Geochemical identification of a source rock affected by migrated hydrocarbons and its geological significance: Fengcheng Formation, southern Mahu Sag, Junggar Basin, NW China

The Fengcheng Formation is a crucial source rock and the primary reservoir for oil accumulation in the Mahu Sag. Crude oils are distributed throughout the Fengcheng Formation, ranging from the edge to the interior of the sag in the southern Mahu Sag. These crude oils originate from in-situ source rocks in shallowly buried areas and the inner deep sag. During migration, the crude oil from the inner deep sag affects the source rocks close to carrier beds, leading to changes in the organic geochemical characteristics of the source rocks. These changes might alter source rock evaluations and oil-source correlation. Based on data such as total organic carbon (TOC), Rock-Eval pyrolysis of source rocks, and gas chromatography-mass spectrometry (GC-MS) of the saturated fraction, and considering the geological characteristics of the study area, we define the identification characteristics of source rock affected by migrated hydrocarbons and establish the various patterns of influence that migrated hydrocarbons have on the source rock of the Fengcheng Formation in the southern Mahu Sag. The source rocks of the Fengcheng Formation are mostly fair to good, containing mainly Type Ⅱ organic matter and being thermally mature enough to generate oil. Source rocks affected by migrated hydrocarbons exhibit relatively high hydrocarbon contents (S₁/TOC > 110 mg HC/g TOC, Extract/TOC > 30 %, HC: hydrocarbon), relatively low Rock-Eval T_max values, and relatively high tricyclic terpane contents with a descending and mountain-shaped distribution. Furthermore, biomarker composition parameters indicate a higher thermal maturity than in-situ source rocks. Through a comparison of the extract biomarker fingerprints of adjacent reservoirs and mudstones in different boreholes, three types of influence patterns of migrated hydrocarbons are identified: the edge-influence of thin sandstone-thick mudstone, the mixed-influence of sandstone-mudstone interbedded, and the full-influence of thick sandstone-thin mudstone. This finding reminds us that the influence of migrated hydrocarbons must be considered when evaluating source rocks and conducting oil-source correlation.