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The El Vado Sandstone Member (late Coniacian) of the Mancos Shale is considered by many to be an important unconventional hydrocarbon resource target (Ridgley, 2001) in the San Juan Basin located in northwestern New Mexico. The shelf sandstones of the El Vado were deposited along the western margin of the Western Interior Cretaceous Seaway. The El Vado is part of a transgressive-regressive wedge of rock that overlies a large, older transgressive wedge which contains the Tocito Sandstone (the largest oil producer in the basin). Despite its productivity in parts of the basin, the nature, origin, and distribution of the El Vado Sandstone is poorly understood. Using well logs and core, the subsurface has been mapped to illustrate the distribution of the El Vado Sandstone across its extent in the San Juan Basin. The El Vado is reflected in logs as a thick (~ 120 foot) interval of low gamma ray and high resistivity. The gamma ray log character reflects highly laminated sand-to-silts interbedded with shales. A total of 13 cross sections were constructed based on the high density of well log control. Isopach maps were then generated for the entire interval and individual cycles of the El Vado. Regional net sand and net-to-gross maps, determined from 104 digitized well logs, highlight the regional distribution of sand content. Two regions in the basin were examined in outcrop, the northeast and the southeast. In the northern outcrops, the El Vado consists of five cycles of similar thickness (~ 4 m each) separated by marine shales with cycles becoming progressively sandier upward. To the south, the El Vado Sandstone appears to transition from a shelf deposited sandstone into a lower shoreface sandstone. Examining the El Vado Sandstone in the subsurface and in outcrops helped the author to improve the understanding of the nature and distribution of the sands throughout the basin.
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