Stratigraphic Controls from a "Basin-Centered" Gas Accumulation, Alberta Basin

Tim McCullagh and Bruce Hart
Earth and Planetary Sciences, McGill University, Montreal, QC, Canada

We integrated core, wireline log, 3-D seismic and production data to examine the controls on gas production from the Lower Cretaceous Cadotte Member in the Deep Basin of Alberta, considered by some to be the “type area” for basin-centered gas accumulations. The Cadotte was deposited as part of a prograding wave-dominated strandplain. Cadotte shoreface and foreshore deposits can be either chert-rich or quartzose. Chert-rich units lack quartz overgrowths, have relatively high permeability and can produce gas, whereas the overgrowths are well developed in the quartzose units which are tight. A sonic and density porosity log crossover technique can be used to distinguish quartz-rich from chert-rich facies. The physical properties of the two lithologies are sufficiently different that they can be distinguished seismically. A map of seismic amplitudes for the Cadotte shows curvilinear high-amplitude anomalies up to a few 100 m wide and several km long, that are parallel to depositional strike. The amplitude anomalies represent stratigraphic compartments in the Cadotte shoreface that are similar in scale to beach ridges of the modern Nayarit coast of Mexico. Seismic, log and pressure data indicate that shale-filled channels incising through the shoreface can compartmentalize the reservoir. Like other basin-centered gas accumulations, the Cadotte produces gas downdip from areas that are wet. The contact between these two phases cuts through our study area. Our work suggests that the location of this contact is controlled by the interplay of regional structure and depositional processes that control the mineralogy, permeability and stratigraphic architecture of the Cadotte.

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