Fracture formation, propagation and interaction history at Raplee Monocline, Utah Ian Mynatt, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 e-mail: [email protected]

reasons an understanding of the association Abstract between the folds and fractures has important applications in both hydrocarbon and Folding and fracturing are intimately related groundwater exploration and extraction. processes with complex interactions between the This paper summarizes observations made in remote stress, local folding related stresses and the field at Raplee Monocline and introduces pre-existing fractures all influencing new conceptual models based on these data for the fracture formation. The first steps to figuring initiation, sequence and interaction of the out these relationships are documenting existing fracture sets seen on and near the fold. Only by examples of folding induced and related documenting and understanding the complex fractures and designing conceptual models interactions between fold evolution, fold based on these data which explain the initiation geometry and fracture characteristics can the and propagation of fracture sets and include the much desired long term goal of fracture effects of pre-existing structures. Here, field forecasting in reservoir modeling be attained. data from Raplee Monocline in SE Utah and conceptual models of fracture evolution based on Field area: Raplee Monocline, these data are presented. Five stages of Utah fracturing are described: 1) the formation of pre-folding E-W Set I fractures by regional Raplee Ridge is a kilometer scale fold located stress, 2) the formation of pre-folding N-S Set II in the Monument Upwarp in southeastern Utah fractures probably by regional stress, 3) the within the Colorado Plateau geologic province formation of tail-cracks off Set II and the flow of (Fig. 1). The fold axis trends north and the fluids through these and the main Set II fold’s cross-sectional form is asymmetric with fractures, 4) right lateral shear induced by beds dipping 20-40° to the west and ~5° to the folding creating NW-SE Set III fractures as tail- east (Fig. 1b), leading to the fold being described cracks of Set I fractures and as independent in the literature as both a monocline and an fractures and 5) the formation of Set II fractures anticline (Ziony 1966; Delaney, Pollard et al. as tail-cracks from slip along bedding planes. 1986; Stevenson 2000). This fold, which will be referred to as Raplee Monocline, is the result of Introduction deformation associated with the Laramide Orogeny and is bracketed in age between the late Raplee Monocline in SE Utah is an excellent Cretaceous and Eocene (Gregory and Moore example of Laramide/Sevier style deformation of 1931). the Colorado Plateau. This geologic province is known for its large, well exposed and visually spectacular folded strata. These folds are usually associated with so called “thick-skinned” orogenic processes and apparently are induced by basement involved thrust faults. Raplee Monocline exemplifies these types of structures and comprises a near ideal field site to examine the relationship between the folding process and both pre-existing and synchronous fractures. The widespread occurrence of fractures on folds has long led researchers to propose relationships between fractures and the folding process (e.g. Gilbert, 1882; Hennings, 2000). Folds often act as reservoirs, and fractures influence permeability and fluid migration (Coward et al., 1998, Aydin, 2000). For these

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Figure 1. Raplee Monocline in southeast Utah folds the marine and continental sediments of the Paradox, Honaker Trail and Rico Formations and is the result of Laramide aged deformation of the Colorado Plateau. Vertical relief of Raplee Ridge is ~500 m. a) Looking NE from near the middle of the folds length. b) Looking N from the southern tip of the fold

While Laramide age monoclines and folds often are cited as the surface expression of large normal or reverse faults, the specific deformational mechanism creating Raplee Monocline is unknown because no underlying faults are exposed. The fold is approximately 14 km long with almost 500 m of structural relief. It is doubly-plunging and its axis trends about Figure 2. Stratigraphic column showing the 355° in the south and 015° in the north, giving units in the Rico Formation. Note resistant the fold an arcuate shape (O'Sullivan 1965; sandstone and limestone layers separated by Ziony 1966). soft shale layers (RB1-6).

The stratigraphic units exposed on Raplee Monocline are the Pennsylvanian Paradox and Honaker Trail Formations, the Pennsylvanian- Permian Rico Formation and the Permian Halgaito Tongue and Cedar Mesa Sandstone of the Cutler Formation. Of particular interest is the ~140 m thick Rico Formation (Fig. 2), which represents a period of regression separating the underlying marine Paradox and Honaker Trail Formations from the continentally derived sediments of the overlying Cutler Formation (Cross, Spencer et al. 1899; Jentgen 1977). The Rico is composed of thick, red, slope-forming siltstone layers separated by eight resistant and Figure 3. Fracture sets in the Unnamed distinctive ledges of sandstone and limestone limestone. Abutting relationships at this and (O'Sullivan 1965; Ziony 1966). The resistant other sites show Set I is the oldest. The layers outcropping at Raplee Monocline presence of Sets I and II and lack of Set III beautifully display systematic joint and fracture away from the fold suggests I and II predate sets both in profile and on large pavements the fold and Set III is folding related and composed of these layers (Fig. 3). The excellent younger. However, abutting relationships (as exposure, presence of multiple sets, and areal seen here) suggest some Set II fractures may have formed after or simultaneously to Set III. extent of these fractures make this an ideal location for studying fold-fracture relationships.

