ABSTRACT

The stratigraphy, facies and porosity of the Golden Spike reef complex, an isolated buildup 575 ft (175 m) thick and less than 2.5 mi (4 km) in diameter, was studied from cores and wireline logs of 10 wells. The total build-up comprises the Late Devonian Leduc Formation (Frasnian) which is divisible informally into lower, middle, and upper Leduc. The middle and upper Leduc are each divisible into two major divisions.

Fore-reef strata consist of detrital coral and tabular stromatoporoid rocks; reef strata comprise the massive stromatoporoid facies; skeletal calcarenites represent the principal reef-flat facies while laminites, dolomitic mudstones, and Amphipora mudstones are locally present in the interior.

The principal subdivisions of the Leduc Formation represent three major depositional events: 1) basal bank or biostrome stage -- lower Leduc (Cooking Lake equivalent), 2) biohermal stage -- middle Leduc, and 3) reef-fringed bank stage -- upper Leduc. The lower Leduc represents a transgressive bank with mostly mud-supported carbonates. The base of the middle Leduc (Division 1) forms extensive boundstones and reef frameworks which grade progressively into a shallow-marine shoal phase (Division 2) with less extensive reef frameworks on the margin. In the upper Leduc (Divisions 3 and 4) a narrow discontinuous reef zone is on the margin with intertidal laminites and mudstones predominating in the interior. This sequence can be correlated with known depositional events at Redwater and other Leduc-age reefs.

Porosity distribution is facies-dependent and related to depositional conditions for each stage of development. Highest porosity and lowest irreducible water saturation occur in grain-supported subtidal massive stromatoporoid and skeletal calcarenite rocks. Nearly all the pore space in these rocks is available for hydrocarbon saturation, and high production can be maintained with little water. Rocks lacking interparticle porosity, including intertidal and supratidal carbonates in the interior and fore-reef coral and tabular stromatoporoid carbonates, have low effective porosity and high irreducible water saturation, and are generally poor reservoir rocks. The best porosity is developed near the margin during mildly transgressive stages of reef-complex development (Divisions 2 though 4).