The Experimental Conditions for Generating Low-Temperature Gas in Marine Shales

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

Marine shales generate little gas below 300^oC under ordinary laboratory procedures and copious amounts of gas below 50^oC under anoxic procedures. Anoxic procedures include three steps: 1) grinding the shales in argon to expose fresh anoxic surfaces, 2) pressure-flushing charged reactors with pure helium to establish anoxia, and 3) passing inert gas (e.g., helium) over the surfaces at reaction temperatures. Omitting any of these steps or otherwise introducing trace amounts of oxygen results in no gas generation or substantially less gas. In one example, a type II marine shale converted 24% of its hydrocarbons (S1+ S2) to gas (C1-C5) at 50^oC in 4 hours under anoxic helium flow. In a duplicate experiment under helium flow with 10 ppm oxygen, the same shale generated 86% less gas.

Marine shales possess natural catalytic activity that is destroyed irreversibly on exposure to ppm levels of oxygen and this may be why most pyrolysis procedures generate very little catalytic gas. Pyrolysis experiments carried out under anoxic procedures co-generate thermal and catalytic gas and they are clearly distinguishable. A New Albany shale generates 90% catalytic gas under anoxic procedure at 350^oC. The broadly accepted view that marine shales generate only thermal gas in pyrolysis experiments can be only partially correct. They generate only thermal gas under oxic procedures, but they generate both thermal and substantial amounts of catalytic gas under anoxic procedures.

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