PSF URTeC 1923429_revised_abstract

URTeC: 1923429

Core-based structural fabrics in mudstones of the WCSB : ‘PSF’ and cleavage.

Graham R. Davies¹*, David Hume², Amy Fox², Steve Haysom³, Glen Nevokshonoff³, and Roger Reinmiller⁴

¹Graham Davies Geological Consultants (GDGC) Ltd., Calgary, Alberta

²Canadian Discovery Ltd., Calgary, Alberta

³Seven Generations Energy Ltd., Calgary, Alberta

⁴Fronterra Geosciences, Denver, Colorado

Copyright 2014, Unconventional Resources Technology Conference (URTeC)

This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Denver, Colorado, USA, 25-27 August 2014.

The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s).  The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein.  All information is the responsibility of, and, is subject to corrections by the author(s).  Any person or entity that relies on any information obtained from this paper does so at their own risk.  The information herein does not necessarily reflect any position of URTeC.  Any reproduction, distribution, or storage of any part of this paper without the written consent of URTeC is prohibited. 

Introduction

Examination of over 1700 carbonate and siliciclastic cores in the Western Canada Sedimentary Basin (WCSB – including northern Montana) and to a lesser extent the Williston Basin by the first author over the last 35 years has revealed the common occurrence of linear structural fabrics along bedding planes in mudrocks (particularly where overpressured), and a related ‘ridge and groove’ fabric in limestones.  In mudstones, these fabrics are manifested as polished slip faces (PSF), typically but not always with slickensides/striae, and cleavage.  The same or similar fabrics have been observed in outcrop in the Utica shale of Quebec (Denis Lavoie, pers. com., 2011), and in core and outcrop in the Marcellus shale in New York State (Fronterra Geosciences).  All of these fabrics are attributed to layer-parallel shortening (LPS).

Figure 1 : Formations in Western Canada Sedimentary Basin (WCSB) with horizontal shear/PSF and/or cleavage. Blue: limestones; grey: fine siliciclastics/mudstones.

Increasing exploration for oil and gas in fine-textured unconventional ‘mudrock’ reservoirs has brought focus on the potential role of these structural fabrics as an expression of subsurface stress conditions, in orientation and behavior of hydraulic fractures, and for anisotropic reservoir permeability implications.  Release of paleomag oriented core data in one of these reservoirs in the WCSB adds many insights to the orientation and significance of the structural fabrics.

Stratigraphic distribution of structural fabrics

Host rocks for horizontal shear fabrics (PSF) and cleavage in siltstones and ‘mudrocks’, and ‘ridge and groove’ structures in limestones (not further documented here: see Davies et al, 2013, figures on p. 38, 39), in the WCSB range in age from Devonian to Cretaceous (Fig. 1).  Siltstone-mudstone reservoirs with these fabrics include the Second White Specks, Fish Scales, Montney, Duvernay and Muskwa, and the Marias River in Montana.  The fabrics are present but not well documented (by the present authors) in the Bakken in North Dakota.

Geographic / basinal distribution of structural fabrics

The distribution of PSF and cleavage in cores in siltstones and mudstones, and of ‘ridge and groove’ fabrics in limestones in the WCSB is illustrated in Figure 2.  Note that the highest frequency of occurrence is within 100 kms east of the surface expression of the main ‘Laramide’ deformed belt, suggesting a genetic link.  The furthest occurrence eastward is about 200 km.

Figure 2 : Distribution of PSF, cleavage, LST fabrics.

Core examination techniques for bedding-orientated fabrics

Quantification and documentation of PSF and cleavage (and related fabrics) requires time-consuming examination of the ends of all core segments, preferably full diameter or at least the thicker core butts if slabbed.  If the core is plastic/saran wrapped, this must be removed if ends of the core segments are to be examined systematically.  Photographic recording of the structural fabrics requires the use of oblique lighting to ‘shadow-highlight’ striae and cleavage lineations.

