The distribution, fractal characteristic and thickness estimation of silty laminae and beds in the Zhangjiatan Shale, Ordos Basin

The Zhangjiatan shale of the Lower Chang 7 member of Yanchang Formation, Ordos Basin is an important exploration target for shale gas. Since the silty beds/laminae in the shale play an important role in gas storage, fluid flow, fracturing and development of shale gas/oil, it is of great significance to ascertain the thickness and the distribution characteristic of the silty beds/laminae. The measurement results from outcrop, core and thin section show that the silty beds/laminae have larger detrital grains than those in the clayey laminae and the silty laminae thickness mainly ranges from 0.5mm to 8mm. Their total thickness can occupy 7%-26% of the shale formation with an average density of 8-40 layers/m, which shows strong vertical heterogeneity. The thickness distribution of silty beds/laminae presents a fractal distribution on a dm to mm scale and the fractal dimension D has scale invariance over the whole thickness range. According to the data from the relatively thick layers of silty beds, the accumulative number of the thinner silty beds/laminae was estimated using the Number-size model. The accumulative thickness of silty beds/laminae and its ratio to the shale formation thickness were calculated. It has been verified that this method can ascertain the thickness of the silty beds/laminae effectively. ©, 2015, Science Press. All right reserved.