Characteristics of silty laminae in Zhangjiatan Shale of southeastern Ordos Basin,China Implications

Characteristics of silty laminae in Zhangjiatan Shale of southeastern Ordos Basin, China:Implications for shale gas formation

Yuhong Lei,Xiaorong Luo,Xiangzeng Wang,

Lixia Zhang,Chengfu Jiang,Wan Yang,Yuxi Yu, Ming Cheng,and Likuan Zhang

A B S T RA C T

Shale oil and gas have been discovered in the lacustrine organic-rich Zhangjiatan Shale of the Upper Triassic Yanchang Formation,Ordos Basin,China.Core observations indicate abun-dant silty laminae in the producing shales.This study documents the stratigraphic distribution of silty laminae and their relationship with interlaminated clay laminae.The type,structure,and charac-teristics of pores and mineral composition of silty laminae were observed and analyzed through thin section and scanning electron microscopy,X-ray diffraction,low-pressure CO2and N2adsorp-tion,mercury porosimetry,and helium pycnometry.Results from silty laminae are compared with those of clayey laminae.The fre-quency and thickness of silty laminae vary over a wide range.The thickness ranges from0.2to4mm and is1.5mm on average;the frequency ranges from4to32laminae/m and is23laminae/m on average.The thickness percentage of silty laminae in the mea-sured segments ranges from6%to17%.Silty laminae consist of quartz,feldspar,mixed-layer montmorillonite,and chlorite.In comparison to clayey laminae,non-clay detrital grains are larger, quartz and feldspar are more common,and clay minerals are less abundant.Pores in silty laminae are primary interparticle,dis-solutional,intercrystalline,and microfracture types.Mesopores (2–50nm in diameter)and macropores(50nm–1μm)are common,whereas,microporese<2nmTare rare;the distribution of pore diameters is multimodal.However,microscopic pores with a diameter commonly smaller than100nm are common in Copyright?2015.The American Association of Petroleum Geologists.All rights reserved. Manuscript received April3,2014;provisional acceptance July11,2014;revised manuscript received September8,2014;final acceptance September30,2014.

DOI:10.1306/0930*******A U T H O R S

Yuhong Lei～Key Laboratory of Petroleum Resources Research,Institute of Geology and Geophysics,Chinese Academy of Sciences, Beijing100029,China;granulitelei@04df3af9be23482fb4da4cae Yuhong Lei is a postdoctoral researcher at the Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing.He received B.S. and M.S.degrees in mineralogy,petrology,and mineral deposit geology from the China University of Geosciences and a Ph.D.in petroleum geology from the Institute of Geology and Geophysics Chinese Academy of Sciences.His interests include hydrocarbon migration in petroleum systems and unconventional oil and gas. Xiaorong Luo～Key Laboratory of Petroleum Resources Research,Institute of Geology and Geophysics,Chinese Academy of Sciences, Beijing100029,China;luoxr@04df3af9be23482fb4da4cae Xiaorong Luo is a research scientist in the Chinese Academy of Sciences and has

B.S.and M.S.degrees in geology from Northwestern University,China,and a

Ph.D.in geophysics from the University of Montpellier,France.His research in the

last30years has been in petroleum geology, currently focusing on numerical modeling,fluid pressure,and hydrocarbon migration and accumulation.

Xiangzeng Wang～Shaanxi Yanchang Petroleum(Group)Co.,Ltd.,75#Keji2Road, Xi’an710075,Shaanxi Province,China; sxycpcwxz@04df3af9be23482fb4da4cae

Xiangzeng Wang received his doctoral degree from School of Energy and Resources of China University of Geoscience.He is currently the vice president of Shaanxi Yanchang Petroleum (Group)Co.,Ltd.He has been involved in oil and gas exploration and exploitation for more than20 years.His main work in recent years has focused on the Ordos Basin.

Lixia Zhang～Shaanxi Yanchang Petroleum (Group)Co.,Ltd.,75#Keji2Road,Xi’an710075, Shaanxi Province,China;zlxcyq999@04df3af9be23482fb4da4cae Lixia Zhang received her bachelor’s degree from Northwest University.She is currently the vice president of Research Institute of Shaanxi Yanchang Petroleum(Group)Co.,Ltd.She has been involved in oil and gas exploration and exploitation for more than30years.Her main work in recent years has focused on shale gas exploration and exploitation.

AAPG Bulletin,v.99,no.4(April2015),pp.661–687661

clayey laminae.Thus,pore volume and surface area of micropores in silty laminae are less than those in the adjacent clayey laminae,and vice versa for meso-and macropores.The porosity of shales increases with the proportion of silty laminae in the shales.The silty laminae provide the storage space and flow conduit for oil and gas,and they play a significant role in the migration,accumu-lation,occurrence,and amount of gas in the shales.INTRODUCTION Shale gas refers to unconventional natural gas in organic-rich mud rocks (Curtis,2002).Shale gas reservoirs are widespread and contain a large amount of oil and gas reserve,which is difficult to produce using conventional technology (Curtis,2002).The success of shale gas exploration in the United States has encour-aged similar exploration activities in many other countries.In the last few years,China has experimented with shale gas exploration and made some major discoveries (Guo et al.,2011;Wang et al.,2011;C.F.Jiang et al.,2013a).The discovery in Zhangjiatan Shale,which is the shale in the lower part of Member 7of the Upper Triassic Yanchang Formation in southeastern Ordos Basin,is representative of those in lacustrine shales (C.F.Jiang et al.,2013a).Compared to sandstones,our understanding of the sedimentary characteristics of shale,such as grain composition,sedimentary tex-ture,and pores is limited (Passey et al.,2010;J.C.Zhang et al.,2011).Recently,many studies have been carried out as spurred by shale oil and gas exploration (e.g.,Curtis,2002;Bowker,2007;Passey et al.,2010),focusing on depositional environments (Kemp,1996;Loucks et al.,2007),organic content (Passey et al.,1990;Derenne et al.,2000)and maturity (Sweeney and Burnham,1990;Jarvie et al.,2007),pore type and porosity (Loucks et al.,2009,2012),characteristics and origin of fractures (Lash and Engelder,2005;Gale et al.,2007),mineral composition,and espe-cially,content of brittle minerals (Yang and Aplin,2007;Rickman et al.,2008),resources evaluation (Jarvie et al.,2005,2007;Kinley et al.,2008;Ross and Bustin,2008),etc.Some of these studies compared the characteristics and parameters of target shales with those with successful discoveries (Nie et al.,2009;Bruner and Smosna,2011;C.F.Jiang et al.,2013a).The Zhangjiatan Shale in southeastern Ordos Basin was deposited in a fresh –brackish,sublittoral to profundal lacus-trine environment (Yang and Zhang,2005;W.Z.Zhang et al.,2008).Intercalated sandy deposits in the shale on the outcrop are 0.01–1.2m (0.03–3.9ft)thick and may have played an important role in shale gas accumulation (Cheng et al.,Chengfu Jiang ～Shaanxi Yanchang Petroleum (Group)Co.,Ltd.,75#Keji 2Road,Xi ’an 710075,Shaanxi Province,China;petrojcf@04df3af9be23482fb4da4cae;1282763150@04df3af9be23482fb4da4cae

Chengfu Jiang received her bachelor ’s degree from Jilin University.She is currently the manager of unconventional resources of Research Institute of Shaanxi Yanchang Petroleum (Group)Co.,Ltd.She has been involved in oil and gas exploration and exploitation for more than 30years.Her main work in recent years has focused on shale gas exploration.

