Sequence stratigraphy and regional context of the Mancos-Niobrara in the northern San Juan Basin

In the northern San Juan Basin, the Niobrara Formation is represented by the upper half of the Mancos Shale (the Smoky Hill Member and Cortez Member). This section is generally equivalent to the Niobrara Formation along the Colorado Front Range. Although the Fort Hays Limestone is absent west of Pagosa Springs, the C Chalk and B Chalk are well-expressed as two resistant bench-forming calcareous units in the northern San Juan Basin. These two calcareous units have also been established as prospective hydrocarbon targets by operators in the area. Calcareous facies equivalent to the A Chalk were not deposited in the northern San Juan Basin due to siliciclastic dilution during the regressive latter half of the Niobrara marine cycle. The overall third-order Niobrara marine cycle includes these members of the Mancos Shale: the Juana Lopez, Montezuma Valley, Smoky Hill, and Cortez members. The Smoky Hill Member sits just above the basal Niobrara unconformity in most of the study area, and the entire section also has greater thickness and siliciclastic content than its equivalent farther east along the Front Range. Several extensive outcrop locations (in and around Pagosa Springs, Piedra, and Durango, CO) along with three new cores along the CO-NM border form the foundation for sequence stratigraphic interpretation of the Niobrara marine cycle in this study. All these locations and cores were tied back to the Mancos reference section at Mesa Verde National Park established by Leckie et al. (1997) which provides detailed description and biostratigraphy for the entire Mancos Shale. Establishing and applying a sequence stratigraphic framework to any section creates consistent reference standards for communication, research, and further correlation. Comparisons of chemostratigraphic data from equivalent strata between the northern San Juan Basin and Denver-Julesburg (DJ) Basin reveal significant differences in the timing and style of source-rock deposition (and associated low-oxygen conditions). The sequence stratigraphic framework also emphasizes tremendous lateral facies changes in the basal Niobrara section (i.e., Fort Hays Limestone to Tocito Sandstone). Once refined and applied, this stratigraphic framework can be used for predicting the distribution of reservoir properties, in addition to enhancing understanding of the Niobrara marine cycle and the Western Interior Seaway.

Modeling geogenic and atmospheric nitrogen through the East River Watershed, Colorado Rocky Mountains

There is a growing understanding of the role that bedrock weathering can play as a source of nitrogen (N) to soils, groundwater and river systems. The significance is particularly apparent in mountainous environments where weathering fluxes can be large. However, our understanding of the relative contributions of rock-derived, or geogenic, N to the total N supply of mountainous watersheds remains poorly understood. In this study, we develop the High-Altitude Nitrogen Suite of Models (HAN-SoMo), a watershed-scale ensemble of process-based models to quantify the relative sources, transformations, and sinks of geogenic and atmospheric N through a mountain watershed. Our study is based in the East River Watershed (ERW) in the Upper Colorado River Basin. The East River is a near-pristine headwater watershed underlain primarily by an N-rich Mancos Shale bedrock, enabling the timing and magnitude of geogenic and atmospheric contributions to watershed scale dissolved N-exports to be quantified. Several calibration scenarios were developed to explore equifinality using >1600 N concentration measurements from streams, groundwater, and vadose zone samples collected over the course of four years across the watershed. When accounting for recycling of N through plant litter turnover, rock weathering accounts for approximately 12% of the annual dissolved N sources to the watershed in the most probable calibration scenario (0–31% in other scenarios), and 21% (0–44% in other scenarios) when considering only “new” N sources (i.e. geogenic and atmospheric). On an annual scale, instream dissolved N elimination, plant turnover (including cattle grazing) and atmospheric deposition are the most important controls on N cycling.

