Introduction

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. As of this writing, the crater rim hosts gas production of 30 Mmcf/d from the Slave Point Formation, but a second gas plant is expected to begin production during 2000. Seasonal petroleum production of ∼1000 BOPD occurs from the Keg River Formation. Approximately one dozen Slave Point gas wells have been drilled with sandface AOF potential up to 95 Mmcf/d, and exploration is continuing around the crater rim. Initial gas production was on the regionally high side, the northeastern rim of the crater. Reserves of ∼70 bcf have been established.

IMPACT ORIGIN

Although Steen River was discovered more than thirty years ago with documented evidence of shock metamorphism 1,2, little has been published about it in the open literature, and its classification as an impact structure has been challenged from time to time. Shock metamorphism was first reported from well 12-19-121-21W5M. An examination of chips recovered from the 3–12 well located ∼3 km SSW of the 12–19 well also reveals abundant evidence of shock metamorphism, and altered and recrystallized melts in a unit once logged as volcanics that immediately underlies the Cretaceous cover. Well 12–19 penetrated impact lithologies immediately below the Cretaceous cover at a depth of 184 m. An ∼120 m thick unit of clay-altered, vesicular rock was recorded on top of the unequivocal crystalline basement rocks 1, establishing a minimum structural uplift of 1000 to 1100 m relative to the surrounding basement surface. The upper unit could be a remnant of crater floor breccias like that found in the 3–12 well rather than representing a dyke contained within the central uplift.

GEOPHYSICAL DATA SETS, WELL CONTROL, AND POTENTIAL FIELD MODELLING

Hydrocarbon exploration companies have acquired more than one hundred and seventy 2-D seismic reflection profiles over the impact structure, and one 3-D seismic survey has been executed over the northwest corner of the crater rim. Approximately sixty 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. Reflection 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. In 1995 an aeromagnetic survey with a maximum of 0.5 km line spacing was flown across the entire structure, revealing large-amplitude central and concentric anomalies.

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