Wan Yang ～Key Laboratory of Petroleum Resources Research,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;present address:State Department of Geological Science and

Engineering,Geology and Geophysics Program,Missouri University of Science and Technology,Rolla,Missouri 65409;yangwa@04df3af9be23482fb4da4cae

Wan Yang obtained a Ph.D.from the University of Texas at Austin in 1995and is currently an associate professor at Missouri University of Science and Technology,teaching sedimentology and terrigenous clastic depositional systems.He has worked on depositional systems analysis,sequence and cyclostratigraphy,

paleoclimatology,and reservoir characterization of marine and nonmarine siliciclastic and

carbonate rocks in China and the United States.Yuxi Yu ～Key Laboratory of Petroleum Resources Research,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;present address:University of Chinese Academy of Sciences,Beijing 100049,China;yuyuxi718@04df3af9be23482fb4da4cae Yuxi Yu got a B.S.degree and an M.S.degree from China University of Petroleum,Qingdao.She is a Ph.D.candidate at University of Chinese Academy of Sciences with interests in pore structure characterization of shale reservoir.Ming Cheng ～Key Laboratory of Petroleum Resources Research,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;present address:University of Chinese Academy of Sciences,Beijing 100049,China;chengmingone@04df3af9be23482fb4da4cae Ming Cheng received his B.S.degree and

M.S.degree from China University of Petroleum,Qingdao.Currently,he is a Ph.D.candidate at University of Chinese Academy of Sciences with interests in basin analysis,basin modeling,hydrocarbon migration,and accumulation mechanism of shale gas.

662Silty Laminae in Zhangjiatan Shale

in press).However,gas has been produced from shale with essentially no sandstone in some wells in the basin center (Cheng et al.,in press).Methane seepage from cores sub-merged in water at the well site indicates that the seepage com-monly occurs from silty laminae that are less than1mm thick; some occur from subhorizontal microfractures and bedding planes but are apparently related to the silty laminae.Further systematic observations indicated the common occurrence of silty laminae in Zhangjiatan Shale,suggesting an important role of silty laminae in shale gas accumulation and production (Cheng et al.,in press).

Textural heterogeneity of shales(Davies et al.,1991;O’Brien, 1996;Bustin et al.,2008;Aplin and Macquaker,2011)and the occurrence of silty laminae and their depositional environments (Kemp,1996),hydrodynamic conditions(Schieber,1990; O’Brien,1996),origin of silty laminae(Carey and Roy,1985; Schieber,1990,1991;Anderson,1996;O’Brien,1996;Reineck and Singh,2006),and provenance(Dean et al.,2002)have been studied.Silty laminae in shales were subdivided into several types,such as thick or thin silty–clayey couplets,wavy–lenticular silty or parallel laminae,massive shale beds,etc.(Carey and Roy, 1985;Schieber,1990,1991).Kemp(1996),Dean et al.(2002), and Potter et al.(2005)interpreted that silty laminae were depos-ited in relatively low-energy hydrodynamic conditions in reducing environments.The studies on sedimentary textures and structures and depositional environments of silty laminae by O’Brien (1990,1996),Shieber(1990,1991),and Anderson(1996)and the physical experiment of Carey and Roy(1985)recognized several processes in the deposition of silty laminae,including eolian,colloidal,sediment gravity flow,and reworking by weak bottom flows.

The common occurrence of silty laminae and their roles in the generation and distribution of shale gas have been noticed in many shales,such as the Mississippian Barnett Shale in north Texas(Slatt and O’Brien,2011),Lower Devonian Ohio Shale in the northwestern Appalachian Basin(Broadhead et al.,1982; Charpentier et al.,1993),Lower Jurassic Haynesville Shale in east Texas and west Louisiana(Hammes et al.,2011),Cretaceous shale in Denver Basin,Colorado(Sutton et al.,2004),and Cretaceous shale in the Green River Basin(Miskimins and Barree,2003).Shales containing silty laminae of a certain thick-ness and abundance have good storage and permeability of natural gas(Davies et al.,1991;Broadhead,1993;Rokosh et al.,2009) and are common good producers(Nuhfer et al.,1979; Broadhead,1993).Thus,it is speculated that silty laminae may form significant migration conduits and contribute significantly to reservoir storage(Schmoker,1993;Davies and Vessell,Likuan Zhang～Key Laboratory of Petroleum Resources Research,Institute of Geology and Geophysics,Chinese Academy of Sciences, Beijing100029,China;zhanglikuan1979@163. com

Likuan Zhang is currently an associate professor

at the Institute of Geology and Geophysics, Chinese Academy of Sciences.He received a B.A. degree in petroleum geology from Jilin University, China,an M.S.degree from Northwestern University,China,and a Ph.D.from the Institute

of Geology and Geophysics,Chinese Academy of Sciences.His research interests include fluid flow

in fault zones,numerical basin modeling,and hydrocarbon migration in petroleum systems.

A C K N O W L E D G E M E N T S

This study was supported partly by the Chinese National Natural Science Foundation(41102078) and Chinese National Major Fundamental Research Developing Project(2011ZX05008-004). The study would not have been possible without the support from Shaanxi Yanchang Petroleum (Group)Corp.,Ltd.,Xi’an,China.We thank Shaanxi Yanchang Petroleum(Group)Corp.,Ltd. for permission to publish this work.We thank Fang Hao,Jack C.Pashin,Roger M.Slatt,Michael L.Sweet,Terrilyn Olson,and Andrea Sharrer for their constructive comments and suggestions that greatly improved this manuscript.

L EI ET AL.663

2002).Some recent studies(J.C.Zhang et al.,2008, 2012;Jiang,2013)focus on intercalated sandstones in shale intervals and their roles in shale gas genera-tion and accumulation.However,no detailed analysis and discussion have been published on the role of silty laminae in shale gas accumulation.Hence,a study on the mineralogy,pore structure,porosity, and gas content of silty laminae will provide insights into the generation,migration,and accumulation of natural gas in shales.The results should aid shale gas exploration and production.

This work describes the microscopic sedimentary characteristics,mineralogy,pore structure,porosity, and gas and oil content of silty and clayey laminae in the Zhangjiatan Shale of the Upper Triassic Yanchang Formation in Ordos Basin,and it docu-ments their physical properties.The results provide the basis for an in-depth discussion on the role of silty laminae in shale gas formation.GEOLOGICAL BACKGROUND

The study area is located on the southern part of the Yi-Shan slope of the Ordos Basin,north-central China(Figure1).The Ordos Basin is located in the western part of the North China craton.The craton was stable during the Paleozoic,when lower Paleozoic platform carbonate and upper Paleozoic sil-iciclastic rocks were deposited(Yang et al.,2006). The basin became an interior lake basin when the Luliang Mountains to the east were uplifted during the Mesozoic.Up to4000m(13,123ft)of fluvial–lacustrine siliciclastic sediments were deposited from Late Triassic to Cretaceous(Figure2);the thickest part of the preserved strata is in the southwestern part of the basin(Zhao et al.,2008).Cenozoic strata occur only locally in the basin(Zhang et al.,1999).The Ordos Basin is structurally simple(Figure1),espe-cially the Yi-Shan slope,which is a west-dipping

o

o

Figure1.(A)Map showing the Ordos Basin in central China.Box is the map area in(B).(B)Tectonic map of the Ordos Basin,modi-fied after C.F.Jiang et al.(2013a).The study area in the southeastern part of Yi-Shan slope is outlined by a box.(C)Isopach map of Zhangjiatan Shale in the lower part of Member7of Yanchang Formation,modified after Yang et al.(2006)and C.F.Jiang et al. (2013a).Contour interval is5m(16ft).

