A Tale of Two "Shales"--Similarities and Differences in the Gas Generation Histories of the Cretaceous Milk River Formation (Saskatchewan and Alberta) and Cretaceous Lewis Shale (San Juan Basin, New Mexico)

Neil S. Fishman¹, T. M. Parris², Donald L. Hall³, and Debra K. Higley^1
1 U.S. Geological Survey, Denver, CO
² Petro-Fluid Solutions, Lexington, KY
³ Fluid Inclusion Technologies, Inc, Broken Arrow, OK

Similarities and differences are present in the gas-generation histories of the Milk River Formation (mostly Campanian), Saskatchewan and Alberta, and Lewis Shale (Campanian), San Juan Basin, New Mexico. Both were deposited in marine environments of the Western Interior Seaway (WIS).

Reservoirs in both are unconventional, typically fine grained (mudstone, siltstone, very fine-grained sandstone), having low porosity (commonly <10%) and permeability (commonly <0.1 md). Organic carbon contents vary (<0.5-3%), with Type III organics dominating in both, although Type II organics also are present in the Lewis. Authigenic calcite in early-formed concretions in both formations has a dominantly marine isotopic signature (d¹³C_PDB -13.50‰ to –23.90‰, d¹⁸O_PDB –0.96‰ to –7.63‰), and concretionary minerals contain primary, methane-bearing fluid inclusions. Calcite cement in thin interbedded sandstones has a meteoric isotopic signature (d¹³C_PDB –4.75‰ to –10.80‰, d¹⁸O_PDB –13.16‰ to –17.48‰) and is texturally similar in both. These similarities indicate a parallel early diagenetic/methanogenic history for both.

Subsequent to the Paleocene, the burial histories of the two formations diverge. Milk River burial ended by middle Paleocene, with maximum formation temperatures (from burial reconstructions) reaching ~45-50^oC. In contrast, the Lewis was buried and heated sufficiently (locally ~170^oC, Ro ~1.5%) for subsequent episodes of petroleum (oil, wet and locally dry gas) generation. Deep burial facilitated quartz cementation and poly-phase fracturing of the Lewis.

These formations might represent the thermal and geographic end members for rocks deposited in the WIS, implying that most organic-bearing rocks deposited in the seaway might have seen either or both biogenic and thermogenic gas generation.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005