Geothermal And Associated Water Soluble Gas Resources Exploration And Prospect Forecast Of Wei He Basin

Weihe basin is a caenozoic fault basin, which lies between Qinling orogenic belt and thesouthern edge of Erdos basin.The earlier explorations show that the basin abounds in variouscomprehensive resources with great industrial value, including geothermal water,associatedwater, soluble helium,associated water dissolved methane gas, a variety of rare elements andmetallic ore. It is a sedimentary basin of great exploiting potential on its resources.Based on the previous researches results, the basic geological data and preliminaryexplorations on Weihe basin in recent years,this article carries on a more systematicexploration and research on its geothermal water resources, associated helium and associatedmethane gas by analyzing the base, the sedimentary strata,types of faults,and sedimentarydistribution facies, etc. Besides, it composites the mutual matching relationship ofhydrodynamic system,seven factors—source, reservoir, cap, trap、migration, preservation andthese following comprehensive points:properties of reservoir and diagenesis. Further, It alsodoes the thoroughly analysis and research on its origin, forming, distribution as well asexplores the exploitable and favorable distribution area with possible industrial value.Weihe basin is cenozoic and rifted which obviously characterized by its double-layerstructures—the base of the under layer is magmatic rocks before the prepaleozoic era, and therock cap is neozoic sedimentary stratum. As a transitional shaping basin between Qinlingorogenic belt and Erdos basin, its fault structures are development of which several east-westdeep and major faults divide the basin into different bases and tectonic elements and itssecondary ones cut the basin into small different-sized fault segments. These small faultsegments influence on the distribution and reservoir of geothermal water, associated watersoluble helium and methane gas.Geothermal water and associated water soluble helium are mainly stored in upper tertiaryBahe group of Lantian County. While dissolved methane gas is in sedimentary stratum ofZhangjiapo group. Among them, geothermal water, associated helium are affected by faultstructure and sedimentary stratum distribution. The source of helium is from Lantian granite,which is locates in the northern Qinling of south part of basin. The helium, emitted by decay and migrated to Bahe group of Lantian County, belongs to the new paleozoic reservoir. Thedistribution of sedimentary facies in Zhangjiapo group has a major influence on associatedwater dissolved methane gas with fault structure following. Marl in Zhangjiapo group, whichis a type of self-generated and self-stored reservoir, is the source of hydrocarbon.According to reservoir factors of the gas and the enrichment rule of the gas, twofavorable exploration target zones are preferentially selected, including the Gushi depressionand xi’an depression. The two zones have well-developed source-reservoir-caprockassociation, reservoir with sound connectivity and transitivity, and fewer Babel fault which isbeneficial to the transmission and the gathering of the water-soluble gas; the fault structureforms good transmission pathway and reservoir trap. Besides, many relatively isolatedstructure traps carved up by the secondary fault are also favorable traps for the water-solublegas to form reservoir. Comprehensive analysis the two favorable zones has preferablereservoir forming conditions and considerable prospect of resource exploration.By studying three different kinds of resources—influencing factor, types of reservoir anddistribution law，combining the present sampling comparison and comprehensive analysis, tofind out the favorable distribution area and reservoir model of different resource types: theforecast that geothermal water and associated water soluble helium’s favorable area mainlydistributed in both sides of Weihe fault zone, of which Bahe group of Lantian shows a goodfeature in its preservation for its closely to the gas source rock. Thus,it predicts to be theperfect area for the new paleozoic reservoir. Water dissolved methane gas features the typicalbiogenic gases in the fixed city depression. The depression in xi’an is local biogenic gases andpartly pyrolysis model. The two main favorable distribution areas are two mid and peripheralareas of two secondary sedimentary depressions. Next to the depression area, near Weihe faultzones, it shows the proper area for forecasting. What’s more, making a comparison byanalyzing the present water and gas, it greatly conforms the distrubition model of forecastingfavorable distribution area from the theoretical points.