Mudstone and Claystone Units: Seals for Ancient Microbial Gas Accumulations in the Upper Cretaceous Milk River Formation, Alberta and Saskatchewan

Neil Fishman¹, John Neasham², Donald Hall³, and Debra Higley^1
1U.S. Geological Survey, Denver, CO
²Poro-Technology, Sugar Land, TX
³Fluid Inclusion Technologies, Inc., Broken Arrow, OK

More than 3 TCF of dry (>99% methane) microbial gas (δ13C -65 to -71‰) has been produced from shallow wells (generally <1500 ft) in the Upper Cretaceous Milk River Formation, southeastern Alberta and southwestern Saskatchewan. The likely source of gas is organic-bearing mudstones (total organic carbon <0.5 to 3.29%) that are interbedded with and encase productive intervals (as much as 90 ft thick). Production is from numerous thin (<<1 ft) silty to sandy layers/lenses within these intervals. Petrologic, isotopic, and burial history studies indicate that methanogenesis commenced during deposition and continued for possibly 20 m.y. Questions remain concerning how early-formed gas could remain trapped in the formation to the present.

Mercury injection capillary pressure data from core samples of the Milk River and overlying Upper Cretaceous Pakowki Formation were collected to evaluate pore apertures and permeabilities of various lithologies. Milk River fine-grained sandstones (n = 11) exhibit generally greater median pore apertures (0.05 to 7.6 μm) and permeabilities (0.01 to 121 mD) compared to mudstones (n = 9), which have pore apertures and calculated permeabilities ranging from 0.03 to 0.17 μm and 0.002 to 0.53 mD, respectively. Pakowki claystones have small pore apertures (0.03 to 0.07 μm) and low calculated permeabilities (0.002 to 0.017 mD). The characteristics of mudstones and claystones point to the likelihood that buoyancy pressure never exceeded their capillary entry pressures, so these units inhibited vertical migration of Milk River gas. Thus, mudstones and claystones helped gas accumulations in the formation to persist for millions of years.

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