| The characteristic of tight carbonate reservoir has low porosity and low permeability, and fracture is the key geological factors to the high production of the gas well. The Ordovician karst carbonate reservoir in Daniudi gas field is the most potential development formation to the Eastern of Ordos basin. The Karst reservoir had experienced the uplifting exposed for a long period of time in Caledonian tectonic movement period. Reservoir suffered intense weathered denudation and karst dissolution, and experienced many times strong tectonic movement. So the reservoir fracture types, causes and the control factors is complex, thus the evaluation of fracture research is more difficult. Predecessors did not carry out fracture research work in the past years, but the capacity of the reservoir fracture system contribution is enormous. Thus,fracture type, characteristics, causes and controlled factors of comprehensive analysis and to establishes a comprehensive evaluation method which are suitable for the fracture in the region have further guiding significance for oil and gas exploration and development.Through the analysis of the basic geological characteristics of reservoir(karst features, landscape characteristics and reservoir characteristics), and the combination of drilling and logging, core, thin section, well logging and rock mechanics parameter test and analysis data, have been made the fracture types and the basic development characteristics(including the filling, fracture width, length, logging response and fracture characteristics of reservoir) clear about. Below the formation of the Ma53, the collapse of rupture is the main weathered fracture types. In the formation of the Ma51 to Ma52, the surface rupture is the main weathered fracture types. The tension weathered fracture development did not steep hills. Compared to the different weathered rupture, the filling characteristics are different, and the degree of the filling reduce gradually, in the order of collapse weathered fracturesurface weathered fracture- steep hills tension weathered rupture.Structural fracture has stronger lithology selectivity compared with the weathered fracture. The structural fractures develop mainly in dissolved pore dolomite and the dolomite containing much gray in Ma51 to Ma52. High angle shear fracture mainly develop in argillaceous dolomite with the low degree of filling. When gray matter increased in dolomite, and the tensile rupture occurred more often below the formation of the Ma53.Shear fracture develop in argillaceous dolomite. And the fracture filling degree is relatively high than that in the overlying strata(Ma51 to Ma52). The tectonic fracture extended along the early microcracks, and Karst collapse weathered fracture and the tectonic fracture formed complex fracture system. The characteristic of fracture filling is obvious, and the fillings is fresh calcite.On the basis of studying fracture development characteristics and using the methods such as structural fractures isotope test and analysis, rock acoustic emission test, rock statics parameters test and the ordovician top surface structure curvature calculation sheet, buckling stress simulation, determine the structural fracture formation time and development controlling factors. Research suggests that structural complex fracture system is the result of Indosinian, Yanshan and Xishan tectonic movement. Yanshan period is the main formation of structural fracture. The effectively tectonic fractures’ main formation period is the late Yanshan and Xishan tectonic movement period. The basic characteristic of the rock reflects the differences of the intensity of rock. Tectonic action is the reason of fracture, and the better of the petrophysical, the more of the solution pores, the thinner of the strata sedimentary thickness, the stronger of the karst dissolution and tectonic action, structural fracture development.Through the vertical zone distribution, karst breccia logging identification, karst landform, and karst reservoir composite analysis and evaluation of the residual strata thickness, so the main controlled factors of weathered cracks and distribution has been made clear. Surface weathered fracture controlled by lithology, the distance from the top weathered crust, and the landscape. Weathered residual hill is such broken main development area. With the increased argillaceous content in dolomite rock, the physical mechanical weathered resistance reduced. Surface weathered cracks mainly development within 15 m away from the earth’s surface.Karst collapse weathered fracture is mainly controlled by landform, and karst dissolution. The stronger the karst dissolution, rock weathered burst occurred more often. Geomorphic soluble pit area is the collapsed fractures’ main distribution area. The tensile rupture near the steep hills has lithologic selective.On the analysis of the structural fracture and the weathered bursts’ development controlling factors, this article establishes a comprehensive fracture evaluation method which is suitable for this area by setting structure fracture zone evaluation index, weathered fracture zone evaluation index, and the composite rock properties. The evaluation results is match to the production test. |
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