Insights into the petroleum geology and stratigraphy of the Dakota interval (Cretaceous) in the San Juan Basin, northwestern New Mexico and southwestern Colorado

—Highlights of an integrated study to evaluate the original and remaining Dakota gas resources in the San Juan Basin are presented. Dakota (Cretaceous) reservoirs contain a major stratigraphic gas accumulation with cumulative production of more than 6 TCF, and include braided and meandering fluvial, deltaic, shoreface, and shelf-ridge sandstones in seven distinct reservoir units. Data from 85 measured sections, 32 core descriptions, and 7,000 wells were integrated to construct a regional stratigraphic framework that correlates outcrops with the subsurface over the entire San Juan Basin. In addition, a 3,300-well digital log database was constructed to compute the petrophysical parameters and volumetric potential of each reservoir. Integration of these datasets and the resulting maps provides methods for determining the significance of various depositional trends, reservoir characteristics, and trapping mechanisms for gas production. Volumetric original gas-in-place and recovery factor maps closely approximate geologic and production trends, providing a basis for additional exploration and development opportunities. FIGURE 1. Dakota resource assessment workflow and primary study components. 435 INSIGHTS INTO THE PETOLEUM GEOLOGY OF THE DAKOTA INTERVAL in the Dakota producing interval has led to confusion, especially where exploration and production occurs in the Burro Canyon, but production is reported as from the Dakota producing interval. The Dakota contains several sandstone beds separated by shales. For at least 50 years, San Juan Basin subsurface geologists have informally lettered these Dakota interval sandstones from the top down as they were encountered in drilling. This practice has been inconsistent and confusing because Dakota sandstone beds wedge out and merge within the basin and different geologists and companies have used the letters differently, especially in separate parts of the basin. Table 1 lists the inconsistent usage of lettered beds from all published papers in which this nomenclature has been used. Formal lithostratigraphic member names have been defined from outcrops around the San Juan Basin by Owen (1966), Landis et al (1973), Aubry (1988), and Owen and Owen (2005, in this guidebook). Only recently have some subsurface geologists (for example, Owen and Head, 2001) begun to correlate the formal outcrop members of the Dakota into the subsurface to replace the numbered sandstones. Table 2 lists these formal members and those of adjacent formations. Note that the informal “Dakota main body” of previous usage has been formally named as the White Rock Mesa Member of the Dakota Sandstone (Owen and Owen, 2005, this guidebook). Key surfaces used in sequence-stratigraphic analysis, (Figure 2) especially marine-flooding surfaces and sequence-bounding unconformities, are also recognizable on outcrops, and well logs and can be used to define stratigraphic units. In the Dakota producing interval, many of these surfaces coincide with formal lithostratigraphic boundaries, so this subdivision was used in this study. The Burro Canyon is bounded by the K1 and K2 sequencebounding unconformities, the Encinal Canyon is topped by the initial marine-flooding surface, the Oak Canyon is truncated by the K3 sequence-bounding unconformity where the White Rock Mesa overlies it and by the K3 correlative conformity or a locally scoured surface where the Cubero overlies it. The White Rock Mesa is topped by a series of marine-flooding surfaces. The Cubero, Paguate, and Twowells are topped by marine-flooding surfaces. The Clay Mesa and Whitewater Arroyo marine shale tongues have gradational upper boundaries, so they were combined with the overlying sandstones into what was informally designated the Paguate interval and the Twowells interval. This gave us 6 (or 7, if the White Rock Mesa and Cubero are mapped separately) intervals for isopach mapping and correlating on crosssections (Table 3). Two prominent bentonites, the A bentonite in the Oak Canyon, and the X bentonite near the base of the Twowells interval, serve as critical key beds that can be used as datums on cross-sections. Other marine-flooding surfaces occur at the top of other parasequences, especially in the Cubero and Twowells interval. Other bentonites occur in the Twowells interval and the Graneros Shale, including an excellent bentonite datum for structural mapping at the top of the Graneros Shale. These bentonites, FIGURE 2. Dakota producing interval stratigraphic cross-section from the Four Corners in the west to the Chama Basin (Heron Dam) in the east (216 km. or 134 mi.), with outcrop sections on each end and well logs in between. Note westward erosional truncation of all lower Dakota and Burro Canyon as well as K-2 and K1 unconformities by K3 unconformity. Also note westward onlap of upper shoreface parasequence of Cubero on White Rock Mesa fluvial wedge. This cross-section is a summary of approximately 100 wells between outcrops. See Plate 7A for a color version of this figure.