Sedimentary Characteristics Of Upper Paleozoic Coal Measure Strata And Coal Reservoir Modeling In Southeast Ordos Basin

In China,Ordos Basin is one of the hottest spots in CBM(coalbed methane)exploration and exploitation.The occurrence of CBM is of complex geological conditions,besides,prototype and tectonic style of basin is diversiform.The coal deposited early,evaluated highly,and experienced long-termed structural changes.The coal reservoirs are characterized by the feature of low porosity,low permeability,low gas saturation and high heterogeneity.To solve the above problem,the paper studies sedimentary characteristics of coal measure strata,physical properties characteristics and distributing characteristic of coal reservoirs,and builds porosity,permeability and water saturation stochastic modeling of No.11 coalseam in Benxi formation and No.3 coalseam in Shanxi formation,and eventually divides the enrichment area.According to the study of sedimentary system and sedimentary micro facies of coal measure strata including Benxi,Taiyuan and Shanxi formation,it finds that Benxi formation mainly developed barrier coast sedimentary system,including shallow marine shelf,barrier island,lagoon and tidal flat deposition from east to west.In Taiyuan period,lagoon and barrier island disappeared with the expansion of transgression.The study area mainly developed shallow sea shelf and tidal flat deposits.In Shanxi period,shallow lake and delta depositional systems in the region was widely developed due to regression,shallow lake deposits was NNW-SSE trending in the central region.Front-delta,which mainly located in the southern and northern regions of the study area,is the major part of Deltaic depositional systemNo.11 coalseams on the top of Benxi formation,with large thickness,distribute wildly and stably in the area.The thickness of coalseams in the middle of study area—Yanchang and Yichuan is rather large.Burial depth increases from 1700m in southeast to 3300m in northwest.No.11 coalseams at the bottom of Shanxi formation distribute rather wildly,but integral continuity is bad.Burial depth is slightly shallower than No11.coalseams.Based on observation and description as well as test analysis,the study finds that the macroscopic coal petrography of the No.11 and No.3 coalseam are semi-bright coal.The main content of lithotypes is vitrain.Metamorphic grade increases from east to west with the increasing of buried depth.Ro,max of the two coalseams generally greater than 1.7%,which belongs to lean coal,anthracite.The pore and fracture of two coalseams develop well,but both pore and fracture are generally filled with mineral,mainly of clay minerals,calcite and pyrite.Taking advantage of software Petrel,the paper builds 6 models of 3 parameters—porosity,permeability and water saturation of the two coalseams.Combined with sedimentary characteristics,the study shows that CBM is rich at the place of broad and gentle structural high part as well as monoclinic zone which is sealed well.Geographically,it indicates that Yanchang in the middle and Yichuan in the southeast of study area are rich in CBM,which are the key points of the next step of research.