Study On Sequence Stratigraphy And Reservoir Of Deep Basin Gas Trap Of The Upper Palaeozoic Strata In The Central-Eastern Ordos

Guided by the principles of sequence stratigraphy, using the integrated data of outcrops, cores and well logs, this paper carries out a high-resolution sequence stratigraphy subdivision and correlation study on the Upper Paleozoic strata in the east-central Ordos. On the basis of the work on this area of many scholars', according to the correlation principles which are from the big orders to the small orders, from the shallow outcrops to the deep burial area, from the southern marine formations to the northern continental formations, a sequence stratigraphic framework is set up with different orders. It is suggested that the Upper Paleozoic is a megasequence. The Carboniferous and the Permian are two mesosequences and can be divided into 5 sequence sets and 16 sequences. The key sequence boundaries with different orders, the lasting time of different sequences and the characteristic of every sequence are studied.During the Upper Paleozoic in Huabei there were 3 main times of sea level eustacy which caused different deposits in research area. The transgression of the Upper Carboniferous formed the discontinuous marine sediment with the characteristic of the Fusulina-Fusulinella and Montiparus-Triticites Fusulinid fossils Zones. With the characteristic of the Pseudoschwagarina-Sphaeroschwagerina fossils Zone, the scale of the transgression of the early Lower Permian was the largest. The characteristic of the transgression episodes of the early Lower Permian is given more attention in this paper. The transgression of the late Lower Permian didn't arrive to the Ordos. But it enhanced the base-level of the Ordos with the sediment of thick and retrograding parasequence sets. So it had the long distance controlling effect on the sediment in the Ordos.It is generally supposed that on the ramp of cratonic basin, the sequence of the epicontinental sea in the Ordos is mainly composed of the transgressive system tract and the high stand system tract. This paper shows that it is possible that the incised valley of the low stand system tract can exist on the continental shelf in the basin. In the research area, the incised valley was formed during the sea level downwards and was provided source by the arenaceous river from Qiaotou. It is stacked by 3 incised valleys formed in 3 different sequences. It is suggested in this paper that the sediment of the high stand system tract is mainly composed of fine-grained sediment, hardly of the large scale of fluvial-delta because of the low dip of the ramp, low subsiding rate and low sedimentary rate in basin.The depositional environment of the Upper Paleozoic in the Ordos evoluted from the Epicontinental sea to the fluvial-delta. The change of the depositional environment and eustacy controlled the main depositional systems in the research area. From Csql to Csq5, the barrier system was the main system. From Psql to Psq5, the carbonate ramp was the main system and from Psq6 to Psq8 the main system was fluvial and fluvial-delta system in brackish basin. From Psq9-Psql2, the main system turned to the fluvial-delta system. It is suggested that the distribution of the depositional systems and the depositional facies have the clear characteristic that in thenorth of the research area, the alluvial fan sediment, the fluvial system and the delta system are the main styles of the depositional system, towards to the direction of basin the lacustrine, the barrier, the carbonate ramp system become the main depositional systems. With the tectonic uplift of the source area and the climate turning dry, the coal forming period was over, the fluvial system and fluvial-delta system became to play a major role.Based on the study of former researchers and the theory of deep basin gas reservoir forming, the deep basin gas trap in the Ordos basin is analyzed and is supposed to form in the ramp of the basin. It is supposed that the hydrocarbon of the deep basin gas trap is mainly composed of coal, carbonate and carbonaceous shale and with the 'upper water seal' in the north of the study a...