Resources Potential And Favorable Areas For Shale Gas In Basins Of Western United States

Great success has been made in commercial shale gas production in the United States and the basins in Western United States have been the current hotspot in shale gas development.Through researching the regional structure and prototype-basin evolution of these basins and the geologic characteristics of Barnett,Fayetteville,Woodford,Eagle Ford and Haynesville,shale formations have been studied to figure out their key geological parameters,shale gas enrichment conditions and enrichment modes.On this basis,three methods were used to evaluate the technical recoverable potential of the five shale formations,andthe characteristics of the five evaluation methods and favorable areas are presented.The key findings are as follows.The Paleozoic and Mesozoic shale gas enrichment modes are established.Both the shales are deposited in TST(transgressive system tract)and HST(highstand system tract)deep water slopes of transgressive mode.For the Paleozoic shale,a large amount of organic matter accumulated in anoxic water in the passive epicontinental tension environment in Devonian and Mississippian periods,piedmont depression subsided rapidly due to Ouachita orogeny,which led to deposition of large-area black shale.The shale is buried continuously later and stable in structure.Under the effect of passive epicontinental tension,the Mesozoic black shale transformed into brown shale at the shelf slope due to transgression,and are under continuous and stable subsidence in later period.Shale oil-gas belts distribute extensively in the basin due to the uplifting and deformation of Lower Jurassic salt-dome and Laramide orogeny.Key factors affecting Paleozoic and Mesozoic shale gas enrichment are found out.Controlled by Ouachita orogeny,Paleozoic shale subsided rapidly in the depressions in the front of thrust fold belt,featuring well-developed natural fractures and high silica content,so the search of sweetspotsshould look for areaswith high silica content and rich natural fractures.Mainly influenced by Jurassic and Cretaceous transgressions,Mesozoic shale is passive epicontinental sediments developed on the basis of rift basin.Affected by salt dome uplifting and deformation,the organic-rich shale features high lime content and well-developed natural fractures.Well-developed natural fractures,big thickness ofhigh-quality shale andposition close to deep part of salt-dome uplift slope are mainly considered in the search of sweetspots.The probability,GIS spatial interpolation and hyperbolic-exponential decline methods are used to evaluate the resource potential of the five shale formations,among them,the latter two can give the abundance distribution of technically recoverable shale gas resource.The technically recoverable reserves of the five shale formations are 12.9307 TCM,in which Haynesville ranks first,followed by Eagle Ford,and Woodford.Based on the resource evaluation,the geologic evaluation and analysis of key factors affecting shale gas enrichment are combined to classify the five shale formations: 5 class I areas highest in abundance,5 class II areas secondary in abundance and 5 class III areas lowest in abundance are identified.Meanwhile,according to the selection criterion of US prospective areas,the class I areas in Haynesville and Eagle Ford basins are selected as strategic areas.The analysis of shale gas enrichment pattern and fine evaluation of technically recoverable shale gas reserves can provide important references for technically recoverable reserve evaluation and sweet-spot selection of shale gas in our country.