Low-temperature gas from marine shales

Abstract

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°C in the subsurface and generally above 300°C in the laboratory. Although there are examples of gas deposits possibly generated at lower temperatures, and reports of gas generation over long periods of time at 100°C, robust gas generation below 100°C under ordinary laboratory conditions is unprecedented. Here we report gas generation under anoxic helium flow at temperatures 300° below thermal cracking temperatures. Gas is generated discontinuously, in distinct aperiodic episodes of near equal intensity. In one three-hour episode at 50°C, six percent of the hydrocarbons (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 gases were compositionally distinct. In sequential isothermal heating cycles (~1 hour), nearly five times more gas was generated at 50°C (57.4 μg C1–C5/g rock) than at 350°C by thermal cracking (12 μg C1–C5/g rock).The position that natural gas forms only at high temperatures over geologic time is based largely on pyrolysis experiments under oxic conditions and temperatures where low-temperature gas generation could be suppressed. Our results indicate two paths to gas, a high-temperature thermal path, and a low-temperature catalytic path proceeding 300° below the thermal path. It redefines the time-temperature dimensions of gas habitats and opens the possibility of gas generation at subsurface temperatures previously thought impossible.

Natural catalytic activity in a marine shale for generating natural gas

“…The system's instability prior to olefin addition (figure 3) might reflect hydrogen fluctuations, since hydrogen plays a key role in gas generation (reaction (1.2)). Propylene could be delivering hydrogen and stability to a hydrogen-deficient catalyst through degradation (Mango & Jarvie 2009a, 2010. We propose two possibilities for propylene dimerization to n-C 6 and MCP.…”

“…Broken lines with filled circle, before propylene addition; solid lines with filled square, after propylene addition. (Mango & Jarvie 2009a,b, 2010. Thermal reactions do not oscillate over time, and first-order kinetics should not generate gas in episodes (Mango & Jarvie 2009a) or the chaotic fluctuations in MCP/n-C 6 shown in figure 3.…”

“…(Mango & Jarvie 2009a,b, 2010. Thermal reactions do not oscillate over time, and first-order kinetics should not generate gas in episodes (Mango & Jarvie 2009a) or the chaotic fluctuations in MCP/n-C 6 shown in figure 3. The transition from chaotic oscillations to invariance shown in figure 3 suggests the creation of a new and stable path to MCP and n-C 6 .…”

“…Neither scheme is hydrogen-balanced. We assume a source of hydrogen through degradation (Mango & Jarvie 2009a, 2010. n-C 6 generation consumes hydrogen while MCP generation is hydrogen-neutral.…”

“…Gas is generated at ambient temperatures under natural conditions without artificial stimulation (Mango & Jarvie 2009a, 2010. Methane, ethane and propane are generated near thermodynamic equilibrium (Mango & Jarvie 2009b) duplicating their molecular compositions in natural gas (Mango et al 2009).…”