Fluid Flow in the Bahamian Platform: Evidence From the Subsurface Great Bahama Bank

Leslie A. Melim, Peter K. Swart, Gregor P. Eberli

Although fluid flow has long been assumed to be common in the near-surface portions of exposed carbonate platforms, many workers are finding evidence for fluid flow continuing into the shallow to deep burial environment. Permeability data and diagenetic patterns from two cores on the western margin of Great Bahama Bank provide evidence for extensive fluid movement during shallow burial (50-500 m) as well as some clues on the direction of fluid flow.

Three diagenetic zones have been identified in these cores: Zone I (Plio-Pleistocene reef to platform facies) characterized by extensive meteoric diagenesis including caliche horizons and abundant cavernous porosity; Zone II (Miocene reef and platform facies) with early meteoric diagenesis overprinted by nearly complete dolomitization; and Zone III (Miocene to Pliocene deeper water facies) characterized by marine burial diagenesis with dissolution, recrystallization and localized dolomitization in Unda above impermeable hardground horizons.

Diagenetic zone I has highly variable permeability (<10-7000 md) but the true permeability follows the cavernous porosity and is unmeasureable using core. Diagenetic zone II is generally permeable (>1000 md) with several low permeability zones (<10 md). The low permeability zones are usually preserved limestones with early cementation. Permeability in diagenetic zone III depends on lithology. Peloidal sediments have consistently low permeability, generally <5 md. In contrast, the sucrosic dolomite in the proximal slope facies of Unda has very high permeability, usually in excess of 1000 md (similar to the sucrosic dolomite in the underlying reef). Skeletal sediments have intermediate permeability that correlates to grain size: silt-sized sediments average 150 md, medium to coarse sand-sized sediments average 500 md.

The diagenetic results combined with the permeability data document at least three types of fluid flow events. These include: 1) meteoric fluids entering the platform during sea level lowstands; 2) a burial fluid, approaching sea water in composition, that was under saturated with respect to aragonite and lead to extensive dissolution of skeletal and peloidal grains; and 3) a dolomitizing fluid, also approaching sea water in composition, that was descending through the platform during burial. The distribution of these fluids was controlled by the permeability at the time of fluid migration as well as the hydraulic head.

This study has found that the subsurface of Great Bahama Bank is an active and dynamic zone with large scale fluid flow continuing to alter carbonate sediments well below the reach of meteoric fluids. At this time, it is not possible to determine the driving forces behind subsurface flow but the sediments certainly record the passage of large volumes of fluids.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995