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

Abstract

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, δ¹³C_CO2 and δ²H_CH4), 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.