664Silty Laminae in Zhangjiatan Shale

monocline with a1to3degree dip angle that has very few seismically identifiable faults and local low-relief folds(Wang and Wang,2013).

The Yanchang Formation contains fluvial–lacustrine siliciclastic rocks ranging from900to 1600m(2953–5249ft)thick,and it can be subdi-vided into10members(Yang,2002).The Zhangjiatan Shale is in the lower part of Member7 (Figure2).The Yanchang Formation records a com-plete cycle of lacustrine basin initiation,develop-ment,and cessation(He,2003).The lowest, Member10,was deposited during the incipient stage of the lake basin.Members9and8were deposited during an episode of major transgression when ther-mal subsidence occurred.Member7was deposited during an episode of accelerated subsidence,when water depth increased to50to120m(164to 394ft),causing significant lake expansion(He, 2003;C.L.Zhang et al.,2011).As a result,the Zhangjiatan Shale is organic-rich and widespread (Yang and Zhang,2005;W.Z.Zhang et al.,2008). Member6signifies a period of constructive deltaic infilling with decreased subsidence rate.Members4 to1were deposited in the period of major lake con-traction;swamp dominated during the deposition of Member1,and the lake disappeared gradually(He,

Figure2.Chart showing chrono-and litho-stratigra-phy,lithology,depositional environments,and occur-rences of shales in the southeastern part of the

Yi-Shan slope of Ordos Basin,central China(modi-fied after Yang,2002;Yang et al.,2005;Zeng and Yang, 2009).The shale in the lower part of Member7of the Yanchang Formation is the Zhangjiatan Shale.

L EI ET AL.665

2003).In the study area,four episodes of major ero-sion had occurred,including Late Triassic(202.5–200Ma),late Early Jurassic(180–175Ma),Late Jurassic(149.5–145Ma),and Late Cretaceous(100–65Ma),resulting in the removal of a total of1800–2400m(5906–7874ft)of sedimentary deposits (Chen et al.,2006).

The Zhangjiatan Shale is one of the most prolific hydrocarbon source rocks(Yang and Zhang,2005; Zhang et al.,2006;Kong,2007)and a potential target for shale oil and gas exploration(Wang et al.,2011;

C.F.Jiang et al.,2013a)in the Ordos Basin.It is 30–100m(98–328ft)thick and contains types I and II1organic matter(Yang and Zhang,2005;Kong, 2007)with a total organic carbon(TOC)of6%to 14%and up to30%(Yang et al.,2005;Kong,2007; Wang et al.,2011).The hydrogen index varies between50and255mg HC/g TOC,with an average 178mg HC/g TOC(Guo et al.,2014a).The amount of free liquid hydrocarbons values(S1)range between0.03and9.6mg/g,and is3.1mg/g on aver-age.The amount of total residual hydrocarbons val-ues(S2)generally varies from0.1to23.4mg/g, with an average8.2mg/g(Guo et al.,2014a).The shale reached its maximum burial depth of～3000m (9842ft)in late Early Cretaceous at～100Ma,with the vitrinite reflectance(R o)ranging from0.7%to 1.3%(Ma et al.,2005;Zhang et al.,2006;Wang et al., 2014).The value of the temperature of maximal hydrocarbon release during the Rock-Eval pyrolysis (T max)ranges from418to474°C,with an average of450°C(Yang et al.,2012;Guo et al.,2014a). Tectonic uplift across the entire basin since Late Cretaceous has caused significant erosion of a maxi-mum of～2000m(6562ft)of strata in eastern Ordos(Chen et al.,2006).At the present,the depth of Zhangjiatan Shale in the study area ranges from 500to2000m(1640to6562ft)(C.F.Jiang et al., 2013a).

DATA AND METHODS

The Zhangjiatan Shale contains intercalated sand-stones,ranging from1to20cm thick(Cheng et al., in press).This study focuses on silty(or siltstone)lam-inae that are thick enough to be identified by the naked eye.Cores from28wells in the study area and100samples from the cores are the main data source in this study(Figure1).The thickness and number of individ-ual clayey and silty laminae and their total thickness were documented.Their mineral composition,pore structure,and porosity were analyzed.

Core Analysis

Lithology,sedimentary structures,geometry,and type of silty laminae were observed from core slabs.

A caliper of a precision of0.02mm was used to mea-sure the thickness of individual laminae with the naked eye.The number and thickness of individual silty laminae were documented.

Thin Section Microscopy

Standard procedures were used in thin-section mak-ing.Thin sections for petrographic observations are 30μm thick;those for fluorescence analysis are 60μm thick.Petrographic studies were done using a Leica DMLP polarizing microscope with a Leica DFC450camera system.Fluoresce analysis was done using a Nikon Eclipse80i upright biological fluores-cence microscope with a light source of a wavelength 330–380nm.

Scanning Electron Microscopy

and Pore Analysis

Pore characteristics were observed using scanning electron microscopy(SEM).The instrument used in the analysis was a Hitachi field-emission SEM with cold emission,which is equipped with low and high secondary electron probes and X-ray spectrometer (EDAX).Samples were cut into0.5cm×1cm×0.2mm chips and polished to0.1mm thick using helium ion beams.Gold coating is20nm.Pore size was measured on SEM images using the Image-proplus software(Bennett et al.,2012;Dong and Harris,2013).Maximum length was measured for isolated pores.In cases of connected small pores with complex geometry,pores were measured separately with the best effort,and the narrowest pore space is regarded as a pore throat.The measuring pore sizes in a two-dimensional plane are pore width,because the maximum dimension is not always intersected. The composition of grains and cements was obtained

666Silty Laminae in Zhangjiatan Shale

by energy dispersive spectrometer(EDS)analysis to infer the mineralogy.

X-ray Diffraction Analysis

Samples were prepared for whole-rock mineralogical analysis.They were crushed and sieved to200mesh size(74μm)and dried at50°C for5hr.The instru-ment is a German Brucker D8Advance X-ray diffrac-tometer.Scanning range is3–85°;aperture1mm,and speed4°/min.

Mercury Injection Analysis

Samples were crushed and sieved into grains of20to 35mesh size(0.85–0.5mm)and vacuum dried at 100°C for24hr before mercury injection.The instru-ment is an automatic mercury injection U.S. PoreMasterGT60.It can measure pore diameters ranging from3.6nm to950μm,with a maximum pressure of60,000psia,and an accuracy of1%in vol-ume.The accuracy meets the requirement by the Chinese Government Standard GB/T21650.1-2008.

A companion instrument,the Ultrapyconometer 1000,measures the grain density at an accuracy of0.03%.

Low-Pressure CO2and N2Adsorption Isotherm Analyses

Silty and clayey laminae were mechanically sepa-rated and crushed respectively into20to35mesh size.They were dried and degassed at80°C.Then, low-pressure CO2and N2isotherms analyses (<0.127MPa)were conducted to obtain the average pore size distributions and specific surface area of the silty and clayey samples at a nanometer scale (Chalmers et al.,2012;Tian et al.,2013).An instru-ment,the Quantachrome NOVA4200e,was used in these analyses that meets the Chinese Government standard GB/T21650.1-2008.Carbon dioxide gas adsorption at0°C is considered to access porosity as fine as0.35nm and was used for investigating micro-porosity(Quantachrome,2008,p.161).The pore size distribution and surface area of micropores were determined over a pressure range of4×10?4–3.2×10?2at0°C by CO2adsorption using the density functional theory model(Klobes et al.,2006).The

nitrogen adsorption/desorption isotherms are consid-

ered to reflect mesopores and macropores.Nitrogen

isotherms were measured at?196.5°C,and the pore

size distribution can be obtained using the Barrett–

Joyner–Halenda model(Klobes et al.,2006).And,

the total specific surface area can be obtained from

nitrogen isotherm data in the1×10?6–0.1relative pressure range(P∕P0,where P is the gas vapor pres-

sure in the system,and P0is the vapor pressure above

the gas at the temperature of interest)using a multi-

point Brunauer–Emmett–Teller(BET)model

(Klobes et al.,2006).

