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Based on core data, physical properties, well logging, gas testing, composition, and helium distribution from 13 newly drilled wells, in combination with regional tectonic evolution, depositional background, and typical gas reservoir profiles, helium-rich gas reservoirs geology characteristics and main controlling factors have been explored. The results show that three gas reservoirs formed from bottom to top, i.e., granite and metamorphite fractured reservoir in the basement, tight sandstone structural reservoirs in the Devonian, and huge continuous carbonate reservoirs in the Lower Carboniferous. Fractures developed on structural points control high-production, and gas is prominently displayed in low structural areas. The dry gas exhibits richness in helium, with decreasing helium content from the basement to the Carboniferous. The pre-Devonian large-scale granite and metamorphite basement serve as the primary helium source, while the high-gamma sandstones of the Devonian and the Carboniferous act as secondary sources. Fractures play a dual role, being filled up and conducting in two ways: gas from the Carboniferous source rocks migrates downward, and helium migrate upward. Thick interbedded gypsum-tight limestone on top of the Lower Carboniferous act as the regional caprock, effectively preserving the helium-rich gas reservoirs. [ABSTRACT FROM AUTHOR]

Abstract: Ordovician karsts are well developed in the Yingmaili and Halahatang areas of the North Tarim Basin. The ancient karsts are an important factor for forming Ordovician hydrocarbon reservoirs in the central and western regions of the North Tarim Basin. They are distributed, like successive broad belts, along the south side of the North Tarim uplift and constitute a large complex ancient karst reservoir system together with the Lunnan bulge. The main controlling factors of the Ordovician karsts are uplift, interlamination parallel unconformity, fault systems, rock physical properties and high-energy sedimentary phases. The Ordovician karsts can be divided into uplift-controlled karst areas, the strata- and fault-controlled karst areas and limited karst areas while karsts of different origins can overlay each other. The ancient karsts are developed mainly in four periods: the Caledonian, Hercynian, Indo-China Yanshan, and Yanshan, and karsts of different periods overlay each other, too. The condition for developing large-scale ancient karst reservoirs exists in the area and the complex ancient karst systems can be rich in oil and gas. Along both sides of the Middle-Upper Ordovician clastic pinch-outs are the more favorable areas for large ancient karst reservoirs. [ABSTRACT FROM AUTHOR]