Characteristics and Origins of Coal Cleat a Review

Ž .International Journal of Coal Geology 35 1998 175–207

Characteristics and origins of coal cleat: A review

S.E. Laubach a,), R.A. Marrett b, J.E. Olson c, A.R. Scott a

a The UniÕersity of Texas at Austin Bureau of Economic Geology, 10100 Burnet Road, Austin,TX 78758-4497, USA

b The UniÕersity of Texas at Austin Department of Geological Sciences, Austin, TX 78758-4497, USAc The UniÕersity of Texas at Austin Department of Petroleum and Geosystems Engineering, Austin,

TX 78758-4497, USA

Received 18 December 1996; accepted 16 April 1997

Abstract

Cleats are natural opening-mode fractures in coal beds. They account for most of thepermeability and much of the porosity of coalbed gas reservoirs and can have a significant effecton the success of engineering procedures such as cavity stimulations. Because permeability andstimulation success are commonly limiting factors in gas well performance, knowledge of cleatcharacteristics and origins is essential for successful exploration and production. Although thecoal–cleat literature spans at least 160 years, mining issues have been the principal focus, andquantitative data are almost exclusively limited to orientation and spacing information. Few dataare available on apertures, heights, lengths, connectivity, and the relation of cleat formation todiagenesis, characteristics that are critical to permeability. Moreover, recent studies of cleatorientation patterns and fracture style suggest that new investigations of even these well-studiedparameters can yield insight into coal permeability. More effective predictions of cleat patternswill come from advances in understanding cleat origins. Although cleat formation has beenspeculatively attributed to diagenetic andror tectonic processes, a viable mechanical process forcreating cleats has yet to be demonstrated. Progress in this area may come from recentdevelopments in fracture mechanics and in coal geochemistry. q 1998 Elsevier Science B.V.

Keywords: coal; fractures; brittle deformation; statistical analysis; mechanics; natural gas

1. Introduction

Fractures occur in nearly all coal beds, and can exert fundamental control on coalstability, minability, and fluid flow. It is therefore not surprising that coal fractures have

) Corresponding author. Fax: q1-512-4710140; E-mail: [email protected]

0166-5162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0166-5162 97 00012-8

( )S.E. Laubach et al.r International Journal of Coal Geology 35 1998 175–207176

Ž . Ž .Fig. 1. Schematic illustration of coal cleat geometries. a Cleat-trace patterns in plan view. b CleatŽ .hierarchies in cross-section view adapted from Laubach and Tremain, 1991 . These conventions are used for

cleat: length is dimension parallel to cleat surface and parallel to bedding; height is parallel to cleat surface andperpendicular to bedding; aperture is dimension perpendicular to fracture surface. Spacing between two cleatsŽ .of same set is a distance between them at right angles to cleat surface.

been investigated since the early days of coal mining, and that published descriptionsŽand speculation on fracture origins date from early in the nineteenth century Mammatt,

.1834; Milne, 1839; cited in Kendall and Briggs, 1933 . Although various miner’s termsfor systematic fractures in coal have been used over the years, they are still generally

Ž .referred to by the ancient mining term: cleat Dron, 1925 .Cleats are fractures that usually occur in two sets that are, in most instances, mutually

perpendicular and also perpendicular to bedding. Although pre-1990 geologic literatureand current mining usage distinguishes these sets on the basis of factors such as‘prominence’ that are difficult to quantify, abutting relations between cleats generally

Ž .show that one set pre-dates the other Fig. 1 . This is a readily quantifiable distinctionbetween sets in most outcrops and cores. Through-going cleats formed first and arereferred to as face cleats; cleats that end at intersections with through-going cleats

Žformed later and are called butt cleats Laubach and Tremain, 1991; Kulander and Dean,.1993 . These fracture sets, and partings along bedding planes, impart a blocky character

Ž .to coal Fig. 2 . A hierarchy — or perhaps a continuum — of cleat sizes typify theŽ . Ž .population of cleats in a coal bed Laubach and Tremain, 1991 Fig. 1 . This is most

easily recognized in cleat heights, but size variations are also evident in cleat lengths,apertures, and spacings.

Although fractures in coal are relatively unimportant in strip mining, their signifi-cance in efficient design and safety of underground coal mines has continued to

Žcommand the attention of the mining industry Hanes and Shepherd, 1981; Esterhuizen,.1995; Molinda and Mark, 1996 . In contrast, the characteristics and origin of fractures in

Ž .coal have been relatively neglected in recent 1966–1996 geological literature. Forexample, only two AAPG Bulletin papers specifically on coal fractures have been

( )S.E. Laubach et al.r International Journal of Coal Geology 35 1998 175–207 177

Fig. 2. Cleats in coal outcrop of Cretaceous Fruitland Formation. Photograph, view to northwest. Rockhammer for scale.

published in that time, and the leading journal on coal geology, the International Journalof Coal Geology, has only published one article that focused on cleat in the last 10years. Other journals and geology textbooks have also paid little heed to cleat in recenttimes.

This neglect partly reflects a historic impasse in the scientific understanding ofcertain types of fractures such as those in coal. Cleats most commonly occur without anyobservable shear offset, and are thus properly termed opening-mode fractures. Studies ofthe origins of opening-mode fractures, such as cleats and joints, are plagued by theproblem of equifinality — the situation where several processes may lead to the sameresult — rendering unique causes difficult to deduce from products. This arises because

Ž .coals and other rocks have low tensile strength, and a wide range of loading paths canŽlead to propagation of opening-mode fractures Engelder, 1985; Engelder and Fischer,

.1996 . Yet such fractures preserve little evidence of the loading conditions that causedthem. A symptom of this is the variety of plausible scenarios that have been advanced


Žfor the origins of cleats Kendall and Briggs, 1933; McCulloch et al., 1974; Ting, 1977;.Close, 1993 and the lack of explicit, testable predictions that can be used to identify the

best interpretations.Owing to the increasing importance of coal beds as gas reservoirs, geologists are

again becoming concerned with the characteristics and origins of cleat. For coalbedmethane extraction, knowledge of the properties of natural fractures is essential forplanning exploration and development because of their influence on recovery ofmethane, and the local and regional flow of hydrocarbons and water. New mapping ofcleat patterns, guided by recent conceptual advances in the description and theoreticalunderstanding of fracture processes, is beginning to bring these patterns into focus. This,together with more rigorous study of the petrology of cleat development, may resolveuncertainties about causes of cleat that now hinder predictions of interwell-scale patternsof fractures in coal beds. This paper highlights recent studies with emphasis on ourresearch in United States coalbed methane basins, and points to areas of potential futureadvancement.

2. Cleat attributes

As many observers have noted, the attributes of cleats closely resemble those ofŽfractures termed ‘joints’ in other rock types Dron, 1925; Kendall and Briggs, 1933;

Williamson, 1967; McCulloch et al., 1974, 1976; Ting, 1977; Campbell, 1979; Laubach.et al., 1992 . Cleats are opening-mode fractures rather than faults; typically there is no

appreciable offset parallel to cleat walls. At surface conditions they generally haveapertures less than 0.1 mm, and these scarcely visible apertures caused some earlyworkers to mistakenly conclude that no opening had occurred normal to cleat walls.They also have surface structures such as arrest lines and hackles characteristic of

Ž .opening-mode propagation Kulander and Dean, 1990 .Many of the prominent attributes of cleats and cleat patterns have been known since

the 1930’s. Individual fractures are generally planar but sometimes are locally curved inplan view. Cleats are subvertical in flat-lying beds and are typically oriented at rightangles to stratification even where beds are folded. In many cases cleats are confined toindividual coal beds, or to layers composed of a particular maceral type. Commonly theyare uniform in strike within an outcrop or core and arranged in subparallel sets that have

Ž .uniform regional trends Kendall and Briggs, 1933 . Yet they locally show abrupt lateralŽ .and vertical shifts in strike Dron, 1925 . Coals containing closely spaced faults rather

than opening-mode fractures are apparently rare. This reflects either proximity to largefaults, reactivation by slip on pre-existing cleats, or anomalous burial conditionsŽ .Ammosov and Eremin, 1963; Laubach et al., 1993 .

One remarkable attribute of cleat formation is the extent to which they are developedin many coal beds of nearly all ranks in maturity. Cleats are typically much moreintensely developed than fractures in adjacent non-coal rocks. The great intensity ofcleat development, in comparison to fractures in other rock types, is an important clue tocleat origins.


2.1. Cleat size

There are little quantitative data on the size of cleats. Because many cleats are onlycentimeters in length and height and typically have nearly indiscernible aperture,modeling studies usually assume cleats are small in terms of length, height, and aperture.In San Juan Basin, New Mexico, Fruitland Formation coal beds cleat lengths and heights

Ž .are reported to range from microns to meters Tremain et al., 1991 . Yet these cleats areŽarranged in a hierarchy of sizes that includes fractures of decimeter size ‘master cleats’,

.Fig. 1 .Ž .Estimates of in situ cleat widths range from 0.001 to 20 mm Gamson et al., 1993 .

However, most information on cleat width is based on outcrop studies andror micro-scopic examination of coal samples not under confining pressure. Little reliable informa-tion is available on cleat apertures in the subsurface. The parallel-plate fracturepermeability model can be used to estimate that cleat apertures range from 3 to 40 mmŽ .Fig. 3 . Where diagenetic minerals have filled cleats, considerably wider fractures arelocally preserved, including fractures with widths as much as 0.5 cm in Cretaceous coalbeds in the western United States. These fossilized cleat apertures show that fracturesmuch wider than most outcrop- and core-based estimates are locally present in thesubsurface. Because cleat apertures may change as effective stress is altered in coalbedmethane reservoirs, lack of reliable information on in situ cleat apertures is a potentialbarrier to effective reservoir management.

Fig. 3. Relation among face-cleat spacing, permeability, and aperture assuming parallel-plate model. Usingmeasured cleat spacing and permeability for highly productive coalbed methane wells in San Juan and BlackWarrior Basins, shaded area shows range of inferred aperture size.


Fig. 4. Cleat aperture size distribution for combined face and butt cleats in cores from San Juan Basin. DataŽ .represent volumetric 3D sampling. Least squares fit made to data in open symbols.

Size distribution of opening-mode fracture apertures have been studied quantitativelyŽ .in a variety of rock types Marrett, 1997 , but there is little published data for cleats to

permit such an analysis. One set of observations, summarized by Close and MavorŽ .1991 , was collected from six wells in the San Juan Basin. Aperture of a typical cleat ineach lithotype layer was measured microscopically without confining stress. From this

Ž .we calculate cumulative volumetric 3D frequency of cleats as a function of each coreŽ .Fig. 4 . This presupposes that all cleats in a specific lithotype layer have the sameaperture, probably a poor assumption.

Apertures observed in three coal cores range from 0.01 to 0.2 mm, although cleatsŽ .with smaller apertures were not measured. In each core, cumulative frequency f of

cleats having apertures of e or larger follows a power law:

fsbeyc 1Ž .

Žwhere b is a general measure of the cleat intensity and c is a constant referred to as the.fractal dimension . A power law forms a line on a log–log plot of f versus e, and the

slope of the line is the negative of the fractal dimension. Values of c determined fromŽ .volumetric sampling of cleat apertures 2.74–2.82 are larger by ;2 than values

Ž . Ž .determined from scanline 1D sampling of apertures in non-coal rocks Marrett, 1997 .


Ž .This is expected assuming aperture, length, and height are linearly proportional due toŽ . Ž . Žthe difference between volumetric 3D and scanline 1D sampling Marrett and

.Allmendinger, 1991; Marrett, 1996 . Thus, size distribution of cleat apertures is consis-tent with size distribution of fracture apertures in non-coal rocks.

Ž .Reports summarized by Close and Mavor 1991 provide the only systematic coredata available on cleat heights. Among cleats with a specific aperture value, heights varysignificantly. However, cleats with large apertures tend to have large heights. Average

Ž .height h of all cleats having a specific aperture value increases approximately linearlywith aperture:

hsae 2Ž .where a is about 1000 for cleats in one well. This linear aperturerheight relationship isconsistent with linear elastic fracture mechanics predictions for opening-mode fractures

Ž .where fracture height is shorter than fracture length Pollard and Segall, 1987 .Ž . Ž .Combining results of Eqs. 1 and 2 implies that cleat heights follow a power law size

distribution. No systematically collected data on cleat lengths are known. Butt cleats arelimited in length by spacing of face cleats, so butt cleat lengths should depend on facecleat spacings. We might anticipate that face cleat lengths will follow power law sizedistributions, by analogy with size distribution of fracture lengths in non-coal rocksŽ .extension fracture data in Marrett, 1997 and references therein . Positive interferencebetween wells shows that interconnected pathways in cleated rocks can be as long as 1km.

2.2. Cleat spacing

Ž .Cleat spacing is sufficiently close on the order of centimeters that numerous visiblefractures are typically present in coal cores. However, core observations rarely distin-guish the hierarchy of fracture sizes that are present, so published cleat spacinginformation is difficult to compare. Spacing measurements reported from core maycount all visible fractures, whereas outcrop and mine studies typically choose fracturesof a given size to measure, but this choice is rarely explicitly stated. Recognizing thewide range of cleat sizes present in coal, quantitative statements of spacing are only

Ž .meaningful with reference to the sizes of cleats included in an analysis Fig. 5 . AnŽ .example where a size criteria was specified is the study of Tremain et al. 1991 . In this

Ž .study, fractures of equivalent height defined by layer thickness were compared.Measurements on long mine highwalls show that spacing between individual cleats of

Ž .similar size ranges from microns to more than a meter Fig. 6 . Yet within a given bed,average spacing of fractures of similar size is remarkably uniform over distances of

Ž . Ž .hundreds of meters Fig. 7 Tremain et al., 1991 . Sampling procedure needs to bespecified in order to obtain meaningful fracture spacing measurements.

Ž .Variability in spacing or intensity of fracture development of cleats in coal beds hasŽ .long been recognized Kendall and Briggs, 1933; Macrae and Lawson, 1954 . Contrasts

in coalbed methane well production also may reflect variable development of cleats, asseveral operators have speculated. Methods for identifying and mapping such variabilityŽ .Esterhuizen, 1995 are important in mining conditions because of implications for


Fig. 5. Cleat spacing and aperture data from Northeast Blanco Unit No. 403, San Juan Basin. Plotted spacingsrepresent averages of all measured spacings having a specific aperture measurement. Least squares fit made toaverage spacings of face cleats.

ŽFig. 6. Cleat spacing versus bed thickness of medium-brightness coals, northern San Juan Basin adapted from.Laubach and Tremain, 1991 .


Fig. 7. Cleat spacing versus traverse distance in a bed of uniform dip, thickness, and composition, San JuanBasin, New Mexico. Center half of data at each station is shown by box, and median by bar. Dashed line andshaded area are mean and one standard deviation of measurements from all stations.

