ABSTRACT: Production and Reservoir Characteristics of the Gallup Producing Interval San Juan Basin, New Mexico and Colorado
Richard J. Bottjer
The Gallup Producing Interval is the primary oil producing horizon in the San Juan basin, with cumulative production of 174 MMBO and 559 BCFG as of 1 January 1989. The NMOCD defines the Gallup Producing Interval as all strata below the Mesaverde and above the Greenhorn Limestone. "Gallup" production is from a stratigraphically complex package consisting of a lower regressive marine and nonmarine sequence overlain by a transgressive marine sequence. Production from the Gallup Interval is related to three main plays, each with distinct reservoir and production characteristics.
Seventy percent of Gallup production is from the Tocito Sandstone, a medium- to coarse-grained glauconitic sandstone deposited above the basal Niobrara unconformity. Tocito production is controlled by lateral pinch-outs of the reservoir facies and is characterized by moderate to high production rates and relatively shallow production declines.
The most active "Gallup" play in the 1980s was the fractured Niobrara play in the southeastern part of the basin. The fracture-prone Niobrara interval is characterized by thinly interbedded, fine-grained sandstones and calcareous black shales. Fractured Niobrara production is characterized by areally variable productivities, early shallow production declines followed by precipitous declines, and a high degree of pressure communication between wells.
The true Gallup Sandstone is a series of progradational nearshore marine sandstones that have produced 20 MMBO south of the main producing area of the basin. Gallup Sandstone production is largely limited to structural closures because the reservoirs are highly porous and permeable and are areally extensive.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990