Development Characteristics And Controlling Factors Of Underground River Karst Reservoirs In The 4 And 6 Areas Of Tahe Oilfield

Multiple periods of tectonic movement and karstification have taken place in Tahe Oilfield,leading to extremely developed karst reservoirs in the area.It is currently the largest karst fracture-cavity reservoir in China.The underground river system is developed in the study area,but the understanding of the cave development status,fracture-cavity filling characteristics and rules of the underground river systems are still unclear,which restrict the reservoir evaluation and prediction of Tahe Oilfield.In this paper,high-precision sequence stratigraphic division and the underground river system of the Yingshan Formation in the 4 and 6 areas of Tahe Oilfield was researched by using drilling,logging,seismic data,and karst outcrops in the Keping area,focusing on the characteristics of karst cave development,filling characteristics and filling rules of the underground river system,clarify the main controlling factors of the development of underground river system,and provide basis for the prediction of favorable cave reservoirs.Summarizing the previous research results,making full use of core,conventional logging,imaging logging and other data in research area,and using INPEFA logging cycle analysis technology,two fifth-order sequence interfaces of SBys1 and SBys2,one fourth-order sequence interface of T₇^5-1and one third-order sequence interface of T₇⁵were identified in the Ying4member.The Ying-4 member was divided into three five-level sequences,including Ying4-1sub-member(Ssqy1),Ying-4-2 sub-member(Ssqy2)and Ying-4-3 sub-member(Ssqy3),established fifth-order sequence comparison framework.Based on field outcrops,cores,drilling logging and seismic data,the karst caves developed in the study area were identified.The karst caves showed the response characteristics of high AC,low density,and low resistivity on the conventional logging curve,and GR value will increase with the increase of cave mud fillings.It appears dark-colored clumps on imaging logging and“bead-like”strong reflection feature on the seismic profile.The study area develops 2underground rivers in the east-west direction and 4 underground rivers in the north-south direction,a total of 6 large underground river systems,with the complex dendritic structures.Vertically,from single well analysis of karst caves to comparison of underground rivers with connected wells,the scale and boundaries of underground river pipeline development are determined.S67 underground rivers and S65 underground rivers have good development continuity and large channel scales,while T403 underground rivers have poor continuity,but the cave is large in scale.The continuity of the underground rivers TK404,TK660 and TK405 is good,but the channel scale is small.The filling materials of underground river karst caves are mainly sandy mud and breccia,and there are few unfilled caves.According to the statistics of the drilled caves,the S67 underground river has the largest number of caves,and the TK405 underground river has the least caves.In the underground river system,the fully-filled caves are the main and densely distributed.The T403 underground river has the highest full filling rate,and TK405 underground river has the lowest full filling rate,the number of semi-filled and unfilled caves is small and scattered.On the whole,the east-west underground rivers have a lower full filling rate and a higher rate of high-yield wells,while the north-south underground rivers have a higher full-fill rate and a lower rate of high-yield wells.Based on the research on the development characteristics of the underground river system,the main controlling factors and the mechanism of the underground river system development are comprehensively analyzed.It is believed that the paleo-landforms,sedimentary microfacies,faults,and unconformities jointly control the development of the underground river system.Paleo-landforms and sedimentary microfacies control the flow direction of the paleo-water system of the underground river system and the distribution of soluble rock layers,which are the key factors to the development of the underground river system.The faults and unconformities have little control effect on the underground river system.