Ž .quality of mined coal and mine stability Brady and Haramy, 1994 . These methodstypically rely on visual estimates of fracture intensity, but recent advances in automatedimaging methods and image analysis promise to make these approaches more quantita-

Ž .tive Djahanguiri et al., 1994 . Some of these mapping methods possibly could beadapted for use in describing core from coal-gas reservoirs or from outcrop analogs.

A variety of factors have been cited as affecting cleat development. These factorsinclude coal rank, coal composition, and layer thickness. To the extent that the impact ofthese factors on cleating has been explored quantitatively, average cleat spacing hasbeen used to characterize the cleats. Other factors, such as mineral fill, degree oftectonic and compactional deformation, and coal age, have received little to no attention.

Researchers have observed that cleat spacing varies with coal rank, decreasing fromŽlignite through medium volatile bituminous coal Ammosov and Eremin, 1963; Ting,

.1977; Law, 1993 , and increasing through anthracite coals, forming a bell-shapeddistribution of cleat spacing. Based on outcrop and core data from North American coal,

Ž .Law 1993 found that face cleat spacing ranges from approximately 22 cm in lignitesŽ Ž . . ŽR vitrinite reflectance values of 0.25–0.38% to 0.2 cm in anthracites R valueso o

.more than 2.6% . The best fit to the data is an inverse exponential equation:

ss0.473P100.398r R o 3Ž .

Ž .where s is cleat spacing in centimeters. Eq. 3 predicts a similar decrease in cleatspacing with increasing rank from the lignite to medium volatile bituminous, but also

Žindicates that spacing remains constant at vitrinite reflectance values above 1.5% Fig..8 . However, only two data points from low-volatile bituminous and higher-rank coals

define the trend, so rank-dependence of cleat spacing remains uncertain.Variation in cleat spacing with rank may reflect competing processes of fracture

Ž .formation and annealing Levine, 1993 . Bedding-parallel compaction fabrics anddeformation or flow of coal around rigid grains shows that flattening predates fracturing


Fig. 8. Cross-plot of vitrinite reflectance versus cleat spacing for coals ranging from lignite to anthraciteŽ . Ž .vitrinite reflectance of 0.25 to 2.6q % . Mean face-cleat spacing. Adapted from Law 1993 . Data werecollected only from bright coal lithotype layers. Ash content varies among coals and affects cleat spacing,however ash content was not reported.

in most coals. Highly strained coals at high rank deform predominantly by flow ratherthan fracture. Because tectonism commonly accompanies high thermal maturation, it isreasonable that tectonism may obliterate previously formed cleats in many anthracitesapparently barren of cleats. It is an open question whether annealing processes are anunrecognized but important process modifying fracture patterns in low-rank coal. If so,regular variation in cleat spacing with rank within the lignite to high-rank bituminousrange might not be expected.

Many authors have noted that cleat spacing varies with coal type and ash contentŽ .Spears and Caswell, 1986; Tremain et al., 1991; Law, 1993 . Bright coal lithotypesŽ . Ž .vitrain generally have smaller cleat spacings than do dull coal lithotypes durainŽ .Kendall and Briggs, 1933; Stach et al., 1982 . Coals with low ash content tend to havesmaller cleat spacings than do coals with high ash content. Organic-rich shales alsocommonly have closely spaced fractures that resemble cleats. This suggests thatgeochemical processes such as shrinkage related to coal composition are key to intensedevelopment of fractures.

The effect of compositional layering on fracture spacing has received much attention.Examples where average spacing is linearly proportional to coal lithotype-layer thick-

Žness have been found Spears and Caswell, 1986; Grout, 1991; Tremain et al., 1991;. Ž .Close and Mavor, 1991; Law, 1993 . Daniels et al. 1996 , in contrast, found no pattern

of cleat spacing with bed thickness. If we interpret cleat height as equal to lithotypeŽ .layer thickness t , and assume a linear aperture-spacing relation the result is a linear

proportionality between cleat spacing and lithotype layer thickness. Equating cleat


Ž . Ž .height and lithotype layer thickness again, the combination of Eqs. 1 and 2 impliesthat lithotype layer thicknesses follow a power law size distribution:

Nsqtyr 4Ž .

where N is cumulative number of lithotype layers having a thickness of t or larger, q isa measure of the total sequence thickness, and r is fractal dimension. Coal lithotype

Ž .layer thickness data collected by Smyth and Buckley 1993 follow a power law sizedistribution. The empirical fit to these data has a fractal dimension significantly greaterthan one, implying that thin layers account for more coal volume than thick layers.

Comparisons of cleat spacings versus bed thickness have typically not accounted forthe range of cleat sizes present. For example, Upper Cretaceous Fruitland coal beds have

Ž .numerous interbedded ash layers tonsteins that define coal bed thicknesses. A cleatspacing-bed thickness relation is apparent if cleats that span the coal layer are used andshorter cleats are neglected, and then only for beds that are thinner than 10 to 20 cm. For

Ž .these, cleat spacing is slightly less than layer thickness Tremain et al., 1991 . Other coalŽbeds in the western United States for example, Upper Cretaceous Adaville Formation

.coal beds at the Elkol mine, Wyoming have a spectrum of cleat sizes and spacings thatresemble those of Fruitland coal beds, but these coal seams lack thin interbeds. Similar

Žhierarchies of fractures in massive beds have been observed in non-coal rocks Nelson,. Ž1985 and in cooling cracks in glass Nemat-Nasser and Oranratnachai, 1979; DeGraff

.and Aydin, 1993 . This implies that the apparent fracture spacingrbed thicknessrelationship in Fruitland coals may be an illusion.

Such observations suggest that, except perhaps for the thinnest beds, interbed-layerthickness is not a primary control on cleat intensity. However, a possible explanation forhierarchical spacing without obvious layer boundaries may be found in the mechanicalsimilarity of cooling, contracting lava flows and dehydrating, contracting coals. DeGraff

Ž .and Aydin 1993 show that cooling rate strongly affects fracture propagation, resultingin closer spaced fractures for higher cooling rates. Possibly variations in contraction rate

Ž .of coal controlled by chemical processes may govern the spacing of cleats of a givensize.

Even cleat spacing is apparent in many coal beds. Although they have been reportedby mine operators, few distinct cleat swarms — zones of anomalously closely spacedfractures — have been described. This may partly result from lack of coal exposures

Ž .where intense localized fracture development can be seen Fig. 9 . Some swarms areŽ .associated with faults Shepherd et al., 1981; Laubach et al., 1991 and locally these

features are evident in seismic records. This is an area where more information isneeded, since these structures potentially are a cause of highly productive areas withincoal beds.

Finally, indirect quantification of cleat spacing comes from data on sizes of minedŽ .coal fragments Bennett, 1936; Turcotte, 1992 . Mass fractions of mined coal fragments

Ž .as a function of fragment size determined by sieve analysis are consistent among theŽseams and collieries analyzed 19 data sets, from United Kingdom coals of unknown

. Ž .rank, composition, and age , and form lines on a log–log plot Fig. 10 . If we assumethat coal fragments in a data set have statistically similar shapes and mass densities, then


Fig. 9. Cleat patterns and potential sources of anomalous cleat attributes: fracture swarms and differentialcompaction zones. Diagram shows schematic fracture patterns in coal.

