Clay mineral transformation as a major source for authigenic quartz in

Research paperClay mineral transformation as a major source for authigenic quartz in thermo-mature gas shaleYasser M.Metwally a ,b ,⁎,Evgeni M.Chesnokov aa University of Houston,Houston,Texas,USA bTanta University,Tanta,Egypta b s t r a c ta r t i c l e i n f o Article history:Received 16November 2010Received in revised form 7November 2011Accepted 9November 2011Available online xxxx Keywords:Clay minerals Illitization Gas-shaleQuartz cementationStudies of laboratory-treated Na +-smectite and Pierre Shale and natural thermo-mature gas shale indicate that,large quantities of silica are released during clay mineral transformation to form quartz cement (within a closed shale system).Two types of quartz cement are recorded in lab treated samples;granular silica,formed with all lab treated samples (Na +-smectite and Pierre Shale)at treatment temperatures (100–150°C)and durations (up to 4months),was seen as rims over the minerals remnance;sheet-like silica,formed on Pierre Shale samples only that was treated at 150°C for more than a month,was seen as very thin sheets covering the mineral remnance or cementing the granular silica.The granular shape silica was seen on natural gas-shale samples as silica rims or silica-rich micro-grains contain signi ficant amounts of inter-grain micropores while the sheet type silica was seen as micro sheets (b 100nm)along the bedding cementing the b diagenesis of Pierre Shale at 150°C for 4months produced minerals and tex-tures similar to those of studied natural gas shale which could be formed through similar conditions,within a closed system.The quartz cement acts as shale stiffening agent while the sheet-like quartz enhances shale anisotropy and fracability.Detailed study of sheet-like quartz will have important implications in modeling the elastic behavior,fluid flow and the gas storage mechanism.Published by Elsevier B.V.1.IntroductionRecently gas shales such as the Barnett,Woodford and Caney Shales (Oklahoma and Texas,USA)have received increasing attention because of their economic importance and dif ficulties in producing economic flow rates of natural gas.Many of these gas producing shales are classi fied as thermo-mature shale which means their or-ganic material turned to gas under the effect of temperature.Temper-ature affected not only the organic matter but it acted on the whole rock including clay minerals,and plays a big role in diagenesis,which in turn controls the rock properties relevant to extraction of in-ternal resources.Boles and Franks (1979),Eberl and Hower (1976),Hower et al.(1976),Lynch et al.(1997)and Perry and Hower (1970)indicated that with increasing depth of burial and temperature in sedimentary basins,relatively siliceous smectite clay transforms to less siliceous illite through the smectite –illite mixed-layer series.Thus,with in-creasing burial depth and temperature,the proportion of smectite de-creases as illite content increases.Meanwhile,thermodynamically more stable muscovite will develop from illite with increasing tem-perature.Merriman and Frey (1999)indicated that ~95%of smectitetransforms to illite between 20and 200°C,while ~95%of illite trans-forms to muscovite between 200and 300°C.Van de Kamp (2008)concluded that the smectite to illite transformation yields 17–28wt.%of free silica as a proportion of the original smectite and the illite –muscovite transformation yields 17–23wt.%of free silica as a propor-tion of the original illite.Other sources of mobilized silica that may be produced from shale are from pressure solution of detrital quartz (Cyziene et al.,2006)as well as the alteration of detrital feldspars such as Na +-Plagioclase and K +-feldspar.In an open system,the silica mobilized from shale has been pro-posed as a major source of quartz cements within sandstones and asso-ciated shales (Boles and Franks,1979;Day-Stirrat et al.,2010;Land and Milliken,2000;Leder and Park,1986;Lynch,1997;Srodan,1999;Van de Kamp,2008).Thus,altered shale formations are more micaceous and less in quartz than their progenitors.However,in a closed system (Thyberg and Jahren,2011;Thyberg et al.,2010),the silica will not be removed from shales and the overall silica content will remain constant.But volumetrically,the rock will contain more authigenic quartz and less clay.We hypothesize that within tight shale systems,diagenesis may be effectively closed to adjacent more porous and permeable formations,effectively producing locally closed systems.Tight thermo-mature gas shales which have not released gas during organic matter maturation may effectively act as a closed system.Lab experimentation on the effect of temperature,pressure,and duration on the chemically active Pierre Shale under a wide rangeApplied Clay Science xxx (2011)xxx –xxx⁎Corresponding author at:University