Distribution characteristics and controlling factors of vertical joint spacing in sand-mud interbedded strata

• Published in: Journal of structural geology 2019-11, Vol.128, p.103886, Article 103886
• Main Authors: Bao, Han, Zhai, Yong, Lan, Hengxing, Zhang, Keke, Qi, Qun, Yan, Changgen
• Format: Article
• Language: English
• Subjects: Fracture spacing to layer thickness ratio; Joint saturation; Joint spacing distribution; Sand-mud interbedded strata; Vertical joint statistics
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2019.103886

Nearly 3,600 vertical joints were measured in sand-mud interbedded rock masses from 30 outcrops in the Yan'an area, China. Information such as joint spacing, joint aperture and the thickness of sandstone formations was obtained. Results show that the vertical joints are well developed in the study areas, and the fracture spacing tolayer thickness ratio (S/T) can reach 0.79. The joint spacing has a strong positive linear relationship with the thickness of the sandstone layer and presents a lognormal distribution in each sandstone layer. However, the morphology of the lognormal distribution is related to the degree of joint development; the higher the degree of joint development, the smaller skewness of the joint spacing distribution. In addition, although the value of S/T for saturate joint is generally 0.8–1.2, the vertical joints may reach saturation at greater S/T by penetrating multiple layers of sandstone when the intervening soft layer is thin. •Sandstone layer thickness and vertical joint spacing have a strong linear relation.•Vertical joint spacing in the study areas follows a lognormal distribution.•The skewness of vertical joint spacing distribution decreases with joint development.•Joint saturation may occur at high FSR when intervening weak layers are thin.•Skewness can be a complementary index to characterize the joint filling degree.