Pore types in the Barnett and Woodford gas shales_ Contribution to understanding gas storage and
GEOHORIZON
Pore types in the Barnett
and Woodford gas shales: Contribution to understanding gas storage and migration pathways in fine-grained rocks Roger M.Slatt and Neal R.O’Brien
ABSTRACT
The identification of“organoporosity”(microscale and nano-scale pores within organic matter in shales),its importance to storage and perhaps transfer of gas molecules through shales, and methods for gathering three-dimensional images of the pores,such as by argon-ion milling and/or field emission scan-ning electron microscopy,have all been well documented and discussed for unconventional gas shales.However,other types of pores exist within shales that can be important to storage and migration of gas(and oil),and other technologies are available for their identification and imaging.The different pore types found in the Barnett and Woodford shales are described and classified in this article.
Scanning electron microscopy revealed the presence of po-rous floccules in both the Barnett and Woodford shales,which appear similar to laboratory-produced floccules and to those in other ancient shales.Published experimental and observational studies indicate that floccules are hydraulically equivalent to coarser grains and are transported by traction processes.Current-induced microsedimentary structures and textures within the Barnett and Woodford shales,as well as the preserved floc-cules,suggest such processes were active during transport and deposition.Pore spaces between the floccules are open and AUTHORS
Roger M.Slatt Institute of Reservoir Characterization and School of Geology and Geophysics,University of Oklahoma,Norman, Oklahoma73072;rslatt@https://www.360docs.net/doc/1b15877030.html,
Roger M.Slatt is the Gungoll Family Chair Pro-fessor in Petroleum Geology and Geophysics and director of the Institute of Reservoir Character-ization at University of Oklahoma(OU).Previous positions were in both academia and the pe-troleum industry.He has published more than 100articles and abstracts on petroleum geology, reservoir geology,sequence stratigraphy,clastic depositional systems,and geology of shale.He is author/co-author/editor of six books,teaches an online course on reservoir characterization for AAPG,has been an AAPG and Society of Petroleum Engineers Distinguished Lecturer,and presents courses internationally for industry or-ganizations,in addition to OU.
Neal R.O’Brien Geology Department, State University of New York,Potsdam,New York 13676;obriennr@https://www.360docs.net/doc/1b15877030.html,
Neal R.O’Brien,a distinguished teaching pro-fessor emeritus in the Geology Department at the State University of New York–College at Potsdam,received his B.A.degree from DePauw University and M.S.degree and Ph.D.from the University of Illinois.Past and present research concerns the scanning electron microscopy in-vestigation of gas shale microfabrics relating to porosity,permeability,and shale properties. ACKNOWLEDGEMENTS
We thank Ted Champagne of Clarkson University, Potsdam,New York;Carol McRobbie,Roberta Greene,Chase Winkler,and Amanda Brewer of the State University of New York,Potsdam,New York,for technical assistance with SEM and FESEM analyses and manuscript preparation; Susan Hovorka,University of Texas for Permian samples;Majia Zheng and Matthew Totten for providing some of the samples and images pre-sented in this article;Younane Abousleiman for discussions on the effect of pore types on geo-mechanical properties;and Devon Energy Co.for partial support of this study.
The AAPG Editor thanks the following reviewers for their work on this paper:Govert J.Buijs, Jack Horkowitz,and Prasanta K.Mukhopadhyay.
Copyright?2011.The American Association of Petroleum Geologists.All rights reserved. Manuscript received September4,2010;provisional acceptance January31,2011;revised manuscript received March11,2011;final acceptance March30,2011.
