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 Stanford Rock Fracture Project Vol. 16, 2005 J-1 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. Stanford Rock Fracture Project Vol. 16, 2005 J-2 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).