13: A Diagenetic Study of the Wolfcamp Shale in the Southeast Midland Basin, West Texas: A Petrographic and SEM Study of Two Cores
Published:January 31, 2020
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Alyssa K. Wickard, R. Douglas Elmore, Gerhard W. Heij, 2020. "A Diagenetic Study of the Wolfcamp Shale in the Southeast Midland Basin, West Texas: A Petrographic and SEM Study of Two Cores", Mudstone Diagenesis: Research Perspectives for Shale Hydrocarbon Reservoirs, Seals, and Source Rocks, Wayne K. Camp, Kitty L. Milliken, Kevin Taylor, Neil Fishman, Paul C. Hackley, Joe H. S. Macquaker
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The lower Permian Wolfcamp Shale in the Permian Basin is a major unconventional resource play composed of organic-rich, siliceous and calcareous mudstones interbedded with carbonate turbidites and debrites. Using two cores that comprise the Wolfcamp Shale near the eastern margin of the Midland Basin, this study reconstructs the complex diagenetic history of both the mudstone and carbonate facies. These cores were analyzed using petrographic and SEM techniques to test if the Wolfcamp Shale was an open or closed system and to characterize diagenetic processes that impact reservoir characteristics, such as porosity types, porosity distribution, permeability pathways, and mechanical brittleness.
Early, middle, and late phases of chemical diagenesis are defined in this study. Mineral precipitation and dissolution events occur from the passage of fluids through both interstitial and fracture pore space. Early authigenic mineral precipitation (calcareous and phosphate concretions, sphalerite, barite, framboidal pyrite, quartz, dolomite, and ferroan dolomite) resulted in destruction of primary porosity within the mudstone facies, before and during the mechanical compaction event. Destruction of porosity in the carbonate turbidites facies occurred through carbonate cementation (calcite, ferroan calcite, dolomite, and ferroan dolomite) during early to middle diagenesis. An episode of dissolution and dolomitization in the carbonate facies resulted in the creation of moldic and intercrystalline porosity respectively. Within mudstones intercrystalline porosity is observed between pyrite framboids and clay sheets of chlorite. Diverse fracture types occur in all facies within the Wolfcamp Shale and play a critical role in the migration of diagenetic fluids and hydrocarbons. Horizontal fractures are filled by “beef”-type calcite, and vertical fractures are filled with equant calcite and/or celestine-barite. Mineralized fractures contain porosity, some of which contain ferroan dolomite rhombs within pores, which supports diagenetic fluid movement through fractures after an initial stage of mineralization. Fluid inclusion data suggest that some mineralized fractures acted as fluid conduits for externally derived, warm, high-salinity brines, suggesting the Wolfcamp Shale was an open system during it burial history.
Diagenesis can affect the producibility of unconventional reservoirs by creating or destroying porosity, increasing brittleness, and influencing permeability (e.g., McHargue and Price, 1982; Beall et al., 1998; Stoudt, 1998; Milliken et al., 2012). To better understand the controls on shale diagenesis and to develop robust predictive models, case studies are needed to characterize the range of diagenetic complexity. The Wolfcamp Shale in the Midland Basin is one of the largest unconventional resource oil shale plays assesses by the USGS to date and has enormous economic significance to the United States (Gaswirth et al., 2016). The objectives of this study are to test if the Wolfcamp Shale was an open or closed system and to characterize diagenetic processes that impact reservoir characteristics, such as porosity types, porosity distribution, permeability pathways, and mechanical brittleness. Devon Energy provided two cores from Irion county, Texas, of the Wolfcamp Shale near the Eastern shelf of the Midland Basin for this study (Figure 1). The cores were analyzed using microscopic techniques to identify diagenetic features and build a paragenetic sequence.
The Permian Basin is a foreland basin formed by the late Mississippian/early Pennsylvanian Marathon–Ouachita collision (Yang and Dorobeck, 1995). In the late Pennsylvanian to early Permian, the Central Basin Platform was uplifted, partitioning the Permian Basin into the western Delaware Basin and the eastern Midland Basin (Yang and Dorobeck, 1995). As movement along the Marathon–Ouachita thrust belt ceased during the early-Pennsylvanian, late pre-Cambrian–early Cambrian basement faults were reactivated and propagated in the subbasins through the Wolfcampian stage (Wuellner et al., 1986). The Wolfcamp Shale underwent accelerated subsidence during the Permian before experiencing more subdued burial rates in the Mesozoic (Yang and Dorobeck, 1995), with maximum burial temperatures of 65–85°C (149–185°F) achieved during the mid-Paleocene in the Midland Basin (Mazzullo, 1994).
