Emily L Stoudt¹
(1) Texaco North American Producing, Midland, TX

Abstract: Summery of ancient carbonate slope outcrops - comparison to modern slope deposits and potential as hydrocarbon reservoirs

Deep-water slope deposits have developed in conjunction with rimmed carbonate shelves in many ocean basins and geologic time periods. Subsequent uplifts of some localities have produced “world class” outcrops that serve as ancient analogs for both modern carbonate slopes and buried hydrocarbon reservoirs. Classic examples include the Devonian of the Canning Basin, Australia, the Guadalupian (upper Permian) of the Guadalupe Mountains (west Texas and New Mexico), the Jurassic of the Dolomites (northeastern Italy), the Cretaceous of the Vercors (southeastern France) and the upper Miocene of southeastern Spain.

Foreslope carbonate deposits include micritic mudstone-packstone lithologies deposited as turbidites and debris flows, mud-lean packstone and grainstone lithologies deposited as grain flows, and cobble to massive blocks in either a micritic or grain-rich matrix that formed as slumps or debris flows. Grain flows seem to form most commonly during sea-level highstands, when the platform top contains uncemented particles that can be swept over the side by currents or tectonic events. Slumping of large clasts is most common during sea-level lowstands, when cemented material can break off the exposed platform edge. The angle of repose (up to 45 degrees) in mixtures of large carbonate blocks and smaller debris is much higher than the angles of repose in clastic units.

Foreslope carbonate deposits can form hydrocarbon reservoirs (examples include parts of the Cretaceous Golden Lane in Mexico, and Wolfcampian and Leonardian units in the U. S. Permian Basin). However, in addition to porosity, slope carbonate reservoirs require both vertical and lateral seals for separation from updip porous carbonates.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana