19 Evolution of the Atlantic and Gulf Coasts from Rift Margin to Passive Margin

Sunday, 14 March 2010
1:30 PM - 4:15 PM
Liberty A

HYDROCARBON POTENTIAL OF THE MESOZOIC SYNRIFT AND POST-RIFT OF THE BAHAMAS

Paper number: 19-4
Program: Theme Sessions
Time: 2:15 PM – 2:35 PM

Authors

Abstract

The evolution of the Bahamas area started with the breakup of the supercontinent Pangea, which started during the Late Triassic and continued into the Jurassic. In the initial stages of the breakup, a rift valley system developed along what is now the east coast of North America and through the Gulf of Mexico. Rifting along the separation zone between North America and Africa occurred during the Early to Middle Jurassic in the area that is now the Bahamas. Rifting and block faulting produced marginal basins that were filled with arkosic volcaniclastics and continental redbeds made up of alluvial fan deposits and lacustrine clastic sediments. The Great Isaac well #1 well, on the northern edge of the Great Bahama Bank, penetrated these red-bed deposits.

With continued rifting, marine water came into the region during the Early to Middle Jurassic and shallow water carbonates and evaporites were deposited over the Bahamas area. This carbonate-evaporite system was an extension of the carbonate-evaporite system that formed off the east coast of North America due to the opening up of the Atlantic Ocean. By the Late Jurassic and into the Early Cretaceous, ocean crust formed under the Bahamas as a result of continued continental drifting. Continued drifting also opened up the Caribbean seaway, which connected the Pacific and newly forming Atlantic Oceans. An extensive carbonate-evaporite megabank formed on a passive margin that covered the Bahamas and South Florida areas during the Upper Jurassic and Lower Cretaceous.

The southern Bahamas was identified as the area that probably had the greatest amount of mature source rock. Thermal maturation (Lopatin) profiles for the Cay Sal well indicated that the Jurassic and Lower Cretaceous were within the oil generation window and the Upper Cretaceous was immature. Dolostone, with estimated porosities of between 10% and 18% were thought to be likely reservoirs. Secondary reservoir objectives were skeletal, oolitic and reefal limestones. Evaporites, mainly in the form of anhydrites, and nonporous carbonates were identified as probable seals.