Polished Slip Faces (PSF)

The first author’s core-based descriptor for the dominantly bed-parallel shear surfaces in siltstones and mudstones is Polished Slip Faces (PSF: Fig. 3) rather than simply ‘slickensides’, as some PSF do not preserve slickenstriae, and there is a range of associated structural and diagenetic fabrics and products that are included in this designation.  Where striae of PSF and cleavage lineation are present in the same core segment, the two lineations are orthogonal (i.e, 90^o) to each other.  PSF occurs most frequently along more argillaceous/micaceous and/or ‘bituminous’ bedding planes or seams (condensed sedimentation boundaries), but also may be oblique to bedding, convolute, or overfolded.  They may occur in ‘swarms’ at spacing in core of less than 2 mm apart.  Where most frequent, and more inclined and deformed, PSF intervals record fault zones, particularly thrust displacement.  PSF increase in frequency in the subsurface westward towards the Laramide deformed belt (Fig. 2).

Figure 3 : Polished Slip Faces (PSF)

Shear and frictional heating along PSF slip boundaries form black or ‘brassy’, highly polished to mirror-like surfaces a few microns to 200+ microns thick.  Steps or microasperities along the PSF surfaces may develop open intercrystalline pore space on the ‘lee’ side (Figs. 4, 5, 6).  Some inclined PSF grade into microbrecciated shear zones with open porosity (Fig. 7).  PSF also forms displacement/slip surfaces for micro-duplex structures and ‘buckle’ breccias (Fig. 8).  Cements in all of these fabrics include calcite, dolomite (including saddle forms), anhydrite, and pyrite.  Less ductile coarser siltstone or sandstone beds bounded by PSF may be disrupted by intrabed fractures, often but not always parallel to striae on PSF surfaces (Fig. 9).  Bentonites, if present, are preferred sites for PSF formation (particularly in Second White Specks, Marias River).

Under reflected light (R_o), all fluorescence of organic matter in PSF layers is extinguished, but normal reflectance may occur in host rocks 20 to 100 microns from the PSF layer.  In some Second White Specks samples, vitrinite/bitumen reflectance analysis (by JP Petrographics) indicates presence of partial graphitization of bitumen within the PSF layer, with temperature calculations suggesting that PSF surfaces may have been exposed to temperatures of 75^oC (order of magnitude) hotter than the ambient temperature of the host rock (Fig. 10).

Figure 4 : PSF fabric with microasperities/steps as observed in Second White Specks, Fish Scales, Marias River, Montney, Duvernay, Muskwa, and Watt Mtn formations.

Figure 5 : PSF fabric with microasperities.

Figure 6 : PSF fabric with microasperities, Second White Specks.

Figure 7 : Inclined PSF and microbreccias

Figure 9 : Intrabed fractures bounded by PSF.

Figure 8 : PSF – controlled ‘buckle’ breccias, fractures.

Figure 10 : Relative temperature implications of VRo/BRo analyses

Cleavage

Vertical parallel linear fabrics observed in mudstone cores by the first author over the last 20 years or so were originally designated ‘pseudocleavage’, but with increasing confidence over the last three years or so have been designated simply as ‘cleavage’ (Fig. 11).  This fabric is interpreted to record early-onset, low-grade metamorphic cleavage, increasing in frequency of occurrence in cores towards the ‘Laramide’ deformed belt (Fig. 2).

Figure 11 : Cleavage in mudstones.

Cleavage in the WCSB is confined to thin, homogenous mudstone beds, not (obviously) extending into or through adjacent coarser-textured siltstone or sandstone beds.  Mudstone beds with cleavage generally show lower TOC than PSF zones.  Spacing of cleavage lineation varies from a few millimeters to greater than 1 cm, with closest spacing apparently in thinnest beds.  Cleavage as observed in core is not an open fracture system: the cleavage lineation records a fabric discontinuity (but a boundary surface at nanoscale?) that is ‘subfissile’ but never mineralized.  In higher grade metamorphic rocks, the cleavage faces are defined by orientated phyllosilicate minerals, but SEM observation of cleavage faces in Second White Specks and Montney cleavage fabrics (to date) do not show recognizable reorientation of phyllosilicates.

Where cleavage and PSF are present in the same continuous core segment, cleavage lineation and striae on PSF are orthogonal (ca. 90^o) to each other (Figs. 12, 13).

It is probable that the cleavage in mudstones in the WCSB is analogous to ‘pencil cleavage’ in Devonian rocks of the Appalachian plateau (Engelder and Geiser, 1979).

Figure 12 : Geomechanical relationship between PSF and cleavage.

Figure 13 : Geomechanical relationship between PSF and cleavage.

Stress field orientation of PSF, cleavage

In earlier reports on PSF and cleavage (Davies et al, 2013), striae on PSF were inferred to be parallel to maximum principal horizontal stress (S_Hmax), with cleavage (orthogonal to striae) parallel to S_Hmin and wellbore breakout (Fig. 14).  This also is the stress orientation of high-grade metamorphic cleavage.  The implication was that, for the WCSB, the striae on PSF probably were formed by shear in a grossly ENE-WSW direction, orthogonal to thrust-borne displacement and shortening along the Laramide deformed belt.

Figure 14 : Inferred orientation of PSF striae and cleavage in WCSB (pre orientated Montney core).

Release of paleomagnetic orientation data for a complete Montney core in the Kakwa area (T64, 5W5) of Alberta by Seven Generations Energy Ltd. (analysis by Applied Paleomagnetics Inc., Van Alstine and Butterworth, 2013), specifically to determine orientation of structural fabrics, now confirms this orientation interpretation (Fig. 15),and provides many additional structural insights.

The analysis confirms that the average difference angle between cleavage and PSF striae is 89^o (range 87 to 92^o), as observed in many other unoriented cores in other formations.  This orthogonal trend is expected if both structural fabrics formed in the same paleostress field and are manifestations of ‘layer-parallel shortening’ (Van Alstine and Butterworth, 2013).  The average cleavage orientation in the Kakwa core is 115^o±7^o; that is, ESE-WNW (Fig. 15).  The cleavage planes in the Kakwa core are nearly vertical to bedding, implying that they formed when the principal stress was horizontal, probably in a strike-slip fault stress regime (ibid.).  The mean orientation of striae on PSF in the oriented core is 28^o, or NNE-SSW (Fig. 15).

Van Alstine and Butterworth (2013) note that the low-angle shear of PSF and formation of cleavage are both manifestations of layer-parallel shortening typically observed in orogenic forelands ahead of a deformation front, with the Marcellus Shale in New York State as a prime example (Engelder and Geiser, 1979; Engelder, 2008).  ‘Pencil’ cleavage in the Marcellus is interpreted to reflect layer-parallel shortening of about 10%, and occurs up to 150 km ahead of the Alleghenian structural deformation front (ibid.), forming prior to the development of first-order folds (note that in the WCSB, PSF and cleavage are most common within 100 kms east of the deformed outcrop belt, but extend as far as 200 kms east: Fig. 2).

Although not discussed in detail here, the paleomag stress analysis of the Kakwa Montney core (particularly orientation of induced petal fractures: Fig. 15) indicates an in-situ S_Hmax of 24^o, essentially orthogonal to the average cleavage direction and close to PSF striae orientation (Van Alstine and Butterworth, 2013.).  However, data for west-central Alberta in the World Stress Map shows an ‘expected’ S_Hmax trend of 42^o.  This apparent discrepancy may be influenced by underlying Leduc reefs (refraction of stress axes), and/or reflecting a late stage in the Laramide orogeny when hydrocarbons were maturing, and layer-parallel shortening was occurring in the Montney (and other formations) and forming PSF and cleavage (ibid.).  As Laramide compressional deformation in this region probably ended by Middle Eocene time, the structural fabrics in the Kakwa core must have developed prior to ca. 42 Ma (ibid., and Wright et al, 1994).  This is further constrained by illite age dating of major thrust faults in the Foothills of the Canadian Rockies (van der Pluijm et al, 2006) that showed that most of the thrusting occurred in two pulses, designated the Rundle (72 Ma) and McConnell (52 Ma) stages, suggesting cleavage in the Montney occurred in the later stage.

