ABSTRACT

The Second White Speckled Shale (SWS; Cenomanian-Turonian) of the Colorado Group is a source and reservoir rock in the Western Canada Sedimentary Basin (WCSB). A sedimentological, paleontological, and geochemical study is underway to determine the depositional environments, mineralogy, and organic geochemical characteristics, and to identify hydrocarbon source and production potential of the SWS. The SWS is distinguished from adjacent shales by mineralogy and organic matter content. Calcite content (0.5 to >30%) is dominantly bioclastic in origin, and consists primarily of fecal pellets composed of nannofossil remains. In this deep water facies, the pelloidal sands were concentrated by current activity. Dolomite is commonly present. Most of the SWS comprises mixed-layer illite/smectite (15-50%), silt- to clay-sized quartz (15-45%), kaolinite (10-20%), and pyrite (3-7%), with minor K-feldspar and plagioclase. Rock-Eval analysis indicates a variable, dominantly Type II OM content (2-12%) with HI from 200-600 mgHC/gOC.

Micropaleontological data indicate an abundant but low-diversity assemblage of planktonic foraminifers within the SWS that suggests a shallow to open marine depositional environment. Marine algal cysts are also common, indicating high organic productivity. Preservation of abundant marine organic matter and the general absence of bioturbation indicate persistent anoxic depositional conditions under a stratified water column.

SWS hydrocarbon reserves produce from fractured, dominantly calcareous, current-reworked pelloidal sands in contrast to siliclastic reservoirs of the Fish Scale Marker Bed. Extrapolation of T_max values suggests that SWS organic matter enters the oil window (T_max = 435°C) at approximately 2500 m (present burial depth). Much of the SWS in the WCSB is therefore immature but may be a source of biogenic gas in eastern Alberta and Saskatchewan.

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