Niobrara Calculation Tool
QUESTION: If a company were to hydraulically fracture the Niobrara formation underneath my house, how deep would this be occurring? How thick would the shale barrier be between my water well and the Niobrara strata? These questions can be answered using this tool.
The Niobrara strata in the Denver Basin are currently being developed for oil production using horizontal drilling and hydraulic fracturing. This development is moving into parts of the Denver Basin where many people depend on groundwater to meet their household needs. Citizens are concerned that this activity may adversely affect their water wells. This calculation tool was developed to help citizens or planners understand the geologic conditions that exist beneath their property.
The tool is designed to help people visualize the spatial relation of hydraulic fracturing in the Niobrara Formation to the important fresh-water aquifers. The tool will show the average depth to the Niobrara Formation at any selected point or address on the map. It will also show the minimum thickness of the shale barrier (Pierre Shale) that separates the Niobrara strata from fresh water aquifers. The tool also provides the depth of the deepest fresh water aquifer at any spot on the map.
Explanation of what the tool is telling you
Click here for larger image.
The above cross section represents what we would encounter if a giant, vertical slice were cut out of the Denver Basin so that we could observe the various layers of rock. Notice that the Niobrara strata are so deep that they are actually below sea level in some parts of the basin.The illustration shows how an oil company would drill a vertical well into the Niobrara strata and then turn the drill bit so that it would drill horizontally in the Niobrara limestone layers. After the horizontal part of the well is drilled (sometimes more than a mile in length), the company pumps liquid and sand out into the horizontal borehole. The pressure of this slurry fractures the limestone so that the oil flows into the well at much higher rates than it normally would without the artificial fracturing.
Controlling where fracturing occurs is important for two reasons. First, if fracturing were to extend into overlying freshwater aquifers it would create a potential pathway for contamination of water supplies. Second, oil production would decrease if fractures extended into non-oil bearing formations.
Fortunately, in the Denver Basin we have a stack of rocks (the Pierre Shale) that separates the Niobrara from shallower aquifers. The properties of the Pierre Shale are ideal for preventing upward migration of fractures or fluids. The Pierre Shale has extremely low permeability and it is very thick (varying from more than a half a mile thick to about a mile and a half thick). These two properties combine to make the possibility of fractures or fluids working their way up through it, essentially nil. The calculation tool will show you how thick this barrier is at any spot in the Denver Basin.
Explanation of how to use the Niobrara Tool
Click here to begin using the tool.
The Niobrara Tool opens with a map of the Colorado area, showing a yellow outline for the area of Niobrara data that is currently available.
You can locate your area of interest within this boundary in two ways: (1) by clicking the left mouse button directly on the map for a specific location (zoom-in first to avoid selecting multiple points); or (2) by first typing the address of interest (including the city and state or ZIP code) into the search bar in the upper right corner (red box in image below), hitting ENTER, and then clicking with the left mouse button on that location.
This will pop-up up a small information box. In this box will be data about the estimated depths to the top of the Niobrara Formation, the estimated depth to the base of the Fox Hills Sandstone (the deepest of the administrated Denver Basin aquifers), and the total thickness of the Pierre Shale (the rock layers between the Niobrara and the Fox Hills). By clicking on the small arrow in the upper right hand portion of that small box, you can toggel through these data. Be aware that if you are zoomed out you may reveal data for more than one location (and multiple redundant data will appear).
Details on how the tool works
The basic data from which the depth and thicknesses in the tool are derived is a recently released publication from the Colorado Geological Survey: Bedrock Geology, Structure, and Isopach (thickness) Maps of the Upper Cretaceous to Paleogene Strata between Greeley and Colorado Springs, Colorado. It uses data from three of the 12 maps in that publication: Structure map on top of the Niobrara (Plate 3), Isopach map from the top of the Niobrara to the Pierre Ash (Plate 8), and Structure map on top of the Fox Hills Sandstone (Plate 5). The subsurface data set consists of 2,951 geophysical well logs that are readily available in the public domain. This data set is made up of 1,800 oil and gas wells, 828 water wells, 321 uranium exploration boreholes, and two cores.
How the tool was created
The tool uses several different datasets to return values to the user. The depth to the Niobrara Formation was determined by subtracting the structural tops of the Niobrara from the National Elevation Dataset 10-meter Digital Elevation Model. The resulting GRID was converted to a raster and reclassified using a 200 foot contour interval.
The depth to the base of the Fox Hills Aquifer was calculated by subtracting an average thickness of 200 feet from the structural tops of the Fox Hills Sandstone. The resulting GRID was converted to a raster and reclassified using a 200 foot contour interval.
The minimum thickness of the Pierre Shale uses the Top Niobrara to Pierre Shale-Ash Isopach dataset included in the publication Bedrock Geology, Structure, and Isopach (thickness) Maps of the Upper Cretaceous to Paleogene Strata between Greeley and Colorado Springs, Colorado. Again, the dataset was reclassified using a 200 foot contour interval.
More detailed depths and thicknesses are possible; however, the tool is limited to 1000 records as determined by the on-line ESRI map server. Thus, we had to generalize the data so that it could be used by the map server.