AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Deep-Water Resedimented Carbonate Exploration Play Types: Controls and Models
(1) Shell Petroleum & Development of Nigeria, Lagos, Nigeria.
(2) Royal Holloway University of London, Egham, Surrey, United Kingdom.
(3) Shell International EP, Houston, TX.
(4) Sarawak Shell, Kuala Lumpur, Malaysia.
(5) Chevron Energy Technology Company, San Ramon, CA.
(6) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.
Deep water, resedimented deposits have been recognized and described in modern and ancient carbonate sequences, many with good reservoir potential, for example the giant Cretaceous Poza Rica field in Mexico (~40 MMBoe), the Mississippian Tangiz field in Kazakhstan, and several fields in the U.S. Permian basin (several Tcf gas). Nevertheless, carbonate slope and basin systems remain poorly understood when compared to their siliciclastic counterparts.
Re-appraisal of legacy published and unpublished work, combined with new work compiling a global database of surface and sub-surface examples of resedimented carbonates, has highlighted that downslope resedimentation of carbonate material is in large part controlled by the evolution of the parent platform margin, which in turn is best characterized in terms of various controlling processes such as carbonate factory type, tectonic setting, eustatic variations, and platform alignment relative to prevailing wind direction and ocean current patterns. Two generic play types are recognized: (i) attached carbonate slope play -developed immediately adjacent to the carbonate platform and dominated by rock fall and platform collapse deposits or in situ boundstone; and (ii) detached carbonate slope play - deposited further from the platform margin via channelized turbidity currents and other mass-flow processes.
High-rising, steep, bypass platform margins with scalloped reentrants and grain-dominated factories have the highest potential to generate channelized and detached deep-water reservoirs with high initial porosity and permeability. Best potential reservoirs are aragonitic grainstones transported from the platform into the adjacent basin, and then subjected to submarine dissolution and early formation of secondary porosity to further enhance reservoir properties.
Any exploration model for identifying potential resedimented carbonate plays should be based on carbonate platform configurations and factory types favorable for re-sedimentation of large sedimentary bodies and preservation or enhancement of high original porosity. Using these proposed conceptual models in combination with global paleogeographic and paleotectonic maps, the explorer may be able to develop predictions for the likely age and location of resedimented carbonate plays with the greatest potential for further evaluation.