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Figure 4. Airphoto with select site locations and rose diagrams of unfolded fracture orientations. The site number, stratigraphic unit, number of measurements and fracture densities (fractures/m) are shown for each location (where measured). Fracture set I (red) appears at all locations as the major E-W striking set. Set II (blue) appears less consistently as the N to N by NE set. A folding related set III (green, NW-SE) is present more in the southern part of the airphoto and corresponds with the area of greatest deformation (highest amplitude folding/greatest curvature). Airphoto is ~3.5 km across, north is up. predominantly bedding perpendicular, with Pre-Folding Fracture Sets I and II unfolded dips generally less than 10° from Three main fracture sets were identified at vertical and no preferential dip direction. A few Raplee Monocline based on orientation (strike important exceptions where fractures are oblique after correcting for fold dip by rotating beds to to bedding are discussed below. horizontal). Measurements of fracture The most prevalent and pervasive of the orientations were taken at various structural fractures sets is classified as Set I. This set locations on and near the fold. Fracture densities strikes from 75° to 105° and is present at all sites were measured where possible using the line and in all stratigraphic layers. It is shown in method (Wu and Pollard 1995). Figure 4 shows figures 3 and 4 and is labeled in red. A second the spatial locations, stratigraphic positions, fracture set (Set II) appears in many of the orientations and densities of fractures measured resistant layers striking between 000 and 050° on the fold. Fracture orientations are shown as (Figs. 3, 4), and is less well represented or absent rose diagrams after unfolding. The fractures are in the soft, red silt and shale layers (RB1-7)

Stanford Rock Fracture Project Vol. 16, 2005 J-3 between the resistant layers (Fig. 2). Aside from shale layer between two grey sandstone or orientation, the sets can be differentiated by the limestone layers. It is important to note the stratigraphically continuous, planar nature of Set assumed differences in material properties I in contrast with Set II, which is localized to between the red and grey layers. The grey layers specific layers, tends to be less planer, and is represent the resistant strata in the field which almost always less pervasive (Fig. 3). In the are inferred to have deformed in a brittle manner parlance of Wu and Pollard (1995), Set I is at or during deformation. We postulated that these near fracture saturation and is well-developed, layers accommodated strain dominantly by while Set II is less well-developed and has fracturing and faulting. The red layer represents usually not reached saturation. Set II is truncated beds in the Rico which accommodated strain by Set I, suggesting Set I formed before Set II dominantly by plastic flow, with fewer fractures (Fig. 3). and bed parallel slip. The interfaces between Several fracture measurement sites were these brittle and ductile layers are not considered chosen away from the fold in slightly tilted (<8° to be bonded, so bedding parallel slip may play dip) to flat-lying sediments (Fig. 5). Both Sets I an important role as outlined below. Associated and II appear at these sites, suggesting their with each block model are arrows representing formation pre-dates the folding event. These the remote principal stress directions. Here data combined with the abutting relationship compression is positive and relative magnitudes between Set I and Set II lead to a relatively are σ1 ≥ σ2 ≥ σ3. Where pore fluid is present uncomplicated picture of the early stages of there could be effective tensional stresses fracture development at and near Raplee Model 6a visualizes the first stage of Monocline. fracturing (Set I). In this earliest stage there is a uniform remote stress field dominated by the vertical most compressive stress (or overburden, σ1) and a horizontal least compressive stress oriented almost due N-S (σ3). Fractures form perpendicular to this least compressive stress which would be an effective tensile stress given sufficient pore pressure. The interemediate compressive stress (σ2) is oriented orthogonally to σ1 and σ3 and is therefore E-W. At this stage there is no folding. Many aspects of the field observations of Set I are captured in this model (Fig. 6a). The spacing of the fractures is at or near fracture saturation, with the spacing distance roughly the bed thickness (Wu and Pollard, 1995). Effects of the interfaces between brittle and ductile layers is evidenced by some Set I fractures either initiating or terminating at layer contacts. At this stage these fractures are mode I (pure opening) Figure 5. Site data for fracture measurements and are classified as joints. Many Set I surfaces in gently dipping to flat-lying sediments currently display rib marks, hackle and plumose approximately 2 km away from the fold. structures, all evidence of mode I opening Lower left number is number of (Pollard and Aydin, 1988). measurements, lower right is bedding dip. Model 6b shows the inferred formation of Set Note presence of sets I and II at all sites and II fractures. Set II formed at some time after Set the near absence of set III. I and was the result of a stress state with σ 3 approximately 90° in the horizontal plane from Conceptual block models were constructed to its orientation during Stage 1, causing the help with the visualization of fracture evolution. fractures to have a roughly N-S orientation. As The first two models (Stage 1 and Stage 2) in Stage 1, these fractures are interpreted as idealize the earliest stages of fracture mode I joints, formed before the folding event. development just described (Fig. 6a, b). The model itself is composed of three layers, a red

