The Accumulation Mechanisms Of Deep Basin Gas Trap

In the elementary clues of the evolution of the slope and the deep depression of the basin and its controlling on the source rocks, the reservoirs, the migration and the accumulation, the accumulation mechanisms of deep basin gas trap have been discussed with the example of the Upper Palaeozoic in Ordos basin. The basic thoughtfulness of this article are as follows: a) The characters of the basin's slope and the deep depression in the sedimentation stage have been studied according to the data of sedimentary context ; b) The characters of the slope and the deep depression of the basin in the subsidence stage and in the erosion stage have been studied according to the data of palaeostructure; c) The characters of the source rocks and the relationship between characters of source rocks and evolution of basin have been studied according to the data of geochemistry; d) The characters of the reservoirs and the relationship between characters of reservoirs and evolution of basin have been studied according to the data of lithology and physical properties analysis and simulation experiments. The mechanisms of the oil and gas migration and accumulation have been analyzed according to the result of geology and simulation experiments; e) The accumulation mechanisms of deep basin gas trap have been analyzed on the basis of studies above.The locality of the slope and the deep depression of Upper Palaeozoic in Ordos basin was alterable when it was in the sedimentation stage, the subsidence stage and the erosion stage.The predominant source rocks of Upper Palaeozoic in Ordos basin are the coal and the dark shale. The seam is mainly forming in the paludal context of TaiYuan formation and ShanXi formation in the area of the slope. The dark shale is broadly forming in all the area of the slope and the deep depression. In the subsidence stage, the transition course of the origins from source rocks to oil and gas occured. The evolution degree of the deep depression was much higher than that of the slope. And the evolution degree of the dipping place of slope was higher than that of its upper place. The predominant reservoirs of Upper Palaeozoic in Ordos basin is sandstone. It can be classified into 5 types, which is intergranular porosity, intragranular porosity, intra-filler porosity, combinational porosity and fracture porosity. By using the parameters such as pore type, porosity, permeability, average radius of pore and throat, etc. the sandstone reservoir of Upper Palaeozoic can be classified into 4 types, which is I(best),II(better),III(bad),and IV(worse).The reservoirs are mainly formed in high energy region of the slope in the sedimentation stage. The depositional compaction is the main factor which made the reservoirs bad. As a result, the sandstone reservoirs which have good storage capacity are mainly forming in the upper place of the slope. The storage capacity can be greatly improved in the erosion stage.Because the greatly descending stage of the porosity of source rocks is consistent with the stage of origins being changed into hydrocarbon, the resistance to water is weak, but the resistance to gas is strong in the compacting stage. With the pressureincreasing, the hydrocarbon is apt to be compressed, the absorption and solubility will increase. So the subsidence stage and sedimentation stage may be not the main stage of gas migrating. Erosion can lead to the source rocks breaking and the sand porosity springing and breaking, so it can greatly improve the migrating channels . In addition, it can induce the gas to expand, break away, dissolve , so the erosion stage may be the main stage of gas migrating. Because the resistance to gas migrating in sand reservoirs is strong, gas often accumulate near the source when it being drived by pressure difference. Because buoyancy can not conquer the resistance to gas migrating, the phenomena of the downdip gas-bearing reservoirs can be formed usually when the gas being injected into the sand reservoirs from the dipping place.The formation of the deep basin gas trap is...