Abstract: Cool-Water Carbonates: Exploration Model for Paleozoic Limestones

Thomas D. Boreen

Cenozoic cool-water carbonates of Australia's southern coastline and continental shelf provide an important actualistic analogue for interpretation of hydrocarbon-bearing Paleozoic limestones. Defining the controls on sediment distribution on the modern shelf, and mapping of lateral and vertical facies transitions in outcropping Tertiary successions gives a high resolution 3-D perspective of sediment packaging on reservoir-prone, grainstone-dominated bioclastic ramps. The basic unit of deposition in cool-water carbonate systems is the metre-scale subtidal cycle. Shallow to mid-shelf deposits are hardground-capped, cross-bedded and bioturbated grainstone cycles and coarsening-upward tempestite cycles formed by eustatically-driven oscillations in critical depth interfacies (abrasion wav base and swell wavebase). Deep shelf deposits are rhythmically-bedded bryozoan marls and bioherms developed through allocyclically-driven productivity and terrigenous dilution responses. Cycles and rhythmic bedding record incompletely-preserved high order eustatic rhythms and are organized into larger scale, 3rd order sequence. Long term accumulation rates are an order of magnitude less than maximum Holocene rates and reflect cumulative non-deposition, and a strong subsidence and accommodation-space control. Sequence-bounding surfaces are complex, multigeneration, amalgamated 4th and 5th order marine hardgrounds, overprinted by meteoric diagenesis, that transform offshore into multiple subtidal paracycle surfaces (and conformable contracts) bounding lowstand wedge deposits.

Long term changes in relative sea level (accommodation space) on cool-water carbonate ramp are reflected in distinctive allostratigraphic packaging. Highstand deposits are highly progradational, wedge-shaped, upward-shoaling, grainstone-dominated successions. Trangressive deposits are thick, wackestone-dominated units associated with backstepping of grainstone facies and abrupt deepening above a sub-horizontal flooding surfaces. Lowstand events may be depositional or erosional, and result in basinward shift in facies belts, bioclastic smothering of deep water mounds, and depositional of lowstand wedge deposits during "forced regressions." There is potential for significant carbonate production and shelf margin progradation through all stages of the eustatic cycle. Continuous aphotic d position and gentle gradients on carbonate ramps contribute to a highly variable depositional signal at both the parasequence and sequence level dependent upon: (1) the rate of relative sea level change, (2) the magnitude of the sea level change, (3) the duration of the event, and (4) the gradient of the shelf. Modern high energy, cool-water shelves are currently accumulating several types of coarse bioclastic sandbodies that offer important information for exploration and development of Paleozoic carbonate reservoirs.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994