Geologic Settings and Controls of Shallow Gas, Rocky Mountain Area
RICE, DUDLEY D., U.S. Geological Survey, Denver, CO
Shallow gas is generated by the degradation of organic matter by anaerobic bacteria and is referred to as microbial or bacterial gas. Bacterial gas can be generated and can accumulate in significant quantities and is an important resource in the Rocky Mountain area. Factors controlling bacterial gas generation after sediment burial are anoxic conditions, low sulfate content, low temperatures, abundant organic matter, sufficient pore space, and rapid sediment deposition. Bacterial gas is distinguished by its chemical and isotopic composition; the hydrocarbon fraction generally contains more than 98% methane, and d13C1 values are generally lighter than -55(sigma).
In the Rocky Mountain area, bacterial gas accumulations occur in three main geologic settings: clastic shelves, carbonate shelves, and coal swamps. In the clastic shelf setting, bacterial gas occurs in thick (200 ft) sequences of sandstone, siltstone, and shale. The siltstone and sandstone occur as thin (a few inches thick), discontinuous lenses and laminae that serve as low-permeability reservoirs because of their small grain size. The enclosing shales are organic rich [average 2% total organic carbon (TOC)], contain type III kerogen, and serve as the source of and seal for the bacterial gas. The carbonate shelf setting is analogous to the clastic shelf with rhythmically bedded couplets (a few inches thick) of low-permeability chalk (reservoir) and shale (source and seal). These shal s are also organic rich (average 3.2% TOC) but contain type II kerogen. In the swamp setting, thick (as much as 200 ft), humic (type III kerogen) coal beds serve as both the source of and reservoir for the bacterial gas. The gas is trapped by the low porosity and permeability of coal and gas adsorption enhanced by hydrostatic pressure.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)