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David J. Sivils1
(1) Texaco Exploration and Production Inc, Midland, TX
Abstract: Outcrop based analog for shallow-water, high frequency carbonate cycles from the Chesterian from Southwestern New Mexico
Chesterian strata in Southwestern New Mexico are composed of 30 cycles ranging from 1 to 10 m in thickness. These cycles are composed of a distinctive suite of eight carbonate and clastic lithofacies including: shales, siltstones, sandstones, lime mudstones, wackestones, packstones, bioclastic grainstones, and oolitic grainstones. Throughout the stratigraphic interval, these facies are ordered in a predictable coarsening-upward sequence, culminating in grainstones, or detrital clastics. Middle Carboniferous glaciation, acting in concert with regional tectonism related to the Ouachita-Marathon Orogeny, controlled fluctuations in relative sea level and cycle development. Based upon biostratigraphic control for this section these meter-scale cycles average 500 k.y. in duration, equivalent to 4th order sequences.
Carbonate grainstones, grain-rich packstones, wackestones, and sandstones serve as potential reservoir facies. Deposited as sheets, these reservoir facies are sub-parallel to the shoreline, similar to the modern grain-rich shoals forming off of the coast of Qatar. The vertical thickness of the grainstones is typically no more than 1 meter. Laterally, these facies are traceable for several km’s in both the strike and dip directions. The lime mudstones and shales are low permeability, non-reservoir facies, Instead, they act as the primary sealing unit, both vertically by variations in stacking patterns, and laterally through facies changes.
This outcrop based depositional model for the Chesterian in the Pedregosa basin has the potential to serve as an analog for modeling reservoir geometry’s of Middle to Late Paleozoic cyclic carbonate ramp reservoirs in the Rocky Mountain region, the Permian Basin, and the Mid-Continent of North America.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana