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

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, d 13 C CO2 and d2HCH4), 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, C 24 tetracyclic/C26 tricylic terpane and C 21 /C 23 tricylic terpane may be indicative of the occurrence of abnormal thermal energy. C 31 /C 30 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, C 28 and C 29 aaa (20R) steranes are probably altered little by the mantle-derived fluids. Based on the ternary diagram of C27, C 28 and C 29 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.<br /> (c) 2024 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).