| Ordos is a large basin of a multi-tectonic systems, multicyclic evolutions and multi-sedimentation types. The faults and folds are very developed in the edge, but inter the basin, the structure are relatively simple. Based on the basement nature, geological evolution and tectonic characteristics, the regional tectonic can be divided into six secondary tectonic units includeing Yimeng uplift, Weibei uplift, Jinxi fault-fold belt, North Shaanxi Slope, Tianhuan Depression and western margin thrust belts. Sulige gas field mainly located in the eastern part of the basin, also in the east side of palaeohigh, and its southern part is connected to Jingbian gas field.Sulige gas field is the most important oil-gas producing areas among Ordos Basin. The 5th member of the Ordovician Majiagou formation is the major gas-bearing series in eastern Ordos Basin.Under the guidance of theories in relevant subjects, such as Carbonate Sedimentology, Sequence Stratigraphy,Petroleum Geology, Logging Geology and Reservoir Geochemistry, this paper analyzed the geological characteristics of the 5th member of Majiagou formation in eastern Sulige gas field by combining with regional geologic settings and reservoir geologic characteristics of the 5th member of Majiagou formation in eastern Sulige gas field, and laid a solid foundation for further exploration and the study of sedimentary microfacies,sedimentary types and sedimentary evolution in the area. By 426 diverse wells covered in the research, such as exploratory wells and appraisal wells, this paper took the Ma55-4 of Majiagou formation as its research objective, since it is the major productive strata in the area.Based on coring datalogging data and physical property data, this paper divided the 5th member of Majiagou formation into 10 layers, namely Ma51～Ma510.Taking the bottom mudstone of Taiyuan formation as the top layer, this paper sets up fourteen standard sectional drawings for comparison. No major change of the formation thickness was detected in the Ma55～-4 of Majiagou formation.Results show that the rocks in Sulige gas field are mainly light-colored dolomite, limestone or dolomitic limestone.The lithology characteristics of Ma55～4 in the study area includes dark-gray micrite-fine-powder dolomite, light-gray fine-silty dolomite with carniole, dark-gray dolomitic breccia, dark-gray micritic limestone,gray gypsum-bearing dolomite containing micritic dolomite and so on.According to core description, single well logging facies analysis, well profile comparison and combining with rock types, sedimentary structures, paleontology features, logging facies and sedimentary cycle analysis and other methods, we identify that the 5th member of Majiagou formation in eastern Sulige gas field mainly developed restricted platform depositional system, and further divide Ma55～Ma54 into one kind of subfacies:tidal flat subfacies. Counting up Ma55～4’s formation thickness,gas-bearing formation thickness,dolomite thickness and limestone thickness,we get the sedimentary microfacies classification standards of Ma55～4 under the compilation principle and methods of preponderant facies.Based on analysing the sedimentary microfacies of single wells and joint wells detailedly,we designated the preponderant facies of Ma55～4 of all wells in the study area,and then produced the sedimentary microfacies horizontal figures of Ma55-4, also did further research on sedimentary microfacies’s distribution characteristics and sedimenatary evolution.By research of sedimentary microfacies horizontal distribution law of Ma55-4, we know that the sedimentation of Ma55 is the intertidal-supratidal zone;the sedimentation of Ma54 is the supratidal zone.There are eight kinds of microfacies such as dolomitic flat, muddy-dolomitic tidal flat, limestone-dolomitic tidal flat, muddy dolomitic flat and gypsum-bearing dolomitite flat, gypsum tuberculosis dolomitic flat, gypsum-salt sedimentary basin, limestone basin sedimentary microfacies.The results show that the 5th member of Majiagou formation is overall in regression environment, the water bodies of Ma56～ Ma510 is relatively open and deep, dolomitic flat developed in large scale.Local area’s water bodies are deeper, and develop limestone flat and limestone-dolomitic flat. During Ma56, the water bodies change rapidly, either in supratidal environment or in intertidal environment.As a whole, the water bodies in the southwest of study area are relatively shallower and supratidal dolomitic flat developed mainly.But the water bodies in northeastern study area are deep and limestone-dolomitic flat developed.Until Ma55’s depositional stage, it turns to a brief transgression phase under the background of a long regressive phase.Water bodies are relatively deeper and salinity is small in general. The rock types are mainly micritic limestone.But, intertidal grain bank developed in local areas.In addition, because of subaquatic uplift in some areas, the terrain is relatively high, the water is relatively shallow,and micritic-silty dolomite containing gypsum pseudomorphic developed. At the end of the sedimentation of Ma55, the water faded gradually and supratidal dolomitic flat mainly developed in northwestern work area.From Ma55’s depositional stage to Ma54’s depositional stage,the sea level decreased gradually.During Ma54’s depositional stage, supratidal dolomitic flat is mainly developed.During Ma 543’s depositional stage, the rock combination is mainly micritic-silty dolomite and large-scale dolomitic flat developed.During Ma542"s depositional stage,the water level is lower than Ma 543’s water level.And the rock combination of Ma542 is also mainly micritic-silty dolomite,but,combined with paper lime dolomite sometimes.The sea level of sedimentation stage of Ma54* is lower than Ma 542’s sea level.Ma541 has higher salinity and a large area of gypsum-bearing dolomitite flat developed.It is characterized by developing a large number of gypsum mould pore.Gypsum dolomitite flat developed on a small scale in southeastern study area.This paper analyzes the sedimentary microfacies distribution,the sedimentary types and the sedimentary evolution of Ma5～4 of Majiagou formation in eastern Sulige gas field. Gypsum-bearing dolomite flat, dolomitic flat and grain bank are thought to be the sedimentary facies which are good for accumulations of oil and gas in the Ma55～4 of Majiagou formation in the study area. The depositional environment from Ma54to Ma5s have good inheritance.Under the background of the overall regression of Ma5,the low sea level began to ascent relativel and the depth of Ma55’s water is deeper than Ma56-10. At the end of the sedimentation stage of Ma55,the sea level fell relatively and the water got shallow again.From the end of Ma5s to Ma54, the water became shallow and the sea level was falling slowly.Until Ma 541, it turned out to be the supratidal evaporative environment with the most shallow water. The sedimentary evolution of Ma5s-4 is closely relevant to the sea level change.With the settings of several secondary transgression-regression cycles and the falling water, the distribution area of dolomitic flat became larger and the scattered distribution characteristics of dolomitic flat evolved to continuous dolomitic flat. Until Ma541,it became the supratidal evaporative environment, produced a wide range of sedimentary microfacies of gypsum-bearing dolomitic flat and formed a distribution feature of sedimentary microfacies that gypsum dolomitic flat was surrounded by gypsum-bearing dolomitic flat.In conclusion,the sedimentary microfacies distribution and evolution laws is in the control of eustatic cycles. |
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