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Dynamic Evaluation on Preservation Condition in Lower Paleozoic and Sinian in Southeast Sichuan Basin, China

Guozhi Wang and Shugen Liu
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China

The lower Paleozoic and Sinian Dengying Formation were located at the base of a superimposed basin and experienced multiphase tectonic activities which had an important effect on seal of cap rocks. The preservation condition of hydrocarbon has become the key point of hydrocarbon exploration in south China. Multi-episode deformation resulted in multi-episode fluid charging and mineral infilling in vug and fissure. The 87Sr/86Sr, δ18O and δ13C of calcite, dolomite infilled in vug and fissure in marine carbonates and their host rocks from Sinian to Triassic, were employed to trace the possible source and migration path of the key fluids related to the hydrocarbon development and its preservation conditions were discussed further. Two-episode petroleum charging in Dengying Formation and multi-episode mineral infillings such as dolomite, calcite, bitumen, quartz, fluorite etc. in lower Paleozoic and the Dengying Formation were identified. Most dolomite and calcite were formed during the second pre-oil window and their isotope of carbon and oxygen suggested that they came from dissolution of carbonatite and reprecipitation. The bitumen and these minerals such as quartz, fluorite, barite and blende were formed during gas window and post gas window respectively.

The research indicated that the hydrocarbon accumulated chiefly in the Dengying Formation and their cap rocks consisted of Cambrian mudstone and gypsum. The mudstone with enormous thickness and 58-meter-thick gypsum in Cambrian suggested that the primary preservation condition was favorable. The first accumulation of hydrocarbon in the Denging Formation indicated that cap rocks remained their primary predominance but the oil pool was destroyed by regional uplift and failure seal during the late Caledonian and early Hercynian. The 87Sr/86Sr, δ18O and δ13C of the calcite and the dolomite formed in late Hercynian suggested that the fluids migrated from the Sinian to Sinlurian and the seal failure appeared during the second pre-oil window. Abundant thermal cracking bitumen in the Dengying Formation suggested that the hydrocarbon accumulated and preservation condition was excellent during late Permian and Triassic. From late Cretaceous to now, the seal failure resulted from regional uplift and the fluids migrated across the strata from Sinian to Permian; the gas pools in the Dengying Formation were destroyed entirely and resulted in development of quartz and later mineral association of fluorite, barite and blende, so that the quartz could be found in Sinian, Siliurian and Permian. Characteristics of quartz with abundant liquid CH4 inclusion and super pressure indicated that the seal were excellent for the thermal cracking gas pools during Jurassic and Cretaceous.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013