Research On Reservoir Characteristics Of The 4th Member Of The Upper Triassic Xujiahe Formation In Xiaoquan-xinchang Area, Western Sichuan Basin

Introduction of the author:LiMin, female, was born in April,1982, whose tutor was Associate Professor Meng Wan-bin and Professor Tian Jing-chun. She graduated from Chengdu University of Technology in sedimentology major and was granted the Master Degree in June,2011.Based on the theory of sedimentology, reservoir geology and oil and gas geology, this paper studies in detail the characteristics of petrology, sedimentary facies, pore structure, physical property, diagenesis and reservoir quality, analyzes the correlation among pore structure and porosity and permeability, and discusses the impact of sedimentary processes, sandstone composition, and diagenesis on reservoir quality.On the basis of lithofacies and sedimentary structures, three sedimentary facies has been recognized, i.e., lacustrine, lake delta and braid river delta. From bottom to top in the 4th member of the Xujiahe Formation, the sedimentary faices evolve from braid river delta through lacustrine to lake delta.The sandstones of the 4th member of the Xujiahe Formation from the study area are mostly fine to medium grained, sub-angular to sub-rounded, moderately well to well sorted litharenite and lithic quartz sandstone. The sandstones of the upper section and the lower section of the 4th member have similar framework-grain composition, characterized by abundant lithic fragments and scarcest feldspathic grains. The sandstones are dominantly cemented by carbonate (mostly calcite), quartz and clay minerals.The 4th member sandstones constitute typical tight gas reservoir with low porosity and extremely low permeability. Nevertheless, it contains some relatively better reservoir intervals that consist of the major reservoir pay zones. Vertically, the upper part of the 4th member has relatively better reservoir property than the lower part, and horizontally northeast and central regions in study area have good reservoir property. In general, the sandstones show a positive correlation between porosity and permeability, of which the upper section is more positive than the lower section.Intergranular porosities enlarged by dissolution, secondary intragranular porosities and moldic porosities which occur within partly or almost completely dissolved feldspathic grains and rock fragments are major pore types. Accordingly, the reservoir is mainly pore type and the sandstones with primary porosities and secondary intergranular dissolved porosities have better reservoir property. The sandstones show complex mercury-injection capillary pressure curve shapes. Poor to medium sorted pore throats indicated by their multimodal distribution suggest that the sandstones have poor reservoir property.Porosity is controlled by the size and number of pore radius, while permeability is controlled by the maximum throat. Large throat radius means wide seepage channel, and large number of throat will provide more seepage passage. Higher pore throat ratio illustrates that the single pore is connected by much more large throats, which results in high permeability, and vice-versa.The study shows that the Xujiahe Formation in study area has undergone complex diagenesis, including compaction, cementation and dissolution. The reservoir diagenesis evolved from syngenetic through early diagenetic (phase A to B) to middle diagenetic phase B at the present time.Reservoir quality is controlled principally by sedimentation and diagenesis of the studied sandstones. Sedimentary facies constrained the distribution of sandstones. Good reservoir mainly developed in delta-plain distributary sandbodies, delta front distributary sandbodies and sandbars. In general, well sorted medium grained sandstones with little clay matrix tend to form good reservoirs. Diagenesis plays a very important role in the development of reservoir. Destructive diagenesis including compaction and cementation is the main cause that destroys primary porosity. The sandstone contains much calcite cement which occupies much pore space is not good reservoir. Pore lining authigenic illite, is helpful in protecting the primary porosity to some extent. Dissolution and tectonic rupture are the main processes that improve the reservoir properties. Microfracture can enhance pore connectivity under certain conditions.