Geochemical characteristics and origin of natural gas in Tarim Basin, China

The Tarim basin, located in northwest China, is one of the largest basins in the world that has not been well explored. In recent years the large scale of petroleum exploration in the basin has led to the discovery of a series of oil and gas fields with an approximate 1:1 ratio of oil to gas resource. This means that the natural gas resource is very important for the hydrocarbon resource in the Tarim basin. In this basin most of the discovered gases are condensate-associated gas and oil-associated gas. Research on chemical components and isotope composition of carbon and hydrogen of natural gases from the Tarim basin shows that all discovered gases in this basin are thermogenic. With respect to source organic matter, there are two types of gases: (1) oil-type gas generated from Sinian to lower Paleozoic marine source rocks with the sapropelic organic matter, with -44.5 to -33.8 per mil of δ13C1, -42.0 to -28.1 per mil of δ13C2, -35.4 to -28.4 per mil of δ13C3, and heavier than -200 per mil of δD1 and (2) coal-type gas generated from Mesozoic terrestrial source rocks with the humic organic matter, with -40.5 to -33.1 per mil of δ13C1, -29.7 to -21.3 per mil of δ13C2, -26.3 to -20.3 per mil of δ13C3, and lighter than -200 per mil of δD1. Based on the geochemical characters and geological setting, Tabei uplift region is regarded to have some reservoired gas pools formed by mixed mature oil-type gas with post-mature oil-type gas. Some reservoired gas pools are formed by mixed mature oil-type gas with mature coal-type gas.