Reservoir Characteristics Of Ma5 Member Of Majiagou Formation In The Lower Paleozoic Of Zhao51 Block And Su77 Block

Zhao51 Block and Su77 Block are located in the northern Ordos Basin, the Eastern Central Paleouplift and the Northwestern Sulige Gas field. The study area is about 2007 km2, which stretches westward to Dalaigou, eastward to Tongganglanggou, northward to Jiabusha and southward to Wushenzhao. Since gas exploration in 2000, it has been confirmed that the 8th Member of the lower Shihezi Formation and the 1 st Member of Shanxi Formation are the main gas reservoirs in the Sulige Gas Field. However, the development practice in recent years shows:the Upper Paleozoic reservoirs are dense, with a large amount of rock debris, highly heterogeneous, which makes it difficult to predict reservoirs; gas-bearing sand bodies are small-scaled, and the effective reservoirs are poorly continued and connected, which result in low reserves of the recoverable wells. Therefore, an important way to develop gas fields cost-effectively is to explore the development potential of the Lower Paleozoic gas reservoirs and reduce the risk of productivity construction. Good development effects are achieved after sticking to the "three-dimensional development of the upper and lower Paleozoic of the gas field " idea.According to the latest statistics, in the eastern Sulige Gas Field, there are more than 460 development wells that are deepened to the Lower Paleozoic and 278 wells which encounter gas layers or gas-bearing layers when drilling. Among them, in the upper combination, there are 236 wells which encounter gas layers or gas-bearing layers, with the average effective thickness of 3.4m; in the middle combination, there are 97 such wells, with the average effective thickness of 3.5m. The average open flow capacity reaches 25.9×104m3/d, and the highest open flow capacity reaches 231 × 104m3/d (SD39-59A). Compared with the Upper Paleozoic gas reservoirs, the Lower Paleozoic gas reservoirs shows a strong gas production ability, which is significant for the per-well production improvement of the area.Despite that the gas reservoirs development of the Lower Paleozoic has achieved some success, there are still some problems on overall gas reservoirs development. The problems include:some strata distribution characteristics are unclear, the formation mechanism and distribution rule of dolomites in the upper and middle combination are uncertain, and the development characteristics, controlling factors and distribution rules of effective reservoirs need to be studied deeply etc. And the problems have affected favorable zones prediction and further development planning of the gas reservoirs. So it is necessary to conduct geological studies on the Lower Paleozoic gas reservoirs of the area, deepen the refined understanding of effective reservoirs, in order to provide reasonable basis for the establishment of the overall Lower Paleozoic gas reservoirs development and the optimal deployment of productivity construction.Based on the statistics of the eastern Sulige Gas Field, by detailed study of more than 140 exploration wells and development wells, and using large amounts of log data combing marker beds identification, detailed classification and correlation of every layer in Ma5 Member have been accomplished. Thus the finishig drilling condition of every layer has been obtained. Also, the stratigraphic correlation chart has been finished, which clarifies the strata distribution rules. Through the observation and study on the cores, rock slices, logging data, and on the basis of single well and well tie study, we conclude that the study area belongs to tidal-flat facies.2 subfacies and 8 microfacies are identified. Using the single factor analysis method, planar graphs of sedimentary facies of every layer are finished.By main reservoir types study of the area, we conclude that the main reservoirs of Ma5 are dominated by penecontemporaneous dolomites and dolomites formed by burial dolomitization. The grains are coarse and some are fine-grained, and intercrystalline pores are developed. The favorable reservoirs of Ma6 to Ma10 Members are micrite-micro crystalline powder dolomites and coarse crystalline powder dolomites. And through diagenesis study of reservoirs in layers, we suppose that the extensively developed diagenesis types are dolomitization, karstification, cementation and metasomatism, recrystallization, pressolution and so on. Dolomitization, dissolution and cavity filling are of the greatest significance to reservoir capability.By studies on the general characteristics and planar variation of the reservoirs of every layer, the main factors of reservoir capability are discussed in terms of lithology, structure, diagenesis, etc. The effective reservoirs distribution rules are also analyzed, which provides basis for favorable zones prediction. According to comprehensive indicators including physical properties of reservoirs, pore texture, sedimentary microfacies and the lithology, combination types of reservoir space, and diagenesis, a reservoir classification system is established. In this way, classification and comprehensive evaluation of every layer are accomplished. Considering the controlling factors of sedimentary facies, diagenesis, physical properties and paleokarst landform etc, and on the basis of favorable zones prediction of every layer, the favorable zones of every layer in the study area are selected.