Geochemistry of Shale Gases from around the World

Abstract

Summary We collected and investigated published data on >1,500 samples of gases recovered from shales in >20 basins in Argentina, Canada, China, France, Poland, Saudi Arabia, Sweden and the USA. Most shale gases are relatively dry (average dryness C1(C1–C1 is 0.9) and contain nitrogen (average about 6.6%) and CO2 (average about 2.5%). Methane has carbon isotopic composition δ¹³C ranging from −82‰ to −27‰ and averaging around −43‰. Most shale gases have thermogenic origin, although gases from New Albany shale in the Illinois basin, Antrim shale in the Michigan basin, and Colorado shale in the Western Canada Sedimentary Basin have predominantly primary or secondary microbial origin. Isotopic reversal of the normal trend in carbon isotopic composition such that δ³C of methane is larger than δ¹³C of ethane is a common phenomenon observed in many studied samples. The vast majority of samples showing isotopic reversal have thermogenic origin and are very dry (ratio C1/(C1–C5) exceeding 0.97 and averaging 0.99) suggesting that isotopic reversal is common among thermogenic gases with very high maturity. However, isotopic reversal of the normal trend in hydrogen isotopic composition such that δ2H of methane is larger than δ2H of ethane is observed among both thermogenic and microbial shale gases.