ABSTRACT

Analysis of well and seismic data from the continental slope and shelf in the Baltimore Canyon Trough allows for comparison of hydrocarbon potential in high-energy shelf-margin carbonate and time-equivalent updip hydrocarbon-bearing siliciclastic trends. Structural closures with reservoir and top seal are present in both updip and downdip trends. The absence of significant hydrocarbon shows in downdip carbonate reservoirs suggests insufficient hydrocarbon charge of objectives located at the shelf-margin.

INTRODUCTION

Exploration in the U.S. western North Atlantic began in 1975 when the COST (Continental Offshore Stratigraphic Test) B-2 well was drilled in the Baltimore Canyon Trough. Since then, 52 wells have been drilled in the U.S. Atlantic offshore. The majority of this exploration activity has been focused on the Baltimore Canyon Trough where 32 of the 52 exploration wells are located.

Exploration of the Baltimore Canyon Trough took place along two subsurface trends, 1) the Wilmington Platform, and 2) an interior shelf structural trend. The Wilmington Platform is the Baltimore Canyon portion of the great Mesozoic carbonate shelf edge which represents the maximum seaward extent of prograded reef carbonates along the Atlantic continental shelf (1, 2, 3). This shelf margin extends for 400 miles from the New England Arch southwestward to the Norfolk/Hatteras Arch. The Wilmington Platform is characterized by southwestward trending down to the basin normal faults. Rotation of Upper Jurassic and Lower Cretaceous rocks into the down-thrown sides of these faults provide the critical west dip component that produces large structural closures beneath the present day continental slope. Shell Offshore Inc. and partners (SOI et al) drilled four exploration wells on the continental slope in water depths greater than 6000' along the southern part of the Wilmington Platform in the Wilmington Canyon Area. These wells tested large structural closures at the culmination of the Mesozoic carbonate shelf margin.

The interior shelf structural trend lies beneath the continental shelf in shallow water (<600') 20 miles landward of the continental slope and updip of the shelf margin trend. This trend is characterized by small throw(<1,000 feet) down to the basin normal faults, associated antithetic faults, down-thrown simple closures, fault closures, and one extremely large dome (The Great Stone Dome 4, 5). The faults and associated structural closures in the interior shelf parallel the southwestward trend of Lower Cretaceous shelf edge for 100 miles. Industry drilled 28 wells in the interior shelf to test in these structural closures (sandstone reservoirs). Hydrocarbon shows have been reported from some of the 28 wells drilled on the shelf-including gas and gas condensate.

This paper offers a comparison of hydrocarbon habitats interpretated from examination and comparison of wells drilled on the continental slope to wells drilled on the continental shelf. Particular emphasis is given to the relationships of structure, reservoir, and top seal, through interpretation of stratigraphic relationships and depositional environments of the Upper Jurassic and Lower Cretaceous in the Baltimore Canyon Trough.

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