-38 -37 -36 -35 -34 -33 -32 -31 -30 -29 -28 -270

54

6789

10

123

-70 -60 -50 -40

-70 -60 -50 -40

?

?

?MICROBIAL

GAS

MIXED GAS

?

-260

-240

-220

-200

THERMOGENICGAS

-180

-160

-140

0.01

0.1

1

10

100

J;]N;SS(F)

mixing

MICROBIALGAS

THERMOGENICGAS

0.5 1.0 1.5 2.0 2.5

SOURCE ROCK MATURITY (Ro %)

? ?

(Tebo & upper Neutral)

(Croweburg )

(Mineral)

(Riverton)(Riverton)

(Tebo & upper Neutral)

(Croweburg)

(Weir-Pitt )

(Weir-Pitt )

(Mulky)

(S. Mound Sh.)

(Lexington)

(S. Mound Sh.)

(Lexington)(Hushpuckney Sh.)

(Hushpuckney Sh.)

(multi-zone)

(multi-zone)

(Weir-Pitt)(Mineral)

(Rowe)

(Riverton)

(Lexington)

(Excello Sh.)

(Mulky)

(Aw)

(Riverton)

(Mulberry)

(Bevier)

(Mineral)(Tebo)

(Dry Wood)

(Mulberry)

(Tebo)

(Dry Wood)

(Mineral)(Bevier)

(Mulky)(Mineral)

(Weir-Pitt )(Rowe)

(Riverton)

(Lexington)

(Excello Sh.)

(Mulky)

(Aw)

(Riverton)

COAL RANK(color-coded to BTU value)Cherokee & Marmaton Gps.

data from Barker and others (1992); Newell (1997)

VITRINITE REFLECTANCE(color-coded to coal rank)

shales inCherokee & Marmaton Gps.

A

B

AB

A

C

C

1.11

0.61

0.470.410.36

0.730.69

0.49

8,3009,500

10,50011,50013,00014,00014,250

15,000

high-volatilebituminous

sub-bituminous

medium-volatilebituminous

lignite

APPROXIMATE RANK

increasingrank

VITRINITE.REFLECT.

(Ro%)

HEATINGVALUE(BTU/lb)

(dry, ash-free)

420

425425

430

435440450460

approximate correlation of coal Rock-Eval Tmax to vitrinite reflectance isfrom Bostick and Daws (1994)

ROCK-EVALTmax

(deg. C))

coal rank classification chart fromMcLennan and others (1995)

thermogenicmethane

lignite sub-bitum. bituminous anthracite graphitefrom Boyer (1989)

ethane+

hi-vol med-vol low-vol semi meta

biogenic methanenitrogencarbon dioxide

GAS GENERATION IN COAL

1

conventional gas (from Jenden and others, 1988)

Mill CreekSchrader

Silver City, ThayerBrewster, Elk City, Mapleton NE, NeoshoFalls, OlatheClinesmith, Easton, McLouth NW, Pomona,Sallyards, Welch-Mohr

KingstonBrewster, Irish Valley, Neosho Falls, Paola-Rantoul,Tucker

Logsden

U. PENN

M. PENN

MSSP

L. ORD

field name and symbol (see map) colored coded according to origin ofgas, as indicated by δ13C and δD isotope crossplot (see right)

(green = microbial gas; blue = mixed gas; brown = thermogenic gas)

Elk City

FIELDAGE of PAY

LOCATION MAPCONVENTIONALGAS SAMPLES

oil oil & gas gas

25 mi

N

ThayerSilver City

Kingston

Mill Creek

Irish Valley

N.E. Mapleton

Olathe

Logsdon

N.W. McLouth

Pomona

Tucker

Welch-Mohr

Sallyards

Brewster

Clinesmith

Elk City

Neosho Falls

Easton

Schrader

Paola-Rantoul

coalbed gas (with coal identified)

LOCATION MAPCOALBED GAS

SAMPLES 25 mi

N

Montgomery Co.samples

symbol colored-coded according to location (see map)

Montgomery County, KSCass County, MOLeavenworth County, KS (from Bostic and others, 1993)

"northern samples" -- Bourbon arch and eastern flank of Forest Citybasin (exact locations still proprietary)

