Influence of tectonic evolution of the northern slope in the central Sichuan paleo-uplift on the Sinian-Cambrian hydrocarbon accumulations

Abstract

The Sinian-Cambrian tight carbonate strata locating on the northern slope (NS) of the paleo-uplift of central Sichuan Basin is the frontier for ultra-deep natural gas exploration. The tectonic evolution of the NS determined the accumulation process and current distribution of oil and gas. In this study, based on integrating drilling, seismic and test data, a 3D basin simulation software was used to carry out the structural restoration. This paper focused on the restoration of the tectonic evolution at the bottom of Cambrian in the NS and Gaoshiti-Moxi (GM) areas, and the tectonic pattern of hydrocarbon accumulation during critical periods was re-constructed. At the end of the Tongwan Movement, the NS area was locally uplifted to the high regions, while paleo-geomorphic depressions formed between the NS and GM areas. In the Late Caledonian, the NS and GM areas displayed a structural pattern similar to 'saddle-domal'. In the Indosinian period, the area to the north of Moxi gradually inclined northwards to form a monocline. In the early stage of Yanshanian Movement, the dip angle of Cambrian in the NS area further increased until it became stable and was consistent with the current tectonic pattern found after Mid-Yanshanian Movement. The paleo-tectonic morphology in the NS area during the Tongwan period laid a solid foundation for the development of large-scale platform margin and inner beach reservoirs in the Sinian Dengying Formation. Controlled by large-scale transgression, the NS area and the Deyang-Anyue rift trough were more conducive to the development of black shale in the Qiongzhusi Formation due to deeper sedimentary water body and higher degree of confinement-hypoxia. The oil-cracking in the Sinian-Cambrian paleo-oil reservoirs of the NS area occurred earlier than that in the GM area. The difference between thermal evolution and source rock characteristics of the Qiongzhusi Formation in the NS and GM areas led to a lighter δ13C1 of natural gas in the NS area. The coupling of local tectonic highs and lithologic traps controlled the formation and preservation of independent paleo-oil and natural gas reservoirs in the NS area.

Keywords

• tectonic evolution
• sinian-cambrian
• evolution process
• northern slope of central sichuan
• sichuan basin