Research On Sedimentary Systems And Hydrocarbons Enrichment Of The Upper Palaeozoic Of The Ordos Basin

Aiming at the subtle gas reservoirs characteristic of low pore, low permeability, low abundance and large area's distribution of the upper Paleozoic in Ordos basin, in the way of studying typical gas reservoirs, in the clue of the high beneficial reservoir distribution and gas migration, and on the emphasis of gas reservoir distribution and high permeability reservoir generation and accumulation regulation, the paper is based on the existing research data, combined with the new theory and progress of the sedimentology, sequence stratigraphy, reservoir sedimentology, petroleum geology, etc, and analyzes systematically geological condition, accumulation model, high beneficial reservoir formation mechanics, distribution regulation of the lithological reservoirs, and points the valid exploration.On the basis of stratigraphic classification and comparison study, the strata from Benxi formation to later Shihezi formation was divided into two I order sequences(super-term stratigraphic base level circle) and eight II order sequences(middle-term stratigraphic base level circle), and 22 III order sequences, middle cycles could been divided into many short cycles. Based on the sequences of sedimentary characteristic, the Upper Paleozoic could distinguish 3 sedimentary system group, 8 sedimentary system(facies), 22 subfacies and 40 microfacies, and discussed detailly the characteristic of all kinds of facies, subfacies and microfacies and spatiotemporal distribution of them.Based on analyzing the regional geologic background, palaeocurrent direction, light mineral, heavy minerals rock trace element characteristics, the author concluded that there had three primary sedimentary provenances in north and four primary sedimentary provenances in southern Ordos Basin, and put forward the new viewpoint that the west provenance began to supply since He-8 stage. Furthermore, the author considered that the structural attribute characters of the south and north sedimentary origins were evidently different by utilizing the rock geochemical characteristics to analyze structural characteristics of sedimentary origins. Whereafter, analyzing the palaeogeography and evolution of Ordos Basin in different periods in neopaleozoic, the author gained the new standpoints that there developed the fan deltas in north in early permian and the high constructive braided river delta in He-8 stage and thought that mainly developed three coal-forming models of the barrier coast, river(or delta plain) and the tidal delta.The source rocks of Upper Paleozoic had gas-forming centers in west and east of Ordos Basin. The gas accumulation models was "widely-covered" coal-formed gas reservoir that had continuous gas generating capacity and the gas generation peak of gas generation in the eastern area is earlier than that in the western area. The reservoir rock bodies had three petrologic characteristic that were high contents of filling materials, high compositional maturity and low textural maturity. The reservoir quality was mainly controlled by provenances and sedimentary facies. The mainly reservoir types had type I,II,III and IV,and the type II and III were the primary reservoir types. The hight effective reservoir mainly developed in coarse grain, gravel-bearing coarse-grained and medium-coarse quartz sandstone that deposited by high-energy water channels, and mainly appeared in the subsequence cycles of rapid marine (lake) transgression to oscillating marine(lake) transgressions and regressions(progradation sandbody), and to relative inactive period of lowstand system tracts, pore assemblage types mainly were the intergranular pores type, the intergranular pores + dissolution pores types and the dissolution pores + intergranular pores types. The regional distribution of hight effective reservoir controlled by sedimentary facies, tectonic, provenances and diagenesis.With the studying of valid hydrocarbon source rock evaluation, oil and gas migration, accumulation, and distribution, the paper puts forward the reservoir formation models of Upper Paloozoic natural gas were the nearest, preferential and multilayer accumulation models and had three types of gas accumulation combination: inside source type, near source type and far source type, and explains the main characteristic and distribution models of different gas reservoirs. The main controlling factors about upper Paleozoic gas accumulation in Ordos basin were listed in the following: A. High efficient reservoir was the main place of gas accumulation; B. Deltaic sedimentary environment and sedimentation mainly contribute to oil and gas's accumulation, and the main accumulation plain, and underwater distributary channel regions were distributary channel sandstones of deltaic sandstones of deltaic front. C. multi-phase structure fracture combination regions that controlled by latitude were the important factors of high production rate gas well.Basing on the analyzing the main controlling factors of forming gas reservoirs, it was pointed that the large lithological gas reservoirs distribution region of Shanbei slope in the middle of basin, was still the main exploration region at present, the west reverse faults and Tianhuan depression of structural pool are the important exploration region, the strategy reserve region was the North Yimeng uplift and South Weibei uplift. The author adviced that deltaic plain sand bodies and the delta front sand bodies of the west of Sulige miao area and the southwestern Ordos Basin as the key areas for further studying and prospecting, and the delta front sand bodies of southern Ordos Basin should strengthen research and exploration.