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Abstract: Hydrocarbon Potential of the Steen River Impact Structure, Alberta, Canada

HILDEBRAND, ALAN R., MARK PILKINGTON, RICHARD A.F. GRIEVE, Geological Survey of Canada; ROBERT R. STEWART and MICHAEL MAZUR, University of Calgary; DON HLADIUK and DEAN SINNOTT, Gulf Canada Resources Ltd.

The ~25 km-diameter Steen River impact structure, (59° 30'N, 117° 38'W) is the remnant of the largest known impact crater in the Western Canadian Sedimentary Basin (WCSB). The eroded crater lies buried under ~200 m of cover with no surface expression necessitating geophysical and drilling projects for its exploration. In this area the WCSB is composed of ~1 km-thick gently SW-dipping strata. The terrain is predominantly poorly drained taiga, necessitating winter operation for most exploration and production activities. The crater rim hosts seasonal petroleum production of ~600 BOPD from the Keg River Formation and shut-in Slave Point gas wells. Gas production infrastructure is approaching this area of northern Alberta stimulating continued searches for hydrocarbon reservoirs in the impact structure Although Steen River was discovered more than thirty years ago with documented evidence of shock metamorphism (e.g., 1,2), little has been published about it in the open literature. A re-examination of recovered drill core from the central uplift confirms the presence of abundant evidence of shock metamorphism.

Hydrocarbon exploration companies have acquired more than one hundred 2-D seismic reflection profiles over the impact structure and one 3-D seismic survey has been executed over the northwest comer of the crater rim. The inferred crater margin is quite irregular on a km scale and broadly polygonal in plan, relatively square comers occur in the northeast and northwest. Approximately forty wells have been drilled in and near the crater, providing generally good control for the coherent seismic data. All known hydrocarbon reservoirs occur in structural closures formed by the rim deformation. Industry seismic data outline parts of the rim uplift in some detail, but most profiles record only chaotic reflectors interior to this. Mapping the impact structure's interior structures has been attempted with magnetic- and gravity-field surveys. An aeromagnetic survey with 0.5 km line spacing has recently been flown across the entire structure (Fig. 1), revealing large-amplitude central and concentric anomalies. A pilot gravity survey revealed associated anomalies with a maximum value of ~3 mGals, slightly smaller than that expected for a crater of this size (3). A positive anomaly of up to ~0.5 mGal was found associated with the rim uplift. A total of ~2,000 gravity stations have now been acquired over the crater. Interpretation of the gravity data is complicated by the high regional gradients (17 mGal decreasing northwestwards across the impact structure), with superimposed regional anomalies of 10 to 20 km scale.

The rim uplift, down-slumped blocks and the central uplift are well to poorly delineated by available seismic, well and potential-field data. Well 16-19 at ~9 km radius preserves the inverted stratigraphy of the overturned flap lying on the inner down-slumped blocks, and establishes a minimum structural downdrop of ~0.6 km. Reflection seismic data usually do not reveal the down-slumped blocks, but occasionally provide vague images, and a slump zone at least 3 km wide is indicated. Wells and seismic profiles reveal an irregular, faulted, crater perimeter with rim uplift of up to ~100 m. The central structural uplift has a radius of ~3 km, based on well control and magnetic-field anomalies and is located off centre. The uplift is displaced eastwards of the rim-uplift-defined centre and the magnitude of down dumping is greater on the west side, base on a limited understanding of the slumped blocks. This indicates an oblique impact from the east formed the crater. Well 12-19 apparently penetrated the central structural uplift immediately below the Cretaceous cover at a depth of 184 m, establishing a minimum structural uplift of ~1100 m relative to the surrounding basement surface. An ~120 m thick unit of clay-altered vesicular rock was recorded on top of the unequivocal crystalline basement rocks (1). This unit could be a remnant of crater floor deposits rather than representing a dyke contained within the central uplift.

Minor relief occurs on the unconformity truncating the impact lithologies, and is thought to represent both differential erosion of the crater lithologies and post-burial differential subsidence within the crater. Pre-burial erosion of up to ~1 km at the crater rim is indicated by the amount of extra stratigraphy preserved in the slumped blocks in well 16-19 and the assumption of a rim-to-crater-floor depth of 0.5 km. This erosion has presumably subdued the crater's gravity expression. Remnants of intracrater melt rocks and/or suevitic breccias likely cause the small- to large-amplitude, short-wavelength anomalies that are preserved adjacent to and detached from the central uplift.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah