| Tahe oil field is the largest karst carbonate oil field in the world,which the average depth of Ordovician fracture-cave reservoir is 5500m,and the petroleum geological reserves are13.8×10~8t.However,due to the complexity of dissolution fracture-cave reservoir space,the karst reservoir has strong heterogeneity,and the oil recovery is less than 20%.The formation process of karst fracture-cave and its reservoir physical property characterization and evaluation are scientific and practical problems that need to be solved urgently.Based on the abundant geological and geophysical data,modern karst observation and paleokarst restoration,this paper,taking the Ordovician karst reservoir in Tahe oilfield as an example,The concept of weathering crust carbonate karst facies was put forward form karst environment and karst products by the theoretical method of the present is the key of the past and karst geology.The karst facies system and facies model are established,and the genetic structure and formation process of fracture-caves in different karst facies were clarified.According to the karst geological conditions and genetic types of fractures and caves,the physical property characterization parameters and evaluation standards of reservoirs in different karst facies and types of fracture-cave were established,A set of karst facies controlled fracture-cave reservoir description and evaluation methods are formed.The results show that the Ordovician paleokarst production in Tahe was very similar to the modern karst production in Hongshilin in western Hunan,both of them are an independent karst system or karst unit composed of two watersheds and a depression under the karstification of a karst drainage.In this karst unit,different geomorphic unit have different flow pattern,and different fracture-cave combinations.So,karst facies is the sum of a series of karst products formed under the karstification of a specific karst drainage.According to karst geomorphic characteristics,drainage distribution and fracture-cave structure,karst facies unit can be divided into 3 karst subffacies of karst hoodoo-upland,karst depression,karst basin.The hoodoo-upland subfacies is located in the local high zone on the karst slope,which belongs to the water-deficient area.In rainy season,the meteoric waters can diffuse and dissolve along the densely developed faults and fracures network channels,forming medium and small-sized fracture-vuggies densely distributed around the karst hoodoo-upland.Due to the lack of argillaceous input in the karst hoodoo-upland,the filling rate of fracture-vug was36%,which preserves a large amount of oil and gas storage space.The karst reservoir thickness and fault density around well are positive correlated with accumulated oil production per well,which can effectively characterize the fracture-vug reservoir physical properties in the karst hoodoo-upland.The karst depression subfacies belong to water-rich area with widely distributed surface rivers,where the surface river has eroded and dissolved the near surface carbonate strata developed by fracture-vuggies.The surface river,in the karst depression subfacies,carried a large amount of argillaceous or clastic materials into the underground river through faults or sinkholes,resulting in the filling rate of karst caves in the underground river reaching 86.8%,and the main reservoir space were unfilled caves and intergranular pore of fillings.The unfilled rate of cave and porosity of fillings can characterize karst cave reservoir physical properties in the karst depression subfacies.Karst basin subfacies was flat in terrain,where the Ordovician limestone was covered by fine-grained sediments,which leads to the almost disappearance of karstification.Only on the local active fault zone,the meteoric waters can be poured into the ground along the fault,and then contact with limestone to form fault controlled dissolution fracture-caves or fault-reservoir,where the development depth of this type of fracture-caves can reach 600m below the weathering crust and distributing within 1km of the fault zone.This fracure-caves were mainly filled by fault breccia and travertine,and the gap between the gravels was developed,which was an important space for oil and gas in karst basin subfacies. |
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