Study On Reservoir-forming Dynamic And Transporting-conducting Of The Natural Gas In Lower Palaeozoic In Yanchang Exploration Area In Ordos Basin

In the recent years, natural gas was discovered in the Ordovician weathered crust inYangchang area located in the east of the Ordos basin. Yangchang area was situated at the karst slope zone of Pre-carboniferous karst landform; fine-crystalline limestone is the main reservoir rock; the Carboniferous-Permian source rock is the main gas source. In this paper, the gas reservoir-forming pattern was studied. The results not only have important theoretical significance, but also have important reference to next exploration.Depending on the theories of reservoir-forming dynamic and transporting-conducting, based on the logging date, the rock cores observing and some testing, combined with thin section analysis, scanning electron microscope and fluorescent testing, overpressure in the period of maximum depth was resumed using equivalent depth method, the power of gas vertical migration was discussed by overpressure and the difference in overpressure in adjacent strata. Characteristics and distribution of sand, fracture and unconformity surface transportation system, the development degree of porosity and fracture and reservoir characteristics of the Ordovician weathered crust were studied. Finally, joint the result of the power of gas vertical migration with transporting-conducting and geologic condition, the gas reservoir-forming pattern was summarized.The results show that, in the key period of the pool-forming, the Permo-Carboniferous formation developed large overpressure was the power of gas vertical migration, for buoyancy and capillary pressure were very small, the difference of overpressure between the adjacent layers was the direct dynamic for gas migration. Because the limit of the power condition, sand body distribution and physical property, natural gas was not suitable for large-scale migration along sand body; fracture conductor department was the important channel for natural gas downward migration; the unconformity between Upper and Lower Paleozoic generally developed three-layer structure, some places lack of bauxite developed two-layer structure; sand conglomerate located above the unconformity surface was the channel for natural gas lateral migration, upper part in pale geomorphic and ancient groove where lack of bauxite was the channel for natural gas entering weathering crust; the holes and suture of semi-weathering rock were very developed; nature gas entered weathering crust the migrated to the high site of palaeo geomorphology with the regulative action of fracture. Due to the stable construct and the poor physical property, the fluid in gas reservoir only had a small adjustment in the vicinity and formed the gas reservoir after the strata uplifting.