Reservoir Trends and Exploration Potential of the El Vado Sandstone Member of the Mancos Shale, Northwestern New Mexico

Tiffany Hedayati and Lesli Wood
Bureau of Economic Geology, Jackson School of Geosciences University of Texas, Austin, TX

The El Vado Sandstone (EVS) Member (upper Coniacian) of the Mancos Shale is considered by many to be an important unconventional hydrocarbon resource target (Rigley and Morton-Thompson, 2001) in the U.S. San Juan Basin. It is part of a transgressive regressive wedge of rock that overlies a large, older transgressive wedge, which contains the Tocito Sandstone. It is thought to be associated with the Dalton Sandstones found along the basin’s south margin. Despite its productivity in parts of the basin, the nature, origin, and distribution of the EVS are poorly understood.

Using well logs and core, we mapped subsurface distribution of the El Vado. It is reflected in logs as a thick (~ 100-ft) interval of high-resistivity, highly laminated sand to silt interbedded with shales. In outcrop the El Vado consists of four cycles of equal thickness (~ 4 m each), with cycles becoming progressively siltier and sandier upward. Some beds appear to be ripple crossbedded, and sands contain substantial oyster and shell hash, showing abundant bioturbation. The El Vado is underlain by a thick shale unit separating it from the stratigraphically deeper Cooper Arroyo Sandstone (CA), a thin (<1-m) interval of coarse-grained, glauconitic, herringbone and sigmoidally crossbedded sandstone. The CA units may have been sourced from lower Laramide structures to the basin’s north and east. Additional outcrop work in spring 2008 will examine facies changes between the El Vado to the north and the Dalton-Gibson to the south. Extensive subsurface data will improve our understanding of log motif facies distributions north to south, improve mapping of the extent of the El Vado, and increase exploration opportunities within these units.

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