Insights into productivity drivers for the Niobrara-equivalent horizontal play in the Piceance Basin, Colorado

Limited horizontal drilling has occurred within the Niobrara-equivalent section of the Mancos Shale in the Piceance Basin, and the results from individual wells are highly variable. Prior studies have suggested that thermal maturity and completion techniques were the primary drivers for the observed production trends, but further analysis of well results indicates there are more variables at play. This study leveraged a comprehensive data set from the Piceance Basin, including core analyses, pressure data, and drilling and completion methods to provide additional context for the production results. From this analysis, several key trends were identified. North/south variations in thermal maturity were confirmed, as well as additional trends were identified revealing later exhumation south of the Rangely fault system resulted in significant depressurization, particularly in the western Piceance Basin. The semi-regional depressurization was the result of decrease in overburden pressures that allowed vertical migration of hydrocarbons out of the Mancos Shale. In addition to the semi-regional depressurization, there were more local depressurization events that resulted from faulting in areas such as the Orchard Unit in the southern Piceance Basin where thrust faults allowed hydrocarbons to migrate vertically into overlying formations. Northwest to southeast production trends are present in the southern Piceance Basin and are interpreted to reflect structurally undeformed areas based on high formation pressures and better producing horizontal wells. Parent-child effects have been observed locally and are linked to lower initial production rates and faster decline rates. The northern Piceance Basin exhibits higher reservoir pressure in the liquids window than was observed to the south due to the relatively low degree of exhumation and/or faulting in areas where horizontal Niobrara wells were drilled. Horizontal well results in the northern Piceance Basin have been mixed, largely due to inefficient completion strategies. By comparing the northern Piceance Basin wells with similar horizontal Niobrara wells in the Powder River Basin of northeastern Wyoming, it is concluded that drilling into the over-pressured liquids rim and utilizing slickwater frac fluid with friction reducer and 100 mesh sand will yield improved economic results over those obtained so far in the Piceance Basin. Though relatively few laterals have been drilled in the Piceance Basin Niobrara play, the basin has great future potential.

Tidal estuarine deposits of the transgressive Naturita Formation (Dakota Sandstone): San Rafael Swell, Utah, U.S.A.

ABSTRACT Thin tidal estuarine deposits of the Naturita Formation (0–23 m) of the San Rafael Swell record the initial flooding of the Cretaceous Western Interior Seaway, Utah, and capture the transition from inland fluvial systems to fully marine conditions over a time period of 5 My or less. A tide-dominated estuarine environment is favored due to the combined presence of mud and/or carbonaceous drapes on ripples and dunes, bidirectional flow indicators, sigmoidal cross-stratification, herring-bone cross-stratification, and bimodal paleocurrent measurements. Facies associations are arranged in a predictable manner. Locally at the base of the Naturita Formation, tidally influenced fluvial channel deposits are present. These are overlain by tidal bars, including subtidal bars and intertidal point bars. Overlying the tidal bars are sand-flat and mud-flat deposits as well as bedded coal and carbonaceous mudstone that represents a supratidal setting in the estuary. The Formation can be capped by a thin transgressive lag composed of shell debris, and/or pebbles, that marks the final transition into the fully marine Tununk Shale Member of the overlying Mancos Shale. Lateral relationships between estuaries and adjacent paleohighs shed light on the influence of foreland-basin tectonics on the location and preservation of tide-dominated estuaries. Estuarine and shoreface deposits are absent along the eastern flank of the San Rafael Swell and eastward for more than 80 km. This zone of nondeposition or erosion is coincident with the location of the forebulge in the developing foreland basin, implying that growth of the forebulge prohibited the development of, or enhanced the later erosion of, estuarine deposits. Conversely, enhanced accommodation in the transition into the foredeep depozone allow the preservation of tide-dominated estuarine deposits along the western flank of the San Rafael Swell. Additionally, the possibility of a pre-Laramide tectonic history for the San Rafael Swell is indicated by a distinct lack of Naturita Formation deposits in an area that is coincident with the modern-day axis of the anticline. Overall, the Naturita records the initial flooding of the Western Interior Seaway in the San Rafael Swell region and provides an excellent case study of the deposits that are laid down in a transgressive system that passes from coastal-plain to offshore deposits.