Žwe can convert mass fraction data into cumulative numbers R. Marrett, 1994, unpubl..rept. and characterize the size distribution of fragments. The spacing of bounding cleats

probably define the size of an individual fragment, so we can interpret the converted

Ž .Fig. 10. Coal fragment size distribution from sieve analysis of coals in ‘Colliery B’ Bennett, 1936 . DataŽ .represent volumetric 3D sampling of mass fraction having linear dimension less than plotted value. For

clarity, 0.5 was added to log mass fractionrnumber values for coal seam 1 and 0.5 was subtracted from logmass fractionrnumber values for coal seam 5.


data as a volumetric sample of the cleat spacing size distribution. This interpretationsuggests that cleat spacing follows a power law of the form:

Nsmsyd 5Ž .where N is volumetric cumulative number of spacings of s or larger, m is a measure ofcoal volume and cleat frequency, and d is the fractal dimension. Best fit values of d

Ž .range from 2.31 to 2.34. Eq. 5 plus a linear aperture-spacing relation suggests thatcsd; however this is not true of the values determined. Although there are a variety ofpossible explanations, the most likely is that the assumption involved in averagingobserved average spacings is incorrect.

2.3. Cleat network geometry and connectiÕity

ŽIf all fractures in a coal bed were isolated, then flow rates observed after draining.those fractures directly intersected by the wellbore would be limited by matrix

Ž .permeability i.e., no fracture enhancement of permeability . Network geometry andconnectivity of fractures in a system clearly are important to the permeability enhance-

Ž .ment that fractures can provide Fig. 9 . For example, coal-bed permeability may be 3 toŽ10 times greater in the face cleat direction than in other directions McCulloch et al.,

.1974 , reflecting the strong preferred orientation and greater length of interconnectedfractures in that direction. On a local scale, cleat connectivity results from cross cuttingand abutting relations. Maps of cleat patterns show that connections within the network

Žmay be accomplished by fractures of vastly different size Tremain et al., 1991; Chen.and Harpalani, 1995 . Vertical connectivity of cleat networks is commonly limited by

the termination of small cleats at interfaces between coal types, and large cleats atcoal-non-coal bed interfaces.

One approach to cleat network characterization is to count the various types ofŽ . Ž .fracture ends i.e., connected, constricted, or dead-end Laubach et al., 1991 . Con-

stricted and dead-end terminations render fractures insignificant to permeability en-hancement, although they might assist flow from the matrix into the fracture network.Analysis of relative frequencies of different fracture-end types was done for two

Ž .exposed bedding surfaces of coals in the San Juan Basin Laubach et al., 1991 . Onecoal-bed surface showed the expected result that cleat systems are well connected, dueto systematic development of butt cleats extending from one face cleat to another. Theother bedding surface indicated poor connectivity among cleats, possibly because only

Žlarge cleats were considered. Uncertainty about timing and origin of butt cleats which.may in some case be restricted to near-surface locations clearly will impact interpreta-

tion of fracture connectivity in coal beds at depth. Cross-cutting relations and contrastsin orientation suggest that not all cleats in a given bed formed simultaneously.

2.4. Cleat petrology

ŽCleat apertures may or may not contain authigenic minerals commonly clays, quartz,. Žand calcite or organic material and resin Spears and Caswell, 1986; Daniels and

.Altaner, 1990 . In coal mining, such minerals have an impact on coal quality, but forcoalbed methane, diagenetic alteration of the cleat network due to precipitation of


authigenic cements can occlude or preserve fracture porosity, and thus the ability offractures to conduct fluid. This is an area that has been relatively little studied. Mineralsin coal cleats have been described by several authors, including Macrae and LawsonŽ . Ž . Ž . Ž .1954 , Hatch et al. 1976 , Cobb 1979 , Hanes and Shepherd 1981 , Spears and

Ž . Ž .Caswell 1986 , Daniels et al. 1996 .In several coal beds, paragenesis of cleat-filling minerals indicates that the develop-

ment of fractures took place during coalification and progressive. Spears and CaswellŽ .1986 showed regional and stratigraphic variations in cleat-fill mineralogy and devel-oped a paragenetic sequence of mineral precipitation for Westphalian A Canock coals in

Žthe United Kingdom. The first minerals to precipitate were sulfides pyrite, sphalerite. Žand galena followed by quartz and clay minerals, then carbonate minerals calcite and

. Ž .ankerite . Hatch et al. 1976 found a similar paragenetic sequence in Pennsylvaniancoals in the Illinois Basin. The presence of multiple mineral phases indicates that cleats

Ž .remained open permeable over extended periods of time or that they were episodicallyreopened, and that the composition of fluid migrating through the cleats was variable.

Fig. 11. Patterns of cleat minerals in Cretaceous Frontier Formation coals, western Wyoming. All diagramsŽ . Ž .show plan-view relations. a Blocky calcite and quartz on opposite fracture walls. b Two generations of

Ž .calcite in face and butt cleat, respectively. c Early-formed quartz along face cleat separated by later crossingŽ .butt cleat having calcite fill. d Intergrowths of early quartz and late calicte, recording progressive cleat

porosity reduction.


Ž .Crack-seal features in cleats Hatch et al., 1976 imply episodic fracture opening.Locally, the mineralogy of coal cleats mimics that of nearby fractures in sandstones. Forexample, calcite and quartz in face and butt cleats correspond to authigenic mineral

Ž .suites in adjacent Cretaceous sandstones in parts of Texas and Wyoming Fig. 11 .Ž .Levine 1993 demonstrated that cleat-filling calcite in Lower Pennsylvanian coals

precipitated early during coalification. Precipitation of calcite or other minerals in cleatsmay preserve the only evidence of early formed cleats because minerals inhibitannealing during subsequent coalification. In the San Juan Basin, calcite precipitationprobably occurred after coalification and subsequent uplift and erosion along basinmargins. Bacteria introduced into Fruitland coal beds by meteoric water metabolized wetgases and organic compounds in the coal to generate secondary biogenic methane and

Ž .carbon dioxide Scott et al., 1994 . Formation waters in the cleat system becamesaturated to oversaturated with respect to calcite resulting in precipitation of isotopically

Ž .heavy calcite in cleats Scott et al., 1991 . Cleat development probably occurs at least inpart during early coalification, thus cleats in some coals may be filled with organic

Ž . Ž .matter during late coalification Crelling et al., 1982 . Rice et al. 1989 reported thatŽ .exsudatinite a hydrogen-rich material , generated during coalification, filled pores or

voids within the vitrain component of Fruitland coal beds. Patterns of regional andstratigraphic variations in cleat-filling minerals could therefore help identify areas ofdiffering cleat openness and target the most conductive parts of fracture systems.

2.5. Local Õariations in cleat style

In addition to size, intensity, and diagenetic variations, cleat permeability could beaffected by shifts in fracture style; that is, changes from opening-mode fractures tofaults. Faults are well recognized in coal beds, and some coal beds have pervasive small

Ž .faults Ammosov and Eremin, 1963 . Such faults may post-date cleat formation,Žalthough faults that accompany early compaction are known Law, 1976; Tremain et al.,

.1991 . Moreover, cleat intensity and size locally vary with proximity to faults andŽ .position on folds Schultz-Ela and Yeh, 1992 so abrupt variations in cleat style, as well

as intensity, could be anticipated in these settings.Coal exposures in Upper Cretaceous Rock Springs Formation in southwest Wyoming

exhibit significant variations in cleat style over distances of a few to tens of metersŽ . Ž .Laubach et al., 1993 Fig. 12 . Closely spaced normal faults abruptly substitute foropening-mode fractures in coal beneath some sandstone lenses having blocky lateralterminations. Because these faults have little or no porosity, the coal that contains themhas low permeability compared to coal having generally porous cleats. These changes in

Žfracture style can be explained by differential, late-stage compaction Laubach et al.,. Ž .1993 Figs. 12–14 . Most differential compaction occurs by ductile flow before

Žconsolidationrcoalification advances enough for fracture to occur perhaps largelyduring deposition of units immediately overlying peatrcoal beds. It is late-stagecompaction of a few percent that occurs from shallow burial to depths of ;1000 m ormore that can influence cleat development.

Ž .In floodbasin settings such as this example, compaction is greatest in peat 10:1 andŽ .mud and least in channel-belt sandstones 1.5:1 . Sand deposition in any given vertical


Fig. 12. Cross section and structural data showing shifts in cleat style, from opening-mode fractures to faults,Žbeneath a sandstone lense in Mesaverde Group, Cretaceous, central Wyoming modified from Laubach et al.,

. Ž .1993 . a Lower hemisphere equal-area projection of poles to face cleats and great-circle arc of bedding andŽ . Ž .average cleat orientation. b Same projection, showing fault attitudes in area beneath lense. c Average

attitudes for pole to bedding, pb; trace cleat, tc; trace bedding, tb; pole to normal faults, nf; pole to cleats, fa.

sequence may be concentrated over compacting floodbasin andror thin channel margindeposits, producing stacked, lenticular sandstone–coal sequences. During subsequentburial loading-late stage compaction-relatively rigid sandstone lenses can distort adja-cent coal beds, creating folds.

Changes in fracture style due to stresses caused by the shape of overlying andunderlying strong rocks, and perhaps even the evolving shape of the basin itself, has todate not been thoroughly investigated. It is interesting to note that shifts in cleatorientations in the San Juan Basin that have been ascribed to separate episodes of

Ž .tectonism see below; Laubach and Tremain, 1991 also correspond to changes insandstone trends.

Fig. 13. Geometry of coal compaction where coal is interbedded with lenticular, and more rigid, sandstoneŽ .modified from Laubach et al., 1993 . Strain during compaction can be resolved into pure shear and simple

Ž . Ž .shear components, determined by layer dip q and compaction ratio R . t, early stage thickness; trR,late-stage thickness. Ellipses show strain at an early and late stages of compaction.


Fig. 14. Finite-element model results after differential compaction of sandstone and coal beds due to 1 km ofoverburden. Shear stress and differential stress are augmented in coal beds below abruptly tapering edges ofsandstone lenses, favoring fault development, whereas under gradually tapering lenses shear stresses are not

Ž .sufficiently enhanced to cause shifts in fracture style. a Rotation of principal stress axes beneath edge of lensŽ . Ž .during elastic deformation. Shear stress is also high everywhere beneath lens not shown . b Strain due to

Ž .nonrecoverable plastic yield equivalent plastic strain . Left-dipping contours are consistent with failure onleft-dipping faults. Model by D. Schultz-Ela.

2.6. Regional cleat orientation patterns

In some respects, cleat patterns on a basin scale are better known than those offractures in non-coal rock types, mainly because the dominant cleat set in an outcrop cangenerally be readily identified. Thus, regional maps of cleat orientation have sharply


Fig. 15. Map showing face-cleat domains and domain boundaries in Pennsylvanian and Permian coals in WestŽ .Virginia. Modified from Kulander and Dean 1993 .

Ždistinguished domains of uniform and variable cleat strike Shepherd et al., 1981;.Kulander and Dean, 1990, 1993, Laubach and Tremain, 1991; Laubach et al., 1992

Ž .Figs. 15–17 . The uniformity of cleat orientations over wide areas was one of theŽfeatures that caught the attention of observers as early as the 1800’s see Raistrick and

.Marshall, 1939 . Regions hundreds of square kilometers in area have uniform cleatŽ .orientations Ver Steeg, 1942; Kulander and Dean, 1990; Laubach et al., 1992 .

However, dominant cleat strike may also shift abruptly from one coal seam to the nextŽ .within an area as small as a single colliery Dron, 1925 . Domains of uniform strike

having a range of sizes, and zones of transition between them, can impede or channelizefluid flow in a cleat system. Such orientation patterns played a prominent role in guiding

Ž .speculation on the origins of cleat Dron, 1925; McCulloch et al., 1974; Ting, 1977 andremain a key attribute to be explained.

Conflicting interpretations of cleat domains have been proposed. They may representŽsimultaneous development of cleat in areas where stress orientations differ Kulander

.and Dean, 1993 or multiple, superposed episodes of cleat development associated withrotation of stress directions, perhaps coupled with coals entering burial conditions that

Ž .favor cleat formation at different times Laubach et al., 1992 . Uniformity of cleatstrikes within domains indicates that fractures responded to regionally coherent stresspatterns, which could reflect plate-scale stresses or stresses related to uplift or basin


Fig. 16. Structure map of San Juan Basin contoured on Huerfanito bentonite, face-cleat strikes in FruitlandFormation coal beds, and cleat-strike domains. Domain boundaries are gradational and approximately located.

Ž .BArIA is Bondad–Igancio anticline. Modified from Laubach and Tremain 1991 .

geometry. On basin scales, transitions between domains of uniform cleat strike rangefrom gradual to abrupt. In some cases, domains are associated with specific folds orfaults. For example, alignment of cleats normal to fold hinges and fault traces was

Ž . Ž . Ž .pointed out by Freiser 1914 , Ver Steeg 1942 , Nickelsen and Hough 1967 , andŽ .Close 1993 . However, fractures that maintain uniform strike across changing fold

Ž .trends may predate folding Kulander et al., 1980; Kulander and Dean, 1993 .The San Juan Basin provides an example of a transition zone between domains of

Ž .differing face-cleat strikes in Cretaceous Fruitland coals Fig. 16 . These are separatedby an east-trending transition zone and can be traced in the basin with data fromoriented coal cores. Interestingly, this zone is coincident with the so-called coalbedmethane ‘high productivity fairway’. In the northern part of the basin, northwest-strikingcleats predominate, but in the south, north–northeast- and northeast-striking cleats aremost common. The transitional boundary between domains is a wide zone havinginconsistent and variable face-cleat strike. Both northwest and northeast face cleatstrikes are evident in this area, and coal beds with mutual abutting and crossing cleatpatterns are prominent. Cleats with opposite abutting relations occur in adjacent beds,and less commonly in adjacent parts of the same bed.


Fig. 17. Map of cleat system in the Piceance Basin, Colorado.

Although convincing mechanical models for development of cleat domains have notyet been tested against well defined patterns and well-dated cleat sets, the PiceanceBasin has evidence of progressive cleat domain formation. A domain of east–northeast-striking face cleat in the south in Cretaceous Mesaverde Group coals is replaced in the

Ž .northern part of the basin by west–northwest-striking cleat Fig. 17 . Thermal historyanalysis shows that the northern coals reached higher thermal maturity, and possibly thethreshold of initial cleat development at vitrinite reflectance values between 0.3 and

Ž .0.5%, later than coals in the south Scott et al., 1996 . Based on kinetic modeling,Ž .Mesaverde peats over most of the Piceance Basin reached the lignite stage R s0.3%o

Ž .at approximately the same time 61.7"1.4 my, Table 1 . However, it took longer fororganic matter to reach vitrinite reflectance values of 0.5% in the north than in the south.Coalification in the southern basin occurred more rapidly and 0.5% vitrinite reflectance


Table 1Summary of geochemical processes and cleat development during coalification

Ž .Volatile components Vitrinite reflectance %aCompactional water loss 0.2 to 0.4

bMaximum rate of structural water loss 0.3 to 0.5bMaximum rate of carbon dioxide loss 0.4 to 0.7

bMaximum wet gasrcondensate generation 0.5 to 1.1b,cMaximum methane generation 0.8 to 1.5

Initial cleat development 0.3 to 0.5Cleat development 0.3 to 2.0qCleat annealing 0.3 to 2.0q

a Ž .Law 1993 .b Ž .Burnham and Sweeney 1989 .c Ž .Tang et al. 1991 .

was reached approximately 50 Ma, or 22 million years after deposition. Coalification inthe north proceeded more slowly and 0.5% vitrinite reflectance was not attained until 41Ma, or approximately 31 million years after deposition.

