The Research Of Relationship Between Diagenesis And Porosity Evolution In Upper Triassic On Xujiahe Formation In Guang’an Area Of Central Sichuan Basin

The purpose of this thesis is comprehensive description of sedimentary characteristics and diagenetic stage of low permeability and low porosity of sandstone reservoir on Xujiahe formation in Guang’an area, central Sichuan Basin. This thesis focus on the relationship between diagenesis and porosity evolution of low permeability sandstone reservoir, formation mechanism of its sandstone reservoir with low permeability and low porosity,and establishs the model of diagenetic evolution of dense sandstone reservoir on Xujiahe formation, objectively evaluates sandstone reservoir on Xujiahe formation in Guang’an area, central Sichuan Basin; points out where the distribution of reservoir for advantageous position is; provide technical support and prospecting theory basis For favorable exploration blocks.Sandstones of the Xujiahe formation in research areas belong to depositional system of the braided fluvial river delta. Braided delta is a kind of coarse grained delta. The most distinctive characteristics are the delta plain, which can be further divided into two parts of upper delta plain and lower delta plain. The sedimentary facies of the most favorable is the upper delta plain river channel.There is the tight sandstone of low permeability and low porosity on the Upper Triassic Xujiahe Formation in Guang’an area ,central Sichuan Basin. It shows the petrographic characteristics of low composition maturity, low cementation content and medium structural maturity,namely "two low and one middle ". The rock types of sandstone reservoir include feldspar lithic sandstone and lithic sandstone and lithic quartz-sandstone, the two former kinds of rock is the main rock types.Generally, there are features of“one strong and two weak”in diagenesis of sandstone reservoirs in the Xujiahe Formation, namely strong compaction effect, weak cementation, and weak dissolution. Diagenetic stage reached period B, the middle diagenesis stage. The reservoir spaces of the sandstone are mainly secondary porosity, seldom primary pores and crack. Mechanical compaction is one of the main factors to control the petrophysical quality. followed byⅡ-Ⅲperiod of quartz overgrowth. Porosity was controlled by the early period of calcite cementation content. The dissolition controled differences reservoir of petrophysical quality.The tectonic setting improved permeability and porosity of reservoir. The chlorite coating of early diagenesis stage inhibited quartz overgrowth and protected the effectiveness of primary pore, while the illite of reservoirs reduced the effectiveness of porosity and permeability. It is the most important reason in the reservoir of low permeability and low porosity on the Xujiahe formation.Based on the analysis of depositional system and diagenesis, the distribution of high-quality reservoirs was predicted. Effective reservoirs of T3x6 mainly concentrated in three blocks: the first block distributed in Well Xinghua 1– Well Guang’an101 - Well Guang’an12-Well Huaxi1 ,which is the best belt in petrophysical property of the studied area; the second block distributed in Well Miao 5– Well Nü4 - Nü107; the third block distributed in Well Ying 22– Well Ying 21 - Well Ying 21.The two latter plays are mainly the class II reservoirs. Effective reservoir of T3x4 mainly concentrated in two blocks: the first block distributed in the area of the Well Chongshen2- Well Guang’an13 , and the second block distributed in the triangle area of the Well Miao 5- Well Nü107- Well Huaxi1. Effective reservoir of T3x2 mainly concentrated in Yingshan-Peng’an areas.