THERMAL MATURITY OF DEVONIAN BLACK SHALE-GAS RESERVOIRS, NORTHWESTERN PENNSYLVANIA – EVIDENCE FROM ORGANIC PETROLOGY, GEOCHEMISTRY, AND MINERALOGY

Christopher D. LaughreyPennsylvania Geological Survey

Statement of Problem• Ro measured in

Devonian black shale-gas reservoirs in northwestern Pennsylvania and adjacent areas suggest the organic matter in the rocks is thermally immature.

Statement of Problem• This observation

contradicts hypotheses suggesting that regional joint sets in these rocks formed as natural hydraulic fractures induced by abnormal fluid pressures during catagenesis.– Lash and others, 2004– Lash and Engelder, 2005– Engelder and Lash, in

press

Photo courtesy of Gary Lash

Statement of Problem• Is Ro suppressed in

these rocks? • Is Ro a reliable

geothermometer in this region?

Purpose

• Compare various geothermometers in core recovered from Devonian black shales located along the Lake Erie shore in northwestern Pennsylvania.

• Provide a reasonable and accurate interpretation of the thermal maturity of the organic matter and origin of the hydrocarbons in these rocks.

Outline

• Geologic setting• The shale-gas petroleum system

– Why is thermal maturity important?• Review selective maturity indices and

methods of measurement• Present the data• Summary and conclusions

Photo courtesy of Gary Lash

Geologic Setting

SHALE-GAS PETROLEUM SYSTEM

SOURCE

TRAP

RESERVOIRSEAL

Key Characteristics of Productive Thermogenic

Shale-Gas Reservoirs• Original organic richness and generation

potential• Primary and secondary cracking of kerogen

and retained oil• Retention of oil for cracking to gas by

adsorption• Porosity resulting from organic matter

decomposition• Brittle mineralogical composition

Shale Gas Petroleum System• Adsorbed petroleum

also trapped by surface chemistry

• Shale diagenesis• Dissolution-

reprecipitation processes may diminish gas reservoir properties by releasing gas from the system at high thermal maturities

Porosity

Reliable interpretations of thermal maturation indicators

(geothermometers) provide critical data needed for resource estimates in thermogenic shale-

gas reservoirs

Photo courtesy of Gary Lash

Vitrinite Reflectance (Ro)• One of the most

commonly measured geothermometers used for mapping regional maturity patterns in the Devonian black shales of the Appalachian basin.

Milici and Swezey, 2006, Figure 10

Ro underestimates thermal maturity in the Lake Erie region.

• Thermogenic gases are produced from Devonian shales along the Lake Erie shore (Laughrey and Baldassare, 1996)

• Numerous geothermometers indicate that these shales are mature (Laughrey and Baldassare, 1997; Obermajer and others, 1997; Lash and others, 2004)

0.6

Problems with Ro• There is more than one kind

of vitrinite; these different types plot in different positions on Ro vs. depth plots at the same actual maturity

• Ro is related more to thermal stress experienced by the vitrinite (complex, irreversible aromatization reactions) than to petroleum generation

• Incorrect maceral identification

• Statistical errors• Oxidized/recycled vitrinite• Poorly polished vitrinite

• Large amounts of oil-prone macerals or bitumen retard the normal progression of Rowith maturity

Selective Maturity Indices• Ro• CAI• Illite Crystallinity• Rock-Eval Pyrolysis

– Tmax– Production Index (PI)

• Atomic H/C; Van Krevelen diagrams

• Bitumen/TOC• Kerogen color; TAI;

pollen translucency• Weight percent C in

kerogen

• Extractable organic matter (EOM) analyses:– Carbon preference index

(CPI)– Pristane and phytane

indices– Gas chromatography

patterns– Biomarker ratios

• Stable isotopes of produced natural gases

• Burial modeling

EGSP PA# 3 Well, Presque Isle, Erie County, PA

Ro in the EGSP PA #3 Well• Mean Ro ranges from

0.43 to 0.55 percent (Huron through Marcellus)

• Thermally immature• Consistent with values

reported for the Lake Erie region by various workers– Lash, 2008 and

references therein– Obermajer and others,

1997 – Milici and Swezey, 2006

and references therein

Conodont Alteration Index• EGSP-PA #3 well:

– Tully/Hamilton– CAI = 1.5 (1041 – 1176 ft.)

• Hammermill #2 well:– Onondaga– CAI = 1.5 (1390 – 1590 ft.)

• Goodwill – Curley well:– Onondaga – CAI = 1.5 – 2 (2330 ft.)

• Ro equivalent ~ 0.8 –1.0+ (peak oil window)

-Repetski and others, 2002, USGS OFR-02-303

Illite Crystallinity Index (ICI)• Symmetry of the 10 Å X-ray

diffraction peak of illite– Variations dependent upon

the amount of mixed-layer illite-smectite associated with illite in the Devonian black shales

– Can be correlated with the intensity of thermal alteration

• Mean ICI = 0.10 in EGSP PA#3 well– ~ CAI 1.5 – 2– T range 50° - 140°C

Hosterman, 1993, USGS Bulletin 1909

Rock-Eval Pyrolysis• Tmax: 439° - 442° C• Production Index:

0.05 – 0.17; mean = 0.13– PI = S1/S1 + S2

• Mature, early oil window

Bitumen Ratios• Ratio of extractable

bitumen to total organic carbon (Bit/TOC) in shales ranges from near 0 in shallow sediments to about 0.25 at peak oil generation.

