Research On TH Macroscopic Heterogeneitye Of Braided River Reservoir

Description and evaluation for reservoir heterogeneity is the key technology of natural in low-permeable tight sand exploration and development. Reservoir macroscopic heterogeneity relate to optimize of gas rich area, well spacing optimization, horizontal well direction, different grades of sand body structure, three-dimensional configuartion,and superimposition relationship. Only by well information’s we can’t get these factors. So outcrop and underground reservoir modeling is the most important means of macroscopic heterogeneity of reservoir research.At early Shihezi stage(the 8th member of Lower Shihezi Formation) the braided river sandstone reservoir distribute widely which is the one of the main layers of gas field with reserves of 10 TCM of upper Paleozoic in Ordos basin.Recently research shows that Qianlishan area which lies in the North of Ordos basin and Zhao30 area developed braided river facies in Ordos basin. This provide the possibility for reservoir heterogeneity study which can combine outcrop with underground. Based on this, this paper chose Qianlishan outcrop and Zhao30 area as the study object, use the theories and technical methods such as sedimentology, sequence stratigraphy,petrology and reservoir architecture to analyse it. Though detailed research of types of sequence boundary and features of sequence,and then build the high resolution sequence stratigraphy framework. At last carried out research of reservoir heterogeneity by three points which are in-layer heterogeneity, interlayer heterogeneity and plane heterogeneity.Main contents and results are as follows:(1) Detailed sedimentary microfacies analysis showed that abandoned channel deposit formed by frequent migration diversion of the river, and the floodplain, crevasse splay deposit whose fine grained sediments formation of seepage barrier between the heart beach sand body, is one of the reasons why the braided river sandstone reservoir macroscopic heterogeneity is strong.(2) On the basis of sequence interface level and characteristics studies, and used the theory and technique of high resolution sequence stratigraphy, divided the lower member 8 of Shihezi Formation braided river deposition into a long-term, two middle and six short term cycles. Among them, short-term and mid-term base level cycles were developed to become "deep-upward" non-symmetrical cycle, the long-term cycle structure was mainly not completely symmetrical type which was the ascending half cycle. In the middle of sequence boundaries and flooding surfaces as stratigraphic framework, with short sequences as isochronous stratigraphic correlation unit, a high accuracy isochronous stratigraphic frame was established. It laid the foundation for the isochronous trellis macroscopic heterogeneity of reservoir research.(3) For the first time analysed the sand body architecture of braided river reservoir in high resolution sequence stratigraphy. Sand body architecture analysis in the middle base level cycle sequence stratigraphy revealed that favorable reservoir sand body of braided river in lower member 8 of Shihezi Formation developed the initial of the medium-term base level rising stage, sand body cut each other in the vertical, and spliced in the horizontal, sand body distribution range and wide. The internal muddy interlayer of composite channel sand body had low development degree, the reservoir continuity and the heterogeneity were weak. Sand body architecture analysis in short-term base level cycle quantified the single channel and river sand body size in lower member 8 of Shihezi Formation. The minimum width of single channel sand body is 46.40m, the maximum width is 494.89m, and the average width is 208.36m. The minimum width of channel belt is 109.76m, the maximum width is 8807.3m, the average width is 2244.12m. Sand body configuration analysis in isochronal stratigraphic frame for the optimization of natural gas in the basin, preferably well spacing optimization and horizontal well orientation provides an objective, scientific basis.(4) For the first time analysed the outcrop section architecture of lower member 8 of Shihezi Formation in northern Ordos basin. Identified fifteen types of lithofacies and six kinds of genetic units, got the three-dimensional parameters of sand body. The single sand body of the minimum thickness is 0.18m, the maximum thickness is 3.2m, length is 232m, width is 68.4m; the average thickness is 0.78m, width is 39.68m, length is 43.59m. Lenth/thickness is 12 to 67, average is 58. Width/thickness is 19 to 59, average is 24. Fitted the length/thickness and width/thickness function relationship in single sand body: L=44.829h+12.484, W=46.5 h0.577, provided a basis for the small layer sand body correlation and downhole sand body extension.(5) Starting from the in-layer heterogeneity, interlayer heterogeneity and plane heterogeneity, and according to the situation of Zhao30 area, the author did the comprehensive evaluation of macroscopic heterogeneity in braided river reservoir. It said the SSC1 interlayer had the highest frequency, intralayer heterogeneity is strongest, reservoir intraformational heterogeneity controlled by the short-term base level cycle. SSC6 developed in the mid-term datum conversion period, sand ratio was the highest, stratification coefficient was the highest, the average thickness of the single layer thin, interlayer distribution frequency of the largest, strongest interlayer heterogeneity, reservoir interlayer heterogeneity controlled by medium-term base level. During the deposition of SSC6, overflow widespread bank deposited in sand body connectivity in the plane, the plane heterogeneity is the strongest, the reservoir plane heterogeneity controlled by sedimentary microfacies.