Abstract
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.
Highlights
Reservoir physical properties of sandstones are mainly controlled by tectonism, sedimentation and diagenesis, among which diagenesis determines the ultimate reservoir physical properties (Rodrio and Luiz 2002; Ali et al 2010; Swei and Tucker 2012)
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
The quantitative analysis of diagenesis shows that diagenetic characteristics of semi-weathered rock layer of the unconformity are as follows: strong compaction, middle cementation and strong dissolution
Summary
Reservoir physical properties of sandstones are mainly controlled by tectonism, sedimentation and diagenesis, among which diagenesis determines the ultimate reservoir physical properties (Rodrio and Luiz 2002; Ali et al 2010; Swei and Tucker 2012). The semi-weathered rock layer of unconformity at the lower Jurassic in Xiazijie area is 407.50 m in thickness with buried depth of 1427.50–1835.00 m It experienced a series of diagenetic evolution. Main diagenetic features of the semi-weathered rock layer of unconformity at lower Jurassic in Xiazijie area are: the grain contact relationship is almost linear, compaction is relatively strong, cementation is mainly siderite cementation and shale matrix filling, dissolution pores and microfractures are well developed, rock alteration is regularly widespread and characterized by the intensity of alteration weakening with the increased depth of unconformity (Fig. 3). The controlling action is mainly affected by buried depth, clastic particle features and the content of plastic and semi-plastic detrital grains plastic in the semi-weathered rock layer, which is relatively high. The detrital grains experience flexible deformation and are partly squeezed in intergranular pores as the overlying formation pressure enhances, resulting in damage to the primary pores
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