Early Grain-Coat Formation in Chaco Dune Field, New Mexico: Insight Into Formation Mechanisms, Distribution, and Implications for Predictive Modeling to Assist in Deep Play Identification

W. L. Esch¹, J. M. Ajdukiewicz², and A. C. Reynolds^3
1New Play Concepts, ExxonMobil Upstream Research Company, Houston, TX
²Formation Evaluation, ExxonMobil Exploration Company, Houston, TX
³Department of Geosciences, University of Arizona, Tucson, AZ

Clay grain-coats preserve favorable reservoir quality in deeply buried eolian sandstones by inhibiting quartz cement. Reported mechanisms for clay-coat formation include:
1. Translocation of airfall/rainout dust into eolian sands.
2. Inheritance of coated sand grains from ephemeral fluvial systems.

Sparse information exists regarding where the coating processes are most effective, the distribution of clay-coated sands in fluvial-eolian environments, and survivability of coats during eolian transport. Such limited knowledge makes pre-drill predictions of coated grains difficult. Consequently, we are conducting field studies in modern eolian environments focused on the genetics and distribution of clay-coated sands. This work will form the basis for predicting the presence, continuity, and composition of clay grain-coats in ancient eolian strata that are potential reservoir targets.

The Chaco dune field, northwest New Mexico, was selected as one of several investigation sites. A transect from the Chaco River to the northeast was sampled that includes sand sheets, barchanoid dunes, parabolic dunes, ephemeral fluvial channels, and Cretaceous age bedrock. Consistent with literature, clay-coated grains are found in Chaco River sediments. These fluvially coated grains appear-to-be transported by wind into the adjacent dune field from the river bed and lose their coats by abrasion. Clay coats are subsequently regenerated in nascent soils by translocation of dust into vegetation-stabilized dunes and sand sheets.

Climate is a key control on vegetation which promotes stabilization of the sands and coat regeneration. Climate cyclicity also is important. Conditions must periodically be dry enough to allow eolian transport, but wet enough at intervening times for stabilization and translocation. The results are being incorporated into a probabilistic predictive model.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas