--> Abstract: Facies Partitioning at the High-Frequency Sequence Scale in Carbonate Ramp Systems, by C. Kerans and L. C. Zahm; #90928 (1999).

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KERANS, CHARLES1 and LAURA C. ZAHM2
1 Bureau of Economic Geology
2 The University of Texas at Austin

Abstract: Facies Partitioning at the High-Frequency Sequence Scale in Carbonate Ramp Systems

One of the goals of high-frequency sequence stratigraphy is the ability to predict reservoir and seal facies distribution at the exploration scale and the geometry and proportion of high and low permeability facies for reservoir characterization. Detailed high-frequency sequence frameworks have demonstrated that changes from transgressive systems tract to highstand systems tract deposition in carbonate systems is not simply a record of landward stepping versus seaward stepping sets of constant environmental belts (i.e. facies tracts). Instead, predictable changes in the relative proportion of key indicator facies, known as facies partitioning, can be defined. The fundamental principle behind facies partitioning is that within a constant water-depth/energy-defined facies tract, the development and preservation of depositional facies will change in response to changing accommodation. From these relationships it is possible to predict changes in facies partitioning from accommodation as given by the sequence/systems tract framework.

The Cretaceous Devils River (Albian) carbonates of the Pecos River Canyon and along Interstate 10, west Texas, and the Permian San Andres (Guadalupian) ramp of the Guadalupe Mountains, New Mexico, illustrate varying degrees of facies partitioning which can be used to illustrate this phenomenon in carbonate ramp systems. The Albian carbonate ramp strata of the Pecos/I-10 area display marked biological and physical sedimentologic partitioning between mud-dominated TST and grain-dominated HST cycles. A complete facies substitution from toucasid-miliolid-chondrodont-radiolitid TST faunas to caprinid-coral HST faunas occurs within the ramp-crest (Figure 1). HST grainstones have continuities of 1-10 miles in dip dimension, with permeability dependent on quantity of interparticle cement. TST grain-dominated facies are isolated to thin sheets and discrete rudist buildups with dip dimensions of a few hundred feet, separated by chert-rich intermound facies. This biologic partitioning is driven by a response of the community structure to changes in substrate, light penetration, oxygen supply, and nutrient level which are in turn driven by changing energy regime/preservation settings.

Data from a detailed study of the ramp-crest facies tract of the San Andres Formation illustrate significant changes in facies proportion while maintaining a relatively constant suite of facies. Accretion-stratified and trough crossbedded grainstone facies of tidal inlet and upper shoreface origin comprise 5 percent of TST cycles as opposed to 38 percent of HST cycles (Figure 2). In addition, the dimension of HST grainstones is an order of magnitude greater in terms of dip-width, in contrast to the characteristically thin sheets of TST grainstone. A direct impact of this change in grainstone facies partitioning is a significantly higher fluid-flow rate in HST cycles in the ramp crest. Once these facies partitioning relationships have been developed through rigorous outcrop/subsurface models, they can be used to predict position within sequence framework, styles of stacking patterns and shingled versus mounded stratal geometry.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas