Investigation on Mixed-Natural Gases from Two Sources or from One Source with Two Generation Maturities: A Case Study of Giant Gas Fields in West China

Jin Qiang and Fuqi Cheng
University of Petroleum, Dongying, Shandong, China

Giant natural gas accumulations have been discovered in basins in west China for recent 20 years, which include the Tarim basin, Ordos basin and Sichuan basin. In these basins, the gases are generated from two kinds of sources: one is gas-prone organic matter in coal-series source rocks, and the other is oil-prone substances mainly in marine source rocks. Gases generated from gas-prone materials are defined as coal-type gas, and those from marine source rocks are called oil-type gas. As the coal-series and marine source rocks were well developed in the three basins, accumulated gases are often mixed from the two kinds of source rocks so that the mixed-gas is called the two source gas, TSG for short. As tectonic movements control hydrocarbon generation from source rocks, sometimes a set of source rocks have generated gases in two different gelogical times so that the mixed-gas with two thermal maturities is accumulated in one trap and is called two maturity gas, TMG for short. TSG and TMG are a characteristic of accumulated gases in the three basins, west China.

To quaitatively identify the sources of mixed-gas and to quantitatively estimate the mixing ratio are of significance in evaluation of the source rocks and in investigation of the gas accumulation. The research framework consists of (1) establishing geochemical models for the coal-type gas and oil-type gas, which described relationship between the source rock maturity (Ro) and δ13C of methane and ethane, and between the Ro and methane and ethane percents in total hydrocarbon gases; (2) identifying the studied gases as TSG or TMG according to the models; and (3) calculating the mixed ratio for the TSG and TMG based on the law of physical mixing of the two different gases, and making comprehensive interpretation to the mixing in the gas accumulation.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005