Research On Filling Structure Features Of Ordovician Large Karst Caves In The Main Area Of Tahe Oilfield

The carbonate reservoir in Tahe Oilfield has complex spatial distribution characteristics,and the karst fractures and caves have experienced long-term filling,resulting in the extremely strong heterogeneity of the reservoir and the large difference in well production,which seriously restrict the effective development of oil fields.Therefore,it is urgent to clarify the structure pattern and filling law of karst fractures and caves in Tahe Oilfield,so as to provide geological basis for oilfield development.To this end,this paper focuses on defining the collective filling structure characteristics of karst cavern carbonate reservoirs,and makes a systematic study on the internal filling structure characteristics,filling rules and filling control factors of karst fractured cavern reservoirs by comprehensively utilizing the theories of karst geology and multidisciplinary resources.The field outcrop divides the large karst cave into vertical shaft-type karst cave,isolated karst cave and underground river karst cave.By means of well logging,drilling and seismic data,the karst cave reservoirs developed in the study area were identified and classified,and finely characterize their internal filling characteristics.On the basis of core calibration logging curve,GR-RD and DEN-AC crossplot were established to identify the type and degree of karst cave filling.By comparing the field outcrop karst cave filling sequence with the karst cave filling sequence in the research area,the comprehensive vertical filling sequence of the karst cave reservoir was summarized,which can be roughly divided into 4 zones vertically,and the reservoir characteristics of the missing core section have been predicted.The ordovician reservoir in Tahe Oilfield has three karst zones: epikarst zone,vadose zone and run-off zone.In the epikarst zone,the water flows along the slope,the surface river along the slope,gullies and other low parts flow,the end of the surface river and the lower part of the terrain form the shaft-type cave,the cave filling degree is high,mainly sand,mud and breccia filling.The vadose zone has a strong vertical seepage effect,and the isolated karst caves are developed.The interior is mainly filled with breccia and calcite,but the overall unfilled degree is high.The seepage well in section is formed when the fracture erosion and widening,and the interior is filled with sand,mud and breccia.In the run-off zone,the underground river karst cave is developed,and the chamber caves is large in scale and has a multi-stage "collapse breccia-sand and mud deposition" combination.The filling characteristics of karst caves are comprehensively controlled by diagenesis,main fault,palaeogeomorphology and palaeodrainage system.The compressive capacity of the cave wall makes the sedimentary fillings in the seepage karst zone and the runoff karst zone in the state of weak compaction,and the primary pores are preserved to form a favorable reservoir space.The main fault is closely related to the development of large karst caves and the proportion of collapse fill,and has a positive effect on hydrocarbon accumulation.The area that karst slope ancient water system flows through and the high point of karst highland are the optimal target for drilling in the future.