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Figure 6. Conceptual models of the five stages of deformation and fracture formation at Raplee Monocline. more complex stress field for the propagating Set However, despite the obvious similarities in II fracture tip. In some places Set II truncated formational conditions between Stages 1 and 2, against the discontinuity formed by Set I there are important differences between fracture fractures and in others Set II propagated through Sets I and II. The first difference is due to the Set I. This could imply that Set I was in effect presence of Set I during the formation of Set II. sealed at some locals and not at others or that the As Set II initiated and propagated, these fractures compressive stress active across Set I fractures encountered pre-existing Set I joints creating a varied spatially at the time of Set II formation

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(Renshaw and Pollard 1995). A second difference is the less developed nature of Set II. Unlike Set I, Set II has not reached fracture saturation and is in fact almost absent at some measurement locations (Fig. 4). This suggests the induced tension was less than during the formation of Set I (Wu and Pollard, 1995). This disparity is exacerbated by the material property differences between the ductile and brittle layers. Set II is much less common or absent in the shale layers and is often limited in vertical extent to a single sandstone or limestone bed (Figs. 4, 6b). At some point after Stage 2, the remote stresses rotated in the horizontal plane again. Figure 7. Stained fractures. a) Set II fractures This new remote stress orientation induced left- show staining and tail cracks with the stains lateral shear on the Set II fractures, causing them forming near the time of the fracture to slip and create tail-cracks (Cruikshank and formation. Set I fractures show only stains with the stains forming from recent meteoric Aydin, 1995). This is shown in figure 7a and water. b) Set II fractures with stains have the model 6c. It is apparent that during this time same abutting relation ship with Set I. some of these fractures were permeable enough to allow fluids to move through them. Many of Folding Related Fracture Set III both the Set II fractures and Set II tail-cracks have a red stain surrounding them in the host The final fracture set is Set III, which strikes rock (Figs 6c, 7a,b). This stain is the result of 115° to 140° and is structurally associated with chemical interactions between the subsurface the fold. Set III is generally absent in the flat- fluids moving along the crack and the host rock lying sediments and in the northern tip of the around it. These stained fractures show the same fold where the fold amplitude and curvature abutting relationships with Set I as other Set II along with bed dip are smallest (Figs. 4, 5). fractures (Fig. 7b), and so even if not forming Farther south on the fold with increasing fold coevally with non-stained Set II’s, theymust amplitude, Set III becomes increasingly have formed successively. Regardless, both are prevalent. At numerous locations it is the set classified here as Set II. Due to cross-cutting with the greatest spatial density (Figs. 5). There relationships discussed below, this stage of are also variations in the presence and density of deformation is again thought to be pre-folding. Set III perpendicular to fold the axis. Figure 8 Figure 7b also shows evidence of staining along shows site 38, where measurements were taken the Set I fractures, although due to the proximity in the Mendenhall sandstone near the top of the of these stains to current ephemeral stream fold. At this location, as the bedding dip channels, they are assumed to be from meteoric decreases from 8° to 3°, the Set III density water. decreases from 0.5/m to 0.03/m. These values can be compared with densities of 1.6-2.6/m in the Mendenhall at sites with ~25-30° of dip.

Figure 8. Site 38 in the Mendenhall sandstone near the top of the fold. Photo looks north. As bedding dip decreases, the density of set III also decreases.

