ABSTRACT: Heterogeneity of Holocene Clinothems: Significance for the Exploitation of Ancient Slope Deposits

G. Michael Grammer, R. N. Ginsburg, D. F. McNeill

Inclined margin deposits (clinothems) of ancient carbonate platforms and reef complexes are known to be important hydrocarbon reservoirs and may function as conduits for the migration of oil from basin to reef. Recent work in the Tongue of the Ocean (TOTO), Bahamas, provides new insight into the depositional anatomy of Holocene clinothems.

Observations from 94 dives in a research submersible in the TOTO indicates that modern carbonate slopes of 30-40° are forming by the synsedimentary amalgamation of linear trains of sediment and talus blocks derived from the margin of the platform. Sand-sized sediment is transported downslope by various gravity induced mechanisms and deposited in topographic lows between linear ridges where it is rapidly lithified by submarine cementation. These sediment lobes are decimeters thick, and extend across and down the slope for a few meters and tens of meters, respectively. Repeated episodes of deposition and cementation result in the amalgamation of individual sediment packages that are linear in shape and discontinuous in both lateral and vertical dimensions along the slope. Progressi e slope accretion of this nature will result in a heterogeneous texture and composition, further complicated by a complex internal facies geometry.

Awareness of the internal heterogeneity of modern slope deposits may be a critical factor when evaluating the potential of ancient slope deposits to act as conduits for the migration of hydrocarbons or to be economically viable reservoirs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990