DOI:10.1306/03301110145
AAPG Bulletin,v.95,no.12(December2011),pp.2017–20302017
can provide storage space as well as permeability pathways for gas molecules through the shales. Pores are also found within organic matter that oc-curs both as discrete particles and as adsorbed coat-ings around clay grains in the shale.They are referred to here as“organopores.”Porous fecal pellets are also common in the Barnett Shale.Preserved fossil frag-ments such as organic-walled spores and inorganic sponge spicules have hollow central chambers, which may remain partially or completely open even after burial.Intraparticle pores occur between grains of various minerals(e.g.,pyrite framboids).Micro-channels within shale matrix,which may be the bounding surfaces of scours or microsedimentary structures,may also provide permeability pathways for hydrocarbon migration.Mircrofractures are also common,and their initiation might be related to mineral crystal structure.When present in suffi-cient quantity,these pore types offer potential gas (and oil)molecule storage spaces and permeability pathways through the shales. INTRODUCTION
The recent popularity of unconventional gas shales as a future long-term energy source has,among other things,led to significant advances in understand-ing shale depositional and diagenetic processes, macroscale to microscale sedimentary structures, both coarse-and fine-scale stacking patterns of dif-ferent lithofacies,and sequence-and parasequence-scale stratigraphy(Hickey and Henk,2007;Loucks and Ruppel,2007;Schieber et al.,2007;Singh, 2008;Schieber and Southard,2009;Bohacs,2010; Slatt et al.,2011a).One such important advance-ment in understanding the gas storage and flow capacity of shales has been the documentation of micrometer-and nanometer-size pores in organic matter within the Barnett Shale(Loucks et al., 2009).Accompanying this documentation has been the perception that this“organoporosity”is the dominant pore type in gas shales and that other pore types are sparse or absent(Loucks et al.,2009).A corollary perception is that organoporosity is best viewed using argon-ion milled surfaces under very high resolution field emission scanning electron microscopy(FESEM),particularly if milled surfaces can be successively generated and photographed to produce three-dimensional(3-D)images of the pore space(Loucks et al.,2009;Sondergeld et al., 2010).Although organoporosity clearly does exist and is indeed best viewed under high-resolution imaging techniques,it is by no means the only type of porosity found in the Barnett or other gas shales (Milner et al.,2010),nor is the3-D high-resolution imagery the only way to identify and view pore types.
The purpose of this article is to document,il-lustrate,and classify the variety of pore types that exist in the Barnett Shale in north Texas and the Woodford Shale in southeastern Oklahoma(and probably other gas shales)and to speculate on their potential control on gas storage and migration path-ways.Different pore types can also influence geo-mechanical properties relevant to wellbore stabil-ity and hydrofracturing,as addressed by Slatt and Abousleiman(2011).
Specifically,microscopic features of the shales discussed here have been documented using well-established relatively inexpensive techniques that combine SEM and FESEM in both the secondary electron mode and backscatter mode(BS),electron microprobe spectroscopy,and energy-dispersive x-ray analysis(EDX).These techniques combine rapid3-D views of pores and grains along with their mineralogical identification by chemical analysis. Results are also substantiated by routine x-ray dif-fraction mineralogical analysis.
Loucks et al.(2009)and Sondergeld et al.(2010) suggested that pores viewed by SEM and FESEM may really be remnant depressions of grains that were artificially plucked during sample preparation and polishing caused by varying mineral hardness. Although such caution in identifying pores is in-deed necessary,we do not believe this to be the general case because remnant depressions generated by plucking(e.g.,silt-or clay-size quartz grains or pyrite grains in framboids)appear circular or ellip-soidal and the holes extend only a few nanometers in depth,revealing the surrounding matrix at their base.The pores we describe here are irregular in shape(except sponge spicules)and extend deeper
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into the matrix,so are easily differentiated from holes produced by grain plucking.
INTERPARTICLE PORES PRODUCED BY FLOCCULATION
Floccules are clumps of electrostatically charged clay flakes that sink toward the sea floor in ion-enriched seawater;the floccule can form a “cardhouse ”struc-ture of individual edge-face –or edge-edge –oriented flakes and/or domains of face-face –oriented flakes (Figure 1)(O ’Brien,1971,1972;Bennett et al.,1991).Laboratory-generated floccules are shown in Figure 1,and floccules in varved claystone from Pleistocene raised terraces at the Great Salt Lake,Utah,and in the Permian Clear Fork Formation are shown in Figure 2.O ’Brien and Slatt (1990)showed examples of floccules dominating the microstrati-fication of several other ancient shales but were unable to explain how this openwork pattern could have survived burial and diagenesis for up to hun-dreds of millions of years!This explanation is still elusive,but the fact remains that preserved
floccules
Figure 1.(A)The freeze-dried floccu-lated illite in distilled water.Note the pores and cardhouse microfabric.Modified from O ’Brien,1971,reproduced with permis-sion of The Clay Mineral Society ’s Clays and Clay Minerals .(B)The freeze-dried flocculated illite in salt water.Note the pores within random floccules.(C)Cartoon of typical floccule microfabric domain
model.
