AAPG Bulletin, V. 84, No. 5 (May 2000), P. 591-606.

Geochemical Characteristics and Origin of Natural Gas in Tarim Basin, China¹

Chen Jianfa,² Xu Yongchang,² and Huang Difan³

©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved.
¹Manuscript received May 18, 1998; revised manuscript received July 2, 1999; final acceptance October 15, 1999.
²State Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China; e-mail: [email protected]
³Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, People's Republic of China.

ABSTRACT

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 -33.8o/oo of d¹³C₁, -42.0 to -28.1o/oo of d¹³C_2, -35.4 to -28.4o/oo of d¹³C₃, and heavier than -200o/oo of dD₁ and (2) coal-type gas generated from Mesozoic terrestrial source rocks with the humic organic matter, with -40.5 to -33.1o/oo of d¹³C_1, -29.7 to -21.3o/oo of d¹³C₂, -26.3 to -20.3o/oo of d¹³C₃, and lighter than -200o/oo of dD₁.