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Major Control Factors of Hydrocarbon Accumulation and Potential Exploration Targets of the Ultra-Deep Carbonate Sequences in China


Searching for oil and gas in ultra-deep marine carbonate sequences is one of the major trends in China. In recent years, significant hydrocarbon exploration progresses have been made successively in deep and ultra-deep sequences in Tarim, Sichuan and Ordos basins. Based on characteristics of the typical ultra-deep Ordovician carbonate reservoirs, e.g. the periphery of Tahe Oilfield and north slope of Tazhong in Tarim Basin, Yuanba and the Leikoupo in Sichuan Basin, it was recognized that the hydrocarbon accumulation was controlled by the following four major factors: (1) High quality hydrocarbon sources, including source rocks with various organic matter assemblages and multiple hydrocarbon generating mechanisms, paleo-oil reservoir and dispersed soluble organic matters. (2) High quality reservoirs. Various factors such as structure, stratigraphic sequence, lithology, fluid and time jointly controlled the development and distribution of different types of quality reservoirs. (3) Favorable sealing conditions. The joint sealing of regional, local and direct cap rocks under relatively stable tectonic setting was favorable for the preservation of hydrocarbons. (4) Favorable migration pathways and traps. Various types of traps on paleo-highs and paleo-slopes combining effectively with unconformities, faults and carrier beds determine efficiency of hydrocarbon migration and accumulation. Both oil and gas of huge potential occur in the ultra-deep marine carbonate sequences, and among them the gas is predominant. Large discoveries and breakthroughs are expected to be made in the ultra-deep marine carbonate sequences in China. In Tarim Basin, a huge “Z-shaped” hydrocarbon enrichment play zone exists in the Ordovician carbonates along paleo-highs, faulted belts and unconformities; And the ultra-deep Cambrian carbonates under evaporites show big exploration potential due to high quality and large distribution of source rocks, favorable reservoir-seal combinations and multiple types of traps. In Sichuan Basin, the potential ultra-deep carbonate reservoirs are mainly distributed in the lower marine assemblage (Silurian and older sequences) in areas out of central Sichuan Basin and the upper marine assemblage (Carboniferous, Permian and Triassic) in western and northeastern Sichuan Basin. In Ordos Basin, the Ordovician carbonates in Tianhuan syncline and the frontal areas of thrust fault zones are potential exploration targets.