47. Petrological conditions affecting porosity in granite, and negative effects of K- and Si-metasomatism on the trapping of oil in layered Precambrian quartz diorite-gabbro sills penetrated by the AOC Granite 7-32-89-10 drill hole near Fort McMurray, Alberta, Canada

The AOC Granite 7-32-89-10 hole, drilled to search for oil in the Precambrian basement below the Alberta bituminous (tar) sands near Fort McMurray, Canada, penetrates layered biotite-hypersthene quartz diorite-gabbro sills that occur between 543 and 2363.3 meters depth. Three episodes of fracturing and cataclasis affected these sills. The first allowed Kand Si-metasomatism to change many parts of the sills into myrmekite-bearing granite and granodiorite. Recrystallization, accompanying this metasomatism, eliminated any porosity. A second episode of fracturing, barren of oil and methane, allowed minor magnetite to fill tiny fractures, cutting both granite and quartz diorite. A third, Tertiary period of deformation caused faulting and fracturing of the sills to produce an anticlinalshaped cataclastic zone in which oil and methane were trapped. Repeated showings of oil and methane occur throughout the 1820-meter-thick series of sills, but insufficient for production. Lessons learned from this study suggest that the search for oil in myrmekite-bearing Precambrian granitic rocks will likely be unsuccessful unless such rocks have been fractured during a time later than any fracturing that permitted the Kand Si-metasomatism. Non-myrmekite-bearing, fractured, primary granitic rocks would not have this problem.