Abstract: Impact of Offshore Exploratory Drilling: Is It Significant? You Be the Judge

E. A. Shinn, B. H. Lidz, C. D. Reich

Twelve exploratory well sites in the eastern Gulf of Mexico were examined using scuba and two research submersibles. When examined, the period of time since cessation of drilling ranged from 15 months to 28 yr. Sites studied using scuba ranged from 5 to 25 m deep and environments included coral reefs, grass beds, and rippled sand bottoms. Biological assessment methods were used to evaluate impacts of drilling. Deeper sites examined using research submersibles ranged from 21 to 150 m of water and environments included tropical hard bottoms, coarse sand (both quartz and carbonate), mud, and pinnacle reefs. This study focused on quantitative measurement and distribution of drill cuttings, barium, and trace metals.

The areas impacted ranged from a few square meters (1/200th of an ac) to 13,352 m² (more than 3 ac). Recovery to predrilling conditions was essentially complete at several sites, but some, especially the deep-water sites, will require many years. Recovery, defined as burial, overgrowth, or removal of cuttings and barium by storm-generated waves and currents, is controlled primarily by water depth, temperature, and water clarity. Shallow-water sites (<40 m) return to predrilling conditions much more quickly (5-15 yr) than do deeper sites, where water temperature is low, water and sediment movement is reduced, and the growth rate of encrusting organisms is slow. Used welding rods, the most common and useful indicator of offshore drilling activity, have a sea-floor life of etween 20 and 30 yr.

Fish abundance and diversity are invariably enhanced at sites having the most debris or structures. Conversely, resident fish were absent at a 17-yr-old site, where the borehole and other debris were obscured by sediment.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994