Abstract: Shunnan area lies in the north slope to the down-thrown blocks of Tazhong No. 1 fault in Katake Uplift, and is adjacent to the hydrocarbon kitchen of Manjiaer Depression. The study area has regionally good cap rocks, and remains a slope for a long geological period, thus it is the main area where hydrocarbons migrate to and accumulate in. Multi-group NE-trending faults developed here were conducive to reservoir development and hydrocarbon accumulation, all such provide the favorable conditions for the formation of large oil and/or gas fields. Multi-period tectonic movements from the Caledonian to Hercynian controlled development of the multilayer and multiphase carbonate reservoirs. With tectonic movements and sea level fluctuations, the Ordovician carbonate rocks experienced multi-period exposure and led to the formation of karst reservoirs. The reservoirs were reconstructed by meteoric water and deep thermal fluids through faults, resulting in fractured-vuggy reservoirs. In brief, reservoirs determined the scale of hydrocarbon accumulation, and the strike-slip faults restricted the hydrocarbon accumulation zones. Geochemical analysis shows that the natural gas is dry gas with high maturity in Shunnan, and its accumulation period was the Himalayan. The natural gas in Shunnan area features in highly variable ln(C₁/C₂)value but low variable ln(C₂/C₃) value, a character obviously differing from that of cracked gas. The inclusions in several wells are dominated by methane gas inclusions, but rare bituminous gas phase inclusions and bitumen inclusions, indicating kerogen cracking origin. Analyses of drilling and test data reveal that the Ordovician carbonate gas accumulation is likely to be a large fractured-vuggy gas reservoir cluster featuring in vertical superimposition, lateral connection, high gas column and strong control of reservoir heterogeneity.

Key words: fractured-vuggy reservoir, carbonate rock hydrocarbon enrichment, Ordovician Shunnan area, Tarim Basin