The Feature And Evaluation Of Sandstone Reservoir In The Fourth Section Of Xujiahe Formation, Xinchang Gasfield

Based on the core observation and by the analysis of log data, we carry out detailed analysis of sedimentary faces of the fourth section in Xujiahe formation located in Xinchang area which belongs to the depression of western of Sichuan. We think that the major sedimentary facies in the fourth section of Xujiahe formation are braided delta, the former delta and shallow lake. Concretely, the upper subsegment of the fourth section develops the braided delta, the middle subsegment develops the former delta-shallow lake and the lower subsegment develops the braided delta. On the base of stratigraphic division and comparison and according to much analysis of faces sequence of single well, combining vertical and horizontal changes of the sequence of sedimentary faces, the vertical and horizontal distribution stability of sandstone, the assemblages features of log and the identification of seismic, we divide the fourth section of Xujiahe formation into 11 sand groups and carry out the cross contrast of sandbody.Based on the previous research and through a large number of analytical methods, such as the identification of slice, SEM, cathodeluminescence and the measurement of physical property, we carry out detailed study of reservoir characteristics of the sand bed of TX4-2,TX4-3,TX4-4 in the upper subsegment of fourth section and TX4-8,TX4-9,TX4-10,TX4-11 in the lower sub-section of fourth section. The types of sandstone are very complicated in the fourth section of Xujiahe formation in the study area. The main rock types are lithic sandstone, which is rich in debris and its maturity of structure is high, followed is lithic quartz sandstone. The main interstitial materials are carbonate cements and clay minerals. The reservoir went through compaction, cementation, filling, dissolution and fracturing. And the compaction and cementation are the main destructive diagenesis, which caused porosity loss. And the dissolution, especially feldspar, rock fragments and other soluble components of the dissolution, is the main constructive diagenesis, it play a important role in reservoir improvement.The main pore spaces are secondary intergranular solution pores, intragranular solution pores, pores in grain films in the fourth section of Xujiahe formation in the study area.The distribution of pore throat is multi-peak shape and its degree of sorting is poor-medium, so the quality of reservoir is poor. And the physical property of reservoir is generally inferior,which is the typical tight reservoir,but there is superior reservoir in the local. The upper subsegment is better than lower subsegment on the relationship between porosity and permeability.At the same time, preferred reservoir distributes in the lower subsegment. By contrasting the geological features between superior reservoir and poor reservoir, we consider that the controlling factors of the development of superior reservoir are the components of scrap, sedimentary environment, the location of dissolution of ancient structures and the development of fractures.So we establish the geological prediction model and forcasting evaluation criteria to each section. At last we evaluate the development of superior quality reservoir within every single sand bed in the fourth section of Xujiahe formation.