Abstract

The Lower to Upper Toarcian Poker Chip Shale is present at outcrop in the central and southern Alberta foothills, and comprises marine, black calcareous shale and thin limestone that yield total organic carbon (TOC) values of 1 to 7 wt%. Such high TOC values suggest that these strata may have generated and expelled economic quantities of hydrocarbons. However, the outcrops are presently overmature with respect to hydrocarbon generation, and hence their initial generative potential is difficult to assess. This study of strata previously presumed to be equivalent in the subsurface of the Plains region was undertaken in an attempt to document the generative potential of this unit at the early to mid-mature stages of thermal maturity. The results of this study are integrated with previous stratigraphic and paleontological studies to support revised correlations of Lower to Middle Jurassic strata from the outcrop belt to the subsurface of west-central Alberta.

New ammonite identifications from the middle and upper units of the “Nordegg Member” (north of Twp. 60) indicate that these strata are of Early Toarcian age. The middle and upper units of the subsurface “Nordegg Member”, north of Twp. 60 (and perhaps also the radioactive shale at the top of the Nordegg Member proper, south of Twp. 60), are thus considered equivalent to the organic-rich, black calcareous shale and thin limestone of the Poker Chip Shale, which yields Early to Late Toarcian ammonites from outcrops near its type area in the vicinity of Turner Valley, Alberta. This stratigraphic interval is identified in this study as Poker Chip Shale A.

Overlying Poker Chip Shale A in the subsurface north of about Twp. 40 is a succession of medium- to dark-grey, or greenish grey, noncalcareous, pyritic, friable and variably bentonitic shales that exhibit moderate gamma ray radioactivity and, based on palynological studies, are Toarcian to Aalenian in age. This interval is commonly identified by various authors and on industry databases as the Poker Chip Shale. In this study, this unit is designated as Poker Chip Shale B, to distinguish it from slightly older, and lithologically and geochemically distinct, strata of the typical Poker Chip Shale as it was originally defined in the Turner Valley region. Poker Chip Shale B is considered equivalent to lithologically similar, imprecisely dated strata in the lower Rock Creek Member of the Fernie Formation at outcrop in the western Alberta Foothills.

Poker Chip Shale Ahas excellent hydrocarbon source rock characteristics (TOC values up to 18.5 wt%; Hydrogen Index (HI) values up to 740 mg HC/g TOC) and is thermally mature to overmature over a large region. It thus must have generated significant volumes of hydrocarbons during thermal maturation. Although some of the generated hydrocarbons are present in conventional reservoirs, large volumes were likely retained in the source rock. Therefore, Poker Chip Shale A may represent a viable target for fractured shale production.

In contrast, Poker Chip Shale B is not a source rock for hydrocarbons, as it commonly yields TOC values much less than 1 wt%. However, Poker Chip Shale B may have acted as a barrier to upward migration of hydrocarbons generated in underlying strata.

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