Diagenesis of unconformity and the influence on reservoir physical properties: a case study of the lower Jurassic in Xiazijie area, Junggar Basin, NW China

Diagenesis process of unconformity is very important in understanding the physical properties of the reservoirs. In this paper, based on a comprehensive analysis of well cores, thin sections, casting thin sections, scanning electron microscopy and other data, we discussed the diagenetic characteristics and the influences on reservoir physical properties of unconformity of the lower Jurassic in Xiazijie area, Junggar Basin. The results show that the semi-weathered rock layers of the unconformity had undergone a series of diagenesis, such as compaction, cementation, dissolution and disruption during burial diagenetic evolution. However, the diagenetic evolution is mainly in the stage A of middle diagenesis, and its alteration degree has become gradually weak as the distance increased from the unconformity surface. The quantitative analysis of diagenesis shows that diagenetic characteristics of semi-weathered rock layer of the unconformity are as follows: strong compaction (average optic compaction is 67.28%), middle cementation (average optic cementation rate is 42.97%) and strong dissolution (average optic dissolution porosity is 65.00%). According to their influence on reservoir physical properties, the intensity of diagenesis is sequenced in the following order: compaction > dissolution > cementation > disruption. Among these processes, compaction, dissolution and cementation can change the porosity by −26.1, 6.0 and −4.3%, respectively. Dissolution is the main controlling factor on reservoir properties of unconformity semi-weathered rock layers.