Figure 2.(A)Floccules in a varved clay,Pleistocene raised terrace,Great Salt Lake,Utah.Note pores.(B)Floccules in mud-stone,Permian Clear Fork Formation,Texas.
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are common in the internal structure of many deeply buried old shales,as they are in the Barnett and Woodford shales (Figure 3).The significance
of this observation is that the open network or cardhouse pattern (Figures 1,2)provides pores between the flocs that are larger than the 0.38-nm diameter of methane molecules.These pores may be interconnected to form permeability pathways.
In addition,the flocculation process might aid in transporting significant volumes of mud for long distances in the marine environment.The myriad of microsedimentary structures in ancient mudstones and shales,including the Barnett Shale (Figure 4),clearly support some mechanism(s)for sediment transport along the sea floor by currents.Experi-mental flume tank studies (Schieber et al.,2007;Schieber and Southard,2009)and subsequent com-parisons of modern mud ripples and scour surfaces with ancient analogs led to Scheiber ’s conclusion that floccules can behave in a hydraulically equiv-alent manner to coarser sedimentary particles and move along the sea floor as components of hyper-pycnal and/or turbidity current flows (Mulder et al.,2003;Mulder and Chapron,in press).The process may be aided by entrapment of silt-size grains within the open internal part of the floccules (O ’Brien and Slatt,1990).A modern hyperpycnal flow has been documented as traveling 700km (435mi)along the floor of the Sea of Japan from its source river (Nakajima,2006).Soyinka and Slatt (2006)
have
Figure 3.(A)Typical flocculated clay microfabric,Woodford Shale.(B)Typical random edge-face clay flake orientation in flocculated clay microfabric,Barnett Shale.Sample floccule pores are shown with
arrows.
Figure 4.Barnett Shale.A thin-section photograph of primary physical sedimentary structures.(A)Irregular (erosional)bottom surface of spiculitic mudstone.(B)Scour surface at the base of the light-colored siltstone bed.(C)Low-angle cross-lamination.
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documented mud hyperpycnites in Cretaceous slope and base-of-slope strata.
Thus,the presence of preserved open or par-tially collapsed flocs in shale can be considered as the locus of interparticle pores between clay flakes (Figures 1–3).However,other silt-poor and clay-enriched shales and mudrocks have been observed in thin section and SEM to exhibit parallel align-ment of individual particles (O ’Brien and Slatt,1990).Such alignment can be the result of “hemi-pelagic rain ”of clay particles onto the sea floor or postdepositional collapse and orientation of origi-nally flocculated particles.Unless aligned particles are completely cemented,nanometer-size inter-particle pores will occur between them.
ORGANOPOROSITY PRODUCED DURING BURIAL AND MATURATION
Organoporosity in gas shales has been well docu-mented since first identified in the Barnett Shale (Loucks et al.,2009).Similar organoporosity occurs in other shales,such as the Lower Silurian
Longmaxi
Figure 5.(A,B)The FESEM images showing pores (black areas)within organic matter:Lower Silurian Longmaxi Shale,Sichuan Basin,China.R o =2.5%.(C)The pyrite framboids with porous (black areas)organic matter between individual pyrite crystals.Longmaxi Shale,China.Figures were provided by M.Zheng.(D)The Woodford Shale FESEM image showing the pore (black area)between organic-clad (coated)clay flakes along the fissile plane surface.The x and y are positions at which EDX analyses were performed.R o =
0.57%.
Figure 6.Modern fecal pellets from Emerald Basin,Atlantic Ocean,Canada.(A)Pellet morphology.(B)Close-up of the in-terior of a fecal pellet.Note pores and random particle orien-tation,including skeletal fragments.
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Shale,Sichuan Basin,China,as shown in Figure 5A to C .These pores are generated during burial and maturation of organic material (Jarvie et al.,2007).Two-dimensonal (2-D)and 3-D models of shale porosity may be built for in-place gas calculations on the assumption that organoporosity constitutes all or almost all of the pores in the shale (Ambrose et al.,2010).However,as noted by Milner et al.(2010),such models are made from FESEM anal-ysis of 1-to 5-m m shale cubes (Sondergeld et al.,2010)that may fail to include larger shale pores such as those described in this article (Figure 5).Also,the degree of connectivity of these pores within the kerogen and among widely dispersed kerogen within the shale matrix at scales larger than a few micrometers has not been ascertained and is undoubtedly quite variable (Figure 5)(as are shale lithofacies;Slatt et al.,2011a),so their contribution to permeability remains speculative.
