Reservoir Characterization Of Tight Sand For The Eighth Member Of Shihezi Fomation In Permian, The South Of Sulige Gasfield

Sulige gas field of Ordos Basin belongs to a typical tight sandstone Reservior. It is an instant problem to make definite sedimentary origin and distribution of gas reservoir, implement favorable gas belts, and process extensive gas development in the southern part of the gas field. Intensive heterogeneity, complex sand body distribution and higher degree of reservoir densification were prominent features of main pay formations of southern Sulige gas field. So, the emphasis and difficulty of exploration and development in Sulige gas field is to find effective reservoir with relatively higher permeability in relatively enriched area within the low porosity, low permeability and low abundance reservoirs of large area. Taking tight sandstone in the 8th member of Shihezi Formation (hereinafter referred to as Hg) in Permian of south Sulige as the object, this paper was carried out on the basis of geology, cores, well logging, slices, analysis and testing data (mercury injection and etc), and etc. According to seismic data in the study area, multi theories such as sequence stratigraphy, reservoir sedimentology and etc, were applied to analyze. According to core observation, well logging analysis, various types of depositional system have been confirmed on the basis of high resolution sequence stratigraphic framework, then the superposition of sandbodies, litholofical characteristics and diagenesis have been studied and conducted to analyze the gensis and controlling factors of tight sandstone reservoir. The development rules of favorable reservoirs under the background of low porosity and low permeability have been summarized, providing target areas for further exploration in the gas filed and basis for the oil/gas exploration & development in similar tight sandstone reservoirs.(1) On the base of High-Resolution Sequence Stratigraphic Framework in the area, the types and distribution rule of sedimentary facies zone were confirmed.2 sub-members (H8x、 H8s) and 4 sand groups (H8x2、H8x1、H8s2、H8s1) were divided through the detailed correlation. The sedimentary facies of the study area is transition phase of braided delta plain. The braided channel within larger and more channel bars is the main microface of northern braided delta plain; while the braided delta front is mainly developed in the southern area with bending small-sized underwater distributary channel and few small-sized mouth bars. Vertically, the channels are large-sized in the east part of H8x sub-member and progressively narrow in the west; the channel developed in H8s sub-member is wider than in the H8x sub-member generally and the channel bars are more developed in the former.(2) Located the Lake Shoreline during the 4 stages when H8 member develops which shows the falling water level and two short stages of water rise. The vertical migration of lake Shoreline effect the sand body size:the water from H8x2 to H8x1 rises and sand body sizes get smaller in H8x sub-member with retrogradation the mainly depositional way; the water from H8s2 to H8s1 rises and body sizes getting smaller in H8s sub-member with retrogradation the mainly depositional way, which is of the same satiation as in H8x. however, the water falls from H8x to H8s,and the sand bodys in H8s sub-member are larger and thicker than in H8x, which show prodominant characteristics of progradation and three kinds of superimposed relationship-Superimposed and continuous, isolated and laterally migrated.(3) Diagenetic stage and types of diagenetic facies has been confirmed in the study area. Four types of diagenetic facies have been identified in plane according to the characteristics of diagenesis. Of which, weak compaction-feldspar dissolution and chlorite cementation-weak feldspar dissolution facies were mainly developed in the northern study area, while silica-carbonate cementation and tight diagenesis facies were mainly distributed in the south. Diagenetic evolution has undergone five stages, which were unconsolidated sediments, early compaction and cementation, strong compaction and cementation in middle period, orgainc acid dissolution and late compaction respectively.(4) The genesis of tight sandstone reservoirs in the study area were controlled by two factors (deposition and diagenesis). The distribution of sedimentary microfaices has divided the major developing area of high-deliverability channels. Diagenesis can improve the physical properties of tight reservoirs. Grain size and sorting difference resulted in high reservoir heterogeneity. The vital reasons for significant reduction of reservoir primary pores is with long time and slow rate compaction and cementation during diagenesis process, while pressolution and development of fractures in the middle and late period made valuable contributions to the formation of secondary pores system.(5) Distribution rules of effective reservoirs in the study area have been summarized. Distribution of favorable reservoirs has been division. Type-I favorable reservoir of high permeability in the study area was defined as porosity more than 5% and permeability over 0.4×10-3 μm2. Target reservoir was the channel bar in braided river delta plain and underwater distributary channel in braided river delta frontier, where the grain size was dominated by medium-coarse sandstone, weak compaction-feldspar dissolution. The type-Ⅱ favorable reservoir was defined as porosity ranged in 3-5% and permeability ranged in 0.1-0.4×10-3μm2, where the target reservoir was the channels in braided river delta plain and river month bar in braided river delta front with medium-coarse sandstone, silica-carbonate cementation.