Porosity Measurement

Grain and gas volume of a sample was measured on

the basis of the helium expansion principle to calculate

the porosity of the sample(Haskett et al.,1988).

Helium molecules are small(0.258nm in diameter,

Li and Fu,2007,p.13)and nonadsorptive,hence,they

can enter pores of variable sizes.The measured poros-

ities include microscopic pores and range from0.01%

to40%.A cylindrical sample of a2.5cm diameter

and3–5cm length with a flat and parallel top and bot-

tom was dried in a vacuum.It was placed in a

PoroPDP helium porosimeter manufactured by Core

Laboratories to measure the sample porosity.The stan-

dard API RP-40of the American Petroleum Institute

was used in the test.

RESULTS

Distribution of Silty Laminae in Shale

Cores of Zhangjiatan Shale from nine wells(W1,W2,

W4,W5,W8,W9,W10,W12,and W17)(Figure1)

were described.The core from W1in the center of

the study area spans the entire Member7(Figure3)

and is133m(436ft)thick.The lower part is the

black Zhangjiatan Shale,58.6m(192.3ft)thick;the

middle part is oil-bearing,fine-grained sandstone;

the upper part is interbedded shale and sandy shale.

The Zhangjiatan Shale consists mainly of black

shale intercalated with sandstone beds15–60cm

thick.Silty laminae are abundant and variably distrib-

uted(Figure3).A total of1880silty laminae or sandy

L EI ET AL.667

Figure3.(A)and(B)Thickness and distribution of silty laminae in Zhangjiatan Shale in Well W1.Per is the thickness percentage of silty laminae(blue),%;Den is the density of occurrence of silty laminae(green),laminae/m;Por is the porosity(red);wireline logs include gamma ray(GR,brown),self-potential(SP,purple),and acoustic(AC,black).Boxes and letters indicate the location of photo-graphs and photomicrographs.(C)Core photograph of an interval without apparent silty laminae.(D)Thin section photomicrograph of sample①,showing silty laminae that cannot be seen by naked eyes.Location is shown in(C).(E)Core photograph showing even and wavy silty laminae.Location is shown in(B).(F)Thin section photomicrograph of sample②,showing quartz,feldspar,and clay min-erals in silty laminae.Detrital grains in silty laminae range from30to235μm,mainly40–160μm in size;those in clayey laminae are commonly less than4μm in size.Fracture is filled by calcite.Location is shown in(E).(G)Core photograph of parallel and lenticular silty laminae.Location is shown in(A).(H)Thin section photomicrograph of sample③,showing clay minerals and minor quartz and feldspar of clayey laminae.Detrital grains are commonly less than4μm in size.Location is shown in(G).(I)Core photograph of even and lenticular silty laminae.Location is shown in(B).(J)Thin section photomicrograph of sample④of silty laminae showing quartz, feldspar,and clay minerals as major minerals with grain size ranging from16to20μm.Location is shown in(G).

668Silty Laminae in Zhangjiatan Shale

beds were identified in50.5m(165.7ft)of Zhangjiatan Shale in Well W1.They have a total thickness of601.13cm,accounting for11.9%of the total thickness of the Zhangjiatan Shale.Average fre-quency of occurrence of silty laminae is37laminae/ m,and individual laminae are commonly0.2–5mm thick(Figure3A,B).Silty laminae are abundant in intervals between1363.3–1371.1,1392.1–1395.8, 1406.4–1409.6,and1411.7–1416.2m,with a frequency of57,45,69,and75laminae/m and thick-ness percentages of11.52%,22.4%,31.3%,and 25.6%,respectively(Figure3A).The intervening intervals between1380.3–1392.1,1395.8–1406.4, and1409.1–1411.7m have relatively limited silty laminae with a frequency of16,11,and16laminae/ m,respectively(Figure3A).

Including the other eight wells(Figure4and Table1),silty laminae range from0.2to4mm and average1.5mm in thickness(Figure4A).Silty lami-nae thicker than10mm account for～2.5%of the total shale thickness.The frequency ranges from4to32, average23,and up to155laminae/m(Figure4B). The thickness percentage of all silty laminae ranges from6%to17%.In general,the frequency of occur-rence and thickness of individual silty laminae increase with increasing distance from the shale depocenter.It should be noted that the trend is documented from cores that have different length and are located in dif-ferent parts of the Zhangjiatan Shale.In addition,the statistical data are from silty laminae that are thicker than0.1mm;in fact,microscopic observations can identify silty laminae that are1–10μm thick(compare Figure3C,G).

Petrographic Characteristics of Silty Laminae Silty laminae are commonly grayish-white and are evidently different from adjacent clayey laminae in grain size and mineral composition(Figure3). Microscopically,grains in silty laminae range from 4to200μm(mostly40–160μm)and average35μm in size with a median of34μm(Figure5A),and they are moderately to poorly sorted(Figure5A). However,detrital grains in clayey laminae are com-monly less than10μm in size,mostly less than6μm, with an average size of3.2μm and a median size of 3.3μm(Figure5B).

Mineral composition of silty laminae also differs from that of clayey laminae(Table2).The former has28%–38%of quartz,with an average of35%,and 23%–51%of feldspar,with an average of38%. However,clayey laminae contain26%–40%of quartz, with an average of33%,and16%–32%of feldspar, with an average of24%.The content of clay minerals, such as chlorite and montmorillonite,varies from20% to32%,with an average of28%in silty laminae,and from30%to53%,with an average of41%in clayey laminae.

Pore Types and Characteristics

Pores were studied under a SEM.They were identi-fied as interparticle,intraparticle,organic pores,and microfractures(Loucks et al.,2012).Silty and clayey laminae differ in pore geometry and size. Interparticle Pores

The pores between particles and crystals are defined as interparticle pores(Loucks et al.,2012).Inter-particle pores are well developed and include primary pores among feldspar and quartz grains and clay platelets,secondary dissolution pores at the edge of rigid grains,and intercrystalline 04df3af9be23482fb4da4caemon brit-tle and hard quartz and feldspar grains in the silty laminae protected the interparticle pores during com-paction(Desbois et al.,2009;Schieber,2010). Hence,primary pore space between brittle grains and between brittle and clay minerals remains (Figure6A,B,D,E).Interparticle pores are roughly polygonal,ranging from several nanometers to tens ofμm in diameter,mainly from tens of nm to several μm(Figure6).The contacts between feldspars and carbonate cements are commonly dissolved to modify and enlarge the interparticle pores (Figure6A–C),where the edge of the pores is com-monly sawtooth-shaped.Some original interparticle pores and dissolution-modified primary pores are filled by authigenic quartz and clay,resulting in a reduction in pore space.For example,the pore outlined by the black line in Figure6D is a large dis-solution pore with irregular ellipsoidal shape.A large part of the pore is filled by authigenic quartz,and the rest is filled by quartz,feldspar,and clay minerals, forming intercrystalline pores and modified intercrys-talline pores by rim dissolution(white dashed arrows

L EI ET AL.669

in Figure 6E).Moreover,pores between crystals are fairly common in between overgrowth quartz,feld-spar,and carbonate cements (Figure 6F).They have variable geometries,are mainly elongate and polygo-nal,and range from 7to 300nm in diameter (Figure 6F).Interparticle pores among clay mineral particles in the silty laminae are common,and they are elongate,triangular,polygonal,or nearly circular,ranging from 5to 100nm and up to 300nm in diam-eter (Figure 6G).

