Accumulation Conditions And Resource Prediction Of Nature Gas And Helium Gas In Weihe Basin

Weihe Basin, a fault basin of Cenozoic Era, located in the middle of Shaan Xi Province. Though researches on Weihe Basin has been proven that it is rich in gas resources, it has been a challenge to locate the gas enrichment region due to the Unclear improper exploitation, Geological conditions methods and the restrictions of the research methods. This article will mainly discuss in the following parts: the types of the gas resources, distributing rules, accumulation model and resource extent in Weihe Basin based on forming conditions of the gas and Accumulation with the methods of the geological investigation outdoors, collection and analysis of the samples, data processing indoor, map compilation, and so on. The main clue of the article is the controlling factors of gas accumulation.Natural gas in Weihe basin can be divided into two types which are helium gas and biogenic gas. The results of proximate analysis of natural gas samples showed that the methane content in natural gas was relatively low, generally concentrated between 10-20%.Only the samples from Weinan showed high methane content and can reach more than 90%. The content of helium in the gas samples ranged from 0.3% to 3% accounted for 78.7% of the total samples, far more than 0.1% of the industry standard. The analysis result of carbon isotope showed that the δ13C1 of the samples was mainly between-43~-10‰, giving priority to coal formed gas mixed with a small amount of inorganic origin gas. The δ13C1 of samples from Weinan were all less than-55% which were called biogenic gas. The analysis result of helium isotope showed that the 3He/4He of the samples was between2.2-78×10-8which was the helium gas from crust. The comprehensive analysis result of the proximate analysis and genesis of natural gas showed that the helium-rich nature gas coexisted with coal formed gas. But the biogenic gas scarcely contained helium gas in this area.The helium gas source rocks were uranium-rich granites in Weihe River Basin. The result on content analysis of the radioactive element showed that the average content of uranium from Lantian Granite and Mu Huguan Granite respectively was 6.9×10-6 and 6.1×10-6 which were the primary source rocks of the helium gas in this area. The helium gas reservoir was Bahe-Lantian Group with middle porosity and permeability. The trap type is mainly structural trap distributed in the southern of the Weihe basin. The helium gas from reservoir dissolved in different kinds of hot fluids, migrated along the faulted structures and accumulated in a condition which reservoir, trap and cap rock concerted well with each other. On the basis of the research, the article concluded four kinds of the helium gas accumulation pattern, there were convection normal fault trap accumulation pattern, convectionconduction fault trap accumulation pattern, conduction fault nose accumulation pattern and conduction stratigraphic trap accumulation pattern. There were three favorable distribution areas of helium gas including Huayin City, Zhouzhi County and Xianyang-Xi’an area. The accumulation of helium gas is 6.58 ×108 m3 by calculation.The source rock of biogenic gas was the mudstone of Zhang Jiapo Group of Gushi Sag in Weihe Basin. The average content of TOC, chloroform asphalt “A”, H/C, Tmax and Ro was respectively 0.46%,0.041%,1.4,427℃,and 0.56%,which illustrated that the source rock was in the immature-low mature stage of kerogen evolution.The kerogen of the source rock belonged toⅡ1 type which included a mass of humic amorphous body, some sapropelic amorphous body and a few of unorganized vitrinite by analysis of maceral of kerogen.The biogenic gas mainly formed in Zhang 4 Segment and migrated up in Zhang 3 Segment to Zhang 1 Segment which were rich in sandstone. The biogenic gas was trapped in the updip pinch-out of sandstone. The biogenic gas accumulation pattern was self-generation, self-seal and lithologic trap accumulation pattern located in the south of Gushi Sag. The biogenic gas generating amount was 28.35×1012m3 and the quantity of resources was1417.5×108m3 by calculation of the gas generation rate.Conclusion demonstrated that Weihe basin was resource compound basin with multicomponent natural gases which had a considerable resource potential. We should strengthen the research on helium gas and biogenic gas in this area so that it would make a contribution to the development of natural gas industry in our country.