Impacts of mantle-derived fluids on source-related biomarkers in crude oils: A case study from the Dongying Depression, eastern China.

Ting Liang; Mei-Lin Jiao; Xin-Liang Ma; Liu-Ping Zhang

Mantle-derived fluids can change the biomarker compositions in oil, with respect to abnormal thermal energy and volatiles input. Identification the reliable biomarkers for oil-source correlation is important in the regions that have been affected by mantle-derived fluids. In the Dongying Depression, deep faults, including Gaoqing-Pingnan Fault in the western part and the Shicun Fault in the southern part, have provided the avenues by which large volumes of mantle-derived fluids have entered this petroliferous depression. For the purpose of comparison, oil and accompanied gas were collected from the active zones with mantle-derived fluids activities, and the stable zones with little mantle-derived fluids. According to isotopic analyses (i.e., helium isotope, δ13CCO2 and δ²HCH4), mantle-derived fluids in the north part of the Dongying Depression have more H2 and less CO2 than those in the south part. The correlations between source-related biomarkers in crude oils and isotopic compositions in the corresponding gases suggest that many biomarker parameters have lost their original signatures due to the abnormal thermal energy, and H2 and/or CO2 derived from the mantle-derived fluids. Pr/Ph, for example, can be modified by both thermal energy and H2 from the mantle-derived fluids. Systematic increase or decrease in the gammacerane index, C24 tetracyclic/C26 tricylic terpane and C21/C23 tricylic terpane may be indicative of the occurrence of abnormal thermal energy. C31/C30 hopane, DBT/TF and DBF/TF, in contrast, may indicate the contribution of hydrogenation as opposed to that of CO2 supply. The relative distributions of C27, C28 and C29 ααα (20R) steranes are probably altered little by the mantle-derived fluids. Based on the ternary diagram of C27, C28 and C29 steranes, the oil samples collected from the Dongying Depression were largely the mixtures derived from source rocks in lower layer of the Es3 member and upper layer of the Es4 member.

HELIUM isotopes; PETROLEUM; ISOTOPIC analysis; PETROLEUM industry; BIOMARKERS

Petroleum Science (KeAi Communications Co.), 2024, Vol 21, Issue 6, p3706

1672-5107

Academic Journal

10.1016/j.petsci.2024.06.002