Our analysis has also found another pore type associated with “organic-clad ”clay flakes.In the Woodford Shale,such pores occur in spaces be-tween overlapping clay flakes on the (top)fissile plane of the shale (Figure 5D ).An organic coating adsorbed onto the clay flakes is indicated by
EDX
Figure 7.(A)Fecal pellet (arrow)in Barnett Shale phosphatic lithofacies (Singh,2008).Note the random porous microfabric within the pellet.(B)Layers of aligned and concentrated fecal pellets (light color)in phosphatic lithofacies.Matrix is dark col-ored.Backscatter (BS)
image.
Figure 8.A fecal pellet in Barnett Shale phosphatic litho-facies.(A)Section within a broken pellet showing nanopores and micropores (dark areas).P =pel-let,outlined by arrows;M =matrix around pellet.(B)The close-up view of nanopores and micro-pores (arrow)in pellet.(C)The close-up view of nanopore (ar-row)in pellet.X and Y are flour-apatite crystals.
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analysis (Figure 5D ,points x and y).Carbon com-poses 46.16wt.%on the platy flakes at point x (Figure 5D ),whereas it is totally absent at point y (Figure 5D )located within the pore.The EDX analysis indicates the presence of Mg,Al,Si,and K at both points,which are typical clay mineral com-ponents.These data suggest a coating of carbon (colloidal carbon?)adsorbed on the clay flakes.Re-sidual organic residue (mucus)is commonly found around pyrite framboids (O ’Brien,1995).
INTRAPARTICLE PORES FROM ORGANISMS Organisms may produce intraparticle porosity by bioturbation of sediments,by generation of fecal
pellets,and by the porous nature of their skele-tons or shells.The relationship between biotur-bation and shale in microfabric has been discussed by O ’Brien (1987)and has not been seen in our samples,so is not repeated here.Fecal Pellets
The excrement of macroorganisms and microor-ganisms,such as zooplankton,produces an impor-tant component of sea floor sediment —fecal pellets (Turner,2002).Modern pellets are composed of randomly oriented fragments of undigested organic matter,clay or silt grains,and commonly,pieces of diatoms or other shelled phytoplankton (Figure 6).Modern pellets vary in size and dimensions;
however,
Figure 9.(A,B)Chemical analyses of typical calcium-and calcium-plus-phosphate-rich fecal pellets in Barnett Shale.(C)Upper left is a secondary electron image of pellets on a polished surface.Other figures are EDX elemental distributions of key elements in the pellets:carbon (C),calcium (Ca),silica (Si),and phosphorus (P).Note the scarcity of Si in the two large pellets and that both contain Ca but only one contains P.Also,C is enriched in the Ca-rich (no-P)pellet.
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many are about 0.1to 0.2mm in diameter,are el-lipsoidal in shape (Figure 6A ),and have a porous random internal fabric (Figure 6B ).The random microfabric of particles within the pellets is most likely a result of their mixing in the animal ’s gut before fecal pellet expulsion.
The distinctive morphology and internal ran-dom microfabric of the modern pellets (Figure 6A )aided in the identification of fossil fecal pellets in the Barnett and Woodford shales (Figure 7).Also,the shale matrix directly in contact with the pel-lets exhibits a different microfabric from the pel-lets (Figures 7,8A ).Pellets in the Barnett Shale are frequently concentrated in millimeter-to centimeter-thick layers,which alternate with layers of clay-silt shale matrix (Figure 7B ).
Interpellet microscale and nanoscale porosity is visible with the SEM (Figure 8A –C ).Pores as small as 300nm are well resolved using the FESEM,and 10-to 30-nm pores are visible at depth within a larger pore opening (Figure 8C ).
