MECHANISM OF NATURAL FRACTURE FORMATION IN THE UPPER PALEOZOIC TIGHT SAND GAS RESERVOIRS IN THE TABAMIAO AREA, NORTH ORDOS BASIN

It is very hard to develop effectively the Upper Paleozoic low-pressure, low-porosity, low-permeability tight sand gas reservoir in the Tabamiao area, northern Ordos basin. The existence of natural fractures in tight gas sand can change the situation dramatically by improving greatly the permeability of the tight sand reservoir. The study of the development and distribution of natural fractures may help to determine the design of hydraulic stimulation and horizontal drilling program in tight gas development. The Tabamiao block is located on a slope in the north of the Ordos basin, and the strata occur as a gentle monoclinal structure with a stratigraphic dip angle of only 1° and it is generally thought that they have no conditions for natural fracture growth. However, many vertical fractures and high-angle fractures have been found through core observations, thin-section analysis and tests of physical properties, as well as well testing and logging. Studies of the shape, attitude, density and distribution of natural fractures indicate that natural fractures result from combined effects of the tectonic stress, differential compaction and very high fluid pressure, whereas faulting is the leading factor responsible for the development of the natural fractures in the area.