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