of Houston,Houston,Texas,USA.Tel.:+14054458452.E-mail address:ykhoris@ (Y.M.Metwally).CLAY-02333;No of Pages 130169-1317/$–see front matter.Published by Elsevier B.V.doi:10.1016/j.clay.2011.11.007Contents lists available at SciVerse ScienceDirectApplied Clay Sciencej ou r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c l a yof P/T conditions within closed lab experiment was conducted by Mohamed(2007).He concluded that,in the presence of K+ions, smectite showed progressive transformation to smectite/illite mixed layer and/or illite beginning at room temperature up to150°C.This transformation is controlled by the concentration of K+,temperature and curing time.He also showed that,at high temperatures(150°C for4months)the smectite and illite structures were altered to form silica instead,most probably asα-quartz producing silicification of shale rock.The rate of mineral dissolution and precipitation was tem-perature and time dependent.This means temperature with long time accelerates smectite/illite transformation and accelerates shale silicification.He proposed that these lab conditions could be similar to thermo-mature gas shales that act as a closed system.This paper is aimed at determining the origin and quantifying the microstructure of quartz cement in gas shales by exploring the similarity between lab-treated shale samples and natural thermo-mature gas shales.In addition,it will study the effect of quartz cementation on stiffness tensor and anisotropy parameters for the natural gas-shale samples.2.Methodology and materials2.1.Mineralogical and microstructure studiesRepresentative natural and lab treated samples were analyzed for their mineralogical characteristics by X-ray powder diffractometer (Rigaku Geigerflux™XRD Diffractometer)and were examined by Scanning Electron Microscope(SEM)observations and EDX measurements.The XRD analyses were performed using CuKa radiation at40kV and35mA,proportional detector,0.1divergence slit,1°receiving slit,scans taken over4to70°2θ,step interval0.02°and a scanning speed of1°2θ/min.Un-oriented mounts of random powdered whole-rock samples were scanned to determine the mineralogy of the bulk samples.10%hydrogen peroxide(H2O2)was used to remove organic carbon.Samples for clay analysis(b2μm)from untreated Pierre Shale and Na+-smectite were prepared by separating the clay fraction using the sedimentation method.The clay particles were dis-persed chemically using sodium hexametaphosphate(4g/1L) (NaPO3).To aid in particle dispersion,the samples were later placed and disaggregated in an ultrasonic water bath for1h.The samples were allowed to sit overnight to ensure that they were thoroughly dispersed.XRD analyses of Oriented specimens of the b2μm fraction were run on air-dried,ethylene glycol solvated at60°C for2h,and heat treated at550°C for2h.Semi-quantitative measurement of the rock-forming minerals was obtained following the procedures given by Brown and Brindley(1980),whereas the relative abundance of clay mineral fractions was determined using their basal reflections and the mineral intensity factors of Moore and Reynolds(1997).Gently broken tiny-pieces(~2by5by5mm)from lab-treated and natural shales were prepared for SEM-EDX analysis by adhering the fresh broken surface of each sample onto a copper sample holder, using double-sided carbon tape.Then each sample was dried for an hour and coatedfirst with carbon(C)and then by-gold(Au)–palladium (Pd).These thin coatings(200Åthick)applied in a Kinney evaporative coater were required to obtain a clear image.The prepared samples were examined with a JSM-880SEM equipped with EDX system under different magnifications from×500to×100,000.The O'Brien and Slatt (1990)and Welton Atlas(1984)were used to assist in interpretation of the SEM images.2.2.Simulated high temperature and high pressure geological conditionsIn order to simulate the deep burial conditions(up to6000ft);a High Temperature High Pressure(HTHP)Shale Apparatus was used (Fig.1).It has the ability to hold a constant temperature of up to 300°C for more than four months.In addition,it has the ability to hold both the confining pressure and the pore pressure up to 10,000psi.Cylindrical disk samples(25mm diameter and3mm thick)were prepared from the Na+-smectite and Pierre Shale then placed on the HTHP Shale Apparatus in the manner described by Mohamed(2007).One mole KNO3was used as porefluid while min-eral oil was used as the confining pressurefluid.The pore pressure was set to2500psi and confined pressure to3000psi and the tem-perature was set to100and150°C.After the designed curing time (1,2and4months)the samples were removed from the HTHP hy-drostatic cell and were tested by XRD and SEM to study the impact of these conditions on mineral transformations and microstructure.2.3.Initial materialsSamples from three different clay-rich formations were used in