The Geologic Characters Of Upper Palaeozoic Gas Accumulation In Zhong Wei Exploration Area And Circumference Regions

The dissertation researches the geologic characters of accumulation in ZhongWei exploration area and its circumference regions during subsidence, uplift-erosion, erosion-resubsidence and classifies the accumulation unit by simulation test and geology analysis, based on the analysis of gas accumulation mechanism during the process of subsidence and uplift-erosion. The conclusions are as follows:1. The exploration area lies in different structure unit on different stage. In the late Cretaceous period, the center of the exploration area formed overthrusting tectonics, Ordos basin was steadily eroded. Yinchuan basin and Bayanhaote basin became rift-subsidence areas.2. The stratum of Upper Palaeozoic in the exploration area belongs to a same set of sea-land transition facies coal measure strata. The predominant source rocks of Upper Palaeozoic are the coal and the dark shale. The evolvement degree of the origins is much high and the primary products are gas. The fluvial-delta depositional facies sandbodies is good reservoir, primary types of the gas pool are deep basin gas trap, anticlinal trap and fault block trap.3. The conclusions of accumulation mechanism in the exploration area are as follows:(l)Because the stage of the porosity of source rocks descend greatly is consistent with the stage of origins changing into hydrocarbon, the resistance of water is weak, but that of gas is strong in compacting, with the pressure increasing, the hydrocarbon is apt to be compress, the absorbing amount and solubility will increase. So the stage of subsidence may be not the main stage of gas migrating.(2)Erosion can lead to the source rocks and reservoirs breaking and the sand porosity springing , so it can greatly improve the migrating channels. In addition, it can induce the gas to expand, break away, dissolve, which make lots of gas migrate from source rocks to reservoirs.(3)Because the resistance to gas migrating in sand reservoirs is strong , buoyancy can not conquer the resistance to gas migrating. The gas often accumulate near the source when it being drived by pressure difference.4. There are possibility of accumulation during the process of uplift-erosion,overthrusting and erosion-resubsidence in the exploration area. They are different about the characters of accumulation. So petroleum accumulation units can be classified according to the characters: erosion-resubsidence accumulation zone, uplift-erosion accumulation zone, overthrusting tectonics accumulation zone.(1) Overthrusting tectonics accumulation zoneIt mostly distributes in the midland of the exploration area. The characters are following: main types of the gas accumulation are fault block trap and anticlinal trap. The distribution of the gas is limited ,but it can enrich gas in local.(2) Uplift-erosion accumulation zoneIt mostly distributes in Ordos basin. The characters are following:the distribution of source rocks is vast and the erosion zone is also vast. So the distribution area of generating gas is vast, but the gas can not be enriched. At the same time, the phenomena of the downdip gas-bearing reservoirs can possibly be formed.(3) Erosion-resubsidence accumulation zoneIt mostly distributes in Bayanhaote basin and Yinchuan basin . The characters are following: main types of the gas accumulation are fault block trap and anticlinal trap. The distribution of the gas is limited, but it can enrich gas in local.