Energy-dispersive x-ray analysis of smooth-sanded (600grit)and polished surfaces that were not gold coated was used to identify pellet elemental concentrations (Figure 9A,B )plus the
distribution
Figure 10.A thin-section photo-graph of Barnett Shale.(A)Macro-fossil shell fragments (note the rounded edges of some grains,indicating transport).(B)Aggluti-nated foraminifera in siliceous noncalcareous mudstone facies.(C)Microgastropods and pelloids within a concretion.(D)Phos-phatic ooids.Classification and identifications are by Singh
(2008).
Figure 11.(A)Elongate cyst (Tasminites?)(s)with opening (pore [p])in the center.(B)The surface of cyst showing micro-pores (mp)on spore surface.2024
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Figure12.(A)The SEM
image of a sponge spicule
in Barnett Shale.(B)The
SEM image of the cross sec-
tion of a broken spicule
with partially filled axial
chamber.(C)The EDX anal-
ysis of material in the central
chamber of B.(D)A thin-
section photograph of a
recrystallized quartz spic-
ule with central chamber.
(E)A thin-section photo-
graph of spicule-rich lami-
nation in Barnett Shale.
Spicules are light,and ma-
trix is dark.
Figure13.Woodford Shale.
(A)Pyrite framboid(f).Organic mucus is in the upper right corner of the framboid.(B)Internal view of the broken framboid. Small pyrite crystals almost com-pletely fill the framboid,and pores (p)occur between the crystals. An organic layer(s)is wrapped around the lower part of the framboid.(C)Organic(black) mucus(m)around a pyrite(Py) grain from the electron micro-probe polished surface.Some quartz(Qtz)is present in the interior of the pyrite.Unpublished picture provided by M.Totten,Jr.
(D)Pyrite crystals with inter-granular micropores(mp). Slatt and O’Brien2025
of the elements in the samples analyzed (Figure 9C ).These analyses revealed an elemental distribution dominated by carbon,oxygen,silicon,phosphorus,and calcium (Figure 9).X-ray diffraction analysis of bulk samples indicated that these elements com-prise calcite,apatite (or collophane),quartz,and clay minerals.
The EDX elemental dot maps (Figure 9C )indicate that two compositionally different types of pellets exist:one containing both calcium and phosphorus and one containing calcium but no phosphorus (Figure 9C ).These two different types of pellets could be a result of excretion by two dif-ferent organisms and/or variations in geochemical environment conditions during pellet formation.Fossil Material
Within some facies of the Barnett and Woodford shales,abundant fossil remains exist,including whole and broken brachiopods and gastropods,arenaceous foraminifera,and whole gastropods within concre-tions (Figure 10).Some of these fragments are https://www.360docs.net/doc/1b15877030.html,pressed algal remains (Tasminites cysts?)
Figure 14.(A,B)The SEM images of microchannels (c)in Woodford Shale.
Figure 15.Woodford Shale.
(A)A thin-section photograph showing zigzag tracks of a bitumen-filled fracture.(B)A thin-section photograph showing partially open fractures (artificially filled with blue epoxy)(arrows)crosscutting the shale matrix.(C)Nanometer-scale pores ap-parently aligned along crystal axes after tensile stress (arrows)was applied.(D)Magnification of fracture tip shown in C.
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(Figure11)also are present that contain pores at both the micrometer and submicrometer scales (Figure11).Elongate siliceous sponge spicules are common in the Barnett Shale(Figure12).They originally had a central chamber that may have contained internal organic matter that decomposed during burial or the chamber may have become partially or completely filled with secondary quartz, clay,or organic particles on burial(Milliken et al., 2007).Similar silicified spicules,sometimes with open chambers,occur within the oil-and gas-producing Mississippian Cowley Formation in Kansas(Mazzullo et al.,2009).In the Barnett Shale, spicules are commonly concentrated in thin bands (Figure12E)in much the same manner as the fecal pellets(Figure7B).
INTRAPARTICLE PORES WITHIN
MINERAL GRAINS
Pyrite framboids are relatively common in Barnett and Woodford shales(Figure13).Their interiors are composed of many small pyrite crystals(Figure13B), between which occur micropores(Figure13D). Residual organic residue(mucus)is commonly found around pyrite framboids(Figure13C)(O’Brien,
1995).
Figure16.Classification of pore types in shales.See text for details.
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Phosphatized pellets,ooids (Figure 10D ),and intraclasts are also present in the Barnett and Wood-ford shales,which can also contain pores.