Interparticle pores are also common in clayey laminae,although they are quite different from those in silty laminae.Most of them are in between clay platelets (Figure 7A –D),but some are between quartz,feldspar,and clay mineral grains (Figure 7E,F).Primary interparticle pores have three types.The first type consists of interparticle pores between clay mineral lumps that were modified by compaction.They are mostly elongate (Figure 7B),some nearly circular and polygonal,and are 1–10s nm in diameter.The second type of pores are well developed in pressure shadows adjacent to larger compaction-resistant grains such as detrital quartz,feldspar,pyrite framboids,and in spaces between such grains (Figure 7C,D).They are commonly triangular and nearly circular,mostly tens of nm in diameter

Table 1.Statistical Data of Frequency of Occurrence,Cumulative Thickness,and Thickness Percentage of Silty Laminae in all Wells Well Depth (m)Measured Core Length (m)

Cumulative Thickness of Silty Laminae (cm)

Thickness Percentage of Sitly Laminae (%)

Frequency of Occurrence (No.of Silty Laminae/m)

W11359.60–1416.6450.5601.1311.937W21376.36–1386.6610.211511.338W41142.9–1153.810.312512.144W51524.3–1532.77.7769.943W81306.9–136049.6312.6 6.311W101105.53–1118.2312.0191.516.025W121014.6–1019.2 4.452.511.948W91187.47–1196.538.978.88.945W17835.6–844.4

8.2

139.4

17

55

Frequency of occurrence (laminae/m)

F r e q u e n c y (%)

Thickness (mm)F r e q u e n c y (%)

Figure 4.Statistical data from nine wells.(A)Distribution of thickness of individual silty laminae.(B)Frequency of occurrence (laminae/m)of silty laminae.

670

Silty Laminae in Zhangjiatan Shale

(Figure 7C,D).The third type of pores are con-structed by flocculated clay minerals to form a “house

of cards ”microstructure (O ’Brien,1971,1972;Schieber,1991;Slatt and O ’Brien,2011).Pores are equant to semicircular (Figure 7A)and tens of nm to several μm in diameters.The third type of pores is rel-atively rare.Clay minerals and some brittle minerals surrounding these interparticle pores are commonly dissolved along the edges to form sawtooth to embayed geometries (Figure 7E).

F r e q u e n c y (%)

40

20

C u m u l a t i v e f r e q u e n c y c u r v e (%)

Diameter( ) Ф

Diameter( )Ф

(A)Figure 5.Statistical data of size of detrital grains in silty (A)and clayey (B)laminae.The mean and median values and standard devi-ations of grain size were calculated by Folk ’s method (Folk and Ward,1957).

Table 2.Major Mineral Content (wt.%)of the Studied Samples,Obtained by X-ray Diffraction Analysis Samples Depth (m)Quartz Feldspar*Chlorite Montmorillonite

Other ?Clayey laminae

W7-11400.6334238341W7-2-11396.76322518233W5-11530.0526308342W3-1-11470.21291810412W1-1-11419.11281712413W1-2-11527.65401615281W11-1-11727.2537279235W11-2-11718.4137327231W6-1-11630.430307313W5-21522.88302053312W1-31315.136225325Silty laminae

W7-2-11396.76285110101W3-1-21470.2134447141W1-1-21419.1138408140W1-2-21527.65312325201W11-1-21727.25382814182W11-2-21718.4138348164W6-1-21630.435336234W4-1

1149.72

39

29

7

21

4

*Feldspar includes microcline,plagioclase,and orthoclase.?

Other includes calcite,dolomite,pyrite,and siderite.

L EI ET AL .

671

Intraparticle Pores

Intraparticle pores,which are located within particles

(Loucks et al.,2012),are common in silty laminae,

including dissolution pores within detrital grains and

cements,intercrystalline pores within crystals,intra-

platelet pores within clay aggregates,and moldic

pores after a fossil or crystal.Feldspar grains and cal-

citic cements are commonly dissolved to form intra-

particle pores (Figure 8A –C),ranging from tens of

nm to several μm.They are elongate,semicircular,

and irregular.Some feldspars have large pores of up

to 2μm in diameter (Figure 8B).The pores may have

been partially filled by quartz and clay mineral

cements,where the cements themselves may have

intracrystalline pores or have been dissolved again

(Figure 8C,D).Irregular dissolution pores also occur

in some phosphatic minerals (Figure 8E).Openings

along cleavage planes of feldspar,mica,and calcite

grains are developed in the silty laminae,some of which may have been subject to some degree of dis-solution (Figure 8F).In addition,rare moldic pores are present,which were formed by dissolution of soluble grains and crystals (Figure 8F).However,intraparticle pores in clayey laminae include mainly intracrystalline and dissolution pores within authigenic clay mineral aggregates (Figure 9A,B),framboidal pyrites (Figure 9C),siderites (Figure 9D),fossil chambers (Figure 9E),and molds (Figure 9F).Dissolution pores within feldspar and quartz grains are also present (Figure 9A),but they are relatively less 04df3af9be23482fb4da4caeanogenic Pores Organogenic pores,which occur within organic mat-ter (Loucks et al.,2012),are less abundant in most organic matter in the samples of Zhangjiatan

Shale Figure 6.Secondary electron images of milled surfaces showing interparticle pores in silty laminae (Q is quartz,F is feldspar,P is phosphate minerals).The mineralogy of grains and cements were identified by energy dispersive spectrometer.Accelerating voltage =15kV.(A)Relict primary interparticle pores (black arrows)and interparticle dissolution pores (white arrows).(B)Relict pri-mary interparticle pores (black arrows),interparticle dissolution pores indicated by the sawtooth edge (white arrows),and intraparticle dissolution pores with irregular shape (white dashed arrows).(C)Interparticle dissolution pores formed by dissolution along the edge of feldspar (white arrows),intraparticle dissolution pores (white dashed arrows),and relict primary interparticle pores (black arrows).

(D)Interparticle dissolution pores (outlined by black line)filled by authigenic quartz,feldspar,and clay minerals.Also present are many relict interparticle pores (black arrows)and intercrystalline pores (black dashed arrows).(E)Intercrystalline pores (white dashed arrows).The feldspar is dissolved,and the margin is sawtooth,as shown by the white arrows.The location is outlined by a box shown in (D).(F)Intercrystalline pores (white arrows)and intercrystalline dissolution pores (black arrows)among quartz crystals.(G)Interparticle pores among clay mineral grains filling an interparticle pore.The largest pore is 283nm,and the smallest is 8nm,with an average of 28nm.

672Silty Laminae in Zhangjiatan Shale

in comparison with the Mississippian Barnett Shale in

north Texas and Devonian Woodford Shale in

southeastern Oklahoma (Loucks et al.,2009;Slatt

and O ’Brien,2011),because organic matter in

the Zhangjiatan Shale has a low thermal maturity

as indicted by R o values that are commonly less

than 1.3%(Ma et al.,2005;Zhang et al.,2006).

