Quantitative evaluation of fracture development in Triassic Yanchang Formation, Ordos Basin, NW China

Abstract

Abstract To uniformly evaluate the fracture development of the Triassic Yanchang Formation in Ordos Basin, a set of evaluation methods for fracture development degree is established by observation and account of fractures developed in the sandstone cores of the Yanchang Formation in the five blocks of Maling, Huaqing, Anbian, Wuqi and Yanhewan in the Shaanbei Slope. The methods include four aspects of qualitative and quantitative indices, i.e., fracture type, fracture strength, fracture grade and fracture assemblage, which are applicable to the fracture assorting, single well evaluation, block evaluation and belt evaluation, respectively. Quantitative evaluation plates of block fracture development degree have been built and applied to evaluate the fracture development degree of the five blocks. The Chang 7 1 Member in Maling block has high angle fractures of three grades (weak, moderate and strong) developed simultaneously and low angle fractures in moderate-strong development degree; the Chang 7 2 Member in Maling block has weakly to moderately developed high angle fractures and moderately to strongly developed low angle fractures; both the Chang 7 1 and Chang 7 2 members in Anbian block have strongly developed high angle fractures, but undeveloped low angle fractures; the Chang 6 3 Member in Huaqing block has undeveloped high angle fractures and moderately to strongly developed low angle fractures locally; the Chang 6 members in Wuqi block and Yanhewan block have undeveloped high angle fractures and low angle fractures.