The Record of Sea-Level Controlled Sedimentation in Lithology, Logs and Seismic 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 sequence 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 seismic data as
a bundle of seismic sequences. In the cores these sequences are recognized as
depositional successions and their boundaries are indicated by changes in facies
and diagenetic overprint. On the slopes the sequence boundaries 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
boundaries as seismic reflectors.
Higher-order sea level changes are recorded in the rocks and in the logs but
only partially on the seismic 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 sequence 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 sequence stratigraphic
framework. Thus, sequence analyses of seismic 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