the current study;a pure Na+-smectite sample from the Newcastle Formation(Cretaceous in age and commercially called Wyoming Montmorillonite(see Chipera and Bish,2001for more details)) obtained by The Clay Minerals Society;Pierre Shale(type I)sample (Upper Cretaceous in age)as a representative example of common chemically active shale,obtained by TerraTek;and Core samples from one of the thermo-mature tight gas shale formation that is a Mississippian age and located in North Texas,USA(Barnett Shale) as a self-contained petroleum system.Potassium nitrate solution was used to make pore solutions for the high temperature–high pressure experiments,because it's not corro-sive to steel and iron alloy of HTHP Shale Apparatus.These solutions were prepared by dissolution of1.00mol of Potassium nitrate in1l of distillated water at room temperature.The different thermody-namic properties of this aqueous electrolyte solution are discussed by El-Guendouzi and Marouani(2003).3.Results3.1.Na+-smectite sampleThe quantitative analysis of Na+-smectite sample indicated that smectite is the main component of this sample(81%)which also con-tains6%of illite–smectite mixed layer.Quartz,feldspar and calcite are also detected as a minor component(Table1).Fig.2A and B shows smectite as thin webby crust which is the common crystal habit of smectite.X-ray diffraction is used for precise identification of smec-tite(Fig.2C).Smectite main peak is seen at1.262nm d-space for the natural sample but the ethylene glycol treatment moves smectite main peak to1.766nm d-space while heating up to550°C shifted the main smectite peak to about1.104nm d-space which is close to illite under normal condition(Fig.2C).EDX analysis used to detect the chemical composition of smectite that shows the major elements of smectite which are Si,Al,Na,Ca,Mg and Fe(Fig.2D).SEM investigations for Na+-smectite treated by1mol K+-nitrate at temperature(100and150°C)pressure conditions(3000psi con-fining and2500psi pore pressures)for one and two months showed changes in the smectite micro-texture.The webby crust of smectite completely disappeared while new phases were seen as discrete shape(rim)coatedflaky smectite(Fig.3A).The honeycomb structure of smectite–illite mixed layer was not observed.The abundance of the rims increases with increasing treatment temperature and time (Fig.3B).Qualitative studies of XRD diffractograms for the treated Na+-smectite samples indicated that smectite is very sensitive to changes in chemical and physical conditions even at low temperature.These effects are controlled by temperature and curing time.In the presence of K+,temperature is able to push smectite peaks from1.766–1.262 to1.262–1.104nm d-space which reflects changes in the mineral structure.The smectite peak intensity is also decreased,while the quartz peak(0.334nm d-space)intensity increases and becomes2Y.M.Metwally,E.M.Chesnokov/Applied Clay Science xxx(2011)xxx–xxxsharper especially for the sample that was treated for 2months (Fig.3C).The increase of quartz peak intensity is supported by EDX analysis (Fig.3D)which showed the pure silica composition of the rims.Meanwhile,the clay flakes showed enrichment on K that could be in-dicative of illite or illite –smectite mixed layer phases.3.2.Pierre Shale samplesThe untreated Pierre Shale composed of quartz,smectite,illite and pyrite.XRD analysis indicated that,the smectite peak position is af-fected by ethylene glycol treatment by the same manner as described for Na +-smectite sample .Ethylene glycol and heating treatments had no effect on illite main peak (0.93nm d-space).SEM studies showed the common minerals and micro-texture usually found in the Pierre Shale sample such as detrital quartz grains,illite and quartz rims coating the detrital minerals and the honeycomb textures of illite –smectite and kaolinite closed-up booklets (Fig.4).Also pyrite framboids,which are spheroidal aggregates of pyritic micro crystal-lites (~0.5μm)and are considered as early diagenetic components in shales (Berner,1970,1982,1984;Love and Amstutz,1966),were seen (Fig.4D).Fig.5showed the impact of 1mol K +-nitrite,temperature and du-rations on clay and non-clay minerals indicated;the change in clay mineral peak positions which re flect changes in the minerals struc-ture;change in clay mineral peak intensities,which re flect change in mineral abundance;change in peak sharpness,which re flects changes in crystallinity and crystal structure;and many peaks start to appear and some others disappear,re flecting a formation of new mineral phases.These changes are directly related to temperature and curing time used in the experiments.Smectite was the clay mineral most affected by temperature/pressure and chemical conditions.Changes in smectite peak position,intensity and sharpness can be easily noticed in Fig.5.The alteration processes induced by