Sedimentary Characteristics And Reservoir Evaluation Of The Duoni Formation,Lower Cretaceous In The Selincuo Area,Tibet

Based on lithology of the Duoni Formation in the Selincuo area of Cuoqin basin, it could be divided into three sections. There are the first section of the Duoni Formation, the second section of the Duoni Formation and the third section of the Duoni Formation. The frist section is main shale, the second section and the third section are main glutenite, and they are the main reservoirs. This paper focuses on the study of the second and third section of the Duoni Formation. They have studied the interior and the field specific, which include the thin section analysis, physical properties analysis and cathodoluminescence analysis.Sandstones of the Duoni formation have developed well in this area during the delta phase, and it can be divided into three subfacies system, namely delta plain, delta front, delta front. During the Lower Cretaceous Period the sandstone of the Duoni Formation in Selincuo area, the regularity of distribution at the vertical direction is that it is gradually upward-coarsening sequence, from the bottom to top in the succession of strata. And colors of this sequence are the gray-green and the black to the white and the purple. It shows that the water becomes shallow gradually the environment transfer into an oxidizing environment with time during the Duoni formation of the Lower Cretaceous period in Selincuo area.The result turned out that the lithologys of reservoirs in this area are mainly feldspathic lithic sandstones, lithic feldspathic sandstones, and little lithic quartz sandstones and feldspar sandstones. The diameter of grains is ranged from 0.15 millimeter to 0.68 millimeter. The separation is medium. The psephicity is sub-angular to sub-round. All of those have manifested that it has low composition maturity and texture maturity, excepting small parts with high composition maturity. In this area, there are carbonate cement and siliceous cement and little clay matrix, which have developed in sandstones of the Duoni Formation. The main type of carbonate cement is calcite cements. And the cement of the second section of the Duoni Formation is significantly higher than the first one.In addition, inter-granular residual primary pore, secondary dissolved pore and micro-fracture are the main reservoir space. The secondary dissolved pore is widespread, which have included inner-granular dissolved pores and inter-granular pores. The micro-fracture has developed in the second section, including structural fracture and diagenesis fracture. Regarding it as a whole, the physical property is poor, with average porosity of 4.47% and average permeability of 0.3575×10-3um.By studying diagenesis of sandstone reservoirs of the Duoni Formation in the Selincuo area shows that it has experienced three stages, involving the stage of buried diagenesis, the stage of early diagenesis and the stage of middle diagenesis. It infers that it has been in the B stage of middle diagenesis. In process of diagenesis of sandstone reservoirs in this area, the controlling factors are mainly connected with diagenesis as follow. Compaction and the late carbonate cementation is the main reason for reducing the number of reservoir porosity. On the one hand, the part of the cementation(such as quartz overgrowth, etc.) has occupied the part of the reservoir space, and on the other hand, it has increased the ability to resist the compaction of the rock, and played an important role of pore preservation in the deep bury condition. With the solution pore and micro-fracture of reservoirs developed, they have greatly improved the reservoir space and reservoir properties.Combined sedimentary facies and reservoir characteristics of sandstone with reservoirs properties analysis, it has pointed out that is is mainly controlled by sedimentation. The channel microfacies of the reservoir property are the best, follow by mouth bar microfacies. Reservoir sand bodies have developed in the same reservoir sedimentary microfacies, the coarse sandstone reservoir properties than the medium sandstone, while rock types are mainly lithic feldspathic sandstones and feldspar lithic sandstones. And the analysis shows that sedimentary microfacies of the Duoni Formation of the Baburi section have effected on reservoir property, and the influence is larger than lithology. The reservoir sandstones of the second section of the Douni Formation have good reservoir properties.