EICHHUBL, PETER, and GARY GREENE
Monterey Bay Aquarium Research Institute, Moss Landing, CA

Abstract: Structural Control on Hydrocarbon and Aqueous Fluid Discharge in the Santa Barbara Coastal Region

Compositional analyses of produced offshore Monterey Formation water and of onshore hot spring water reveal two distinct hydrodynamic systems along the northern flank of the Santa Barbara basin. The elemental and isotopic composition of Monterey Formation water is that of diagenetically altered Tertiary connate water, whereas onshore hot spring water is meteoric with an inferred circulation depths of 3-4 km. Near Gaviota W of Santa Barbara, these two distinct fluids are found within a horizontal distance of 15 km without evidence of fluid mixing. We infer that faults striking parallel to the basin margin separate these two fluid regimes by acting either as barriers or as preferred conduits deflecting formation-parallel flow.

Extensive bathymetric and sidescan mapping of the Santa Barbara Basin, followed by ROV ground-truthing, shallow sediment coring, rock sampling, and shallow pore water sampling were performed to identify possible locations of offshore fluid seepage and to compare location and composition of seeps in light of their position to structure. In addition to extensive hydrocarbon seepage south of Goleta, widespread hydrocarbon seeps along the northern shelf appear to be distributed along anticlinal trends. Hydrocarbon seepage was typically observed as methane release during push coring, as bacterial mats, and as tar extrusions. Based on the alignment of canyons, carbonate deposits and bitumen along fault traces, we speculate that preferred fluid flow along faults is responsible for canyon formation.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California