Low-angle shear, polished slip faces in the Marcellus Shale

As noted in the preceding discussion of cleavage, the Marcellus Shale in New York State is characterized at least locally by ‘pencil’ cleavage (Engelder and Geiser, 1979; Engelder, 2008).  Fronterra Geosciences has also documented low-angle shear zones, some with ‘polished slip faces’, and associated fractures (Figs. 16, 17) in Marcellus core, and also in outcrop (Fig. 18).  The relationship of shear surfaces to cleavage is not defined.  Wellbore image analysis ground-truthed by core and illustrated in some of these figures emphasizes the value of image analysis logs in interpretation of these types of structural fabrics.

Figure 15 : Paleomagnetically oriented induced & natural fractures in the Lower Triassic Montney and Doig Fm cores from 7Gen HZ Kakwa 2-28-64-5.  The average strikes of the major fracture sets are Cleavage = 115o, Slickenlines on subhorizontal shears = 28o, Set 1 natural = 109o, Set 2 natural = 3o, Set 3 natural = 77o, Listric normal microfaults = 114o, and Induced petal fractures = 24o. Courtesy Seven Generations Energy Ltd.

Figure 16 : Core-image log comparison of bed-parallel shear in Marcellus Shale. Fronterra Geoscriences.

Figure 17 : Polished slip faces in Marcellus Shale.  Fronterra Geosciences.

Figure 18 (Above) : An outcrop example of broken beds of carbonate surrounded by more ductile shale “mushwad” near the roof thrust in a faulted zone of the Union Springs member of the Marcellus Formation exposed at the Hanson Aggregates Quarry in Oriskany Falls, N.Y.  (Left) : The image snapshot on the right shows a broken carbonate bed that has developed a small-scale rollover anticline (red arrow) having been thrust on top of the bed in front of it.  This geometry is that of an individual horse within a cleavage duplex.  The outcrop and the image log from the sub-surface have similar characteristics.  Fronterra Geosciences.

Interpretation and reservoir implication of PSF and cleavage

PSF and production in Second White Specks

Some wells in the Upper Cretaceous Second White Specks shale in Alberta have produced in the order of 1 MM barrels of oil without an obvious fracture connection.  A composite plot of multiple cored wells in the region (Fig. 19) shows that a zone of high frequency of PSF occurs in the transition from siltier, more calcareous and dolomitic mudstones (less ductile, lower Poisson’s ratio, higher Young’s modulus) into higher TOC shales (more ductile, higher Poisson’s ratio, lower Young’s modulus).  A similar geomechanical control of PSF distribution is observed in the Duvernay, Muskwa and Montney.  In the Pine Creek composite (Fig. 19), the upper cluster of PSF corresponds closely to the majority of perfed intervals.  The implication is that horizontal shear faces have contributed to preferential reservoir anisotropy and enhanced production.

Figure 19 : TOC – log – facies – structural fabric correlation, Upper Cretaceous lower White Specks, Pine Creek area, Alberta.

PSF and fractures in the Muskwa shale

PSF (and some cleavage) is abundant in the lower 15 m of the Upper Devonian Muskwa shale (ca. Duvernay equivalent) in NE BC (Fig. 20).  Core control allows documentation of frequency of PSF per metre of core.  The frequency increases downward toward the top of the underlying regional Slave Point carbonate reef/platform from 2 or 3 per metre about 15 m above the Slave Point, to 8 per metre, to uncountable swarms in the lower few meters.  High-angle fractures, some cemented and terminating at PSF surfaces, also increase downsection with increasing PSF frequency.  This PSF distribution records extensive shear and lateral displacement at the Muskwa-Slave Point boundary and transition, and thus probable thrust faulting and décollement.

Lateral displacement of PSF

Figure 21 is a summary of the implications of lateral displacement within a core segment with multiple PSF surfaces.  The largest unknown is the actual distance of offset at each PSF face.  It is probable that thermal properties of different faces as determined by vitrinite/bitumen reflectance analysis might indicate relative degree of displacement, but only a few such analyses have been run to date.

Figure 20 : PSF and fractures, Muskwa – Slave Point boundary, Upper Devonian, NE BC.  Courtesy Spectra Energy.

Figure 21 : Implications of PSF in WCSB.