The Midland Basin is bound by the Central Basin Platform to the west, the Matador Arch to the north, the Marathon–Ouachita Thrust Belt to the south, and the Fort Chadbourne en echelon fault zone on the eastern margin (Ewing, 1990). A carbonate platform shelf outlines the margin of the Midland Basin (Figure 1). Sediment transport into the basin was controlled by a combination of tectonism and rapid subsidence rates, coupled with glacioeustatic sea-level fluctuations during the late Paleozoic icehouse (Brown Jr., 1972; Eberli, 1991). The highstand shedding model has been adopted to explain high-order cyclical shedding of carbonate platform material, cumulatively comprising an overall progradational shelf system during the early Permian (Montgomery, 1996; Mazzullo, 1997).
The Wolfcamp Shale is comprised of carbonate turbidites and debrites interlayered with organic-rich siliceous and calcareous mudstones (Montgomery, 1996; Beall et al., 1998; Mazzullo, 1998; Stoudt, 1998; Baumgardner et al., 2016). The matrix of mudstones is composed of illite/mica, smectite, chlorite, kaolinite, and illite–chlorite mixed layers (Sivalingam, 1990). Additionally, ash beds composed of mixed layers of smectite and illite (Sivalingam, 1990) are observed within the Wolfcamp Shale from volcanism with the Delicias Volcanic arc, Mexico (Centeno-Garcia, 2005). Previously, the Wolfcamp Shale has been stratigraphically divided into cycles A–D from its base to top (Silver and Todd, 1969) and the lower, middle, and upper Wolfcamp (e.g., Mazzullo and Reid, 1989; Montgomery, 1996; Mazzullo, 1997). However, this study adopts the petroleum standard stratigraphic designations, including Wolfcamp A, B, C, D, E, and F from top to bottom.
Samples analyzed in this study were obtained from two vertical cores, designated as the WC and WP cores, which collectively represent 1000 ft (304 m) of the Wolfcamp Shale. The WP core spans the complete Wolfcamp sequence (A–F), whereas the WC core represents a partial E–F sequence. The WC core is about 5 mi (8 km) closer to the eastern shelf margin of the Midland basin than the WP core (Figure 1).
Thin sections (292) were examined with a petrographic microscope to identify mineral components and examine textural relationships with respect to the relative timing of events. Thin sections were created at ~4 ft (1.2 m) intervals and were equally distributed between all facies. Alizarin red S and potassium ferricyanide stains were applied to 50% of each thin section to distinguish calcite, ferroan calcite, and ferroan dolomite. Fluorescence images were captured with an external ultraviolet (UV) light source. Thin sections were analyzed with a FEI scanning electron microscope (SEM) fitted with an electron-dispersive spectrometer (EDS). Carbonates were classified using Dunham’s classification scheme (Dunham, 1962). The Lazar et al.’s (2015) descriptive methodology was used in this study; however, no attempt was made to classify the mudstones (e.g., Milliken, 2014; Camp et al., 2016).
X-ray computed tomography (XRCT) of specimens were performed to investigate fracture morphologies. The XRCT analyses detect density contrasts between minerals that can be used to evaluate different minerals morphology and distribution (Cnudde and Boone, 2013). Analysis of the samples was conducted at the University of Minnesota using a X5000 high-resolution micro-CT system with a twin head 225 kV FeinFocus FXE-225.99 X-ray tube and a Dexela 2923 area detector. The grayscale image slices that are output by this process were segmented and analyzed for identification of different mineral assemblages using the efX-CT Lite 3D© software package (from North Star Imaging Inc.) and compared with thin sections of the samples.
Doubly polished thin sections containing calcite- and celestine/barite-filled fractures were used for fluid inclusion analysis to determine the temperatures (Th) of formation and fluid compositions. A Zeiss Universal microscope, with an attached Linkham THMSG 600 heating and cooling stage and TMS94 controller, was used to identify and analyze fluid inclusion assemblages. Single- and two-phase primary fluid inclusions were observed using standard heating and freezing microthermometric procedures outlined by Goldstein and Reynolds (1994). Fluid inclusions within barite can stretch during fluid-inclusion microthermometry, thereby increasing homogenization temperatures (e.g., Ulrich and Bodnar, 1988). Considering this, mean homogenization temperatures were used to mitigate any error. Pressure corrections were performed using Bodnar and Vityk’s (1994) method, which uses estimated pressure, Th, and salinity of the fluid within inclusions, adjusting for overpressuring of the Wolfcamp Shale during inclusion formation.
Four facies were identified through visual core description and characterized using microscopic techniques: siliceous mudstones, calcareous mudstones, carbonate debrites, and carbonate turbidites (Figure 2). Similar facies descriptors have been used to describe the Wolfcamp Shale by Montgomery (1996), Mazzullo (1997, 1998), Beall (1998), Stoudt (1998), and Baumgardner et al. (2016).
The siliceous mudstone facies (Figure 2A) contains mudstone clasts and wispy organic matter/pyrite laminae resulting in an overall lenticular to laminated fabric. Disseminated framboidal pyrite (5–10 μm) and aggregated framboids (20–50 μm) are common (Figure 3A). The aggregates commonly contain hydrocarbons between the individual framboids (Figure 3A). Partially collapsed agglutinated foraminifera are commonly composed of chert although dolomite, ferroan dolomite, and/or calcite can also occur. Radiolarians, algal cysts, and sponge spicules are composed of single minerals or combinations of microcrystalline quartz, chert, calcite, and dolomite. Silt-size grains of quartz (10–20%), crystals of dolomite (10–30%), feldspar (~10%) 4–10 μm, and muscovite (<5%) are dispersed throughout the mudstone matrix. Some detrital quartz grains have overgrowths. Some dolomite grains display apparently rounded corners inside of ferroan dolomite rims (Figure 3B). Ferroan dolomites (10–30%) dominantly have rhombohedral habits (5–20 μm). Chlorite (<5%) occurs dispersed in mudstone matrix with intercrystalline porosity (2–20 μm) commonly containing interstitial hydrocarbons. Sheets of chlorite with interstitial hydrocarbons also occur in fractured celestine-barite in the matrix adjacent to a mineralized fracture (Figure 3C).
In one siliceous mudstone sample, prolate barite grains (1–10 mm) (Figure 3D) are clustered in a horizontal layer and occurs with amorphous spherical sphalerite (500–1000 μm). Sphalerite (Figure 3E) also occurs in rare 1 mm–1 cm laminae accompanied by quartz, calcite, and ferroan dolomite in siliceous mudstones and has also been observed in distorted laminae around concretions within the mudstone matrix (Figure 3F). Phosphate (Figure 3F) and calcite concretions, 1–4 cm in diameter, are common in siliceous and calcareous mudstones. Concretions commonly incorporate matrix material within their structure. Barite and celestine also occur in some phosphate concretions (Figure 3G). The core of the calcite concretions commonly contains iron-poor calcite surrounded by ferroan calcite on the margin. Ferroan dolomite occurs between ferroan calcite crystals and extends out into the mudstone matrix.
The calcareous mudstones (Figure 2B) contain 20–40% calcite fossil fragments, 10–30% silt-sized quartz, ~10% feldspar 4–10 μm in size, and intact fossils (algal cysts and sponge spicules). Allochems are commonly composed of one or more of the following: calcite, dolomite, ferroan carbonates, or chert (Figure 3H). Articulated fern-like plant fossils and Chondrites trace fossils are present in the mudstone facies.
The carbonate debrites (Figure 2C) are composed of carbonate rock fragments (CRFs) (fossiliferous grainstones and packstones) and mudstone rip-up clasts supported in a clay matrix that contains less than 10% silt-size quartz, feldspar, dolomite, and ferroan dolomite. The CRFs have distinct erosional boundaries and contain allochems and cements, such as fibrous calcite, equant calcite, dolomite, ferroan dolomite, chert, and authigenic albite. The reader is referred to other studies for detailed descriptions of the diagenesis in the CRFs (Mazzullo, 1997, 1998; Beall et al., 1998).
