DISCOVERY AND EXPLORATION SIGNIFICANCE OF MICROFISSURE GRAIN LIMESTONE RESERVOIRS IN THE ORDOVICIAN YIJIANFANG FORMATION OF TAHE AREA

Tahe area is located in the southern part of the Shaya Uplift,the Tarim Basin. A large-scale stratigraphic unconformable-paleokarst trap was formed on the background of the large-scale Ordovician fossil buried hill. The further exploration of the trap showed that reservoirs in the main part of the trap were the karst fissure-typed reservoirs of the Lower Ordovician Yingshan Formation,and the grain limestone pore-typed reservoirs of the Lower Ordovician Yijianfang Formation were newly discovered in the southern part of the trap. The storage space of this type of reservoirs was dominated by intergranular openings ang pinpoint-like solution openings,which were connected by microfissures and sutures to form 20-30m thick and 200 km²spread more wide-ranging layered pore-typed reservoivs. The further interpretation and treatment of imaging logging data revealed that the porosity of solution openings was about 5%. This type of reservoirs were mainly controlled by the distribution of tableland marginal bioclast-bank facies,and paulopost solution was advantageous to the improvement of their storage performance. The grain limestone reservoirs and the upperlying regional caprocks of the Middle-Upper Ordovician argillaceous limestone and the Lower Carboniferous mudstone constituted a good reservoir-caprock assemblage with good sealing performance. Under the multiple periods of high-mature hydrocarbon injection by hydrocarbon source areas,medium-light oil pools with good exploration benefits were formed. In some prospect wells completed,high-productive oil flow was obtained through acid fracturing treatment. This expressed tremendous potential of the exploration field.