Research On Sedimentary Facies And Sequence Stratigraphy In The Palaeozoic In The Eastern Part Of Ordos Basin

Based on the synthetic research of field outcrops, drilling section, well log and office compilation, combined with analyses of geological background, lithological association, sedimentary texture, sedimentary structure, geochemical and palaeobiological features, the researched area in the Palaeozoic can be divided into deltas, braided deltas, lacus, barrier coast, continental shelf and carbonate platform etc. six kinds of sedimentary facies. Furthermore, sixteen sedimentary subfacies are identified. Respectively, the sedimentary subfacies are delta plain, delta front, prodelta, braided delta plain, braided delta front, braided prodelta, coastal lake, saucer lake, tidal flat, barrier island, lagoon, carbonate continental shelf, muddy continental shelf, open platform, local platform and evaporate platform.On the basis of the analyses of peripheral ancient land, types of sand stone, light and heavy mineral associations and directions of palaeoflow, it is considered that detrital material of terrigenous origin of the Upper Palaeozoic in this area come from south provenance and north provenance, respectively, Yinshan ancient land in the northern and East Qinling ancient land in the southeastern. The north provenance provided main detrital material, which can be delicately divided into two different sections, Taipusiqi, Ji'ning area in the east and Zha'ertai Mountain, Wula Mountain in the middle of Yinshan ancient land.The distribution of gas pool in the eastern part of Ordos area was obviously controlled by sedimentary facies belts. Evaporate platform and local platform developed in this area during Ma5 stage. Sedimentary environment in Benxi stage from east to west were respectively shallow-sea muddied continental shelf, barrier island, lagoon and tidal flat. Most area in Taiyuan stage was distributed by shallow-sea continental shelf, but the northern and southeastern part were scattered by clastic tidal flat and carbonate tidal flat. Sedimentary environment in Shanxi stage, in the middle and late Shihezi stage was deltas and lake. Braided deltas and lake developed extensively in early Shihezi stage. Coal, carbargilite in Permo-Carboniferous and carbonate in Ordovician system were the main source rock of natural gas in the researched area. Important gas-bearing reservoirs included sand bodies of distributary channels and subaquatic distributary channels in deltas and braided deltas, dolomitite in carbonate platform, and sand bodies in tidal flat and barrier island.The filling evolutionary process in the Palaeozoic era in this area was extremely complex. During the Majiagou stage of Early Ordovician period, the researched area went through three tertiary eustatic cycles. With negative movement in Ma; stage, deposits of evaporate platform and local platform were developed. Caledonian movement made the researched area exposed tothe air and was eroded more than 100 million years. Started from Benxi stage of late Carboniferous, deposits of epiric sea were developed in this area along with the spreading of west-east direction and gentle depression of the basement. After late of Early Permian period, deposits of deltas and lacus took shape extensively along with up-lifting of Yimeng area in the north and marine regression of this area.Regarded regional unconformity, transformation basement of tectonic regime, incised water channels, discontinuous surface of sidementary facies and exposed surface of lowstand as boundary of sequence, the Palaeozoic in the researched area can be divided into three supersequence and fourteen sequence. It is considered that the sequence evolution in the Palaeozoic went through the following three stages, the filling of evaporate platform, the filling of carbonate and detrital rock mixed deposits in epiric sea, and the filling of detrital deposits of lake-deltas in paralic inland sag. It is concluded that there is a very closed relationship between sedimentary sequence and gas pool. Sequence controlled the association of source beds, reservoirs and cap beds, controlled the dis...