The carbonate turbidites commonly exhibit a fining-upward sequence (Figure 2D) and are classified as fossiliferous wackestones, packstones, and grainstones. Pervasive cements include chert, calcite, ferroan calcite, dolomite, and ferroan dolomite in the fossiliferous grainstones with microcrystalline cements in the fossiliferous packstones and wackestones. Allochems are commonly partially replaced by calcite, pyrite, ferroan calcite, dolomite, ferroan dolomite, chert, minor chalcedony, and rarely barite. Intergranular porosity (10–50 μm) occurs in association with carbonate beds composed of chert and dolomite (Figures 2D, 4A), commonly near the base of turbidite sequences. Additionally, some turbidites contain sucrosic dolomite with moldic porosity (100–300 μm) (Figure 4B).
Mineralized fractures are common in all facies. Horizontal fractures are filled with calcite “beef” textures (Cobbold et al., 2013; Figure 4C), where fibrous calcite is oriented perpendicular to fracture walls. These mineralized fractures occasionally contain kerogen or relic hydrocarbons in the center of the calcite fracture fill (Figure 4C). Additionally, some vertical calcite-filled fractures fluoresce (Figure 4D).
Complex celestine-barite filled and calcite-filled fracture morphologies (Figure 4E, F) are present in the XRCT scans. Fracture thickness varies from ~10 μm to ~10 mm with large euhedral barite crystals (~1–3 mm) occurring within larger, subvertical fractures. One inch (25 mm) to 15-ft (4.5-m)-long linear, continuous, vertical fractures with rough fracture boundaries (Figures 3G; 4D, G–I) commonly occur in the siliceous mudstones but are present in all facies. Vertical fractures commonly terminate against carbonate intervals or have complex anastomosing patterns within debrites (Figure 4G). Some vertical calcite-filled fractures have ferroan carbonates on the edges (Figure 4H). Pores occur within healed fractures and may contain ferroan dolomite rhombs (Figure 4H). In some samples, vertical celestine-filled fractures cross cut barite in phosphate concretions (Figure 3G). Some fracture fills transition from celestine on the outer edge to barite in the center (Figure 4I). Pores are also present along the contact between celestine and barite, some of which host dolomite rhombs (Figure 4I) with ferroan rims. In some celestine–barite-filled fractures (Figure 4F), calcite occurs along the fracture-matrix boundary and in fracture porosity.
Fluid inclusion microthermometry was performed on calcite- and barite-filled fractures in the WP core and calcite-filled fractures in the WC core (Figure 5). Homogenization temperatures (Th) for fluid inclusions in calcite-filled fractures display a bimodal distribution and range from 41 to 65°C (106 to 149°F) (mean: 57°C [135°F]) in the WP core and 53–104°C (127–219°F) (mean: 69°C [156°F]) in the WC core (Figure 5). Barite-filled fractures in the WP core range from 77 to 125°C (171 to 257°F) (mean: 102°C [216°F]) (Figure 5). Salinity of the fluids within barite-filled fractures, based on ice-melting temperatures (TmI) and first ice-melting temperatures (Tme) (Goldstein and Reynolds, 1994), suggest a CaCl2-rich brine of ~25 Wt. % (Figure 5). Because the Wolfcamp Shale is currently overpressured (e.g., Engle et al., 2016), a pressure correction was applied. Based on 25 Wt. % CaCl2 content and utilizing Bodnar and Vityk’s (1994) method, the temperature of entrapment (Tt) for barite fluid inclusions increases by ~3°C (~6°F) to 81–128°C (178–262°F) (mean: 105°C [221°F]) (Figure 5). TmI or Tme for the calcite-filled fractures were not obtained because of inadequate inclusion size.
The diagenetic events identified in this study are interpreted as early, middle, and late based on textural relationships as discussed below. Insufficient data were available for the determination of absolute age or burial depth. Paleomagnetic dating (Elmore et al., 2012), or other absolute dating approaches, and burial history modeling would be necessary to attempt more specific interpretations. Diagenetic events discussed below are framed within interpreted facies. However, the paragenetic sequence (Figure 6) constructed for this study aggregates the diagenesis associated with the matrix, allochems, and fracture-fill mineralogy of all facies. A conceptual model was constructed of the paragenetic sequence, showing the interpreted diagenetic progression within the Wolfcamp Shale (Figure 7).
Siliceous and Calcareous Mudstone Diagenesis
Within mudstones, primary porosity was reduced by mechanical compaction, growth of concretions (Figure 3F), and precipitation of authigenic minerals in the matrix such as pyrite (Figure 3A), barite, sphalerite (Figure 3D), dolomite, ferroan dolomite (Figure 3B), calcite, and chert (Figure 7). Based on the distorted mudstone fabrics around the carbonate and phosphatic concretions (Figure 3F), they likely formed as pre- or syn-compactional features during early diagenesis (Figure 7). Compositional zoning from iron-poor calcite in the center, to ferroan calcite on the outside, suggests carbonate concretions formed in the sulfate reduction zone during early diagenesis (Hesse and Schacht, 2011). Many of the phosphate concretions contain barite with quartz crystals along the edge (Figure 3G), interpreted as occurring after the formation of the phosphate concretion. The barite may have originated from remobilized barium and sulfate during early-to-middle diagenesis (e.g., Hanor, 2000), whereas the quartz may have derived from biogenic silica (Schieber et al., 2000; Hesse and Schacht, 2011; Milliken and Olson, 2017), or shale dewatering could have mobilized silica, magnesium, and iron as described by Marshall (1982) and Cobbold et al. (2013). Alternatively, the quartz may be derived during middle diagenesis from silica released during the conversion of smectite to illite as described by Coniglio and James (1988), Lumsden (1988), and Sivalingam (1990). Quartz overgrowths on detrital quartz grains in mudstones, which presumably filled primary porosity, and quartz and chert in replaced fossils could have been derived from the same sources as quartz in concretions listed above.
Distorted sphalerite laminae around concretions (Figure 3F) within the mudstone matrix suggest that sphalerite most likely formed before compaction during early diagenesis (Figure 7). Sphalerite also appears to have displaced barite crystals (Figure 3D), suggesting that barite formed before sphalerite. Barite and sphalerite could have formed during bacterial sulfate reduction when zinc, barium, and sulfur were produced by bacterial activity in the sediment (e.g., González-Muñoz et al., 2003; Hesse and Schacht, 2011; Peltier et al., 2011; Selleck, 2014).
Rounded corners of some dolomite grains (Figure 3B) suggests they could be detrital and derived from the platform (e.g., Mazzullo, 1997). The ferroan dolomite overgrowths occluded primary porosity in mudstones (Figure 3B). Ferroan dolomite precipitated into the mudstone matrix as calcite concretions grew and became incorporated into the outer part of calcite concretions. During early diagenesis bacterial sulfate reduction and/or shale dewatering could have mobilized magnesium and iron, allowing for rhombic dolomite and ferroan dolomite to form (McHargue and Price, 1982; Coniglio and James, 1988; Hesse and Schacht, 2011; Blättler et al., 2015).
The textural relationship between chlorite and celestine-barite suggests that chlorite formed after celestine and barite filled fractures (Figure 3C). Chlorite is interpreted as forming during middle to late diagenesis when iron and magnesium would be available from shale dewatering or the smectite–illite conversion. The presence of hydrocarbons between the clay sheets (Figure 3C) suggests that chlorite likely formed before peak hydrocarbon migration.
Carbonate Turbidites and Carbonate Debrites Diagenesis
Carbonate cementation of the matrix and fossils in carbonate turbidites is pervasive (Figures 3H, 7) and occurs to a lesser extent within carbonate debrites (Figure 4G). Textural relationships in carbonate grainstones suggest that ferroan dolomite, dolomite, and ferroan calcite cementation over printed calcite and microcrystalline chert cements (Figure 3H). Calcite cementation followed by chertification are interpreted as occurring early because of the lack of mechanical compaction within the turbidite facies (e.g., Beall et al., 1998). Dolomite, ferroan dolomite, and ferroan calcite are interpreted as forming during the early to middle diagenesis when iron and magnesium would have been available in the system during shale dewatering or smectite converting to illite. These cementation events could coincide with the equivalent precipitation of dolomite, ferroan dolomite events in mudstones facies.
Dolomitization is interpreted to have occurred after chertification based on the preservation of intercrystalline dolomite porosity (Figure 4A) and cross-cutting relationships. Chertification and dolomitization in the carbonate turbidites could have occurred concurrent with the chert and dolomite associated with mudstones during early diagenesis. Alternately, chertification and/or dolomitization of carbonate turbidites could be a later separate event sourced by the smectite to illite conversion in mudstones when silica, iron, and magnesium were remobilized during middle to late diagenesis. This interpretation is reinforced by the juxtaposition of siliceous mudstones with the pervasive chert in the turbidite facies. The moldic porosity (Figure 4B) in carbonate turbidites suggests that a dissolution event occurred after the cementation of the carbonate facies, otherwise the pores most likely would have been filled by one of the cements.
