Oil cracking under closed-system pyrolysis: Implications for deep oil occurrence and related source determination

The cracking and stability of liquid oil is of critical importance in the exploration of deep petroleum accumulations. Numerous studies have been conducted on oils to investigate the cracking process. However, limited research has been directed to the cracking of oil with different maturities. Here, gold tube pyrolysis experiments were conducted on lower maturity normal oil (LMO) and higher maturity condensates (HMC) samples to investigate the compositions of pyrolysis products and to estimate the stability of oil during continuous deep burial. The results indicated that oil cracking can be generally divided into two stages, namely light oil generation stage and gas generation, with a maturity boundary approximately EasyR(o) similar to 1.5%. Most of oil cracking gas was generated at EasyRo similar to 2.4 %, with maximum mass yield of C1-5 is 553.8 mg/g and 609.4 mg/g for LMO and HMC, respectively. The difference in gas yields between HMC and LMO indicates the higher gas potential during the cracking of light oil components. An obvious increase in the C-19/C-23 tricyclic parameters was observed with enhanced thermal maturity, which indicates the interpretation of tricyclic parameters of high mature oils should be made with caution. Although high maturity also influenced the ratios of gammacerane/alpha beta C-30 hopane (G/H) and 4-methylsteranes/C-29 steranes (4MSI), the altered corresponding values still fall within the suggested discriminating zones for the Dongying and Shahejie Formation source rocks. Thus, the G/H and 4MSI parameters can be utilized to characterize the high mature oil with Ro <1.3% in the Bohai Bay Basin (BBB) or other basins. The calculated activation energy of C1-5 generation for LMO displays a more discrete and lower distribution compared to the HMC, which may indicate relatively lower stability heavy hydrocarbons in LMO. Based on previous studies of petroleum charge, the kinetics of C1-5 generation of LMO were extrapolated to geological conditions, yielding a maximum depth of 5750 m for the occurrence of liquid oil in the BBB. This study underscores the high thermal stability of light oil fractions, which indicates that deep high mature oils may represent the preservation of directly charged light oil or the transformation by in-reservoir cracking of normal oil associated with continuous burial.