Source Rock Potential of Nanpanjiang Basin, South China--Preliminary Report

Judith Totman Parrish, Su Zhongrui, Zhang Yirong, Hu Wenhai

The Nanpanjiang basin of south China occupies 100,000 km² in southern Guizhou and eastern Yunnan Provinces and northwestern Guangxi autonomous region. The basin contains primarily Paleozoic and Triassic rocks. As yet, no producible hydrocarbons have been found in Nanpanjiang basin, although oil seeps exist in margins of the basin and producing wells are present in adjacent basins. We analyzed Ordovician through Permian carbonate rocks and Cambrian through Triassic clastic rocks from within the margins of the Nanpanjiang basin for total organic carbon (TOC), vitrinite reflectance, and extractable hydrocarbon (HC) content and composition. TOC values ranged from 0.02 to 3.91%, total HC (S₁ and S₂) ranged from 0.004 to 0.11 mg/g (S₁ 0.003-0.048; S₂: < 0.003-0.07), and vitrinite reflectance values were mostly above 1.0%. Permian carbonates throughout the basin contain solid bitumen in cavities and fractures; the one available sample of this solid bitumen contained no chloroform-soluble bitumen.

Previous studies by several groups of scientists in this region have found very low TOC values and evidence of overmaturity. Our studies found higher TOC values, but otherwise confirm the findings of previous workers. Our preliminary conclusion is that hydrocarbons have been generated but not preserved because the basin is now thermally overmature. This conclusion is supported by fluid inclusion studies and by the conodont alteration index, which ranges from 3 to 5, both of which indicate high temperatures. Generated liquid hydrocarbons may be represented by the solid bitumen in the Permian carbonates; the presence of gas is not ruled out by the geochemical evidence.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.