Organogenic pores occur in a few samples of silty

laminae.For example,pores in kerogens in silty lam-

inae of wells W1and W8have mainly a circular,

some triangular,polygonal,semicircular,irregular or

elongate shape and range from 5to 150nm in diam-

eter (Figure 10A,B).Organogenic pores in kerogens

are more common in clayey laminae,and mainly

range from 3to 347nm and 20nm on average in

diameter (Figure 10C –E).Some of the organogenic

pores connect with each other to form a complex pore

network (Figure 10A,C –E).Other pores appear iso-

lated in two dimensions,whereas,most of them are

connected and form an effective pore network in

three dimensions (Figure 10A –E),which has been demonstrated by Ambrose et al.(2010)and Sondergeld et al.(2010).Some of the organic matters in both silty and clayey laminae show no organogenic pores (Figure 10F),which may be due to low thermal maturity or kerogen type (Schieber,2010).Microfractures Fractures are well developed in the Zhangjiatan Shale.They have several origins,structural,diagenetic shrinkage,and bedding-plane shearing (L.Jiang et al.,2013b).They are either high angle and nearly perpendicular to the bedding plane or low angle and subparallel to the bedding plane.High-angle fractures may have been related to regional tec-tonic stress that has a direction nearly perpendicular to the bedding plane (L.Jiang et al.,2013b).Subparallel fractures observed in cores and thin section are tens of mm to several cm long and com-monly occur at the interface between silty and clayey laminae (Figure 11A),some partially in the silty lam-inae (Figure 11B).Some of these are filled

by

Figure 7.Secondary electron images showing interparticle pores in clayey laminae.(A)“House of cards ”structure formed by colloidal clay mineral flakes with roughly circular and oval-shaped pores.Broken surface.Accelerating voltage =15kV.(B)Interparticle pores (black arrows)in between clay mineral lumps in an unpolished sample.Broken surface.Accelerating voltage =15kV.

(C)Uncompacted interparticle pores (white arrows)preserved among clay mineral grains and interparticle dissolution pores (black arrows).Milled surface.Accelerating voltage =10kV.(D)Preserved interparticle pores (white arrows)among clay mineral 04df3af9be23482fb4da4caeled surface.Accelerating voltage =10kV.(E)Interparticle dissolution pores (white arrows)and interparticle pores (black arrows).Milled surface.Accelerating voltage =20kV.(F)Interparticle pores among detrital grains (black arrows)and clay mineral grains (white arrows).Milled surface.Accelerating voltage =15kV.

L EI ET AL .673

Figure8.Secondary electron images showing intraparticle pores in silty laminae.The mineralogy of grains and cements were identi-fied by energy dispersive spectrometer.(A)Dissolution pores with embayed boundaries in feldspars.(B)Pores similar to those in(A), with a minimum diameter of8nm,maximum1.9μm,and average37nm.(C)Authigenic quartz and clay minerals filling dissolutional intraparticle pores in feldspars.(A)–(C)are images of milled surfaces.Accelerating voltage=15kV.(D)Intraparticle dissolution pores in quartz grains,which are partially filled by quartz and clay minerals.Broken surface.Accelerating voltage=15kV.(E)Abundant pores in a phosphatic mineral grain.Broken surface.Accelerating voltage=20kV.(F)Cleavage pores(black arrows)and moldic pores(white

arrows).Broken surface.Accelerating voltage=15kV.

Figure9.Secondary electron images of intraparticle pores in clayey laminae.Accelerating voltage=15kV.(A)Intraparticle(black arrows)and interparticle(white arrows)pores of clay mineral 04df3af9be23482fb4da4caeled surface.(B)Intraparticle pores within clay mineral aggre-gates,with a minimum diameter of11nm,maximum162nm,and average29nm.Broken surface.(C)Intraparticle pores(black arrows)within framboidal pyrite 04df3af9be23482fb4da4caeled surface.(D)Intraparticle dissolution and intercrystalline pores(white arrows)inside a siderite grain and interparticle pores(black arrows).Milled surface.(E)A fossil chamber pore.Broken surface.(F)Moldic pores(white arrows).Broken surface.

674Silty Laminae in Zhangjiatan Shale

epigenetic minerals or liquid hydrocarbons ensuring

they are natural fractures and not stress-release part-

ings induced during retrieval of the core or sample

preparation.The abundance of these fractures is

closely and positively correlated with the abundance

of silty laminae in the shale.

Only a small amount of subparallel and per-

pendicular to bedding microfractures in silty laminae

are open;others are filled by epigenetic minerals and/

or bitumen.The epigenetic minerals are generally cal-

cite,siliceous,and clay minerals.In some fractures,

epigenetic cements are dense,large,and euhedral;

they are mainly mosaic or isopachous calcite,local

siderite,some locally concentrated siliceous miner-

als,and rare epigenetic clays (Figure 11C).Moldic

and intercrystalline pores are common in the fracture

cements,and relict bitumen is rare.In the other

fractures,cements are less abundant,mainly

microcrystalline calcite and clay minerals,and a

variety of pores occur (Figure 11D).Calcite cement

is commonly 5–10mm,and epigenetic clay minerals

are 1–2mm,filling between calcite crystals.The calcite has random crystal orientations and abundant intercrystalline pores.Carbon atoms were detected with EDS in these fractures,which are interpreted as relict bitumen filling in fractures or contaminating mineral surfaces (Figure 11D),sug-gesting that these fractures were hydrocarbon migra-tion pathways.Three types of bitumen in microfractures were recognized on the basis of their fluorescent character-istics.The first type is solid or colloidal bitumen,with brown or no fluorescent color;the second is liquid bitumen with yellow or yellowish-white fluorescence (Figure 11A);the third also is a liquid bitumen,with blue or bluish-white fluorescence (Figure 11B).The third type is relatively abundant.Fractures filled with bitumen are much narrower than those cemented and commonly 1–6μm in width.They are also relatively more abundant and discontinuous but commonly per-sistent across the entire thin section.The three types of bitumen may occur together in a single fracture,indicating multiple episodes of hydrocarbon migra-tion,which conform to previous conclusions of

three

Figure 10.Secondary electron images of nanopores in organic matter.(A)Pores in organic matter in silty laminae,with a minimum diameter of 5nm,maximum 64nm,and average 04df3af9be23482fb4da4caeled surface.Accelerating voltage =15kV.(B)Organic matter pores with a minimum diameter of 10nm,maximum 116nm,and average 40nm in silty 04df3af9be23482fb4da4caeled surface.Accelerating voltage =10kV.

(C)and (D)Organic matter pores in clayey laminae.The largest pore is 61nm,and the smallest is 6nm,with an average 20nm.Accelerating voltage of image C is 10kV,that of image D is 15kV.(E)Organic matter contains large nanopores,with a maximum diam-eter of 04df3af9be23482fb4da4caeled surface.Accelerating voltage =20kV.(F)Organic matter without organogenic pores in clayey 04df3af9be23482fb4da4caeled surface.Accelerating voltage =15kV.

L EI ET AL .675

episodes of oil charging into adjacent sandstone res-

ervoirs (Wu et al.,2006).

Clayey laminae also have some open fractures

(Figure 11E,F).Fine epigenetic quartz and clay min-

erals (Figure 11E)on the fracture wall or across the

fracture (Figure 11F)suggest that the fractures are

not artificially induced.These fractures are parallel

to bedding,continuous,and relatively narrow with a

width of 1to 2μm.

Pore Size and Distribution

Laboratory tests and analysis were conducted to char-

acterize the pore structure of silty and clayey laminae

to understand the effect of silty laminae on the physi-

cal properties of the shale.

