ABSTRACT

The Gilwood sandstone (Devonian), a member of the Watt Mountain Shale, is one of several sandstone units which were deposited on the flank of the Peace River Arch where it now forms an excellent stratigraphic trap containing hydrocarbons. The sandstone has an average composition of 75 per cent quartz, 16 per cent microcline, 1 per cent plagioclase, 1 per cent biotite, 15 per cent cementing material and an average porosity of 10 per cent. It exhibits several varieties of sedimentary structures such as cross-bedding, both trough and tabular, ripple drift cross-laminations, graded-bedding, rip-up clasts, laminations, convolute laminations and other minor structures. Grain size distribution of the sandstone is largely unimodal, except in certain places where it is bimodal. Most samples are positively skewed and moderate to poorly sorted.

Paleocurrent analysis, texture of the sand and its geometry suggest that the Gilwood sandstone is, largely, a deltaic deposit derived from the west and modified slightly by long shore currents flowing from the north-northeast. The roundness of grains, suturing in quartz particles, high concentration of quartz and the remarkably low concentration of heavy minerals suggest that the sand is at least a second cycle deposit, indicating that the Peace River landmass had a sedimentary cover during Middle Devonian.

Depositional environments, as inferred from texture and structures of the Gilwood sandstone, are divided into three realms: fluvial, transitional and nearshore marine. Several types of sedimentary structures are characteristic of each realm.