Characteristics and Stage Analysis of Hydrocarbon Migration Along Fault Zones

ABSTRACT Faults are well developed in the Lishu Depression, and their transport capacity strongly governs differential hydrocarbon enrichment. However, the phases and boundaries of hydrocarbon migration along these faults remain poorly constrained, hindering a quantitative evaluation of fault transport capacity. In this study, well‐logging parameters were used to characterize fault zone architectures. Microscopy, fluid inclusion analysis, and bitumen Raman spectroscopy were performed on fault zone samples to identify internal hydrocarbon migration pathways and phases and ultimately to establish threshold activity rates for distinct transport stages. The Lishu Depression hosts two types of fault zone architecture: one‐unit (damage zone only) and two ‐ unit (fault core and damage zone). The damage zone exhibits high fracture density and large pore‐throat radii, resulting in more favorable flow conditions than the fault core and protolith and thus serves as the principal hydrocarbon transporting pathway. Observations of bitumen and hydrocarbon inclusions within the fault zone reveal that asphaltic bitumen formed from migration differentiation; its formation temperature is consistent with the homogenization temperatures of aqueous fluid inclusions coeval with hydrocarbon inclusions. Both indicate that the main phase of hydrocarbon migration along the fault zone occurred during the deposition of the Quantou Formation. On the basis of the fault activity rate corresponding to this migration period, the threshold activity rate between the intermittent and static periods is determined to be 1.5 m/Ma. Furthermore, on the basis of the relationship between fault activity rate and the hydrocarbon fullness degree of associated traps, the threshold between the active and intermittent periods is 4.9 m/Ma.