Thermal and cementation histories of Permian shelf-edge carbonate rocks in the Nanpanjiang Basin, South China

Abstract

As of 1984, the Nanpanjiang Basin of South China has had almost no exploration by drilling although oil seeps exist among its margins and producing wells occur in adjacent basins. however, cooperative studies by petroleum geologists and geochemists of the United States and the People's Republic of China (1982-1984) show that calcite-cemented reef and fore-reef carbonate rocks near Ziyun contain bitumen in Upper Permian shelf-edge sediment. the cementation history consists of three episodes: (1)precipitation of syndepositional marine cement (formerly botryoidal aragonnite and fibrous magnesian calcite); (2)precipitation of post-depositional early cement (radiaxial calcite); and (3)precipitation of late burial cement (white calcite spar). Hydrocarbons were introduced into the rocks between cementation episodes 2 and 3. Fluid inclusion analysis of secondary inclusions in the burial cement indicates that the rocks were heated to nearly 200 C after hydrocarbon migration and cementation episode 3. Bitumen remains in the rocks as evidence of the earlier presence of liquid hydrocarbons. During this study, Ordovician through Permian carbonate rocks and Cambrian through Triassic clastic rocks were analyzed for total organic carbon (TOC) vitrinite reflectance (R 0 ), and extractable hydrocarbon (HC) content and composition. Within the basin, TOC values ranged from 0.02% to 3.91%, total HC ranged from 0.004 to 0.11 mg/g, and R 0 values were mostly greater than 2.0%. Previous studies by several groups of scientists in this region have found low TOC values and evidence of overmaturity. Our studies found higher TOC values, but otherwiese confirm the earlier findings. We conclude that portions of the basin have been heated beyond the level of liquid hydrocarbon generation and retention. Although our observations do not favor the occurrence of liquid hydrocarbons in the Paleozoic strata along the western maring of the basin, the possibility of significant gas accumulations in fractured reservoirs or of other hydrocarbon accumulations in the basin cannot be ruled out.