Geochemistry and accumulation model of the Ordovician hydrocarbon in the Sichuan Basin, China

Abstract

Oil and gas discoveries in the Ordovician in the Sichuan Basin are mainly concentrated in the compact limestone of the Nanjinguan Formation and the Baota Formation at the bottom and at the top of the Ordovician, respectively; this is characterized by fractured hydrocarbon. It is difficult to obtain samples and to further study the reservoir-forming conditions. Recently, oil seepage has been obtained from the Ordovician rock samples, which provides important pieces of evidence for the further study of the Ordovician reservoir-forming conditions. Through the analysis of liquid oil seepage found in bioclastic limestone of the Ordovician Baota Formation in the Maobahe section of northern Sichuan Basin, saturated hydrocarbon chromatography, characteristic analysis of inclusions, and carbon isotope analysis have been comprehensively applied. By comparing the Ordovician reservoir-forming conditions of Well Heshen-1 in northern Sichuan, paleo-uplift in central Sichuan, and Well Dongshen-1 in southeastern Sichuan, it is clear that the hydrocarbon of the Baota Formation in northern and southeastern Sichuan originated from the overlying source rocks of the Wufeng Formation and the Longmaxi Formation. The aforementioned two formations have the generating mode in the underlying formation and storing in the overlying strata. The Ordovician hydrocarbon, mainly situated in the paleo-uplift in central Sichuan, receives hydrocarbon from the underlying Cambrian source rocks, forming a generating mode in the overlying formation and storing in the underlying strata. These conclusions would provide a reference for the Ordovician hydrocarbon exploration prospects.