The impact of measurement scale
These bedded Pennsylvanian carbonates dip consistently towards the east at about 45 degs. This outcrop is in Arrow Canyon, in southern Nevada. It presents a view of the effects of mechanical and fracture stratigraphy on the occurrence of joints.
The structure of the joint system and bedding is simple and orthogonal. The two joint sets are normal to bedding and to each other. One set strikes nearly parallel to bedding dip, whereas the other set is about parallel to bedding strike.
At a glance the characteristic relationship between layer thickness and joint spacing is evident. Don Medwedeff is sunning himself on a joint surface that crosses a thick layer defined by predominantly tall, widely-spaced joints. In contrast, looking up the ridge on the right side, thinner layers show serrated edges due to their more-closely-spaced joints. Qualitatively, the commonly observed spacing-thickness relationship is evident. But it’s not quite so clean and final.
The fracture surface behind Don shows decimeter-scale bedding of alternating light limestone and dark chert-rich beds. These lithologies tend to have very different mechanical properties, and the fracture behavior of them differs profoundly, as may be seen in the nearby outcrop described in photo #31 of this collection.
Laubach and others (Laubach, S. E., Olson, J. E, and Gross, M. R., 2009, Mechanical and fracture stratigraphy: AAPG Bulletin, v. 93, no. 11, p. 1413-1426) discuss three distinct bases for defining layering: depositional stratigraphy, mechanical stratigraphy, and fracture stratigraphy. The limestone/chert layering is depositional (actually, the chert is a diagenetic product of the transformation of depositional silica within the layering). This limestone/chert layering might be considered the mechanical stratigraphy, which is defined based on variation in rock properties due to depositional or diagenetic processes.
If we zoomed in we would see a distinct difference in fracture character between the limestone and the chert (a la photo #31). At the zoomed-in scale the fracture stratigraphy mimics the decimeter-scale mechanical stratigraphy. The fracture stratigraphy is defined based on the fracture attributes, which includes their spacing and collective termination at consistent layer boundaries.
The large-size fractures in the current view apparently ignore the fine-scale mechanical layering. For them the thick interval behaves like a continuous material. As the large fractures formed they responded to the package of limestone/chert that occurs across this thick layer. The fine detail of the decimeter-scale mechanical layering has been averaged out. Evidently, the fracture stratigraphy manifests itself differently when viewed at different scales. Scale of observation matters. Perhaps this is one reason that fracture spacing measurements made by different researchers are often difficult to reconcile. Different people may be observing distinctly different, or mixed, scales of observation that are influenced by different processes or boundary conditions. Understand and comparison of results of different workers calls for attention to the scale of the fracture and mechanical stratigraphy.
A final consideration regarding this topic. We commonly consider correlation of fracture spacing to layer thickness as a fundamental relation to be investigated in the study of fracture systems. In fact, the mechanically significant relationship is the correlation of fracture spacing with fracture height, not with layer thickness. Fracture height is the parameter that controls the size of the stress shadow zone that envelopes extension-mode fractures. The relationship with layer thickness is a consequence of fracture stratigraphy (fractured layer thickness). The mechanically significant relationship is that of fracture spacing vs. fracture height. Layer thickness is just a proxy for fracture height.
The sales pitch – This photo, and others among the 50 Photos for Fracture Aficionados, were taken in the splendid exposures of Arrow Canyon, Nevada. I’ve engaged with the folks at Knowledgette.com, an eLearning company for the O&G industry, to create a user-interactive virtual geological field trip there for reservoir engineers & geologists. Below is a description of it. Or you may want to just jump straight to the Fractures of Arrow Canyon link to preview the trip (see Promo link), and to enjoy some free content at stop 7 and stop 9. The full trip is available for a small enrollment fee. I hope you enjoy it.
Naturally Fractured Reservoir Analogs in the Field & Online
A geological field trip can be informative, inspirational, and invigorating. It can also be hazardous, uncomfortable, and expensive. I put together an online geological field excursion to examine some aspects of natural fractures in carbonates in an exceptionally well-exposed flank of an anticline in the Mojave Desert of southern Nevada. This video excursion capitalizes on the benefits of a field trip while avoiding its risks.
In a series of short videos I venture down-section in Arrow Canyon, exploring the fracture system as I go. During an actual field trip participants can look around for context, and approach the outcrops to see features close up. We’ve tried to mimic these practices. Along with the videos there are panoramas of key outcrops as well as a collection of high-resolution still photos that allow you to look around and to zoom in on the details.
The rocks and their fractures are an analog for a naturally fractured reservoir. Reservoir engineers and geologists that are actively engaged with an NFR will derive the greatest benefit, because I discuss many implications of fracture characteristics in terms of reservoir flow behavior. This is not a specialist-focused trip although I expect fracture specialists will particularly appreciate these exposures.
This video field-excursion does not seek to replace an actual field trip, but hopefully makes the fracture system of Arrow Canyon accessible to people who would not normally be able to explore it on their own.
So take off your hiking boots, forget about the sunscreen, and take a trip with me into Arrow Canyon.
Clicking on the “Fractures of Arrow Canyon” link takes you to a page where the Promo link takes you to a stop-by-stop overview of the entire trip. That’s a good place to begin. There’s an interactive map that lets you put the full trip into context. Finally, stop 7 and stop 9 are open for unrestricted viewing, as an example of our visits to specific stops, to help you decide if the trip is right for you.