Results of Exploration in Mesozoic Shelf-Edge Carbonates, Baltimore Canyon Basin

John F. Karlo

The Late Jurassic-Early Cretaceous carbonate shelf margins of the Baltimore Canyon basin were drilled in three exploration wells by Shell Offshore Inc. and partners. Drilled during 1983-1984, the wells were in record-setting water depths (maximum of 2,119 m). Pre-drilling seismic-stratigraphic evaluation revealed four main stages in the evolution of the paleoshelf margins with four corresponding trends of carbonate objectives: (1) Oxfordian-early Kimmeridgian calciclastic(?) shelf progradation with growth fault related rollover anticlines; (2) late Kimmeridgian-Berriasian shelf aggradation culminating in pinnacle reefs at the shelf edge; (3) Valanginian drowning of the shelf with, as part of the transgressive interval, erosionally sculpted plateaus with mesa-like shapes; 4) Neocomian deltaic(?), clastic shelf progradation culminating in carbonate shoals.

The most updip well, OCS-A 0336-1, tested two structures. It penetrated thick oolitic grainstones of an Aptian bar complex and, deeper in section, a rollover anticline of lower Kimmeridgian, lime mudstone-dominated, shelf-interior facies. Intercalated sandstones in the lower structure gave minor gas shows, which were the only hydrocarbons found in the three wells. The OCS-A 0337-1 well, 5.5 km farther downdip, tested a mesa, penetrating sponge and tubiphytes-rich chalky (allodapic?) wackestone. Drilling below the mesa penetrated upper Kimmeridgian-Berriasian back-reef and lower Kimmeridgian foreslope deposits. The most downdip well, OCS-A 0317-1, 1 km seaward in strike projection, tested a pinnacle reef. The well was sited on the back side of the reef, 49 m below the reef crest. It encountered a cap of sponge and tubiphytes-rich limestone overlying hexacoral and stromatoporoid skeletal grainstones, and boundstones.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.