Sequence stratigraphy and geologic reservoir characterization of the Niobrara-equivalent section: northern San Juan Basin

Abstract

The natural gas in the San Juan Basin (SJB) is a world-class resource. Significant contributions to both oil and gas production have recently come online from horizontal wells targeting calcareous mudstones and marls of the Upper Mancos Shale in the northern San Juan Basin (NSJB), which is lithologically similar and generally equivalent to the Niobrara Formation. Sandier tight-oil reservoir units (such as the El Vado Sandstone) within equivalent strata are unconventional targets for operators further to the south. In the northern San Juan Basin, the Niobrara Interval contains a Smoky Hill Member and an overlying Cortez Member which is very thick and grades upward into the Mesaverde Group. Two horizontal target zones in the lower Smoky Hill and lowermost Cortez have been recently established and tested to show significant production potential. The majority of the Smoky Hill Member in the northern SJB study area is in the dry gas window, except to the west on the Four Corners Platform where it is less thermally mature (oil to wet gas generation windows). The Smoky Hill Member sits just above a regional unconformity, below which reservoir and completions quality is drastically reduced. Numerous horizontal wells targeting the San Juan Basin Niobrara Interval have been very successful (initial production rates can exceed 600 BOPD or 12 MMCFD). These unconventional mudrocks are not limited to distinct reservoir types, and their production is influenced by lithology, pore style/geometry, fractures, fracture type, fracturability, subsurface pressures, structural context (affecting fractures and entrapment), and stratigraphic context (affecting organic richness, potential migration pathways, entrapment, and previous factors listed). This study characterizes the Niobrara Interval with respect to factors relating to production potential using outcrop, selected cores, and associated data. Establishing a sequence stratigraphic framework was a key part of this study that vastly enhances the utility of data and predictive potential for operators. The sequence stratigraphic framework also enables correlation of the Niobrara in the northern San Juan Basin to other paleoenvironments of the Western Interior Seaway. Consideration of lateral changes within stratigraphically equivalent units is also promoted by the framework. Outcrop investigations took place near the northwestern and northeastern margins of the San Juan Basin (near Durango, CO and Pagosa Springs, CO). Primary research cores for this study form an E-W transect across the northern San Juan Basin, just above the CO-NM state border. Characteristics from these data and observations were calibrated to well logs to develop detailed cross-sections intended to enhance understanding of the stratigraphic variability and distribution of the Niobrara Interval in the northern San Juan Basin. This area is markedly further from the paleoshoreline than the well-studied areas to the southwest and is host to a still-emerging Mancos unconventional play. Understanding stratigraphic variability, distribution, and characteristics facilitates the identification of ideal target zones and production “sweet spots” in the San Juan Basin.