Split Mountain
A single vertical fracture raises questions about flow character in a billion-barrel oil field
Erosion-resistant sandstone of the Pennsylvanian Weber Formation crops out in the south-dipping limb of Split Mountain Anticline in Dinosaur National Monument, Colorado.The Weber Formation is the reservoir of a number of oil and gas fields, including Rangely, the largest oil field in the U.S. Rocky Mountains, which lies just 35 miles SE of this location.Fractures can play a significant role in fluid productivity at Rangely and other Weber reservoirs, so understanding their character can be of economic significance.
This image, taken where the Green River cuts through the southern limb of the anticline, shows an outcrop that is about 50m high.Bedding dips moderately to the right (south).Most joints are perpendicular to bedding, so can be seen dipping moderately towards the left (north).Also, most joints are constrained mechanically by bedding surfaces.However, in the center of the image is a tall, opening-mode fracture that is vertical and through-going.
Is this vertical fracture an isolated anomaly, or are widely-spaced vertical fractures significant features in the Weber Formation?Rangely field is undergoing enhanced oil recovery, including water and CO2 injection, thus understanding of the nature of fractures in the reservoir could be important financially. Such fractures, if present and open in the reservoir, would potentially have a first-order impact on the flow of fluids due to enhancement of permeability and the efficient mobility of fluids both along and across bedding.It would also impact their drainage characteristics, so gravity drainage, for example, would likely be very effective in the presence of such tall fractures.
Rangely field has more than 1100 wells, all but a few of them vertical, and most of them drilled before 1970.What is the likelihood of hitting widely spaced vertical fractures with a vertical well?Understanding the presence and distribution of such potentially important features could be assessed in outcrop relatively easily in these well-exposed strata, whereas they might be almost unsampled in data from vertical wells.
Another question to ponder is when the fractures formed relative to the folding of the strata.We might assume the vertical fracture formed relatively late in the sequence of fracture development, after the beds had rotated to their present dip, however its relative age is not clear here.Regarding the age of the bed-perpendicular joints, did they form before, after, or synchronous with folding of beds?Or perhaps they formed over a long period, starting before folding and culminating after the folding was complete?It will require more than a single photograph to answer this question of fracture system genesis.