These observations, combined with the lack of Set III in the flat-lying sediments, suggest that

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Set III was induced by the folding. It also fractures and isolated in the host rock in appears that areas of greater dip have greater Set orientations sub-parallel to Set I (Fig. 11b). III densities. Preliminary curvature analysis (as outlined in Bergbauer, 2003) also suggests that it is in fact bed dip and not bed curvature that correlates with the presence of these fractures. There appear to be two mechanisms involved in the initiation of the Set III fractures. Many seem to have formed in a similar manner to Sets I and II, i.e. in response to the remote stress field and initiating at a relatively arbitrary point, such as a pre-existing flaw in the host rock. However, while having the same orientation (strike and dip) as those just mentioned, some Set III fractures have geometries relative to Set I that suggest they formed as tail-cracks in response to shear along Set I (Fig. 9). Figure 10. Right-lateral offset of stained Set II fractures caused by slip along Set I.

Figure 9. Set III fracture forms as tail-crack induced by right lateral slip along Set I.

In addition to the fracture geometry, other stress-field indicators and structural relationships suggest some Set III fractures formed as a result of slip along Set I fractures. The main evidence comes in the form of multiple expressions of right-lateral shear along Set I fractures, consistent with the shear sense necessary to form Set III as tail-cracks. An excellent example is the right-lateral offset of stained Set II fractures along Set I cracks (Fig. 10). Note that this structural relationship constrains the age of the stained fractures. Given that Set III formed Figure 11. a) Right-lateral calcite slicken during folding and that slip along Set I is fibers on a Set I fracture surface. b) Right- similarly folding related (hence forming Set III), lateral en echelon veins forming along Set I the stained Set II cracks must have formed fracture. before the folding event, or early in it. Other evidence of right-lateral shear along Set All of these structural components are I is found on Set I faces in limestone layers captured in the conceptual model for Stage 4 which display right-lateral shear in the form of (Fig. 6d). The model shows right lateral shear calcite slicken fibers (Fig. 11a). Right lateral en causing en echelon veins, slicken fibers, offset echelon calcite veins also appear along Set I markers and the formation of Set III as tail cracks of Set I and as independently forming

Stanford Rock Fracture Project Vol. 16, 2005 J-7 fractures. It is worth noting that in this model while the gretest compressive stress (σ1) is still inferred to be perpendicular to the bedding (as evidenced by the bed perpendicular orientation of Set III fractures), the bedding itself is no longer horizontal and σ1 and the overburden stress are not collinear. This is an interesting aspect of the deformation and merits further study. Other interesting questions include why the fractures formed in the orientation they did and why this orientation seems uninfluenced by structural location on the fold.

Figure 12. Non-bed perpendicular Set II Folding Related Fracture Set II, fractures form as tail-cracks from slip along Infilling? bedding planes. All of the field data described above implies that Set III is the youngest and formed during the The Stage 5 model requires σ1 to be oblique to folding event creating Raplee Monocline. bedding. It follows that the stress state was However, while outcrops often show Set III different from that which formed Set III in Stage abutting against Set II, Set II can also be seen 4. This was probably the result of changes in the abutting against Set III (Fig. 3). These geometry of the fold. These fractures have only observations may imply that additional members been found in areas of high bed dip, implying of Set II formed by infilling in response to they formed in the later stages of fold folding and is locally coeval with Set III (e.g. development. However this, like most of the Bergbauer, 2002). If this infilling model were model of this stage of deformation, is the case, Set II densities on the fold should be speculative. greater than in the undeformed sedimentary strata. More data in the flat-lying areas are Conclusions needed to test this particular hypothesis. This paper outlines the synthesis of the field While infilling of Set II may be one viable data on the fracturing at Raplee Monocline. The mechanism, a second mechanism may present conceptual models based on these data define 5 itself in the form of fractures with strikes parallel stages of deformation, beginning with fractures to Set II, but with orientations that are not formed before the folding event and ending with perpendicular to bedding (Fig. 12). These fractures formed at perhaps some of the latest fractures strike N-S, but dip around 70° to the E stages of folding. The data and models suggest (after unfolding), unlike the consistently vertical that pre-existing fractures play an important role Set II fractures seen elsewhere. The conceptual in the initiation and evolution of later fractures model for these fractures (Stage 5) again utilizes by both inhibiting the propagation of some and tail-cracks, but here resulting from slip along acting as the initiator of others. There are also bedding planes between lithologic units instead certainly more complex interactions between of along pre-existing fractures (Fig. 6e). As the pre-existing fractures and propagating ones that fold grew in amplitude and bedding dips may explain current mysteries such as the increased on the limbs, slip along bedding planes uniform orientation of the folding related Set III and layer contacts was initiated. These contacts across the fold. These conceptual models will be moved in a thrust sense (base of upper layer used to design and constrain mechanical models moving toward the fold hinge, top of lower layer for folding and fracturing in the next phase of away) and created the stress state necessary to this research. propagate Set II oriented tailcracks (Fig 6e, 12). Other future work has the goals of How prevalent these tailcracks are and their documenting the fracture characteristics of the structural relation to other fractures remains to be rest of the fold as well as more clearly defining determined. For this reason, model 6e is the the relationship between the known fracture sets, most speculative of the five. such as how the non-vertical Set II fractures interact with the other fracture sets. The ultimate goal is a complete understanding of the timing, fold geometry, and local and regional stress

