Formation Of Carbonate Reservoir And Evolution Simulation Of Lower Paleozoic Paleo-karst During Exposed Period In Ordos Basin

Ordos basin is the second largest sedimentary basin and an important energy base with a wealth of oil, gas and coal resources. The distribution patterns of oil and gas in layers are as follows: oil in Mesozoic shallow layer and gas in Paleozoic deep layer, oil-rich in the southern, and gas-rich in the north-central. Paleozoic carbonate rocks are the major oil and gas reservoir layers in the Ordos basin with character of entire layer, big thickness, widespread distribution, various combination of reservoir. Although people have carried out exploration in different blocks with different degrees,the level of oil and gas exploration being conducted is very uneven because of the high degree of heterogeneity in karst medium. To address the theoretical and practical issues which constraint exploration of oil and gas in Lower Paleozoic, it is necessary to carry out in-depth study about the mechanism of karst development, evolution process of reservoirs, and?characteristics of oil and gas accumulation in the Ordos Basin,?so as to provide a theoretical basis and data to ensure the rational development of resources in study area.In the full understanding of the depositional history,?tectonic evolution, and the background of the ancient geography and ancient climate in Ordos basin, this paper then in-depth analysis the paleohydrogeologic evolution and features of ancient Paleozoic karst using disciplinary research methods in geology, ancient hydrogeology, ancient karst science, quantitative?geology, computer technology and carbonate reservoir geology. Also, in order to provide a reliable theoretical basis on oil and gas exploration, in this paper, research methods about quantitative?geology were used to construct the model of the evolution of karst systems under dissolution, to simulate the development process of the Lower Paleozoic carbonate rocks among the exposed period after Ordovician, and to reveal the mechanism of ancient karst evolution and the characteristics of reservoir formation and evolution.The development process of groundwater in carbonate aquifer ,which belong to Cambrian and Ordovician of lower Paleozoic at the central and western of Ordos basin ,experienced 5 hydrogeological periods and 9 hydrogeological sub-periods, including diagenesis, leaching, three times burial leaching and burial enclosing . At the eastern, it experienced 2 hydrogeological periods and 4 hydrogeological sub-periods, including diagenesis, leaching and one time burial leaching.?From the horizontal plane,?the evolution of flow field in?Lower Paleozoic karst aquifer system mainly depends on the evolution of the ancient landscape, which can be divided into 3 stages distinctly:?flowed from the central to around, from the south to north, from east to west. From the vertical profile, the flow field was controlled by the occurrence conditions, which can also be divided into 3 stages: open, closed and stagnated. The ordovician carbonate rocks at Ordos basin has experienced many periods of geological processes, which can be expressed as Sedimentary diagenesis→Exposed weathering→Sedimentary filling→Dissolution and alteration.?According to diagenetic stages and diagenetic environments, the carbonate karst can be divided into three types:?syngenetic karst, supergene karst and buried karst. And the carbonate karst experienced four-step evolution: prototype stage, developmental stage, transformation stage and maturity stage.The main storage space of reservoir performances as the secondary gap, whose distribution is extremely uneven, and so the karst reservoir media field showed a high heterogeneity. The evolution of karst basin is always accompanied by differences in dissolution. Whether the basin is homogeneous or heterogeneous, it becomes more and more non-homogenized. Area with strong dissolution mainly distributed near the phreatic zone and the edge cracks. Good reservoirs with high porosity and high permeability are always formed in the base level of erosion, tectonic fractures, bedding and other large cracks.During the exposed period, the Paleozoic karst aquifer is an open system. The strength of karstification and the overall pattern of the carbonate reservoir development are determined by topography. Due to the reaction of heterogeneity of the media field, groundwater level in karst highlands declined with the system evolution. Strong corrosion at the water table dropped continuously with the water level dropping, and then made karst highlands change into oil and gas reservoirs with vertical cracks. The horizontal runoff was active in karst slopes, where usually formed large area of reservoirs with horizontal cracks. The plain areas of karst basins were located at the downstream of groundwater runoff, where the dissolution was slow, and was bad to reservoir development.The hydrodynamic conditions determined the dissolution intensity in the media field. The distribution pattern of seepage field affected the migration characteristics of solid materials. Controlled by the topography, the seepage of two models about the basin can generally be described as follows: The karst highlands are recharge area, karst slopes are strong runoff area, and the karst basins are drainage area. From recharge area to discharge area, the concentration of chemical field in groundwater increases gradually, as the dissolution process weakened, and the sedimentation process increases gradually. During burial period, by the groundwater, Solid sediments migrated from the karst slopes and karst highlands to karst basins, and deposited with the weakening of hydrodynamic. Sediments in the basin area played a backward role in carbonate reservoir development. Overall, the karst slopes and karst highlands in the Lower Paleozoic stratum helped to the formation of carbonate reservoirs with good quality.