• Bit/TOC in the Devonian shales in the EGSP PA# 3 well ranges from 0.06 to 0.072 indicating a maturation level equivalent to the early oil window

Extractable Organic Matter (EOM)

• Carbon preference index (CPI):– Relative abundance of odd

versus even carbon-numbered n-alkanes

– CPI values significantly above or below 1.0 indicate an extract is thermally immature

– Organic matter input affects CPI

605.4 ft.

623.6 ft.

Huron Shale, EGSP PA# 3 well:•CPI = 1.10, a value consistent with an early oil window maturation level

•Relatively high hydrocarbon yields and high proportion of hydrocarbons (HC/EOM = 0.74)

•Relatively low concentration of isoprenoids relative to n-alkanes

Smooth n-alkane curve typical of a thermally mature extract

EGSP PA# 3, Huron Shale, 623.6 ft.

Pr/Ph = 2.20Pr/n17 = 0.74Ph/n18 = 0.42

Pr/Ph = 2.14Pr/n17 = 0.86Ph/n18 = 0.52

Biomarkers (molecular maturity parameters)

• Individual organic constituents of sediments, sedimentary rocks, and petroleum which are derived from biological precursors.

• Undergo systematic and sequential transformations during diagenesis; changes in composition used as measures of thermal history

Distinguishing characteristics of a biomarker

• The compound shows a structure indicating that it was, or could have been, a component in a living organism

• The parent compound is in high concentration in the organisms, which show widespread distribution

• The principal identifying structural characteristics of the compound are chemically stable during sedimentation and early burial

Thermal conversion of a C29-monoaromatic to C28 triaromatic steroid:Aromatization of the A and B rings – loss of a methyl group and 7 hydrogen atoms attached to these rings.The ratio increases from 0 to 100% during maturation.

Example:

Adapted from Peters and Moldowan, 1993 and Hunt, 1996

Biomarkers (molecular maturity parameters)

• Critical factors:– Sensitive to maturity change– Useful in a relevant range of sediment maturity

• Tmax limit for biomarkers ~ 450° to 460°C (peak – late oil window)

– Based on widely occurring compounds– Easily measured in GC-MS analyses

• The most useful parameters are measured from a single mass chromatogram and span both the oil generation threshold and the oil window:

– 20S/20R C29 steranes– Proportion of monoaromatic to triaromatic steroidal

hydrocarbons

Biomarkers in the Huron Shale

Steranes

• Mature sterane distribution– Isomerization of C29

steranes (20S)/(20R + 20S) approaching equilibrium values (early stage of oil generation)

Huron Sh.

605.4 ft.

m/z 217

m/z 217 Huron Sh.

605.4 ft.

Terpanes• Relatively high values

for C31 (22S)/(22S + 22R) - 59.0 to 61.5 –indicate early oil window

m/z 191

m/z 191

605 ft.

623.6 ft.

Adapted from Peters and Moldowan, 1993 and Hunt, 1996

Huron Shale Biomarker Summary

Oil window59.061.5%C31 (22S)/(22S +

22R)

Oil window63.264.9Ts/Ts + TmHopanes

Oil window0.910.895α20S/5α20R C29

Oil window54.455.9ββ/( ββ + αα) C29

Oil window47.547.120S/(20S + 20R) C29

C29 sterane

Thermal Maturation

623.6 ft. 605.4 ft.Biomarker

Natural Gas Geochemistry

Milici and Swezey, 2006, Figure 27

Individual gas components (CH4 ,C2H6 , etc.) can be characterized by their stable carbon (13C/ 12C) and hydrogen (2H/1H) isotopic compositions

δ 13C (permil) = [ (13C/ 12C)sample / (13C/ 12C)PDB – 1 ] 1000

δ D (permil) = [ (D/H)sample / (D/H)SMOW – 1 ]] 10001000

Huron Shale Gas Isotopes• δ13C1: -53 to -55.74 permil• δDCH4: -301.2 to -307.6 permil• C2: 3.33 to 6.1 percent

– δ13C2 = -41.79 permil• C3: 1.8 to 2.9 percent

– δ13C3 = -36.27 permil

• Wet thermogenic natural gases generated in the early oil window– Possible mixing with

bacterial methane

Burial and Thermal History Modeling

• “All models are wrong, but some are useful.”– Dr. Bruce Wienke,

Director, Computational Nuclear Physics Laboratory, LANL

Summary and Conclusions• Ro measured in the Devonian shales in the EGSP

PA#3 well in Erie County, Pennsylvania ranges from 0.43 – 0.55 percent suggesting that the rocks are thermally immature.

• Stable isotope data suggest that the hydrocarbon gases produced from these rocks are thermogenic and the geochemical characteristics of extracts from the shales, determined by EOM analyses, indicate generation and expulsion in the oil window. Several additional geothermometers, including CAI, ICI, Rock-Eval pyrolysis, and bitumen/TOC, indicate that the rocks are thermally mature.

Summary and Conclusions

• Burial and temperature history modeling demonstrate that the rocks may have entered the oil window approximately 190 Ma and were exposed to temperatures as high as 120°C over a period of 50 million years before uplift retrogressed organic maturation in the rocks.

Summary and Conclusions• Ro might be suppressed

in the rocks, but not by very much.– There are other possible

sources of error in the Romeasurements.

• Biomarkers are ideal for determining thermal maturity in the Lake Erie region.

• Correlation of several geothermometers is the best approach.