--> Diagenesis of Unconformity Semi-Weathered Rock Layer and Its Influence on Reservoir Physical Property: A Case of Unconformity on the Top of Triassic in Xia 9 Wellblock, Junggar Basin, NW China

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Diagenesis of Unconformity Semi-Weathered Rock Layer and Its Influence on Reservoir Physical Property: A Case of Unconformity on the Top of Triassic in Xia 9 Wellblock, Junggar Basin, NW China

Abstract

The unconformity semi-weathered rock layer on the top of Triassic in northwestern margin of Junggar Basin is one of the most important hydrocarbon-producing layers in Xia 9 wellblock. The influence of diagenesis on the reservoir physical property of unconformity is obvious and serious. Based on systematic observation of cores, normal sections, cast sections and scanning electronic microscope, the diagenetic characteristics of the unconformity semi-weathered rock layer and its influence on the reservoir physical property were analyzed in order to expand understanding of unconformity and guide fine exploration and development of stratigraphic hydrocarbon reservoirs. The semi-weathered rock layer had undergone a series of diagenesis, such as compaction, cementation, dissolution and disruption in burial diagenetic evolution. Among them, compaction and cementation were the main factors for destroying pore of reservoir, while dissolution and disruption were constructive for pore of reservoir. The semi-weathered rock layer is mainly remained in the stage A of middle diagenesis, which alteration degree has become weak gradually along with the increase of distance from unconformity surface. The quantitative evaluation of diagenesis shows that the semi-weathered rock layer has diagenetic characteristics as follows: strong compaction(average optic compaction 67.28%), middle cementation(average optic cementation rate is 42.97%) and strong dissolution(average optic dissolution porosity is 65.00%), which relative strength of diagenesis were arranged according to their influence on reservoir physical property in following order: compaction> dissolution> cementation> disruption. Therefore, dissolution is the main factor controlling good reservoir properties of unconformity semi-weathered rock layer on top of Triassic.