MICROCHANNELS IN SHALE MATRIX Microchannels of various sizes and shapes occur within the matrix of shale (Figure 14).If abundant,they could provide significant permeability path-ways,in addition to microporosity.They commonly are sinuous and discontinuous and subparallel to the bedding plane.When viewed in SEM,they commonly do not extend entirely across the view-ing area of the shale sample,which is generally less than 0.5-cm (<0.2-in.)distance.These features sug-gest that the microchannels are not artifacts pro-duced by pressure release when the core is retrieved from a well,nor by fracturing the sample during handling and preparation,but represent original microchannel openings preserved in the undis-turbed shale matrix.Microchannels are commonly less than 0.3m m in width,which is wide enough to provide a permeability pathway for gas molecules.The origin(s)of such microchannels are difficult to
determine,but may be remnants of bioturbation,microerosion surfaces between laminae,or the top surfaces of microripples;the latter two of which are commonly seen in Barnett Shale thin sections (Figure 4).Narrow microchannels also occur at the boundary between pellets and shale matrix (Figure 8A ).
MICROFRACTURES
Fractures in the Barnett and Woodford shales oc-cur at a variety of scales (Gale et al.,2007;Portas,2009;Gale and Holder,2010;Slatt et al.,2011b)and are significant in any fabric investigation of shale properties,particularly those related to the drilling of wells and artificial fracture treatment.In the Woodford Shale,small natural fractures may be filled with bitumen (Figure 15A )or partially open (Figure 15B ).At the micrometer scale,application of tensile stress to samples has produced artificial microfractures that are aligned within mineral grains (Figure 15C,D ),which suggests that there may be a crystallographic control on microfracture initia-tion (Slatt and Abousleiman,
2011).
Figure 17.The SEM images of oil droplets produced from the Woodford Shale during hydrous pyrolysis experiments,where samples were heated to 350°C for 3to 4days.(A)An oil droplet emerging from a pore in the matrix after 3days.(B)Oil drop-lets in pores and microfractures in matrix after 4days.(C)Oil ribbon emerging from a micro-fracture (arrows)and swelling into a droplet (X)in a pore space after 4days.(D)Oil droplets emerging from pores and micro-fractures after 4days.All surfaces were broken perpendicular to bedding.O ’Brien et al.(2002);republished with permission of the Gulf Coast Association of Geological Societies,whose per-mission is required for further publication use.
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PERMEABILITY PATHWAYS AND PRIMARY MOVEMENT OF HYDROCARBONS THROUGH SHALES
Figure16presents the pore types in classification format.Of these pore types,those that are most likely to contribute to permeability are(1)floc-cules,(2)organoporosity,(3)microfractures,and (4)microchannels.If fecal pellets and fossil remains such as sponge spicules and spores are touching and aligned in sufficiently thick laminae or beds,they too may provide some permeability.Intraparticle pores in grains and crystals such as pyrite,which are dispersed within the shale matrix,probably could not contribute much permeability.
Although it is not possible to view the migra-tion of gas molecules through shales,it is possible to capture stages in movement of oil droplets through shales.O’Brien et al.(1996)documented with SEM photos the morphology of oil droplets during their primary migration from within the upper Miocene Monterey Shale,California.Similar SEM and hy-drous pyrolysis techniques were used to view stages of oil migration in the Woodford Shale(Figure17). Migration of gas molecules should in fact be easier than migration of larger viscous oil droplets. CONCLUSIONS AND APPLICATIONS
In this article,we present examples of a variety of pore types that are present in the Barnett and Woodford shales and present a classification for them.These pores are all large enough to store gas molecules.Porosity caused by flocculation of clays has perhaps the greatest potential—in addition to microfractures and organoporosity—of provid-ing storage places as well as permeability pathways for migration of hydrocarbon molecules.A combi-nation of hand-sample observation,thin-section description,SEM and FESEM,EDX,backscatter imaging,and electron microprobe spectroscopy provide the tools for identifying and documenting the occurrence of these pore types.Although we have constrained our discussion in this article mainly to the Barnett and Woodford shales,similar pore types are present in other unconventional oil and gas shales.They should be included in attempts to build realistic2-D and3-D fluid-flow models of shales.
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