Ž .If cleat development occurred during the peat to lignite transition R s0.3% , thenoŽheterogeneous directions of paleo-maximum horizontal stress west–northwest and

.east–northeast were active in the basin at this time. However, if cleat developmentoccurred at 0.5% vitrinite reflectance, the two separate cleat domains could represent atemporal shift in homogeneous paleostress directions. Evidence for a temporal shiftincludes different face-cleat orientations in Cretaceous and Tertiary coals in the samearea.

3. Cleat origins

Cleats without any observable shear offset are opening-mode fractures, allowingallows us to relate cleat orientation to past stress fields. However, driving mechanismsresponsible for controlling stress fields and for causing cleat formation are debatable. Acentral question is whether cleats form during burial, at depth after lithification, orduring uplift. More specifically, we could ask the following questions: Are cleats theresult of contraction of coal beds during dewatering or chemical metamorphosis? Arethey caused by thermoelastic response of coal to regional tectonic extension androruplift? Are cleats merely a response to local stress perturbation caused by folding andfaulting? Is elevated pore pressure the key parameter, due to either water expelled fromcoal structure or perhaps hydrocarbon gas generation? We cannot answer all of thesequestions, but we can shed some light on the issues and point to observations that mayhelp resolve the problem of the origin of cleats.

3.1. Opening-mode fracture mechanics

At the turn of the century, the origin of cleats and joints was the subject of muchŽcontention Dron, 1925; Kendall and Briggs, 1933; McCulloch et al., 1974; Ting, 1977;

.Pollard and Aydin, 1988 . One aspect of this debate was whether these fractures were of


shear origin or not. Although it was recognized that fractures could form in rock due totension, this type of fracture was considered to be limited to the near surface as a resultof folding, cooling, and contraction of igneous rocks, dehydration, or shear zone

Ž .formation Engelder, 1987 . Such fractures were not expected to show the planar,sharp-sided aspect, and regionally persistent orientations that typified fractures in coalsŽ .Kendall and Briggs, 1933 . It was thought that since tensile stress was unlikely to existat depth, all fractures at depth had to be of shear origin.

Ž .Secor 1965 helped resolve this controversy by showing that the concept of effectiveŽ .stress as presented by Hubbert and Rubey 1959 could account for opening-mode

fracture development in a compressive stress state if the pore pressure was sufficientlyhigh. However, debate persisted as some continued to interpret many joints as ‘shear’

Ž .fractures due to their conjugate geometry in relation to other fractures Hancock, 1985 .Later work showed that plumose structure on fracture surfaces and lack of shear offset

Žprecluded a shear origin for many conjugate fractures Barton, 1983; Dyer, 1983;.Nickelsen and Hough, 1967; Pollard and Aydin, 1988 .

Cleats primarily accommodating opening displacement propagate along a plane ofzero shear stress, specifically the plane perpendicular to the least compressive principal

Ž .stress Lawn and Wilshaw, 1975; Pollard and Aydin, 1988 . This makes such fracturesindicators of past stress orientations, where vertical cleats include the maximumhorizontal stress direction at the time of their formation. Conditions under which cleatsŽ .or other opening-mode fractures form can best be described using fracture mechanics.

Ž .K is called the opening mode or mode I stress intensity factor and measures theI

magnitude of the stress concentration at the crack tip. For a vertical, uniformly loaded,Ž .planar crack, whose length is much greater than its height Fig. 18 , it takes on a simple

relationship:1r2K sDs pPhr2 6Ž . Ž .I

Žwhere Ds is termed the driving stress and h is the height of the crack Lawn and.Wilshaw, 1975 . When K exceeds a critical value, K , the fracture will propagate. ForI Ic

Fig. 18. Pore pressure effects promoting a fracture. See text for explanation.


stress concentration to occur, there must be crack-opening displacement, and openingdisplacement requires a positive driving stress. Driving stress is defined as:

Dss pys 7Ž . Ž .H min

Ž .where s is the minimum in situ stress compression is positive and p is magnitudeH min

of pore pressure acting inside the fracture. Driving stress can be positive under twoconditions-the local minimum stress acting on the fracture is tensile or the pore pressurein the fracture exceeds the minimum stress. Absolute tension may be possible at or nearthe surface, but for fracturing at depth, where even the minimum stress is compressive,there must be some contribution from pore pressure.

Most subsurface rocks experience a negative driving stress most of the time. Thespecial situation of a positive driving stress at depth can come about under three basicconditions — pore pressure increases to exceed minimum stress, minimum stressdecreases to fall below the magnitude of the pore pressure, or a combination of both.This is a limited set of conditions, but there are numerous processes that can beresponsible for the occurrence of these conditions. Some processes that might cause pore

Žpressure increase in coals are compaction, dehydration, and devolatilization Ting,.1977 . Mechanisms that cause a decrease in minimum stress are uplift, folding, cooling,

and either dehydration or devolatilization. All of these factors could potentially inducevertical cleats.

Another result of analyzing cleats with fracture mechanics is that a mechanicalmaximum for cleat aperture can be estimated. Assuming a coal behaves approximatelyelastically when fracturing occurs, maximum aperture, e, for cleat should depend onfracture height, driving stress, and coal elastic properties according to this relationshipŽ .Pollard and Segall, 1987 :

e s4 1yn 2Ds hr2 rE 8Ž . Ž . Ž .max

where E is Young’s modulus. Thus aperture should be linearly proportional to drivingstress and fracture height, and inversely proportional to rock stiffness as measured by E.

The process by which fractures propagate under the influence of pore pressure hasŽ .been termed natural hydraulic fracturing Engelder and Lacazette, 1990 , and there has

been controversy over how applicable this mechanism really is. Pore pressure and in situstress are not independent variables, due to poroelastic effects in rock. So care must betaken in determining stress and pore pressure conditions under which opening-modefracture propagation occurs. However, by properly combining the driving stress equationwith an expression for minimum in situ stress, we can make generalizations aboutconditions that might be necessary for cleat propagation. If we follow the approach of

Ž .Engelder and Lacazette 1990 , we can show that natural hydraulic fracturing is possiblefor low pore pressure relative to overburden stress.

For illustrative purposes, and in recognition that the critical stress intensity factor canŽbe small under geologic conditions for saturated rocks Atkinson and Meredith, 1987;

.Segall, 1984; Olson, 1993 , we will assume a positive driving stress is sufficient forcrack growth. A more complete treatment of the problem can be found in Engelder and

Ž .Lacazette 1990 . For a positive driving stress, the following condition is necessary:

p)s 9Ž .min


The expression for minimum stress due to gravitational loading only is:

s snr 1yn s ya p qa p 10Ž . Ž . Ž .min overburden

where n is Poisson’s Ratio, a is Biot’s poroelastic constant, and s is verticaloverburden

stress from overburden. Combining these two equations gives an expression for porepressure required for fracturing in terms of material properties and overburden stressrequired for positive driving stress:

p) ns r 1ynq2naya 11Ž . Ž . Ž .overburden

Ž .Plotting this expression Fig. 18 shows that fracturing may require pore pressurefrom as small as 0.1 times the overburden for ns0.1 and as0 to equal the overburdenstress for any Poisson Ratio and as1. Typical numbers for rock are ns0.2 andas0.6, which would predict a pore pressure of 0.5 times overburden for fracturing tooccur, which is just slightly over hydrostatic in most basins, without reducing minimumstress due to tectonics or coal shrinkage during compaction. If we include the othereffects that can reduce minimum stress, fracture mechanics relations predict fracturing atsub-hydrostatic pore pressures, which imply that all coals should be fractured to someextent.