Implications and conclusions from Montney oriented core

The pervasive structural fabrics (mainly PSF and cleavage) in the Montney Kakwa core almost certainly have major reservoirs permeability implications (Van Alstine and Butterworth, 2013).  A strong directional anisotropy is expected, with maximum permeability along an 115^o – 295^o axis defined by the average cleavage trend.  In published reports, permeability across cleavage planes can be orders-of-magnitude lower than permeability parallel to cleavage planes (ibid.).

The cleavage in the Montney in the Kakwa area (and elsewhere?) gives rise to a 24^o S_Hmax axis reflecting ‘structural anisotropy’ rather than ‘stress anisotropy’ (ibid.).  Drilling horizontal wells perpendicular to 24^o (i.e., azimuth of either 114^o or 294^o) may be more reasonable (ibid.) than perpendicular to the 42^o S_Hmax based on the World Stress Map (latter based mainly on wellbore breakouts, mainly carbonates?).

An analogy may be made with the Barnett Shale, where the predominant set of natural fractures strikes at a high angle (60^o) to S_Hmax (Gale et al, 2007, 2010; ibid.).  When hydraulic fractures encounter natural fractures in the Barnett, the energy is dissipated over a much wider area than if no fractures were present, creating a diffuse ‘cloud’ of microsiesmic events (Van Alstine and Butterworth, 2013).  (Note, however, cleavage in the Montney and all other formations with cleavage in the WCSB do not show open fracture geometry under present post-coring conditions).

Van Alstine and Butterworth (2013) note that in the Montney horizontal wells in the Kakwa field, hydraulic fractures striking parallel to the inferred 24^o S_Hmax might be expected to cross and split open millions of cleavage planes striking perpendicular to the hydraulic fracture.