Pore Size Measured on SEM Images

Pore size of silty laminae samples was measured and

estimated from SEM images;pore size distributions

are multimodal and similar to each other.Pore width

ranges from 8nm to 7.7μm (Figure 12A –D),mainly from 20to 200nm,some from 200nm to 1μm,and a small amount greater than 1μm (Figure 12).Individual silty laminae samples differ in mode,mean,and median pore width.The mean width of all samples ranges from 107to 309nm,median width from 74to 199nm,and commonly,the median is smaller than the mean.For example,the pore width of a silty lamina sample W4-3of Well W4has a mean of 107nm,median of 74nm,and three modes of 60–80,240–260,and 380–400nm,respectively (Figure 12A).The pore width of a silty lamina sample W7-1from Well W7has a mean of 139nm,median of 100nm,and three modes of 40–60,100–120,and 220–240nm,respectively (Figure 12B).Finally,pores with a size range of 200nm –1μm of a silty lamina sample W4-1from Well W4are much more abundant than those in the other samples.In the same sample,pores greater than 1μm in width account for ～5%of total pores,with a maximum width of 7.7μm (Figure

12D).Figure 11.(A)Bedding-parallel microfractures along the bedding planes between silty and clayey laminae are filled with blue and yellowish-white fluorescent bitumen and cements.(B)Low-angle microfracture filled with blue fluorescent bitumen.(C)A microfracture filled with calcite,quartz,and clay minerals within a silty lamina,resulting in low porosity.(D)Cements of calcite,quartz,and clay min-erals filling a microfracture in a silty lamina are loose with common intercrystalline pores filled commonly with bitumen.The surface of cements is contaminated by crude oil.(E)An open microfracture lined with epigenetic quartz and clay minerals.(F)Epigenetic clay min-erals grew across an open microfracture.(C)–(F)are secondary electron images of a broken surface.Accelerating voltage =15kV.The mineralogy of grains and cements were qualitatively identified by energy dispersive spectrometer.

676Silty Laminae in Zhangjiatan Shale

Pore size distribution of clayey laminae has a sin-gle mode (Figure 12E –H).The pore width is com-monly less than 100nm,mainly from 6to 60nm,with a minimum pore width of 4nm.Clayey laminae have a mean pore width of 26nm and a median of 42nm.There are only a few pores larger than 100nm and almost no pore larger than 200nm.

Thus,the pore size of clayey laminae is significantly smaller than that of silty laminae.

Pore Size Measured through Laboratory Tests

Three methods,mercury porosimetry,low-pressure CO 2,and N 2isotherm analyses,which can measure a wide range of micropore diameter,were used to

N= 1047Mean:107nm Median:74nm Min:11nm Max:849nm

100200400500300600900800700>10000

5101520

25

0N=452

Mean:168nm Median:112nm Min:39nm Max:2.1μm

10020040050030060090080070050101520

>1000

N=5460

Mean:309nm Median:199nm Min:8nm Max:7.7μm

0100200400500300600900800700>1000

5

10

15

N= 1347Mean:139nm Median:100nm Min:25nm Max:1.3μm

0100200400500300600900800700>1000

51015200

(C)

(D)

(E)

(F)

(G)

(H)

(A)

(B)

N=1934Mean:29nm Median:24nm Min:4nm Max:232nm

N=690

Mean:43nm Median:24nm Min:4nm Max:480nm

102030040

50

10204030050)

m n ( h t d i w e r o P )

m n ( h t d i w e r o P F r e q u e n c y (%)

)

m n ( h t d i w e r o P )

m n ( h t d i w e r o P F r e q u e n c y (%)

)

m n ( h t d i w e r o P )

m n ( h t d i w e r o P F r e q u e n c y (%)

F r e q u e n c y (%)

F r e q u e n c y (%)

F r e q u e n c y (%)

100

200400500300600900800700>10000

100200400500300600900800700>1000

N=4665Mean:47nm Median:27nm Min:4nm Max:963nm

N=392

Mean:47nm Median:36nm Min:10nm Max:337nm

10200

3040

1020300

40

)

m n ( h t d i w e r o P )m n ( h t d i w e r o P F r e q u e n c y (%)

F r e q u e n c y (%)

0100200400500300600900800700>10000100200400500300600900800700>1000

Figure 12.Histograms of pore sizes measured from scanning electron microscopy images.(A)Sample W4-3is from a silty lamina.(B)W7-1from a silty lamina.(C)W4-4from a silty lamina.(D)W4-1from a silty lamina.(E)W1-3from a clayey lamina.(F)W4-4from a clayey lamina.(G)W5-2from a clayey lamina.(H)W4-2from a clayey lamina.

L EI ET AL .

677

estimate the pore diameter and distribution of the same samples(Figure13).

Test results on pore diameter and volume indicate that silty laminae and adjacent clayey laminae differ significantly in pore structure(Figure13).Pore diam-eter distribution of silty laminae is multimodal;pore diameter ranges from0.3nm to10μm(Figure13). Micropores smaller than2nm are not well developed, and meso-and macropores with a diameter greater than2nm dominate.The abundance of micropores in silty laminae is lower than that in clayey laminae. The former has an average pore volume of0.10cc/ 100g,contributing to～4%of total pore volume, which is much lower than the9%–25%,average 18%contribution in the clayey laminae.A peak between2and100nm in the pore size distribution of silty laminae is evident,similar to that documented using SEM images(Figure12A–D).Pores ranging in size from2to100nm are mainly interparticle and intraparticle pores,among which those ranging from 2to50nm in diameter are best developed and have an average pore volume of0.63cc/100g and account for～30%of the total pore volume of the silty 04df3af9be23482fb4da4caerge pores with a size range of50–100nm have an average pore volume of0.19cc/100g,accounting for～10%of total pore volume.In addition,two peaks between100nm and1μm and between1and 10μm are evident(Figure13);the mode of the former peak is similar to that documented using SEM images (Figure12A–D).These pores are dominantly large dissolution and intraparticle pores(Figures6,8). Pores ranging from1to10μm in size are probably mainly fractures and dissolution pores.Pores greater than100nm in diameter are the main contributors of pore volume and have an average pore volume of 1.10cc/100g,accounting for～56%of the total pore volume.

Pore size distribution of clayey laminae is similar to that documented using SEM images.That is,pores less than100nm in diameter are dominant;micro-pores smaller than2nm are well developed (Figure13).The volume of micropores in

clayey Pore diameter (nm)

d

v

/

d

l

o

g

(

D

)

c

c

/

g

Pore diameter (nm)

d

v

/

d

l

o

g

(

D

)

c

c

/

g

Pore diameter (nm)

d

v

/

d

l

o

g

(

D

)

c

c

/

g

Pore diameter (nm)

d

v

/

d

l

o

g

(

D

)

c

c

/

g

Pore diameter (nm)

d

v

/

d

l

o

g

(

D

)

c

c

/

g

Pore diameter (nm)

d

v

/

d

l

o

g

(

D

)

c

c

/

g

2

2

22

Figure13.Pore-size distribution(pore diameter D against d V∕d log D,D is pore diameter,d V∕d log D is pore volume)of silty lam-inae(solid black lines)and adjacent clayey laminae(dotted lines)of Zhangjiatan Shale defined by low-pressure CO2and N2adsorption and mercury porosimetry.Presentation method is modified after Yu et al.(in press).