As with fractures in non-coal rocks, cleats may vary in orientation in the vicinity ofother fractures or structures. Local fracture hooking or curving may occur due to

Ž .mechanical interaction of overlapping, en echelon cleat segments Pollard et al., 1982 .Crack-path hooking is expected to be stronger when in situ stress is nearly isotropic andinhibited when there is a strong differential compressive stress in the plane of the

Ž .fracture Olson and Pollard, 1989 . Patterns should exhibit less curving in thin rocklayers due to a diminished stress perturbation caused by three-dimensional effectsŽ .Olson, 1993 . Surface roughness of the fracture might also inhibit crack path hooking.The overlapping segments of face-cleat traces are commonly straight and parallel ratherthan hooked and curved even in thick coal beds, suggesting that significant crack-paral-lel differential stress was present during fracture propagation.

3.2. Cleat formation

The ubiquity of cleats in a coal suggests that they result from processes common toall seams. Regularly invoked causes for the formation of cleats are the extremely large

Ž .compaction and desiccation of peat coalification during burial relative to theseŽ .processes in other sediments Ting, 1977 . Dehydration and associated shrinkage of

carbonaceous material during coalification create stresses that allow fractures to form.

Ž .Fig. 19. Numerical model GEOSIM-2D of maximum and minimum horizontal stress trajectories in westernŽ .United States for late Tertiary Laramide deformation. Shaded boxes show ‘basement uplifts’ that are eight

times stiffer than surrounding elastic medium. Model assumes 1 km northeast-directed shortening applied toŽ . Ž . Ž .left overthrust side of model. a Pattern for entire region. b Part of model near Wyoming salient,

Ž .comparing stress trajectories to measured face-cleat strikes solid dark lines in Cretaceous and Tertiary coals.


However, contraction alone is insufficient to account for the strong preferred orientationof cleats over wide areas, since shrinkage tends to produce polygonal patterns like

Ž .mud-cracks or cooling cracks in lava flows Kendall and Briggs, 1933 . Preferredfracture orientations suggest the existence of consistent differences in principal stressesover wide areas. This suggests that the coal has been buried below the near surface andhas acquired strength in order to sustain the differential stress required for the regular,

Ž .planar geometry to the cleats Olson and Pollard, 1989 .Tectonics can impart a significantly anisotropic stress field, and many workers have

attributed uniform patterns of cleat strike over large areas to the effect of tectonicallyŽcontrolled stress orientation on fracture growth direction Kaiser, 1908; Freiser, 1914;

McCulloch et al., 1974; Price, 1966; Ting, 1977; Laubach and Tremain, 1991; Laubach.et al., 1992; Kulander and Dean, 1993; Close, 1993 . Cleats can then be used as

indicators of the kinematics of past tectonic events, or conversely, knowledge of tectonichistory in a basin or region can be used to predict cleat orientation. Recent examples of

Ž . Ž .this approach include Laubach et al. 1992 and Kulander and Dean 1993 . Fig. 19shows a comparison of face-cleat strikes in Cretaceous coal beds in the western United

Ž .States to stress trajectories predicted by a numerical model Laubach et al., 1994 .Agreement between prediction and observation is exact in some areas, but highlydivergent elsewhere.

Such models are obviously simplifications of the tectonic loading history of a region.Plate-scale anisotropic stress fields exist for long periods, and do not require tectonicevents, as conventionally understood, for their formation. Yet a principal cause ofdiscrepancy between such models and measured cleat orientations is uncertainty in

Ž .timing of cleat formation and thus the identity of relevant tectonic stress field s towhich cleat patterns should be compared. Timing of cleat formation is challenging tospecify precisely, and thus strong tests of this mechanism of cleat formation remainelusive. If cleat patterns reflected a simple scenario of compaction and dewateringwithout fracture as burial increases, followed by an abrupt change in brittleness andfracture toughness properties at maximum burial and maturity, a plausible model wouldbe fracture due to a combination of pore pressure and minimum stress reduction caused

Ž .by exhumation Price, 1966 . Stress trajectories recorded by cleat patterns would bethose existing at uplift through some critical threshold.

Exhumation probably accounts — at least in some instances — for formation of buttcleats, which are typically at right angles to face cleats, and are analogous to cross-joints

Ž .as described by Engelder 1987 . Uplift and erosion increases driving stress for fracturepropagation due to a decrease in all of stress components. Pre-existing face cleats tend torelieve induced stresses perpendicular to them and inhibit growth of additional parallelfractures. However, stresses parallel to face cleats are not relieved, and these stressespromote growth of secondary fractures perpendicular to pre-existing face cleats. Facecleats that lack mineral fill will be open in response to stress changes caused by upliftand will act as free surfaces against which younger butt cleats end.

However, in addition to unloading effects, coal experiences significant shrinkage dueto progressive devolatilization reactions, so uplift is not required to drive cleat forma-tion. Moreover, core studies show that face and butt cleats exist at depth and are thus notentirely due to uplift and unloading. Mineral suites in some cleats suggest formation


Žunder conditions of increasing temperature and progressive burial Spears and Caswell,.1986 . Many cleat sets in outcrops of folded rocks evidently formed prior to folding

Ž .Tremain et al., 1991 . Confirming that cleats can form in basins that have not hadepisodes of uplift would require drilling in a suitable location such as the deeply buriedTertiary coals of the ancestral Mississippi delta.

Time-temperature modeling, based on burial and thermal history, can be used toestimate the time required to reach the thermal maturity at which cleat genesis is mostlikely if desiccationrdevolatilization occurs over a given vitrinite reflectance or rank

Ž .range Scott et al., 1996 , because coals undergo systematic chemical changes duringcoalification that are marked by peaks in shrinkage and expulsion of volatiles. Althoughmoisture andror volatile matter loss could contribute to coal shrinkage, it is therearrangement of coal structure that is responsible for most shrinkage that couldcontribute to cleat development. Loss of inherent moisture, which is free moistureassociated with micropores and capillaries andror physically sorbed to organic com-pounds, results in significant volume shrinkage during the peat to lignite transition.However, these early formed shrinkage cracks close through annealing or repolymeriza-

Ž .tion Levine, 1993 . During progressive coalification, cleavage of cross-linked, oxygen-bearing functional groups in the coal structure results in additional shrinkage and cleat

Ž .development. Based on kinetic data from Burnham and Sweeney 1989 , the rate ofmoisture loss increases significantly between vitrinite reflectance values of 0.3 and 0.5%and reaches a maximum rate at 0.5%.

Based on when maximum shrinkage occurs, early cleat development probably occursbetween vitrinite reflectance values between approximately 0.3 and 0.5% or the peatthrough subbituminous B coal. This is consistent with field observation of cleats in

Ž .lignites Law, 1993 . Although initially formed cleats at vitrinite reflectance valuesaround 0.3% may become annealed, progressive desiccation at vitrinite reflectancevalues between 0.4 and 0.5% in the presence of either local or regional stressesrepresents potential for fracture formation due to shrinkage. Thus, using burial historyand thermal modeling to identify when peat reaches vitrinite reflectance values of 0.3 to0.5% gives an estimate of the time of cleat development. Petrologic study andradiometric dating of minerals within cleats is the most reliable approach to testing such

Ž .interpretations Daniels and Altaner, 1990; Daniels et al., 1996 .Compaction takes place during progressive coalification, but fracturing may only be

associated with latter stages of coalification, perhaps after nearly all compaction hasoccurred. Few, if any, fractures are formed during peat compaction and associated lossof non-structural adherent moisture. Shrinkage during the peat to lignite transitioninvolves loss of inherent moisture from micropores and capillaries and that physicallysorbed to organic compounds. Potentially, this could lead to fracture formation. Laterstages of coalification involve cleavage of cross-linked, oxygen-bearing functionalgroups and additional shrinkage.