Summary and conclusions

• Polished slip faces (PSF) and cleavage are common in mudstones of Devonian to Cretaceous age in the WCSB.
• Striae on PSF and cleavage lineations are orthogonal to each other.
• Both fabrics are interpreted to be the product of Laramide-related layer parallel shortening (LPS).
• Limestones in the WCSB show related linear structural fabrics.
• PSF often is concentrated at the geomechanical boundary or transition between less ductile (lower Poisson’s ratio, higher Young’s modulus) and more ductile (higher Poisson’s ratio, lower Young’s modulus) rock units, including formational and intraformational facies boundaries: for examples, the Duvernay-Ireton boundary.
• In at least one productive interval in the Cretaceous Second White Specks in Alberta, higher concentrations of PSF correlate to a geomechanical boundary, increasing TOC, and highest frequency of perfed intervals.
• Slip along PSF bounding less ductile siltstone or sandstone beds allows formation of open intrabed fractures, with at least some of these fractures parallel to striae of associated PSF (thus parallel to S_Hmax) – observed in Montney, Second White Specks, Marias River.
• Slip on PSF has also created ‘mini-duplex’, ‘buckle breccia’ and microfold structures, variably with calcite, quartz, anhydrite and occasionally saddle dolomite cements.
• Inclined PSF may grade into dilational/transtensional breccia fabrics, some with open cement-lined cavities.
• Crystal-lined (calcite, quartz) cavities on the lee sides of steps or microasperities on PSF faces are a localized source of intercrystalline porosity.
• Frictional heating and pressure along some PSF slip surfaces has created differential temperature gradients in the order of 50 to 75^oC, based on vitrinite/bitumen reflectance analysis.
• Graphitized bitumen is recognized in some of these PSF layers.
• The thin (20 to 200+ micron) black polished layer of PSF shows no reflectance (extinguished) under R_o examination.
• XRD analysis of the black PSF layer indicates a higher percentage of illite and other clays compared with the surrounding host rock (but limited data), supporting a component of ‘clay smear’.
• The PSF are non-coherent boundaries, or geomechanical discontinuities: unloaded core often separates or disintegrates along PSF surfaces.
• PSF probably are preferential sites for bedding-plane interfacial slip during hydraulic fracturing.
• A low angle of internal friction for higher TOC, higher Poisson’s ratio, lower Young’s modulus intervals may favour ease of shear displacement along PSF surfaces.
• Reduced effective stress under overpressured conditions driven by burial-thermal conversion of kerogen to bitumen (or bitumen to oil?) also may favor formation of PSF.
• Most cores with recorded PSF (particularly Montney, Second White Specks) lie within the Tmax oil window.
• Fluid inclusion analyses and/or organic petrology data (including evidence for light oil inclusions in re-sheared mineral phases on PSF surfaces) confirm formation of PSF within the oil window.
• Prediction of PSF beyond core-based ‘ground truthing’ may be based on confluence of higher TOC, lower Young’s modulus, and higher Poisson’s ratio values calculated from shear sonic log data.
• Bed-parallel ‘ridge and groove’ shear fabrics in limestones in the WCSB are parallel to the striae of PSF, thus parallel to maximum horizontal principal stress.
• Cleavage lineation is not an obvious open fracture system, but is a geomechanically and diagenetically-controlled expression of a westward-increasing, early onset, low-grade metamorphic overprint.
• Cleavage lineation parallels the azimuth of wellbore breakout (minimum horizontal principal stress).
• Open crystal-lined cavities in microasperities in PSF and in associated inclined microbreccias, and intrabed dilational fractures (IDF), contribute meso- to macro-pore components to a mainly nano- to micro-porosity system.
• PSF and cleavage (and limestone ‘ridge and groove’) structures are most common within a belt about 100 km east of the outcrop boundary of the Laramide deformed belt, but have been recorded as far as 200 kms east.
• Polished slip faces and cleavage (pencil cleavage) also are recognized in the Marcellus Shale of New York State in the subsurface and in outcrop, with slip faces and fractures in core documented by image analysis logs.
• Paleomagnetically-oriented core in the Montney Formation in the Kakwa area of Alberta now confirms that cleavage is parallel to S_Hmin (and thus wellbore breakout) with a consistent trend of 115^o (WNW-ESE), approaching but not paralleling the dominant NW-SE trend of frontal structures of the Laramide deformed belt west of the Kakwa area.
• Striae on PSF on the Montney Kakwa core are essentially parallel to S_Hmax, and orthogonal to cleavage.
• Although other factors may come into play, it is reasonable to assume that cleavage and by association striae on PSF in other formations in the WCSB will show a similar orientation to principal stress axes as in the Kakwa Montney.
• Cleavage in the Montney Kakwa area may have influenced the orientation of S_Hmax, resulting in an 18^o directional difference between ‘stress-induced anisotropy’ as defined by the World Stress Map for west-central Alberta and the ‘structural anisotropy’ measured in orientated core.
• This difference in trend of stress axes may be influenced by sedimentary wedge geometry, basement structure, and possibly Leduc reef and platform margin geometries.
• Interim SEM examination of cleavage faces does not show recognizable reorientation of phyllosilicate minerals as expected for higher-grade metamorphic cleavage (but may be incipient).
• A strong directional permeability anisotropy with maximum permeability along a 115^o to 295^o axis defined by the average cleavage trend may be expected in the Montney in the Karr area, based on paleomagnetic structural data; similar conclusions may be drawn for other mudrock formations with cleavage.

Figure 22 : An interpretation of interaction between hydraulic fracture and bedding-plane interfacial slip.

• Other data sources suggest that permeability across cleavage planes may be orders-of-magnitude lower than permeability parallel to cleavage.
• Based on the paleomagnetic structural data, horizontal drilling in the Montney in the Kakwa area might be directed orthogonal to the 24^o S_Hmax derived from the World Stress Map,
• Hydraulic fractures striking parallel to the inferred 24^o S_Hmax axis for the Kakwa Montney may be expected to cross and split open millions of cleavage planes striking orthogonal to the hydraulic fractures, creating a ‘cloud’ of microseismic events.
• Non-cohesive PSF surfaces may also influence propagation and geometry of hydraulic fractures, creating preferential sites for bedding-plane interfacial slip (Fig. 22).
• Most of these structural fabrics are missed in conventional surface-only core imagery, and surface-focused examination of unslabbed or slabbed core.  Initial examination of bed-parallel or horizontal core surfaces before slabbing is recommended to maximize detection and quantification of these structural fabrics.

Acknowledgements

We wish to thank Anschutz Exploration Corporation, Spectra Energy Ltd., Applied Paleomagnetics Inc., and Judith Potter of JP PetroGraphics for contributions.

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