(Weir-Pitt)

North

5S 10S 15S 20S 25S 30S 35S0

0.5

1.0

1.5

2.0

TOWNSHIP

%Helium

200400600800

100012001400

Douglas, Shawnee Gps. (Virgilian)

Lansing, Kansas City, PleasantonGps. (Missourian)

Cherokee, Marmaton Gps.(Desmoinesian)

Leavenworth Co. coal desorption gases(after Bostic and others, 1993)

1060 BTU/scf ≅100% methane

950 BTU/scf ≅90% methane

BTU and He Content for Eastern KS Pennsylvanian Gases(projected onto a north-south crossplot)

BTU/scf

methaneethane

90.52%

propanebutane+

nitrogencarbon dioxideheliumhydrogen

0.09%0.01%(trace)

0.12%0.92%

8.34%

(trace)

Assay of OsbornEnergy coalbedproduction gas

(unpublished) fromnorthern Miami Co.

(913 BTU)

Leavenworth Co.coal desorptiongases (Bostic andothers, 1993)

Miami Co. coalbed production gas

Miami Co. coalbed production gas

25 mi.

(Nebraska)

(Oklahoma)

Cass Co. coal desorption gasMontgomery Co. coal desorption gas

Cass & Montgomery Co.coal desorption gas

Cass & Montgomery Co.coal desorption gas

0

5S 10S 15S 20S 25S 30S 35S3000

2000

1000

0

470460450440430420410

(oilwindow)

NorthTOWNSHIP25 mi.

(Nebraska)

(Oklahoma)

Douglas, Shawnee Gps. (Virgilian)

Lansing, Kansas City, PleasantonGps. (Missourian)

Cherokee, Marmaton Gps.(Desmoinesian)

Tmax maturity for eastern KS shales(projected onto a north-south crossplot)

Rock-EvalTmax

maturationscale

data from Barker and others (1992); Newell (1997)

Eastern Kansas conventional gas(from Jenden and others, 1988)

Coalbed gas

Ethane δ13C vs. % Ethane in Total HydrocarbonsEastern Kansas coalbed and conventional gases

Ethane δ13C ä (PDB)

Eth

ane

%

(increasing biogenesis)

REGIONAL TRENDS IN THERMALMATURATION

Thermal maturation, as displayed by the vitrinite reflectanceand coal rank maps above, increases southward in the Kansaspart of the Western Interior Coal Basin. The most prolific gasgeneration in coals occurs at medium-volatile bituminous rank.Kansas coals are less thermally mature (generally high-volatilebituminous ranks) and hence contain less gas.

A north-south projection of the Rock-Eval Tmax maturationparameter for shales from well cuttings and cores in the ForestCity basin, Bourbon arch, and Cherokee basin (see diagram atright) also indicates a southward increase in thermalmaturation. At a given depth, there is less maturation in theForest City basin than further south in the Cherokee basin.This may be caused by higher heat flow in southeasternKansas, or northward movement of higher-temperature watersout of the Arkoma basin onto the cratonic platform during thelate Paleozoic Ouachita orogeny.

These trends in maturation indicate that operators attemptingto produce coalbed gas in the marginally mature strata in theBourbon arch and Forest City basin should concentrate on thedeepest coals, which should have better gas content.

100 200 300 400 5000

1000

2000

0

3000

medium

-vol bituminous

high-vol Abituminous

high-vol Bbitum

inous

high-vol Cbitum

inous

subbituminous

C

ESTIMATED MAXIMUMPRODUCIBLE METHANE

CONTENTBY DEPTH AND RANK

(after Saulsberry and others, 1996)

gas content (scf/ton)

250

0

500

750

1510gas content (cc/g)