678Silty Laminae in Zhangjiatan Shale

laminae is4.6times more than that in the silty lami-

nae.The specific surface area of micropores in clayey

laminae estimated by CO2adsorption is～7.95m2∕g on average and is3.8times greater than that of

2.1m2∕g on average in silty laminae(Table3). Mesopores with a diameter of2to50nm have an average pore volume of0.94cc/100g,accounting for～66%of total pore volume in clayey laminae, and they are more abundant than those in the silty laminae.The pore volume of these mesopores is evi-dently much greater than that of0.63cc/100g in the silty laminae.The volume of pores ranging from50 to100nm in diameter is0.22cc/100g on average and accounts for～16%of the total pore volume in clayey laminae,which is slightly greater than that in the silty laminae.Finally,the N2BET specific surface area of clayey laminae is8.64m2∕g,greater than that of5.90m2∕g of silty laminae(Table3).

In general,silty laminae differ from clayey lami-

nae in pore-size distribution,structure,volume,and

specific area.Micropores are not well developed in

silty laminae,but pores ranging in size from2to

50nm and100nm to10μm are abundant,and mac-

ropores with a size greater than100nm contribute

the most in pore volume.However,pores with a size

of2to50nm are abundant in clayey laminae,and those greater than100nm are rare.Micropores are better developed in clayey laminae than in silty lami-nae.Furthermore,the porosity of silty laminae varies from3.2%to7.2%and is5.3%on average,greater than that of clayey laminae,which ranges from2.6% to7.2%and is3.6%on average.

The effect of silty laminae on the shale porosity was assessed through nitrogen porosity tests of shale samples with varying content of silty laminae from Well W1.The results indicate that shale poros-ity increases with the frequency of occurrence and total thickness percentage of silty laminae (Figures3,14).Shale intervals with0,1–20,20–80, and100thickness percentages of silty laminae have an average porosity of2.5%,2.9%,3.3%,and5.0%, indicating a significant role of silty laminae in shale porosity.

DISCUSSION

Silty laminae in Zhangjiatan Shale differ from strati-graphically adjacent clayey laminae in many petro-graphic and diagenetic aspects,including pore type and size distribution,specific surface area,and poros-ity.Shale intervals rich in silty laminae also have abundant subparallel fractures.These results suggest

Table3.Specific Surface Area and Pore Volume of Silty and Clayey Laminae Determined by Low-Pressure Gas(CO2,N2) Adsorption and Mercury Injection

Sample No.and Type Depth

(m)

CO2

Specific

Surface

Area

em2∕gT

N2BET

Specific

Surface

Area

em2∕gT

N2

Mesopore

Volume

(cc/100g)

CO2

Micropore

Volume

(cc/100g)

Grain

Density

(g/cc)

Macropore

Volume

(cc/100g)

Calculated

Porosity(%)

W3-1Silty laminae1470.21 1.36 3.550.420.03 2.77 1.44 4.96 Shale8.62 5.290.610.29 2.590.17 2.71

W1-1Silty laminae1419.11 1.627.310.860.03 2.81 1.56 6.46 Shale9.517.620.890.30 2.590.14 3.33

W1-2Silty laminae1527.65 2.93 5.310.530.07 2.800.97 4.19 Shale 6.397.790.760.17 2.750.22 3.07

W11-1Silty laminae1727.25 4.48 5.320.630.12 2.80 1.37 5.60 Shale9.7713.40 1.530.29 2.690.38 5.57

W11-2Silty laminae1718.41 1.15 2.910.310.03 2.700.88 3.20 Shale 3.647.900.740.09 2.680.18 2.63

W6-1Silty laminae1630.4 1.0611.00 1.110.33 2.75 1.547.15 Shale9.779.82 1.010.30 2.700.24 4.40

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that silty laminae played a significant role in the for-mation,migration,and production of shale oil and gas.

Causes for Different Pore Types and

Structures between Silty and Clayey Laminae

The aforementioned differences in various pore prop-erties between silty and clayey laminae may be closely related to their composition,grain size,and diagenesis.In comparison with clayey laminae,the relatively abundant brittle quartz and feldspar grains in silty laminae promote the preservation of primary interparticle pores and retard the development of micropores.The preservation of primary pores resulted from the support of brittle minerals,hamper-ing deformation of soft particles in between (Desbois et al.,2009;Schieber,2010).Calcite and quartz cementation in silty laminae also slows down compaction.

A large volume of carbonic and organic acid

(Surdam et al.,1984,1989;Surdam and Yin,1994)will be generated during the maturation of kerogen in source rocks.That will decrease the pH of fluids,decrease the stability of both carbonate and aluminosilicate minerals,and inhibit the precipita-tion of authigenic clay by complexing aluminum and then increasing the carbonate and aluminosili-cate solubility (Surdam et al.,1984).Dissolution pores in silty laminae may have been formed by dis-solution of feldspars by carbonic and organic acid,which was generated during maturation of organic matter in the adjacent organic-rich clayey laminae and expelled into the adjacent silty laminae.For example,sample W4-1has abundant pores of a diameter between 200nm and 1μm and some greater than 1μm,in addition to those smaller than 200nm (Figure 12D).In comparison to samples W4-3,W4-4,and W7-1(Figure 12),W4-1has more pores of a diameter greater than 200nm.This is probably related to the grain size of detrital grains and degree of dissolution.Sample W4-1has a mean grain size of 50μm,a median size of 57μm,and abundant relict primary interparticle pores (Figure 6A).Feldspar dissolution is common,form-ing large interparticle dissolution pores (Figure 6A,B).In addition,intraparticle dissolution pores inside feldspars were enlarged to connect with each other or interparticle pores (Figure 8B),resulting in large pores.Sample W4-4has a mean grain size of

Thickness percentage of silty laminae per meter of shale (%)

P o r o s i t y (%)01

2

3

4

5678Figure 14.Positive

correlation between mea-

sured shale porosity and

the thickness percentage

of silty laminae per meter

of shale in Well W1.680Silty Laminae in Zhangjiatan Shale

44μm and a median size of 38μm,both smaller than

those of W4-1,and thus,has some micrometer-scale

dissolution pores (Figures 8A,12D).On the other

hand,samples W7-1and W4-3have a mean size of

35and 25μm and a median size of 31and 21μm,

respectively,which are smaller than W4-1.Thus,

dissolution was weak,resulting in a small amount

of macropores and small mean and median pore

diameter (Figure 12A,B).

Micropores are much more abundant in clayey

laminae than in silty laminae,and the micropore vol-

ume trends to increase with increasing clay mineral

content in general (Zeng et al.,2013)because clayey

laminae have much more clay minerals and organic

matter and a smaller grain size.Primary interparticle

pores between clay minerals and detrital grains and

among clay minerals (Figure 7B –D,F)are preserved under pressure shadows formed by feldspars and quartz grains.However,they are smaller than those in silty laminae because feldspars and quartz in clayey laminae are not abundant and their grain size is smaller,commonly less than 10μm (Figure 5)and,as a result,compaction is more intense in clayey laminae.These pores could have been filled by cements or dissolved,forming pores with a rela-tively small size.Small feldspars in clayey laminae would also result in small dissolution pores along grain boundaries and inside the grains.Finally,interparticle and intraparticle pores of clay minerals and intercrystalline pores in pyrite framboids (Figure 9A –C)are major types of pores in clayey laminae but are small.As a result,pores of a diam-eter smaller than 100nm are dominant in clayey

laminae.

Figure 15.Secondary electron images of silty laminae.(A)Liquid-hydrocarbon-filled pores at Point 04df3af9be23482fb4da4caeled surface.(B)Liquid-hydrocarbon-filled pores at Point 2.Broken surface.(C)and (D)Energy dispersive spectrometer spectra and charts of elemental weight percentages of points 1and 2,respectively.Accelerating voltage =15kV.

L EI ET AL .681

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