Low Temperature Gas Generation in Marine Shales

Frank Mango¹, Daniel M. Jarvie², and Steven Garcia^1
1Petroleum Habitats, Houston, TX
²Humble Geochemical Services, Humble, TX

Marine shales generate two distinct gases in the laboratory, one at high temperatures (> 300^oC) from kerogen cracking, and the other at low temperatures (< 50^oC) by a catalytic process probably promoted by transition metals. Low-temperature gas forms at reservoir temperatures, but only when there is gas flow under anoxic conditions. This is achieved in the laboratory by grinding the shales in pure argon to expose inner anoxic surfaces, and then passing a purified inert gas (e.g., helium) over the surfaces at constant temperature. In a typical example, a type II marine shale from the Black Warrior Basin generated 2.0 mg gas/(g shale) (C1-C5, 24% of its hydrocarbons) in three hours of anoxic helium flow at 50^oC. The same shale generated 99% less gas in a closed reactor at the same temperature over 20 hours and 86% less gas under helium flow containing 10 ppm oxygen.

Low-temperature gas generation is unique in all respects. Generation rates are orders of magnitude higher than thermal cracking rates, product compositions are dynamic, kinetics of generation are non-linear, and gas generation terminates on exposure to trace levels of oxygen. Equally surprising, different shales generate gases having different compositions. Barnett Shale, Fort Worth basin, generates a gas enriched in methane and near thermodynamic equilibrium in C1-C3 (K = [(C1)(C3)]/[C2)2]), while New Albany Shale, Illinois basin, generates a gas with mainly propane; it is not at equilibrium, but it approaches equilibrium over time, consistent with catalytic gas metathesis.

Significant amounts of low-temperature gas could be generated during conventional gas production from marine shales. The reservoir conditions and shale properties conducive to such a process will be discussed.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas