Development characteristics of joints in the Chang 6-Chang 9 reservoirs of the Yanchang Formation in southern Ordos Basin, China

As the most important source rocks of the Ordos Basin, China, the Chang 6-Chang 9 reservoirs of the Yanchang Formation have developed an abundance of joints. Understanding the formation, geometry and distribution is very important for flow modelling and shale gas exploration in the reservoirs. To this end, we investigate the development characteristics of joints in the Chang 6-Chang 9 reservoirs based on field joint mapping and statistical analysis. A total of 1574 joints were mapped across 43 outcrops, which can be divided into three groups: NW-SE, north-south and NEE-SWW. The distributions of joint spacing (s) and trace lengths (l) and their dependence on bed thickness (t) are statistically analysed. We find that the distributions of joint trace lengths follow exponential distributions, while joint spacing distributions are well expressed by lognormal distributions. Besides, both the s-t and l-t relationships fit linear distributions. Here, a total of 12 sandstone layers are selected as representative layers to study the condition of joint saturation. Statistical analysis shows that the joint spacing distribution of the sandstone gradually transforms from a skewed lognormal distribution to a normal distribution as s/t decreases from 1.51 to 0.54, indicating that the development of joints is gradually approaching saturation.