The Characteristics And Its Control On The Reservoir Development Of The Key Unconformity Of Palaeozoic In Tazhong Area,Tarim Basin

In recent years,the reservoir of Ordovician carbonate in Tazhong area becomes the focus of Tarim Basin petroleum exploration,and the karst reservoir is one of the most important types.Under the guidance of sequence stratigraphy,seismic stratigraphy,sedimentology,structural geology and reservoir geology,the four key surfaces of unconformable,T74?T72?T70??T60,are identified and analyzed through the study of outcrop,well drilling,well logging and seism.And the function how the unconformities developing control carbonate reservoir is discussed.The key unconformable surfaces have obvious logging and seismic responsive characteristics.The unconformable surfaces,T74?T72?T70.T60,show that the peak value of TH/K curve is higher than 7.And the INPEFA curve presents a forward turning point in unconformable surface,which means that and the sedimentary environment is from drought to moist climate gradually.The strong partial negative value of isotope and the difference of isotope value at both sides of the interface indicate the unconformable surface well.The T74 interface,which is the top interface of the Yijiangfang Formation,is unconformable contact with the lower formation.In seismic section,there is obvious truncation and oniap phenomenon on T74 interface.The unconformable composite area is around Tazhong 19 Well and the scope is small.The T72 interface is the top interface of the Lianglitage Formation and the sedimentary stratum is mainly uniaxial continuous.There is seldom truncation phenomenon in underlying strata,while the onlap phenomenon is obvious in upper formation.The unconformable composite area of the T72 is larger than the T74 and is distributed along the line of Tazhong 19-Tacan 11-Tazhong 1.The T70 interface is the top interface of the Sangtamu Formation and there is truncation and onlap phenomenon on the interface.The unconformable composite area of the T70 is distributed in the Guchengxu uplift in the southeast of Tazhong Area.The T60 interface is the top interface of the Silurian and there is obvious truncation and onlap phenomenon in the seismic section.The palaeogeomorphology of key unconformable interface has been resumed by strata trend extending metnod and onlap point method and the results are shown below:Ancient landform highland forms from west side of Tazhong area to central area of Tazhong upheaval like a shape of bird head.Its high elevation part developed near the Tazhong II fault zone.There are two landform high spots figured in the T72 ancient landform respectively around the Well 4 and Tazhong Well 19-Tacan Well 1 ·The morphological characteristics of T70 ancient landform presents high in the south part and low in the north part on the whole.And it shows that there is mainly platform sedimentary in the south part of Tazhong area while basin facies sedimentary in the north.T60 ancient landform high spots only contain two higher position which respectively are the area with Well 4 as the center and local region of the ancient city,when the sea level compared with Ordovician period had a further rise so that the basin was mostly covered by the sea and to form a mixed marine-terrestrial sedimentary environment.Each ancient landform high spot of the interface is the initial place where the karst reservoir development.But both the amount and area of the relict landform high spot are reduced by the tectonic movement with the time goes by.These ancient landform high spots development specially near the Tazhong II Fracture,and they are not disappeared until the early Hercynian.It is concerned that these landform high spots are the favorable area for the development of karst reservoir.