Low-temperature gas from marine shales

摘要

Thermal cracking of kerogens and bitumens is widely accepted as the major source of natural gas(thermal gas). Decomposition is believed to occur at high temperatures, between 100 and 200°Cin the subsurface and generally above 300°C in the laboratory. Although there are examples of gasdeposits possibly generated at lower temperatures, and reports of gas generation over long periodsof time at 100°C, robust gas generation below 100°C under ordinary laboratory conditions isunprecedented. Here we report gas generation under anoxic helium flow at temperatures 300°below thermal cracking temperatures. Gas is generated discontinuously, in distinct aperiodicepisodes of near equal intensity. In one three-hour episode at 50°C, six percent of thehydrocarbons (kerogen & bitumen) in a Mississippian marine shale decomposed to gas (C1–C5).The same shale generated 72% less gas with helium flow containing 10 ppm O2 and the two gaseswere compositionally distinct. In sequential isothermal heating cycles (~1 hour), nearly five timesmore gas was generated at 50°C (57.4 μg C1–C5/g rock) than at 350°C by thermal cracking (12 μgC1–C5/g rock).The position that natural gas forms only at high temperatures over geologic time is based largelyon pyrolysis experiments under oxic conditions and temperatures where low-temperature gasgeneration could be suppressed. Our results indicate two paths to gas, a high-temperature thermalpath, and a low-temperature catalytic path proceeding 300° below the thermal path. It redefinesthe time-temperature dimensions of gas habitats and opens the possibility of gas generation atsubsurface temperatures previously thought impossible.