Fracture Genesis and Morphology
Several episodes of overpressuring or tectonic activity could have generated fractures within the Wolfcamp Shale (e.g., Marshall, 1982; Yang and Dorobeck, 1995; Cobbold et al., 2013). Horizontal fractures commonly occur in shale basins that experience high subsidence rates (Marshall, 1982; Yang and Dorobeck, 1995). Horizontal “beef” calcite-filled fractures (Figure 4C) are interpreted to have formed during early to middle diagenesis when shale dewatering and hydrocarbon generation could have produced overpressure (Figure 7; e.g., Marshall, 1982; Cobbold et al., 2013). Vertical fractures filled with equant calcite cement are interpreted to have formed in lithified rock during middle to late diagenesis (Figure 7). Homogenization temperatures from fluid inclusions within calcite fracture fill (WC core mean of 57°C [135°F] and WP core mean of 69°C [156°F]) supports precipitation during middle to late diagenesis. The bimodal distribution of homogenization temperatures (Figure 5) suggests the possibility of multiple generations of calcite fracture fill. Textural relationships suggest that calcite fracture fill events occurred before and after celestine–barite fracture fill. An isotopic study is necessary to establish more detailed interpretations.
Fracture networks within the cores appear to have acted as migration pathways for external fluids to enter the Wolfcamp Shale. The salinity of the fluid within the inclusions (~25 Wt. % CaCl2) supports this scenario. Engle et al. (2016) determined the Wolfcamp Shale pore fluids to be less than 8.5 Wt. % salinity. This suggests that the fluid inclusions containing high-salinity brines could be externally derived. The mean entrapment temperatures of fluid inclusions in barite (105°C [221°F]) are also approximately 20°C (36°F) above inferred maximum burial temperatures (~85°C [~185°F]) calculated by Mazzullo (1994), which is consistent with external fluids.
Most of vertical fractures contain equant calcite fill, and some vertical fractures contain alteration to ferroan carbonates along their edges (Figure 4H). This suggests that fluids were able to migrate along the fracture–matrix boundary. Mineralized fractures that change from celestine on the margin to barite in the center (Figure 4I) suggest that brine fluids evolved during precipitation and reflects a common solid solution process for healing fractures (Hanor, 2000). The presence of equant calcite within Ce–Ba-filled fractures may indicate fracture reactivation. Additionally, the calcite and celestine–barite-filled fractures with open pores and dolomite occluded pores (Figure 4H, I) suggest that fluids continued to migrate through fractures even after they were mostly healed.
Hydrocarbon Migration and Mechanical Brittleness
A hydrocarbon migration event may have occurred after dissolution events, based on the presence of hydrocarbons in intragranular pores in allochems and in moldic pores. The presence of hydrocarbons in horizontal fractures (Figure 4C) suggests that hydrocarbon migration was likely contemporaneous with fracturing. The hydrocarbons that fill porosity between chlorite sheets (Figure 3C) may be relatively late because they were emplaced after fracturing of celestine/barite and formation of the chlorite. The vertical-fracture networks that fluoresce under ultraviolet light (Figure 4D) suggest that fractures may have facilitated hydrocarbon migration. An organic geochemical study of the Wolfcamp Shale kerogen types would be necessary to determine if the hydrocarbons were produced solely from the Wolfcamp Shale or were externally derived. The relative timing of hydrocarbon migration during middle/late diagenesis is broadly consistent with Engle et al.’s (2016) assertion that peak hydrocarbon generation occurred during the Late Cretaceous.