K +,and T/P conditions changes positions and sharpness of peaks.Smectite peak intensity decreases with increasing temperature,because temperature destroys smectite crystals.By in-creasing temperature from 100to 150°C,the smectite peak intensityTable 1Average bulk composition of pure Na +-smectite,natural Pierre Shale and the studied gas-shale (in weight percentage with 5%error).The gas shale 9samples re flect the nine different lithofacies.SampleSmectitePierre Shale Gas shale 123456789Quartz 74336374355111573627Orthoclase 42102121111Plagioclase 07422324633Pyrite0310*******Total carbonate 24343932652361123541Calcite 2227322252185472731Dolomite 004371024145Aragonite 00111111220Siderite 022********Anhydrite 00212212121Celestine 00333636235Apatite 00122313123Total clay 8741181613141511191717Smectite 8120101111111Illite069854951078Mixed layer 44433323433Kaolinite 011122211122Mica 00221211222Chlorite 00111210121Other 2Fig.1.The High Temperature High Pressure (HTHP)Shale Apparatus schematic design.3Y.M.Metwally,E.M.Chesnokov /Applied Clay Science xxx (2011)xxx –xxxnearly disappeared.This is because the KNO 3starts to push the smec-tite towards smectite/illite mixed layer,and eventually towards illite by re-crystallizing the smectite crystals destroyed under the effect of temperature and,less importantly,pressure.For a 150°C samples,the formation of smectite/illite mixed layer seems to be greater than the formation of a discrete illite phase.In the experiment where the sam-ple was treated for 4months with 1mol KNO 3and 150°C,smectite peak almost disappeared (Fig.5E).KNO 3seems to be able to recrystallize smectite and mixed layer smectite/illite or discrete illite crystals to form a new phase ofillite.Fig.2.Natural Na-smectite sample.A)The webby morphology,a common crystal habit of smectite,B)close up of the smectite webby morphology,C)XRD diffractograms for natural and ethylene glycol and 550°C treated samples,and D)smectite EDX spectrum showing the major element of smectite (Si,Al,Ca,and Na).Note the collapsed appearance of the coating is due to dehydration of the clay in the SEM vacuum system (Wilson and Pittman,1977).b-treated Na-smectite sample (1mol KNO 3,150°C,3000psi C p and 2500psi P p for 2months).A)Silica dust line (silica rim)coated flaky smectite,B)close-up view of the silica dust lines (granular quartz shape),C)XRD diffractograms for black natural and lab treated samples,and D)EDX spectrum for the rims showing Si the main element of silica rims.4Y.M.Metwally,E.M.Chesnokov /Applied Clay Science xxx (2011)xxx –xxxThis is indicated by the increase of the illite peak intensity and “sharp-ening ”of the peak.At 150°C,illite peak's intensity does not show the same rapid increase as at 100°C.This may be because the rate of illi-tization of smectite is less than the rate of destruction of initial illite itself.This clearly appears when time of the same condition is in-creased to 4months,the illite and smectite peaks disappear while the quartz peak sharply increases,most probably because of the for-mation of a new α-quartz phase.The disappearance of illite may be partially because the new silica phase masks clay minerals from being seen by X-ray.Kaolinite is the clay mineral least affected by the study conditions.The fluid and temperature that was used in the current study had an impact especially with samples that are treated by 150°C on kaolinite.Fig.5showed a slight shift in kaolinite peak position towards the high 2θat 150°C for the 2month treated sample,possibly indicating a slight Ost-wald ripening of kaolinite crystals (Eberl and Srodon,1988;Eberl et al.,1990;Eberl et al.,1993;Srodon et al.,2000).The kaolinite peak intensities also showed a reduction with increasing curing time.It's important to say “many investigators claimed that,in the presence of K +source,kaolinite thought to be sensitive to temperature higher than 80°C which is in agreement with the current laboratory investigations ”.The intensity of quartz primary peak (0.334nm d-space)and sec-ondary peak (0.425nm d-space)increased for treated Pierre Shale sam-ples in all the studied conditions.Meanwhile,new tertiary peaks at (e.g.at 0.182nm d-space)started to appear.The new formed peaks matched with a synthetic α-quartz.These peaks started to appear within the first month of treating Pierre Shale even at 100°C while they were not seen with treated Na-smectite sample at similar conditions which means that the whole-rock's mineral composition (illite,kaolinite,chlorite,quartz,feldspars pyrite and other minerals)as they exist in nature shales in fluences the clay mineral's transformation.The newly formed materials cover the pre-existing components and precipitate in micro-pores as seen in Figs.6and 7.The new silica phase was seen either as discrete shape (rim)or sheets.This rim is observed in all treated samples (even in the treated