Study On Basic Hydrocarbon Accumulation Conditions In Caohu Areas Of Tarim Basin

Caohu area is located northeast of the Tarim Basin, which belongs to Yuli in Bayingolin Mongol Autonomous Prefecture. This study area includes four blocks :Korla, West Korla, Northeast Taheâ… and East Manjiaerâ… . The Korla and West Korla mainly belong to Caohu Depression and Korla Nose, Northeast Taheâ… and East Manjiaerâ… belong to Manjiaer Depression. Although this region does not get a major breakthrough in oil and gas exploration, but from the facts that some wells of the area have tested out oil and gas, and the surrounding adjacent area has industrial oil and gas, so in its geological evolution process there must have the hydrocarbon generation, migration and accumulation. Therefore, the research of geological conditions of the area will provide a scientific basis for the exploration of the area.Based on the previous research results, taking Geological background and two-dimensional seismic data as the basis, Petroleum geology, sequence stratigraphy, seismic stratigraphy and other basic theory as the guide, the main controlling factors of accumulation as the goal, the author studied the geological conditions of Caohu Depression in Tarim Basin and exploration potential, in order to provide a basis for optimization of exploration targets and the deployment of oil and gas exploration of the area.After analysis, there are three sets of source rocks in Caohu area, which are marine dark limestone and dark mudstone of Cambrian-Middle/Lower Ordovician in Lower Paleozoic, source rocks that are mostly paralic mudstone of Carboniferous in Neopaleozoic, marine coal bed, bass and dark mudstone of Triassic-Jurassic in Neopaleozoic. The petroleum generative center is located in the concave area of Caohu Depression. There are four sets of reservoirs. They are: Cambrian-Middle/Lower Ordovician reservoirs, Silurian sandstone reservoirs, Devonian East-River sandstone reservoirs and Triassic sandstone reservoirs. Mudstone cap rocks are in the Upper Ordovician, Silurian, Carboniferous, Triassic and Jurassic. In the study area there are totally three sets of reservoir-cap combination which are the reservoir-cap combination at Caohu Depression and Southwest edge of Korla Nose which reservoirs are Cambrian-Middle/Lower Ordovician carbonate rocks and caps are Upper Ordovician mudstone, the reservoir-cap combination at Caohu Depression which reservoirs are Devonian East-River and Silurian sandstone and caps are Carboniferous mudstone, the reservoir-cap combination at Caohu Depression which reservoirs are Triassic reservoirs in the three oil groups and caps three Triassic mudstone.The traps of the study area can be divided into structural traps, depositional traps, porosity traps and combination traps. Overall, the type of traps for the Cambrian-Middle/Lower Ordovician carbonate rocks is buried hill weathering crust trap, the Devonian East-River sandstone is overlap pinchout trap, the Triassic reservoirs in the three oil groups are anticlinal trap and pinchout trap, the east slope and Tectonic zone in northern of Caohu depression is structural trap, and the the west slope of Caohu depression is nonstructural trap. The thickness and quality of the caps have great influence on the preservation of oil and gas, and the fracture's active time and scale and the distribution of the unconformity also have great ifluence on the preservation of oil and gas.There are two critical oil and gas accumulation moment for Caohu area which are the late Hercynian and the late Yanshan - Early Himalayan tectonic movement. The migration pathway is the vertical fracture and crack, and the horizontal unconformity and sand. The accumulation process is divided into the Caledonian, Hercynian early, late Hercynian, Indosinian, Yanshan and Himalayan. The conservation conditions of the area is the key of accumulation, A good matching of traps and sources necessary condition for the oil and gas accumulation, and the aims of the migration of oil and gas are succession of paleo-uplift and ancient slope.On the basis of the above, this article evaluated the distribution of profitable blocks in Ordovician, Silurian, Devonian East-River sandstone and Triassic of the four exploration strata.