--> ABSTRACT: The Record of Sea-Level Controlled Sedimentation in Lithology, Logs and Seismic Data, Late Cenozoic, Great Bahama Bank, by Gregor P. Eberli, E. Robert Warzeski, Robert N. Ginsburg, Flavio S. Anselmetti, Jeroen A. M. Kenter; #91020 (1995).
[First Hit]

Datapages, Inc.Print this page

The Record of Sea-Level Controlled Sedimentation in Lithology, Logs and Previous HitSeismicNext Hit Data, Late Cenozoic, Great Bahama Bank

Gregor P. Eberli, E. Robert Warzeski, Robert N. Ginsburg, Flavio S. Anselmetti, Jeroen A. M. Kenter

The record of several hierarchies of sea-level fluctuations is identified in the lithology and log signature of two core borings (Unda and Clino) on western Great Bahama Bank. The data shows that sea-level controlled changes in sediment composition and early diagenesis determine the petrophysical behaviour of the rocks within the Previous HitsequenceNext Hit stratigraphic framework. This calibration helps to extract information about facies and diagenesis variations, and fluid pathways from geophysical data sets.

Along the prograding western margin of Great Bahama Bank 3rd-order sea-level fluctuations produce a pulsed progradation that is recognized on Previous HitseismicNext Hit data as a bundle of Previous HitseismicNext Hit sequences. In the cores these sequences are recognized as depositional successions and their Previous HitboundariesNext Hit are indicated by changes in facies and diagenetic overprint. On the slopes the Previous HitsequenceNext Hit Previous HitboundariesNext Hit are marked by major discontinuity surfaces all of which are discernible on the logs. Two surfaces are formed by hardgrounds and overlain by sand-sized mixtures blackened lithoclasts, planctonic foraminifera and minor amounts of platform derived grains. Increased diagenesis at and below the hardgrounds, combined with the change of facies, produces the necessary impedance contrasts for imaging these Previous HitboundariesNext Hit as Previous HitseismicNext Hit reflectors.

Higher-order sea level changes are recorded in the rocks and in the logs but only partially on the Previous HitseismicNext Hit section. On the platform top, these changes are recorded in shallowing-upward cycles bounded by exposure horizons. On the slopes, higher-order sea level drops are recognized by the occurrence of intervals of coarser-grained sediments in the peri-platform ooze. The change in sedimentation rate and hydrology during these intervals results in firmgrounds with increased cementation and velocity but reduced porosity and permeability. These intervals are well recognized as sharp peaks on the y-ray and velocity logs. The low permeability on top of these intervals is likely to separate the fluid flow on several levels within each Previous HitsequenceNext Hit and influence both late diagenesis and hydrocarbo migration. The next higher order of cyclicity is represented by alternations (0.3 - 1m) of coarser and finer grained beds within the coarse-grained intervals. Because of their small-scale and low contrast in rock properties, these high-frequency cycles are not recorded in the logs.

In summary, due interlayering of intervals of different sediment composition and diagenesis, the log properties do not follow any simple downhole pattern of increasing velocity and density, or decreasing porosity. These variations are sea-level controlled and, consequently, occur within a Previous HitsequenceNext Hit stratigraphic framework. Thus, Previous HitsequenceNext Hit analyses of Previous HitseismicTop sections and logs can help predict heterogeneities within platform margin reservoirs.

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