Depositional Character And Control Factors Of Reservoir Sandbody In Major Gas-bearing Interval Of Upper Paleozoic In North Ordos Basin

Abstract: On the basis of predecessor's achievements, the paper take major gas-bearing sandstones of upper Paleozoic in North Ordos basin as the object of study, According to the theory and methods of Sedimentology, reservoir geology, geophysical, geochemistry and so on multi-disciplinary knowledge, and considering the geological background of the whole North China, In combination with outcrop,the well drilling and logging information, macroscopic and the microstudy, taking full advantage of modern test technology, source direction and the mother rock nature, character of sedimentary system, sedimentary facie, sedimentary modle, palaeogeographic evolution, distribution regulation and genesis type of reservoir, main controlling factors of the reservoir were studied systematically, and several achievements as follow were obtained.Combining the ancient land features around basin, Source is discussed under the use of ancient flow direction, detrital component and detritus distribution, heavy minerals, sandstone cathodoluminescence features, rare earth elements and structural patterns and other study. There are many sources in the North and South of basin, which are main sources of North, Northeast, Northwest direction. There is minor variety of source properties from Shanxi group to He8 in vertical.According to the regional deposition pattern and deposition characteristics, throughing the study of the exposure profile in surround of basin and core analysis, it can been found three depositional systems group of continental, marine and transition. At the same time, it can been found six depositional system of alluvial fan, rivers, lakes, lake delta , delta and coast from Shanxi formation to He8 formation.Based on the analysis of lithological characters, formation distribution, sedimentary structure characteristics, fossils and geochemical characteristics of boron, barium, strontium-boron, gallium, rubidium three elements Relations, it is concluded that Shan23 and Shan22 in Shanxi formation has marine transgression characteristics, but the effect is obviously limited.Obtained from the deposition evolution of late Paleozoic Era in Nortern China block, lithic facies ancient physical character and the evolved rules are established from Shanxi age (Shan23, Shan 22, Shan 21, Shan 1) to lower Shihezi age (upper He8, lower He8), at the same time, several age's sedimentary modles are established from Shan2,Shan1 to He8,which also reflects the North China plate tectonic pattern. Based on the systematical study on genesis type and forms of reservoir sandstone, it can been concluded that the main reservoirs have braided river channel in the middle of fan, Channel Bar of Braid river, point bar of meandering river, Distributary Channel of Delta plain, underwater distributary channel of Delta front, debouch bar and so on in Shanxi formation. In He8 Formation, there are main reservoirs of river channel of fan, Channel Bar of Braid river, point bar of meandering river. In plane, there are mainly three sandstones distribution of lenticular, banded, leaf-shaped. In vertically, there are different types of sandstones configuration of the single profile.Through the systematical study of character of reservoir mineralogy, pore type, pore configuration, physical properties, it can been concluded that the reservoir has seriously heterogeneity characteristic. The sandbody whose physical properties is good exit on Channel Bar of Braid river, point bar of meandering river, Distributary Channel of Delta plain, underwater distributary channel of Delta front and so on. In the Meanwhile, regression equations on quartz content of sandstone and logging parameters is established after calibrating well logging with core analysis.Based on the forenamed studies, the paper summarizes the main control elements of reservoir sands, lithological characters is controlled by source, sand forms is controlled by facies, sand distribution is controlled by tectonic setting and ancient landforms, structure and distribution of sand is controlled by hydrodynamic conditions.