A devil of a time propagating fractures at Devil’s Slide
The well-layered sandstone and mudstone that comprise these Paleocene turbidites is located in California’s San Mateo County, perched several hundred feet above the Pacific Ocean in an area called Devil’s Slide. The view from here is delightful. When coupled with the fresh air and incessant calls of the common mures that inhabit the nearby islands, it can distract one’s attention from the cliff-side outcrop. But the fractures are compelling.
One set of fractures, or joints, are well developed and oriented approximately along the direction of view, thus we are able to see the true spacing of joints in this set, as well as their true dip. A second set of joints is approximately parallel to the plane of the photograph and thus its characteristics are not easily seen.
The joints dip steeply to the right, and are normal to bedding. It is not clear whether the bedding in this image is upright or overturned, but that is not important as far as the fracture character is concerned. Did vertical fractures develop when these beds were horizontal, and were subsequently rotated to their present orientation during folding of the strata? Or were the rocks folded and the joint systems developed subsequently? Distinguishing which of these possibilities is correct is not straightforward. However, the presence of many barren fracture surfaces suggests that they formed relatively late, during near-surface erosion and unloading of the strata. Based on this I judge that the majority of these joints formed after the strata attained their current structure.
The spacing of joints – the distance between adjacent joints – is proportional to thickness of the bedding layer that contains them. Joints are more widely spaced in thicker layers (I used the term “layer” rather than “bed” because a bed is defined based on sedimentology, whereas the layers evident here are defined on the basis of their fracture character. However, in this case either term works!). This thickness/spacing relationship is clear here, as it is in most well-exposed turbidites or other strata where distinct, brittle, fracture-prone layers are separated by weak mudstones or shales. Mechanically, each layer is decoupled from the overlying and underlying layers, thus it can form its own fracture system largely independent from that in adjacent layers.
One implication of the independent development of fractures in turbidites is that the fractures rarely crosscut multiple layers. Thus if we were trying to drain a turbidite reservoir we might be able to efficiently tap into a fracture system in one layer, but the fractures will typically drain only that layer. Typically, fractures in turbidites are bedding-confined. Thus producing fluids from a fractured turbidite may require tapping into the fractures in multiple layers. A well deviated to follow a single bed may disappoint. The propagation of cross-formational induced fractures will be hindered in the same manner as the natural fractures. An induced fracture will encounter the same mechanical decoupling of successive layers that the natural fractures encounter, thus potentially failing to cross-cut much of the reservoir and resulting in fractures of limited height.