Intragranular fractures within tight sandstone reservoirs in the Hangjinqi area, northern Ordos Basin, China: Morphology, intensity and controlling factors

Microfractures play a critical role in the evaluation and exploitation of unconventional reservoirs. Tight sandstones of the He-1 Member in Shihezi Formation in Hangjinqi area are rich in intragranular fractures while morphology, intensity and factors controlling the development of these fractures are still unclear, which limits the exploration and development of tight gas in the study area. Therefore, thin sections observations, porosity and permeability measurements, minerals composition, along with intragranular fracture areal density of the target layer was carried out to obtain a comprehensive understanding of the formation. Based on this data, by considering effective pressure, mineral composition and grain size as main controlling factors, 16 synthetic cores were made and their fracture density was also documented. Integrating burial history and sedimentary facies as well as comparison between the natural cores with synthetic ones, showed that both natural and synthetic cores developed two types of fractures: simple and complex. Most fractures in natural cores are filled with clay minerals while in the synthetic cores fractures are open with a large aperture. Furthermore, average fracture density of natural and synthetic cores is found 5.27 x 10-3 mu m/mu m2 and 0.61 x 10-3 mu m/mu m2, respectively. Additionally, it was found that intragranular fractures are the result of intense compaction during eodiagenesis. Moreover, fracture density is positively correlated with grain size and effective pressure but negatively correlated with clay mineral content. The correlation between fracture density and effective pressure as well as clay minerals is almost logarithmic. Considering the characteristics of sedimentary facies, reservoirs developed in braided bar and channel should be rich in fractures and those in floodplain having fewer fractures. It was concluded that intragranular fractures are generally found in layers that subside quickly and have had low temperature gradient causing an indirect improvement of the porosity but directly enhancing the permeability. As a result, reservoirs should have better petrophysical properties in braided bar and channels. Collectively, this study enables us to illustrate the controlling factors on the development of intragranular fractures and their implication for tight reservoirs in the Ordos Basin and other basins with similar settings around the world.