Forming Of The Epikarst Zone In Ordovician Karst Reservoirs Of The Tahe Oilfield And Its Reservoirs Space Prediction

The Ordovician carbonate fracture-cavity reservoirs in the Tahe Oil Field experienced tens of millions of years of karstification.The distribution of karst fractures and caves is highly heterogeneous in both vertical and horizontal directions.The epikarst zone in the upper part of the Ordovician is an important part of the Ordovician karst reservoir in Tahe Oilfield,and it is also the primary object of the oil field to describe the structural characteristics and reservoir geology of the karst reservoir.In order to establish an effective reservoir geological model and efficiently develop crude oil,this paper combines the data of karst outcrops,cores,and geophysics,and studies the formation and evolution of surface karst zones and their favorable reservoir space.Through the geological survey of seven karst geologic profiles of Yijianfang,Sanjianfang,Xikeer etc,it was found that the karst landforms in the Bachu-Keping area of Tabei can be divided into six types: high-steep-Rong-Feng type,low-steep-Rong-Feng type,Rong-Qiu type,fault-controlled type,and karst-trough type.Karst-valley type.Using the impression method to recover the paleogeomorphology of the former Carboniferous in Tahe Oilfield,it was found that the former Carboniferous karst paleogeomorphology is similar to the outcrop karst topography in Tabei,and the karst paleo-geomorphology is mainly in the form of Rong-Feng type and Rong-Qiu type.Through the detailed interpretation of the core,imaging logging,conventional logging,and seismic data in the study area,the basic features of the surface karst zone were clarified.The Ordovician epikarst zone in the study area has developed extensively surface rivers and sinkhole systems,and summed up four typical sinkhole forms: symmetrical sinkholes,asymmetric sinkholes,fault-control sinkholes and river erosion sinkholes and three kinds of surface river evolution patterns.At the same time,it was found that the development of surface rivers and sinkholes were controlled by the initial karst paleo-geomorphology and affected by faults.The physical space data of the core were used to evaluate the reservoir space in the epikarst zone.The principal component analysis method and multivariate regression analysis were used to establish a karst sedimentary and limestone porosity interpretation model.It was found that the karst deposits in the surface karst zone have poor reservoir properties,and the porosity is usually less than 2%,and the permeability is less than 0.1mD.However,the fractured and dissolved pores in limestone develop a favorable reservoir space,and the well with high liquid production capacity is usually close to the location of the karst valley.It may be related to the degree of development of fractures and dissolution caves.The evaluation method has guiding significance for the exploration and development of karst carbonate reservoirs.