3.3. Annealing

Ž .The possibility that cleats, once formed, could be destroyed annealed has not beenwidely investigated, although annealing has been called on to explain the lack of


fractures in some anthracites. Yet in the structural evolution of a complex organicŽ .compound coal under the influence of burial and thermal histories, annealing poten-

tially plays a role in governing resultant fracture patterns. The evidence for annealingcomes from considering the interplay of compaction and fracture formation. The largeamount of vertical shortening associated with the early stages of peat consolidation

Ž .imply that early-formed structures such as early cleats become distorted with progres-sive compaction. Processes that promote fracturing, such as dehydration, devolatiliza-tion, and regional and local tectonics, affect organic deposits throughout the lignite toanthracite rank. Yet similar-appearing cleats are in coals of various ranks, including

Ž .lignite and low-rank bituminous coal Law, 1993 . In a typical coal bed, cleats appearnearly perpendicular to bed boundaries and compaction fabric, and are themselves notdeformed. Although at this time we are aware of little or no direct maceral ormineral-matter petrographic evidence for cleat annealing, if compaction overlaps to asignificant degree with fracturing, the planar character of cleats in lignites implies

Ž .operation of processes that destroy anneal these early formed cleats in higher rank andmore compacted coals. The observation that face and butt cleats are nearly always planarand are not visibly rotated or deformed indicates that they must have formed relativelylate in the compaction history of the coal bed.

An annealing process such as repolymerization can efficiently remove early formedŽ .cleats Levine, 1993 . Quantitative work has been sparse on geochemical parameters that

govern both coal shrinkage, which can drive fracture formation, and ductile coal flowand repolymerization, which might lead to fracture annealing. Cleat annealing occurswhen the coal structure is rearranged, and results in increased density, structuralanisotropy, and hardness. High-rank coals have undergone devolatilization so that coalshrinkage is a less likely driving force for fracture formation.

4. Cleat and coalbed methane

Ž .Over 95% of the gas in coal is stored in micropores Gray, 1987 that are estimated toŽ .have diameters ranging from 0.5 to 1 nm Van Krevelen, 1981 , values so small that the

matrix may have no effective permeability. Cleat-fracture porosity in coal is estimated toŽbe between 0.5 and as much as 2.5% Puri et al., 1991; Gash, 1992; Chen and Harpalani,

.1995 . Although small amounts of free gas may exist in coal fracture systems, coalbedmethane is mainly adsorbed on the large internal surface area of the impermeable coalmatrix and fracture surfaces, and most produced methane may be adsorbed onto fracturesurfaces. Releasing adsorbed methane is accomplished by lowering reservoir pressure,generally through removal of water and reducing hydrostatic pressure on the coal bed.Therefore, coal beds generally must be partly dewatered to initiate gas production. Afterdesorption from coal, gases must diffuse through the coal matrix until encountering openfractures. Interconnected, open fractures are conduits for gas and water flow to thewellbore, and fracture surface arearcoal volume is critical to producible gas volume.

For development of coalbed methane, important natural fracture attributes contributeto permeability pathways for gas and water flow to wells. Also significant are fractureproperties that enhance or detract from coal stability and success of well completion and


stimulation techniques such as horizontal drilling, open-hole cavitation, and hydraulicfracture treatment. Horizontal wells and hydraulic fractures induced to enhance produc-tion ideally should be oriented to maximize contact with the permeable natural fracturesystem. For example, in United States Bureau of Mines degassification experiments inshallow Appalachian Pennsylvanian coal beds, horizontal boreholes drilled perpendicu-lar to face cleats yield 2 to 10 times the production rate of gas as holes drilled parallel toface cleat.

Changes in the fracture system during production are also important. Unlike moreconventional fractured reservoirs, during gas drainage from coal beds, absolute perme-ability may vary by several orders of magnitude with changing effective stress, gas

Ž .pressure, and matrix shrinkage Sparks et al., 1995 . Key fracture attributes include size,spacing, connectedness, aperture and degree of mineral fill, and patterns of preferredorientation on local and regional scales. Progress in understanding cleat origins fromrecent developments in fracture mechanics and chemical changes in the molecularstructure of coal during burial could lead to better predictions of these attributes inadvance of drilling.

Cleat intensity can affect coal stability and thus success of cavity stimulations.Dynamic open-hole cavity stimulations involve high pressure build up by naturalaccumulation of methane or air injection followed by sudden release of pressure,inducing coal to cavitate into the wellbore. This procedure is successful in parts of theSan Juan Basin, but elsewhere in the basin conventional stimulation is more effective.To date, cavity wells have not been highly effective in other basins worldwide. It is

Žbeyond the scope of this paper to review cavity stimulation see Mavor and Logan,.1994; Palmer et al., 1996 , but it is clear that strength of coal significantly influences the

Ž .spatial extent of permeability enhancement Choi and Wold, 1996 and that closelyspaced low-strength natural fractures can cause failure and deformation of coal overlong distances.

Areas of the San Juan Basin where open-hole cavity stimulations are most successfulcoincide with a domain where two cleat sets overlap and interfere, and cleat is closely

Ž .spaced due to relatively high coal rank Laubach and Tremain, 1991 . This area of thebasin probably has higher overall fracture connectivity as a consequence of variablecleat strike, changes in dominant cleat strike over short lateral distances and betweennearby beds, and greater abundance of small faults. Compared to other parts of thebasin, prevalence of two directions of strongly developed cleats in this transition zoneresults in qualitatively greater coal friability. This intense fracture development may

Ž .enhance cavity formation Laubach and Tremain, 1991 . This example shows that,together with other information, new cleat observations can help development of coalbedmethane resources.

5. Conclusions

Much quantitative observational work remains to be done on characterizing cleat sizepatterns, network geometries, and the response of cleat systems to changing effectivestress conditions. Measurements of size distributions of apertures, in the context of


rigorous scaling studies, can provide the basis for improved cleat porosity, permeability,and elastic shear wave anisotropy calculations. A challenge will be obtaining reliabledata on these parameters at in situ conditions. Few data are available on apertures,heights, lengths, connectivity, and the relation of cleat formation to diagenesis, charac-teristics that probably are critical to permeability. Recent studies of cleat orientationpatterns and fracture style suggest that new investigations of even these well-studiedparameters can yield insight into coal permeability patterns.

More effective predictions of cleat patterns will come from advances in understand-ing cleat origins. Although cleat formation has been attributed to a vague combination ofdiagenetic and tectonic processes, a viable mechanical process for creating cleats has yetto be demonstrated. Progress in this area may come from recent developments infracture mechanics and in appreciation of chemical changes in the molecular structure ofcoal during burial and coalification.

Acknowledgements

We are grateful to Romeo Flores for inviting us to prepare this paper, to J. Close, R.Flores, and B. Law for reviews for the International Journal of Coal Geology, and to W.Ambrose for comments. S.E.L. and A.R.S. acknowledge collaborative studies with W.B.Ayers, Jr., W.A. Ambrose, W.R. Kaiser, D.D. Schultz-Ela, C.M. Tremain, and R. Tyler.R.A.M. thanks Amoco Production Co. for permission to publish work begun whileunder their employment.

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