Controls on Ramp-to-Rimmed Shelf/Slope Transition and Implications for Slope Sedimentation and Reservoir Development

The transition from low-angle carbonate ramps to steep reef-rimmed debris-shedding platform margins is a predictable evolutionary path recorded in 2nd-order supersequence stacking patterns. While widely documented in platforms ranging from Proterozoic to Tertiary, the driving mechanisms for this transition and impacts on linked slope deposition have yet to be fully integrated. Ramp slopes are mud-dominated and largely prospective as unconventional targets. Transitional margins begin to exhibit organized architecture and feed channelized grain-rich sediment-gravity flows hosting moderate-sized toe-of-slope and basin-floor-fan reservoirs. True reef-rimmed margins and associated debris aprons provide abundant reservoir capacity, but do not occur with the necessary trap/seal, such as in the Permian Basin, to foster hydrocarbon accumulation. Fundamental controlling parameters for the ramp-to-rim transition include (1) position of precursor margins/paleotopographic breaks, (2) accommodation-controlled stacking patterns/P/A ratios, and (3) the positive feedback loop between sediment bypass, early lithification, and organism colonization .

In order to evaluate the role of these controls on reservoir development in the ramp-to-rim transition, data from Lower-Middle Permian platforms of the Guadalupe and Apache Mountains is synthesized. An evolution is observed from (1) low-angle (0.5-2o) ramps dominated by mud-rich hemipelagites, to (2) steep (2-8o) ramps with gullied slopes , mound colonization, and mud-rich debris flows, to (3) transitional ramp/rim profiles with 8-20o slopes, detachment/slump surfaces, channelization, and mud- and grain-dominated debris flows and turbidites, and finally (4) true reef-rimmed margins with 30-50o dipping debris-dominated aprons.

A critical slope angle for the transition from slump/soft sediment dominated sediment gravity flows to debris flows and mass wasting varies from platform to platform depending on the composition of slope sediments and P/A ratios . However, a gradient range of 8-15o appears to be a minimum threshold for significant coarser sediment gravity flow deposition and carbonate reservoir development in the Deepwater.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California