Study On Controlling Effects Of Faults And Fractures On Ordovician Carbonate Fracture-cavity Bodies In Tahe Oilfield

Influenced by superposition of multi-period tectonic movements and stress field,faults and related fractures with different stages,groups and scales are well developed in the Ordovician carbonate of Tahe Oilfield,Tarim Basin,which have important controlling effects on the formation and distribution of karst fracture-cavity reservoirs.Taking the fracture-cavity reservoirs of Ordovician carbonate in the 4^th,6^th and 7^th typical units of Tahe Oilfield as the research object,this paper studied the distribution law of caves,development characteristics of multi-scale faults and fractures,and relationship between tectonic stress field and fault-fracture,by using outcrops,cores,thins,drilling,logging,seismic,and testing data.On this base,the controlling effect of multi-scale faults and fractures on the development of caves was systematically analyzed.Furthermore,the development models of fault-controlled karst were established.The Tahe Oilfield has experienced six stages of tectonic stress field,including the early Caledonian,middle-late Caledonian,early Hercynian,late Hercynian,Indosinian-Yanshanian and Himalayan.Faults with four stages were formed in the early Caledonian,middle-late Caledonian,early Hercynian and late Hercynian regionally.Faults developed in Ordovician of the 4^th,6^th and 7^th typical units of Tahe Oilfield were formed in the middle-late Caledonian,early Hercynian and late Hercynian.Faults combining with strike-slip and reverse characteristics were the main fault type,which were developed in the directions of NNE,NNW,and nearly E-W.The scales of faults can be divided into four levels,including?,?-1,?-2 and?-3 level.The tectonic fractures were formed in the middle-late Caledonian,early Hercynian and late Hercynian,which were consist with the formation period of faults in the study area.Tectonic fractures with high angle were the main fracture type in reservoirs,which developed in directions of NNE,NNW,nearly N?S,and nearly E?W.The longitudinal extent of tectonic fractures was vertically limited by rock mechanics layer.Faults had obvious controlling effects on the formation and distribution of tectonic fractures in surrounding rocks,resulting in significant distribution difference of the fault-related fracture development zone(FRFDZ)in different scales and orientations faults.In the study area,the average width of the FRFDZ for the?,?-1,?-2 and?-3 fault were595 m,460 m,390 m and 185 m,respectively.And the average width of FRFDZ for the faults in directions of NNE,NNW,and nearly E-W were 560 m,530 m,and 380 m,respectively.In addition,the distribution of the FRFDZ in different positions of faults vary dramatically.The FRFDZ in the intersection,overlap and end position of faults were widely distributed.The plane and vertical distribution of caves were mutually controlled by the multi-scale faults,fractures and the rock mechanics layer.There were two types of basic fault-controlled karst,including the fault core type and fault damage zone type.The karst development degree and scale of them were controlled by the scale and direction of faults.The larger the scale of fault was,the more the number of caves could develop,with wider distribution,larger scale,and lower filling degree.In the study area,the plane distribution range of caves controlled by the?,?-1,?-2 and?-3 faults were 450 m,350 m,250 m and 150 m,respectively.Compared with the nearly E-W trending faults,caves controlled by faults in NNE and NNW direction were more developed and had wider distribution.The caves were mainly distributed within 350 m from the NNE and NNW trending fault.While,the plane distribution range of caves controlled by nearly E-W trending faults was less than 250 m in general.Moreover,the intersection,overlap and end of faults were favorable for the development of large-scale caves.Furthermore,six types distribution models in different structural styles and the formation-evolution mod of fault-controlled karst in the Ordovician carbonate of typical units in Tahe Oilfield were established.