Stanford Rock Fracture Project Vol. 16, 2005 J-8 conditions for all stages of fracture formation. reconnaisanc of parts of Utah and Arizona." U. S. The excellent exposures and relative simple Geological Survey Professional Paper 164. fracture history at Raplee Monocline make this Hennings, P. H., J. E. Olsen, et al. (2000). "Combining an achievable goal. outcrop data and three-dimensional structural models to characterize fractured reservoirs: an example from Wyoming." American Association of Acknowledgements Petroleum Geologist Bulletin 84(6): 830-849. Financial support for this work was provided Jentgen, R. W. (1977). "Pennsylvanian Rocks in the by an Office of Technology Licensing Stanford San Juan Basin, New Mexico and Colorado." New Mexico Geological Society Guidebook 28th Field Graduate Fellowship, the Shell Fund, NSF Conference, San Jaun Basin III: 129-132. Colaborations in Mathematical Geosciences and O'Sullivan, R. B. (1965). "Geology of the Cedar the Stanford Rock Fracture Project. Thanks to Mesa-Boundary Butte Area, San Juan County, Nicolas Bellahsen, Yukiyasu Fujii, Tricia Fiore Utah." Geological Survey Bulletin 1186. and Mom and Dad for field assistance. Special Price, N. (1966). Fault and joint development in brittle thanks to Dave Pollard for his patience with me and semi-brittle rock. Oxford, Pergamon Press. and my work. Renshaw, C. E. and D. D. Pollard (1995). "The Development of Fracture Connectivity by Propagation Across Unbonded Frictional Interfaces: References An Experimentally Verified Criterion." Aydin, A. (2000). "Fractures, faults, and hydrocarbon International Journal of Rock Mechanics and entrapment, migration and flow." Marine and Mining Science and Geomechanical Abstracts. Petroleum Geology 17(7): 797-814. Rogers, H. D. (1841). "Origin of overturned folds in Bergbauer, S. (2002). The use of curvature for the Pennsylvania." American Journal of Science. analysis of folding and fracturing with application Smith, D., A (1966). "Theoretical considerations of to the Emigrant Gap Anticline, Wyoming. sealing and non-selaing faults." American Department of Geological and Environmental Association of Petroleum Geologist Bulletin 50(2): Sciences, Stanford University: 216. 363-374. Bergbauer, S. and D. D. Pollard (2003). "How to Stearns, D. W. (1969). Certain Aspects of Fracture in calculate normal curvatures of sampled geological Naturally Deformed Rocks. Rock Mechanics surfaces." Journal of Structural Geology 25: 277- Seminar. R. E. Riecker. Bedford, Mass., Air Force 289. Cambridge Research Laboratory: 97-118. Coward, M. P., T. S. Daltaban, et al., Eds. (1998). Stevenson, G. M. (2000). "Geology of Goosenecks Structural Geology in Reservoir Characterization. State Park, San Juan County, Utah." Utah Geological Society Special Publication. London, Geological Association Publication 28: 433-447. The Geological Society of London. Wu, H. and D. D. Pollard (1995). "An experimental Cross, C. W., A. C. Spencer, et al. (1899). "La Plata study of the relationship between joint spacing and Folio." U.S. Geological Survey Atlas Folio GF- layer thickness." Journal of Structural Geology 0060. 17(6): 887-905. Delaney, P. T., D. D. Pollard, et al. (1986). "Field Ziony, J. I. (1966). Analysis of Systematic Jointing in relations between dikes and joints: emplacement Part of the Monument Upwarp, Southeastern Utah, processes and paleostress analysis." Journal of Univeristy of California in Los Angeles: 152. Geophysical Research 91(B5): 4,920-4,938. Gregory, H. E. and R. C. Moore (1931). "The Kaiparowits Regions, a geographic and geologic

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