Structure of a pre-Triassic unconformity and its hydrocarbon transporting characteristics, Wuerhe-Fengnan area, Junggar Basin, China

Abstract

This study focuses on the qualitative and quantitative evaluation of the transporting capacity of a pre-Triassic unconformity and its influence on hydrocarbon migration in the Wuerhe-Fengnan area, Junggar Basin, China. Based on the lithological and geophysical characteristics of rocks and the geochemical parameters of formation water and hydrocarbon fluids from the unconformity, we identified the structure of the unconformity, analyzed the lithological assemblage through its vertical profile, calculated its transporting efficiency for oil in the Triassic Baikouquan Formation (T1b), and tracked the direction of oil migration along it. Four conclusions can be drawn. First, the pre-Triassic unconformity has a complex three-layer structure, including a basal conglomerate, weathering crust, and leached zone from the top to the bottom. Second, different lithologic assemblages form different types of migration pathways, mainly comprising a dual-channel with a sealing layer and a single channel of basal conglomerate. Third, the relative transporting efficiency (RTE) of the basal conglomerate for T1b oil migration is primarily controlled by its thickness and porosity. The RTE decreases from the northwest slope and uplifted area to the southeast sag area, with a mean value of 36%. Finally, the quantitative grain fluorescence, quantitative grain fluorescence on extract, and grain containing oil inclusion values confirm that the unconformity is an effective migration pathway for T1b oils. Horizontal changes in the oil group components and pyrrolic nitrogen compounds indicate that the T1b oils migrated from the southeast sag to the northwest slope and uplifted region along the pre-Triassic unconformity.