Diagenesis can increase the brittleness of rock by precipitation of quartz, chert, and dolomite (Jin et al., 2015), which can result in more efficiently breaking rock when hydraulic fracturing methods are used (Taleghani and Olson, 2014; Li et al., 2015). Petrographic observations suggest that there are at least qualitative differences in brittleness among the four main facies. In carbonate turbidites, pervasive carbonate cementation and chertification increased brittleness. High amounts of quartz and chert increased the brittleness of siliceous mudstones. A lower brittleness is suggested in the calcareous mudstones and debrites owing to their relatively high clay content and low amount of quartz, chert, and dolomite (e.g. Jin et al., 2015). Natural fractures extend continuously through siliceous mudstones and terminate against or bifurcate in calcareous mudstones or debrites, suggesting that the former may fracture more efficiently than the latter facies. In addition to fluid conduits, fractures may also behave as preexisting planes of weakness, which may reactivate during hydraulic fracturing (Gale et al., 2014; Taleghani and Olson, 2014; Li et al., 2015). Fractures filled with calcite, dolomite, barite, and quartz are prone to fracturing because of their brittle nature (Dehandschutter et al., 2005); however, host rock properties are also a factor. For example, Gale et al. (2014) asserted that in shales reactivation tends to be seen in steep fractures in low-clay facies. This suggests that preexisting vertical mineralized fractures in the Wolfcamp shale that are within lower clay composition facies would refracture efficiently when hydraulically stimulated.
Internal and External Fluids
Observations from this study suggests that most authigenic minerals in the Wolfcamp Shale were derived from internal fluids. For example, during initial burial, bacterial interactions caused decomposition of organic matter and the formation of minerals such as barite/sphalerite (González-Muñoz et al., 2003; Peltier et al., 2011), framboidal pyrite, concretions, and ferroan dolomite (Hesse and Schacht, 2011; Blättler et al., 2015). Dissolution and replacement of silica microfossils probably provided the silica for authigenic quartz and chert (Hesse and Schacht, 2011). Subsequent shale dewatering in the mudstone facies may have caused the movement of pore fluids (derived from Mg-rich aragonite seas) and supplied magnesium, silica, and iron, which could be used to form ferroan carbonates and chert. The smectite-to-illite conversion could have provided silica, iron, and magnesium (Coniglio and James, 1988; Lumsden, 1988; Sivalingam, 1990), which were used to form chlorite, in addition to cementing carbonate intervals with chert and ferroan carbonates. Celestine in some of the fractures, for example, can be explained by the Sr released from replaced aragonite (Hardie, 1996). Similarly, the calcite in fractures could have been internally derived from allochems in carbonate facies. Marshall (1982) argued that in shale basins with high rates of subsidence, such as the Midland Basin, differential overpressuring can aid pore fluid migration not just through fracture systems, but also by increasing permeability intervals within the mudstone matrix. However, the Wolfcamp Shale did not remain exclusively a closed system. As discussed above, the presence of high-salinity brines and relatively high homogenization temperatures in fluid inclusions within barite fracture fills suggests the Wolfcamp Shale became an open system at some point during its paragenesis.
The complex diagenetic history of the Wolfcamp Shale results from precipitation and dissolution events during burial. Early diagenesis is characterized by precipitation of precompactional framboidal pyrite, phosphate and calcareous concretions, barite, sphalerite, and quartz, all of which occluded primary porosity and enhanced brittleness in mudstones. Early to middle diagenesis carbonate cementation (calcite, ferroan calcite, dolomite, and ferroan dolomite) and chertification in carbonate turbidites, and to a lesser extent in carbonate debrites, also occluded primary porosity and enhanced brittleness. Intercrystalline and moldic porosity occur within dolomitized carbonate turbidites. Intercrystalline porosity occurs within framboidal pyrite and middle to late diagenetic chlorite within mudstones. Horizontal fractures are interpreted to have developed from early to middle diagenesis, whereas vertical fractures formed during middle to late diagenesis. Petrographic analysis and XRCT scans reveal anastomosing and linear equant calcite and celestine–barite-filled fracture sets, some of which contain porosity that may have been migration pathways for fluids, including hydrocarbons. Additionally, fractures filled with brittle minerals could represent preexisting planes of weakness that could be refractured during hydraulic stimulation.
Authigenic mineralization can be explained by internal fluid mechanisms, such as bacterial sulfate reduction, shale dewatering, and illite converting to smectite. Some mineralized fractures, however, acted as conduits for external high-salinity fluids to enter the Wolfcamp Shale. Although the Wolfcamp Shale may have been a closed system for the majority of its diagenetic history, high salinities and relatively high homogenization temperatures in fluid inclusions suggest the system was open at least one time during its burial history.
The authors thank Devon Energy for funding the research and providing data and, Dr. Erik Kvale for his guidance, lending his expertise, and reviewing an early draft. Additionally, thank you to Dr. Mark Evans for imparting his knowledge about fluid inclusions and Brian Cardott for his consultations on organic petrology.