THE SPATIAL AND COMPOSITIONAL DISTRIBUTION OF THE NATURAL GAS SEEPAGES IN SOUTHWESTERN TAIWAN

As a hydrocarbon indicator of a working petroleum system, gas seepages reveal underground reservoirs, and possible migration pathways of hydrocarbons. Compiling the results of recent seep surveys and mud gas logging, here we present the spatial and compositional distribution of the natural gas in southwestern Taiwan, and explore the natural gas resources of different origins.

The natural gas in the foothills belt of southwestern Taiwan is characterized by distinct thermogenic features, with the δ¹³C_CH4 values ranging from -32‰ to -35‰ along the major thrust faults. However, in some minor mud volcanoes and the westernmost thrust belt, the isotopic compositions of methane vary from -45‰ to -57‰. Farther westward to the coastal plain, the microbial methane is marked with the δ¹³C values less than -58‰. Therefore the transition is interpreted as mixing with biogenic gas during upward migration into the shallow level. This observation is well consistent with the mud logging result that the thermogenic gas is distributed among the formations lower than 920m, while the microbial methane only exists in the shallow level. In addition, relevant studies on the cold seep carbonates indicate an almost similar trend of the spatial-compositional distribution. Based on the correlation between onshore and offshore subdivision zones of biogenic, mixture and thermogenic gases, we point out that there will be high potential in the east of Kaoping Valley.