pure clay sample)at increasing abundance with increasing temperature and curing time (Fig.6).The rim is similar in shape to silica overgrowth phases recorded during sandstone diagenesis and causing a porosity and permeability reduction.The silica sheet coated the preexisting minerals (Fig.7).The silica sheet is not observed with treated Na +-smectite samples or the Pierre Shale that is treated by 100°C.The abundance of silica sheet increases with increasingtemperatureFig.4.SEM photographs show some of natural Pierre Shale features.A)Thin illitic and quartz dust line (R)or sometimes ribbons coated flaky smectite (S),B)close-up view of the silica dust lines (R),C)webby or highly-crenulated smectite –illite mixed layer (honeycomb structure),and D)pyrite framboids,note the clearness of the pyritic micro crystallites,E)EDX spectrum showing the major element of smectite –illite mixed layer (Si,Al,Ca,and Na and K),and F)EDX spectrum showing the major element of pyrite framboids (S and F).5Y.M.Metwally,E.M.Chesnokov /Applied Clay Science xxx (2011)xxx –xxxand curing time.This is in agreement with the above mentioned XRD studies.Pyrite framboids also showed extensive effects caused by the treatment conditions,because of oxidizing conditions that samples were subjected to during the experiments.Euhedral crystals changed to anhedral ones,and crystal edges dissolved to form an iron-silica phase covering the original crystals.Also,a reduction in pore size be-tween the pyrite crystals as a result of new cementation is clearly seen (see also Mohamed,2007).The cement material composition is recorded by EDX as a Fe –Ca-silicate,suggesting mass transferofFig.5.X-ray diffractograms for both natural and lab treated Pierre Shale (whole-rock pattern).Note increase of the main illite peak intensity of sample that treated for 1month by 100°C,illite and kaolinite peak intensities decrease for samples that are treated by 150°C,and quartz peak mean intensity increases from the upper diffractograms to the lower one,and the intensity of the higher 2θquartz peak (tertiary peak)also increases from up to down.6Y.M.Metwally,E.M.Chesnokov /Applied Clay Science xxx (2011)xxx –xxxCa and silica from adjacent materials as a result of the destruction of the clay minerals under the in fluence of chemical solution chemistry and elevated temperature conditions.3.3.Gas shale samplesThe studied gas shale is classi fied into nine different lithofacies based on the high resolution lithological characteristics of the core (Singh and Slatt,2006).These nine facies correspond to nine different depositional environments.XRD and SEM studies for the nine differ-ent lithofacies indicated that Quartz (detrital,and authigenic),clay minerals –mostly illite-and calcite and dolomite are the most abun-dant minerals.Minerals such as plagioclase,orthoclase,pyrite,sider-ite,apatite,anhydrite and mica (muscovite)are less abundant.Table 1shows the average composition for the nine different lithofa-cies.The most common minerals and their micro-texture of the cur-rent studied gas-shale are seen in Fig.8.Based on mineralogical composition the studied gas-shale can be classi fied into two major different lithofacies:siliceous facies and carbonate-rich facies.Clay-rich facies are characterized by quartz and clay,while carbonate ones are rich in carbonate.Study of the XRD diffractograms of the nine different lithofacies of studied natural gas shale formation (Fig.9)indicated that all nine dif-ferent lithofacies were similar in complete disappearance of the main smectite peaks re flected by very low smectite content as in Table 1,and illite is the predominant clay mineral (~10%of the whole rock).Smectite/illite mixed layer peak is also found,kaolinite main peak did not show,and the main and secondary quartz peaks are very sharp for all the lithofacies.Each lithofacie is characterized by different abundances of quartz,carbonate (calcite,dolomite)and phosphate (apatite)content.Close look into the internal composition by SEM showed two types of quartz:the detrital quartz which is usually seen in sand-and silt-size euhedral grains and the authigenic quartz which is usually seen within clay size anhedral particles.The latter type of quartz is recorded as α-quartz in XRD pattern (Fig.9)and seen as silica coating the clay platelets and also as small grain and rim on the remnance of clay platelets (Fig.10).Fig.10A shows a clear example for transformation of clay minerals into granular authigenic silica-and Al-rich gains.This could be the answer for the disappearance of smectite from the studied gas shale formation.The sheet-like structure seems to be a layered structure in between remnance of clays.In this case the remnance materials are rich in silica and have a lot of micro pores in between (Fig.10D).Signi ficant amount of mica is also seen as in Fig.8C.The origin of