Geochemical Characteristics and Genetic Types of Natural Gas from Tazhong Area in the Tarim Basin,NW China

Plentiful natural gas has been discovered in Tazhong area in the Tarim Basin, and is mainly reservoired in Ordovician at the northern slope and in Carboniferous at the central fault horst. The analyses on natural gas components and carbon and hydrogen isotopes indicate that the natural gas in Tazhong area is mainly composed by alkanes with methane contents generally over 60%. The dryness indexes and N₂ and CO₂ contents are regionally different with low contents of H₂S. The δ¹³C₁ and δD₁ values of alkane gas from the western part of northern slope are generally lower than −45‰ and −160‰ respectively, whereas those from the central and eastern parts are mainly higher than −45‰ and −160‰ respectively, and those from the central fault horst are mainly centralized in the ranges of −46.3‰∼−42.3‰ and −163‰∼−154‰ respectively. The alkanes display positive carbon and hydrogen isotopic series, and a few gas samples are partially reversed. The geochemical characteristics indicated that the alkane gas in Tazhong area was oil-associated and derived from the sapropelic organic matters. Natural gas from the western part of the northern slope was mainly generated in the early stage of oil cracking and was mixed by kerogen-cracking gas, and that from the central fault horst was mainly the products of oil cracking to a higher extent, whereas the gas from the central and eastern parts of the northern slope was mainly derived from the primary cracking of kerogen. The mixing analog calculation indicated that the high δ¹³C_CO2 values was caused by the mixing of inorganic CO₂ which had been generated in the dissolution reaction of reservoir carbonate rocks under acidic conditions.