Study On The Carbonate Reservior Of Ordovician In Block 2 Of Tahe Oilfield

Ordovician reservoirs of block 2 of Tahe Oilfield are in the transitional slope from high land to depression of the karst. Based on the character of Ordovician reservoirs, the adoption on contrasting study of similar outcrops with underground and the application on well-log information, logging information and seismic data, etc, the paper had a redefinition to the Ordovician strata, and the stratigraphic framework accurately was modified in the research area. According to the thinking of single well facies—section facies—plane facies in the stratigraphic framework, the paper studied on the Ordovician sedimentary facies and evolution. In research area, sedimentary facies had a great significant influence on the development of reservoirs.Physical properties of Ordovician resevoirs matrixs in the research area were poor in all, and porosity and permeability had basically no contribution to accumulation and permeation.The reservoir properties were decided by dissolution fissure and pore, fracture and sizable dissolution cave and pore. The reservoir spaces determined by the observation of cores, thin sections and SEM and the information of engineering logging and well-logging,etc, was divided into three main types (pore, fissure and cave)and other 16 ones. According to the means that these reservoir spaces formed reservoirs type and scale, the reservoirs could be divided into dissolution pores type of reef-bank facies, fracture type, karst cave type and dolomite limestone type. Dissolution pores type of reef-bank facies was developed in Yijianfang formation in the south, and it belonged to the complex body of multi-cyclicity. The multi-episodic reservoirs of reef-bank facies superimposed vertically and continued laterally. It developed widely along the margin of the southeast, and had a large scale. Its development degree and range were controled by the sedimentary microfacies. Additionally, tectonization and early exposed honeycomb dissolution were also the important factor for reef-bank facies to form the high quality reservoir of pore type. Lower-Middle Ordovician reservoirs of fracture and cave types developed in the north, which formed the complex fractured-cave carbonate reservoir. The 95.6 % of karst cave developed in the depth of 0～100m that was below the T74, and decreased gradually as the increase of depth. By the analysis of C, O stable isotopes, Sr isotope and rare-earth elements of neutron activation, it thought that the karst cave formed mainly in the early Hercynian. The fillings type in the karst cave included: (1)collapse breccia; (2)arenopelitic fillings; (3)calcites. In combination with the collapse breccia, the reservoirs filled with pelite and the giant calcites were poor. The fracture and karst cave could be vertically divided into four combined types: upper-cave and lower-fracture, upper-fracture and lower-cave and fracture-cave-fracture. The range and develop degree are controlled by tectonization, lithology, palaeogeomorphology and palaeodrainagepattern, etc. The reservoir space of dolomitic limestone was inter-granular pores of dolomite formed by burial dissolution. The cause of formation included:(1)metasomatism of dolomite based on the burial dissolution;(2)hydrothermal fluid dolomitization controlled by the structural fracture. The dolomitic sand-clastic block mass formed by the first reason was the most common. Development of the reservoirs was mainly influenced by stylolite, lithology, diagenesis and hydrothermalism, etc.Based on the knowledge of well history, the application of dynamic producing data and important changing event of production operation and tracer test data, this paper carried out the connectivity analysis to inter well-groups of research area. According to the synthesis of fracture-cave distribution, the pressure system of strata and reservoir connectivity, referring to the karst topography of T74 top surface and dynamic producing data of Ordovician reservoirs, the research area was divided into 15 units of fracture-cave so that the dynamic research of following development would be more convenient. The matching of fracture-cave was better near the pinch outlines of Upper Ordovician and inside the fracture-cave unit of the north denuded zone, the well yield was higher in the area that had good connectivity. The matching of fracture-cave was worse inside the fracture-cave unit of the south, and the well yield was lower in the area that had worse connectivity.