this mica needs further studies.It could be detrital or authigenic in origin.If it is authigenic mica,signi ficant amount of authigenic quartz will be produced during mica formation.In this case and according to the traditional way of thinking,the current formation could be heated to more than 200°C if we ignore the duration.We searched for any other mineral assemblage that is characterized by very low grade metamorphism (Dunoyer de Segonzac,1970;Frey,1978and Hoffman and Hower,1979)but unfortunately we did not find any of these mineral assemblages.This means the studied gas shale for-mation is still in the diagenesis windows.Pyrite framboids are common in the current studied gas-shale as rigid grains and platy minerals such as clay and mica are bended and wrapped around them (Fig.8F).The rigidity is a result of silica and clay cementation.These cementations reduce the pore amount and size that are seen between pyritic micro crystallites (see Figs.4D and 8F forcomparisons).Fig.6.SEM pictures of granular silica formed during high temperature high pressure treatment of Pierre Shale.A)Sample treated for 1month using 100°C,B)sample treated for 2months using 150°C,C)sample treated for 4months using 150°C,and D)EDX spectrum shows Si as the main element for the granular bodies.Note the increase of the abundance of granular bodies with increase of temperature and treatment time.7Y.M.Metwally,E.M.Chesnokov /Applied Clay Science xxx (2011)xxx –xxx4.DiscussionSEM and XRD investigations for natural and lab treated clay rich samples indicated that:temperature and K +-nitrate altered smectite crystals to form new illite and quartz phases instead,even at low temperatures (100–150°C);temperature and duration of treatment are important factors in clay mineral transformation and new quartz phase texture;illite and kaolinite are less affected while pyrite framboids textures are altered due to the treatment conditions.Chemically silica is required to be removed from the smectite –illite transformation reaction either by using mass balance equation written by Hower et al.(1976);K þþAlþ3þSmectite →Illite þNa þþCaþ2þFeþ2þMgþ2þSiþ4þH 2O(this reaction is nearly mass conservative and would lower the pH of the pore fluid),or using the other reaction written by Boles and Franks (1979);K þþSmectite →Illite þNa þþCa þ2þFeþ2þMgþ2þSiþ4þOH−þH 2O(this reaction decreases the mass of clay by more than 30%,produces large amounts of quartz,and raises the pH).However,the role of that released quartz on mud rock cementation is neglected.The source of potassium is another openly debated question in illi-tization of smectite,whether it comes from the shale formation (close system)or migrates into shale from the surrounding formations (open system)(Weaver and Beck,1971).During burial diagenesis,this could accompany the “albitization ”or “dissolution ”of potassium feldspars in adjacent sandstones in (open system)or in shale itself (close system),as suggested by several authors,such as Cuadros and Linares (1995),Day-Stirrat et al.(2010)and Lynch et al.(1997).We believe that the thermal heat that led to maturation of organic matter to produce gas was able to alter and dissolute clay minerals (smectite,illite and kaolinite)and non-clay minerals such as k-feldspar,and the newly formed gas and elements including silica and potassium were unable to escape from this very tight rock.Disso-lution of K +-feldspar could be indicated from low K +-feldspar con-tent of the studied gas-shale (Table 1).It is widely accepted that illite –smectite (I –S)mixed layer,which is the most abundant mineral class by volume in sedimentary rocks (Land et al.,1997),is an intermediate product of smectite –illite trans-formation.However and based on the lab treated sample for 4months at 150°C and also the gas-shale sample the amount of detected illite and smectite/illite mixed layer is less than expected.Further studies are suggested to solve this problem.4.1.Quartz cement morphologyOur studies indicated that,the released silica precipitated in-situ to form the recorded authigenic quartz.The quartz new phase has two shapes;First one is the granular authigenic quartz which is similar to quartz rim and grains observed within the entire lab treated sam-ples (100and 150°C).Also granular quartz has been seen in nat-ural gas-shale samples as micro-gains (less than 1μm)and contains a lot of micropores (~0.1μm)in between (Fig.10D).Fig.7.SEM pictures of sheet silica formed during high temperature high pressure treatment of Pierre Shale.A)Sample treated for 1month using 100°C,B)sample treated for 2months using 150°C,C)sample treated for 4months using 150°C,and D)EDX spectrum shows Si as the main element for the sheets.Note the increase of the abundance of sheets with increase of temperature and treatment time.8Y.M.Metwally,E.M.Chesnokov /Applied Clay Science xxx (2011)xxx –xxx。