Alluvial Processes and Sandbody Architecture in the Raton Basin

By

Paul R. Clarke

The University of Birmingham, Birmingham, United Kingdom

Chris Cornelius

Evergreen Resources Inc, Denver, CO

Peter Turner

The University of Birmingham, Birmingham

The late Cretaceous / Tertiary coal bearing strata in the Raton Basin continues to develop into one of the Rocky Mountains’ principal CBM plays. Current production is primarily from the laterally extensive delta plain coals of the Cretaceous Vermejo Formation. The overlying humid alluvial plain deposits of the Raton Formation locally contain thick coals, although seam geometries are commonly discontinuous (<1000 ft) reflecting the strong influence of alluvial processes, with peat accumulation restricted within poorly drained interfluves of a broad fine grained high sinuosity meandering fluvial system.

Throughout the Raton Formation, channel sandbody architecture markedly affects coal seam geometry and distribution. Sand bodies are dominated by two principal channel types which frequently bear intimate relationships with each other within a regional east to north-east paleodrainage system. Point bars are characterized by well-developed lateral accretion surfaces. Typically these are 2-5m thick, heterolithic with variable sand/mud ratios and relatively steep depositional dips of up the 32^o. There are complex lateral changes and a characteristic feature is the collapse of cutbanks during the initiation of point bar development followed by the development of relatively low angled accretion surfaces as the channel migration stabilizes. The second main channel sandbodies are avulsion related and typically show marked erosional contacts with the underlying point bars. Channel dimensions are variable but in general, the avulsion related sandbodies are much larger, locally reaching 500m wide and up to 25m thick (width/depth ratios of 20). These deposits comprise fine to coarse grained clean sandstones that are generally massive or cross-stratified.

The large external geometries and potential reservoir quality of avulsion-related channel sandbodies, suggests that significant additional reserves could be present within intercoal lithologies throughout the Raton Basin.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming