Low-temperature gas from marine shales

Area of Science:

• Geochemistry
• Organic geochemistry
• Petroleum geochemistry

Background:

• Natural gas is primarily attributed to thermal cracking of kerogens and bitumens at high temperatures (100-200°C subsurface, >300°C lab).
• Low-temperature gas generation (<100°C) under laboratory conditions has been considered unprecedented, despite some geological evidence suggesting otherwise.

Purpose of the Study:

• To investigate and report robust gas generation at temperatures significantly below established thermal cracking thresholds.
• To explore the potential for a low-temperature catalytic pathway in natural gas formation.

Main Methods:

• Experiments were conducted using anoxic helium flow over Mississippian marine shale at temperatures as low as 50°C.
• Gas generation was monitored in sequential isothermal heating cycles.
• Comparative analyses were performed with helium flow containing trace oxygen (10 ppm O2).

Main Results:

• Significant gas (C1-C5 hydrocarbons) generation was observed at 50°C, with 6% of hydrocarbons decomposing in a 3-hour episode.
• Gas generation occurred discontinuously in aperiodic episodes.
• Gas generation was substantially reduced (72%) and compositionally distinct under slightly oxic conditions.
• Low-temperature generation at 50°C produced nearly five times more gas (57.4 µg/g rock) than high-temperature thermal cracking at 350°C (12 µg/g rock).

Conclusions:

• A low-temperature catalytic pathway for natural gas formation exists, operating approximately 300°C below the conventional thermal cracking path.
• This discovery redefines the time-temperature parameters for natural gas generation.
• It opens possibilities for gas formation in subsurface environments previously considered incapable of generating gas.