Sequence Stratigraphic Framework of the Cretaceous Gallup and Tocito Sandstones, San Juan Basin, New Mexico

VALASEK, DAVID W., Colorado School of Mines, Golden, CO

The Gallup (Turonian) and Tocito (Coniacian) sandstones are hydrocarbon reservoirs in the San Juan basin, northwestern New Mexico. The Gallup is a regressive shoreface system capped by Dilco Member coastal plain deposits. The Tocito is a transgressive system of valley-fill, estuary, and shelf deposits. All of the units were deposited in a ramp setting.

The Gallup is comprised of seaward-stepping (10-20 km) genetic units. Each genetic unit is also comprised of smaller scale (1.5-3 km) parasequences. Both scales are separated by flooding surfaces. Each genetic unit has an independent fluvial feeder system. The most well-developed, widespread, and amalgamated fluvial system correlates to the downward shift of facies basinward at the top of the shoreface. The base of the amalgamated fluvial, downward shifted facies and the valley fills is interpreted to be a sequence boundary.

The Tocito fills the incised valleys, lies on the shoreface deposits, and is capped by the maximum flooding surface. Near the shoreface pinch out, the transgressive surface is amalgamated to the sequence boundary surface. These estuary/shelf deposits are the primary hydrocarbon reservoir. Landward, the transgressive surface lies on coastal plain deposits and stratigraphically climbs. The transgression was interrupted by a progradational event consisting of ebb and flood tidal delta deposits of the Borrego Pass Member. These genetic units form a landward-stepping geometry.

Availability of outcrop exposures, well logs, and seismic data makes the Gallup/Tocito sequence an excellent example of the development of new exploration scenarios in well-exploited hydrocarbon reservoirs using sequence stratigraphy.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)