Abstract

In this study, pyrolysis experiments were conducted with a saturate-rich Tertiary source rock-derived oil from the South China Sea basin using a fixed-volume pressure vessel at temperatures from 350 to 425 °C for 24 h (0.92–1.85% Easy R 0) to investigate pressure effects up to 900 bar on the generation and stable carbon isotopic fractionation of light hydrocarbons in the C6–C7 range. The results demonstrate that the pressure retards oil cracking to light hydrocarbons, but the retardation depends on the thermal evolution. In the peak oil to early wet gas stage (350 and 373 °C, 0.92–1.15% Easy R 0), the light hydrocarbon generation is low but it is still suppressed by increasing pressure. In the late stages of the wet gas window (390, 405, and 425 °C, 1.35–1.85% Easy R 0), the light hydrocarbon generation is suppressed significantly from 200 to 470 bar, followed by promotion and promotion-suppression as pressure is increased up to 900 bar. Meanwhile, the distributions of branched alkanes, cycloalkanes, and aromatic hydrocarbons are pressure-dependent. The medium to high pressures result in increasing Mango K 1 values and toluene/n-C7 ratios and decreasing n-C7/methylcyclohexane ratios, suggesting that pressure benefits the occurrence of cyclization and aromatization during oil cracking, probably involving bimolecular reaction pathways. Preferential aromatization and isomerization with increasing pressure lead to significant carbon isotopic fractionations of aromatic hydrocarbons and branched alkanes as up to 4‰ and 2‰, respectively. However, stable carbon isotopic compositions of cycloalkanes show almost no fractionation under pressurized cracking. Therefore, caution must be taken with respect to the application of light hydrocarbon-derived parameters in deep petroleum reservoirs usually at high temperatures and pressures. The carbon isotopes of branched alkanes and aromatic hydrocarbons could be potential measures to identify the pressure effects, while carbon isotopes of cycloalkanes could be an effective index for oil–oil/oil–source correlations.

Stable carbon isotopic compositions of individual light hydrocarbons in the C5–C7 range in natural gas from the Qaidam Basin, China

“…However, the maturity effect on the carbon isotopic variation of individual light hydrocarbons is under debate. Based on the results of pyrolysis experiments, researchers proposed that carbon isotopes of n -alkanes, iso -alkanes and aromatics in light hydrocarbons become heavier with increasing maturity (Rooney et al, 1998; Xie et al, 2019). While crude oil analysis suggested that mainly carbon isotopes of n -alkanes in light hydrocarbons become heavier with increasing maturity (Chung et al, 1998; Li et al, 2017).…”

“…First, Figure 5(a) exhibits a positive correlation between 13 C nC6 and R o , indicating that the carbon isotopes of n -C 6 increase with increasing maturity. This phenomenon is commonly interpreted as the preferential breakage of the 12 C– 12 C bond under thermal stress (Xie et al, 2019). Figure 6(a) displays that the contents of n -C 6 decrease gradually with increasing maturity, which implies the cracking of n -C 6 as maturity increases.…”

Light Hydrocarbon (C5–C7) Geochemistry in the Northern Margin of the Qaidam Basin, Northeastern Tibetan Plateau: Gas Mixing and Hydrocarbon Charge History

“…C 1 –C 3 hydrocarbons were mostly employed to investigate their origins . Besides, the composition of C 5 –C 7 light hydrocarbons may also provide important information about the genetic type of natural gas, gas maturity, and gas–gas correlation. − Furthermore, methylcyclopentane (MCC 5 ) and methylcyclohexane (MCC 6 ) are considered to inherit kerogen geochemical characters, and the carbon isotopes of MCC 5 and MCC 6 are commonly used to distinguish the organic matter type of oil. − So far, carbon isotopes of C 5 –C 7 light hydrocarbons have primarily been used to analyze mature oil, with little application to highly mature gas.…”