50

EasternKansasCoals

anthracitelow-vol bituminous

subs

urfa

cede

pth

(feet

)

subs

urfa

cede

pth

(met

ers)

volatiles driven off

POSSIBLE THERMOGENIC AND BIOGENIC ORIGINS OF KANSAS COALBED GASThe desorption diagrams above are from two wells in adjacent counties in the Cherokee basin. The deepest coals in Montgomery County (to the west) register gascontents from 250 to 300 scf/ton. The same coals in Labette County (to the east) are buried less deeply, and they have gas contents considerably less than theMontgomery County coals. However, the Iron Post coal at 382 ft (116 m) depth in the Labette County well has an unexpectedly large gas content (>100 scf/ton),exceeding that of the deeper coals. A microbial or mixed thermogenic-microbial origin for this gas is suggested. Pennsylvanian coal-bearing units crop out at thesurface in Cherokee County (the county immediately east of Labette County). Downdip movement of fresh water from the outcrop may augment biogenicproduction of coalbed gas in shallow coals along the eastern flank of the Cherokee and Forest City basins. A possible consequence to this model is that separatethermogenic and biogenic production fairways in the same coal may be present. The thermogenic fairway would be deeper in the basin where there is sufficentburial and confining pressure. The biogenic fairway would be updip and closer (and likely parallel) to the outcrop where basinal brines would be diluted by meteoricwaters carried downdip from the outcrop.

MontgomeryCounty

LabetteCounty

CherokeeCounty

(from Lange and others, 2003)

GAS ISOTOPIC DATA

A crossplot of methane δ13C and the δD can be used to infer gas origin. Thermogenicmethane carbon is typically isotopically heavy (i.e., less negative) whereas microbialmethane carbon typically is isotopically light (i.e., more negative). Microbially-derivedgas is also dry and largely void of heavier hydrocarbons (i.e., ethane, propane, etc.).

A data set on isotopes of conventional gases from eastern Kansas (from Jenden andothers (1988) can be compared to coalbed gas samples. The conventional gases(squares in the above diagram) range from biogenic to thermogenic in origin. A map ofthe data (see above, left) shows most biogenic and mixed biogenic-thermogenic gasesare on the shallow eastern flank of the Forest City and Cherokee basins, whereasthermogenic gases are farther west in the deeper portions of the basins. There is nostrong stratigraphic differentiation of these gases in which younger, less thermallymature formations display a stronger biogenic signature (see key for location map forconventional samples). This suggests that some conventional and coalbed gases ineastern Kansas could be what Scott (1999) termed "secondary biogenic gases" in whichmethanogenic bacteria modify existing hydrocarbons.

Coalbed-gas methanes (circles in the above diagram) show no strong thermogenicsignature. Gases from the Bourbon arch and eastern flank of the Forest City basin tendto be isotopically lighter than Cherokee basin gases, which is consistent with lesserthermal maturation northward.

Bacterial modification of eastern Kansas coalbed and conventional gases is alsosuggested by the crossplotting of ethane δ13C with % ethane. Methanogenic bacteriamore easily consume isotopically lighter carbon. In such circumstances, the residualethane will become isotopically heavier (i.e., less negative) as it is consumed. Similareffects of microbial oxidation of heavier hydrocarbons in gas have been observed withDevonian shales in the Michigan basin (Schoell and others, 2001; Walter and others,2001; Martini and others, 2003) and with Fruitland coal in the San Juan basin (Schoelland others, 2001).

(diagram after Jenden and others, 1988)

(diagram after Jenden and others, 1988)

incr

easin

gm

atur

atio

n

REGIONAL TRENDS IN GAS QUALITY

Conventional gases have higher BTUs in southeastern Kansas, indicating agreater proportion of heavier hydrocarbons -- a trait that is consistent withthe inferred greater maturation in this region. Shallower Pennsylvaniangases from the Missourian and Virgilian part of the section have greaterpercentages of noncombustable gases, which significantly lowers theirheating value. Heating values for coalbed gases mimics the trendestablished by the conventional gases. Inasmuch as helium is not easilyretained by adsorption, coalbed gases generally have low helium content,but the presence of helium in some coalbed gases suggests leakage fromconventional reservoirs with well completion.

CONCLUSIONS

1. A marked increase in drilling for coalbed gas has occurred insoutheastern Kansas in the last three years, with a commensurate increasein coalbed gas production.2. Most of the activity for coalbed gas has been in southeastern Kansas inthe Cherokee basin, but isolated projects farther north in the Bourbon archand Forest City basin are in progress.3. Most Kansas coals are thin (