Origin and Distribution of Large Asphaltite in South China

Large quantities of Triassic solid asphaltite were discovered in the Guangyuan area, northwest Sichuan. The asphaltite is formed in layers with a vertical thickness between 0.3 and 2.8 m and is stably distributed with intrusive contact with surrounding rocks. This study aims on the genesis and distribution of asphaltite through trace element, biomarker, and Re-Os isotope analyses. Trace element analysis shows the enrichment of V and Cr in the asphaltite, indicating that it is derived from relatively deep hydrocarbon sources. The carbon isotope and biomarker results suggest that the asphaltite originates from Cambrian paleo reservoir. The Re-Os isotope analysis determines a formation age of 220 ± 6 Ma, which corresponds to the late Triassic, indicating the cracking of paleo reservoirs in late Triassic. Therefore, the origin of asphaltite is epigenetic-reservoir asphaltite. The generation of oil from Cambrian source rocks began at the end of Silurian and ended after Caledonian orogeny. At the end of Permian, the fracture system was well developed due to the influence of the Hercynian movement, which provided favorable conditions for the migration of Cambrian oil. By the end of Triassic, hydrocarbons generated from Cambrian source rocks were mainly distributed in fractures and reservoirs, thus forming paleo oil reservoirs. Afterward, the paleo reservoirs were adjusted to the surface or near the surface during the Indosinian movement and thus have cracked into asphaltite. The distribution of asphaltite is closely related to the tectonic activities, and the asphaltite is preferentially stored in the anticline axes, fissures, and some interlayer fracture zones.