Basic Characteristics of Natural Fracture Systems in the Upper Paleozoic Tight Sand Reservoirs in Tabamiao area, North Ordos Basin and Its Role in the Process of Gas Reservoir Formation
塔巴庙气田上古生界致密储层裂缝系统基本特征及其在天然气成藏中的作用

The Tabamiao block is located in the northeastern part of Yishan slope in northern Ordos basin. Its tectonic setting is of gentle monoclinal structure with dip angle only 1° and hence is commonly regarded as unfavorable for natural fracture growth. However, many natural tectonic fractures in reservoirs of Upper Paleozoic were found on the basis of such means as core analysis, thin section analysis, well testing and imaging logging interpretation. The natural fracture systems in the area include two types, namely regional fractures and local fractures, related respectively to the regional stress field and the stratigraphic flexibility. These regional fractures, formed in the same period of regional deformation, are mostly vertical fractures and high-angle tectonic fractures. Comparatively, local fractures are mostly low-angle tectonic fractures. Based on studies of shapes, attitudes, densities and distribution areas of natural regional fractures as well as stress researches made by previous researchers, the authors consider that regional tectonic fractures related to repeated lithosphere upheaval resulting from regional and horizontal compressed stress have played double important roles in the process of gas reservoir formation. That is to say, regional fractures function as gas-migration channels in the combination of self-production and self-conglomeration, and as gas accumulation in the combination of under-production and upper-conglomeration. According to the double roles played by regional fractures, a model for gas-forming reservoirs has been established, which will undoubtedly provide a good guidance for gas exploration and development of the H- 2+3 segment in Tabamiao block.