Diagenesis and Reservoir Quality in the Montney Formation in British Columbia — A Major Siltstone Reservoir in Western Canada

Vaisblat, Noga; Harris, Nicholas B.; Zonneveld, John-Paul

Diagenetic controls on play quality in sandstone reservoirs types are well understood, and an understanding of reservoir quality in shale reservoirs is emerging. Diagenetic models for siltstone reservoirs, however, have yet been developed. Are siltstone reservoirs more similar to sandstones or shales?

The Lower Triassic Montney Formation is a westward-thickening accumulation of carbonates and fine, well-sorted silisiclastics in the West Canada Sedimentary Basin, with as much as 400 TCF of recoverable gas.

The formation is thought to record deposition in shoreface to offshore settings on the western margins of Pangea in arid conditions. The subcrop of the Montney extends from the AB - BC border to the Rocky Mountains deformation front in British Columbia.

The Montney unconformably overlies the Permian Belloy Formation, or older Carboniferous units, and underlies the Doig Formation. Analysis of this low permeability siltstone reservoir may reveal the mineralogical and diagenetic processes that affected the quality of this type of hydrocarbon accumulations.

This study is based on the interpretation of XRD analysis, SEM imaging and thin section petrography and will incorporate petrophysical analyses included porosity, permeability and mercury injection porosimetry. Our results to date indicate that primary mineralogy, whether quartz or carbonate dominant, exerts a strong control.

The porosity of the Montney formation is estimated at about 4%. Carbonate-dominant sections appear to be slightly more porous than silisiclastics-dominant sections. Diagenetic processes, as cementation and authigenic clay minerals growth clearly reduced primary porosity. Cementation and dissolution patterns have been recognized for carbonate minerals, feldspars and quartz. Quartz dissolution suggests basic conditions in the formation water, for at least a short period of time. Dissolution is rather superficial, and much of the secondary porosity seen is filled with either cements or clay (authigenic and/or detrital).

While the Montney Formation is generally thought to contain little clay, our results show that clay minerals varies from 15% in siliciclastic sections to nearly 0% in carbonate sections. Authigenic clay minerals, chiefly illite and mixed layered illite smectite bridge or fill pores. Detrital clay minerals coat grains, and at times the texture is consistent with clay infiltration.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013