E

C

/R Incorporated Providing Environmental Technical Support Since 1989

501 Eastowne Drive, Suite 250  Chapel Hill, North Carolina 27514

Telephone: (919)484-0222  Fax: (919) 484-0122

DRAFT 

MEMORANDUM

DATE: August 23, 2011

SUBJECT: Composition of Natural Gas for use in the Oil and Natural Gas Sector 

Rulemaking

FROM: Heather P. Brown, P.E.

TO: Bruce Moore, EPA/OAQPS/SPPD

The purpose of this memorandum is to document the development of a representative 

natural gas composition for use in the oil and natural gas sector rulemaking. This composition 

will be used to determine hazardous air pollutant (HAP) and volatile organic compound (VOC)

emissions from several segments of the oil and natural gas sector.

Gas composition data was compiled from several sources across the industry. The 

following is a list of the sources of data used for this analysis:

CENRAP database. “Recommendations for Improvements to the CENRAP States’ Oil 

and Gas Emissions Inventory”, November 13, 2008. Covers the following States: Texas, 

Louisiana, Arkansas, Oklahoma, Kansas, Nebraska, Missouri, Iowa, and Minnesota

GTI Database. “GTI’s Gas Resource Database, Second Edition – August 2001”

TX Barnett Shale. “Emissions from Natural Gas Production in the Barnett Shale Area

and Opportunities for Cost-Effective Improvements”, January 26, 2009

INGAA/API Compendium. “Greenhouse Gas Emission Estimation Guidelines for 

Natural Gas Transmission and Storage Volume 1 – GHG Emission Estimation 

Methodologies and Procedures” September 28, 2005

GOADS Offshore. “Year 2005 Gulfwide Emission Inventory Study” December 2007

NREL LCA. “Life Cycle Assessment of a Natural Gas Combined-Cycle Power 

Generation System” September 2000 

Union Gas. Chemical Composition of Natural Gas found online at 

http://www.uniongas.com/aboutus/aboutng/composition.asp

Marcellus. “Supplemental Generic Environmental Impact Statement On The Oil, Gas 

and Solution Mining Regulatory Program - Well Permit Issuance for Horizontal Drilling 

And High-Volume Hydraulic Fracturing to Develop the Marcellus Shale and Other LowPermeability Gas Reservoirs” September 2009

Wyoming DEQ. Speciation of Natural Gas and Condensate. Courtesy of Cynthia 

Madison, Wyoming DEQ
2

Tables 1 and 2 present a summary of the Methane, VOC, and HAP contents provided in 

the above data sources for the production and transmission sectors, respectively, along with an 

identification of the basins/areas of the country covered by the gas composition.

In addition to the above, gas composition data were collected from the industry in 1995 

during the development of the original maximum achievable control technology (MACT)

standards for this sector. These data are presented in Tables 3 and 4 for production and 

transmission, respectively. The original submission from GRI is contained in Attachment 1 of 

this memorandum. This 1995 GRI data represents gas samples from across the United States. 

Tables 1 and 2 also present a comparison of the 1995 GRI data to the other data sources. 

As shown in the tables, the 1995 GRI data is well within the ranges of the other data sources and 

in fact for Production, the VOC volume percent is equal to the average of the other data sources. 

Further, of the data sources that provide data on HAP emissions, the GRI data represent gas 

compositions across the United States, while the CENRAP, TX Barnett, and Marcellus data are 

specific to the regions specified in Tables 1 and 2. Therefore, it was determined that the 1995 

GRI data was appropriate to use to develop a representative gas composition. It should be noted 

that the attached memorandum in Attachment 1 contains a statement that the BTEX data are 

“skewed toward high BTEX and VOC content gases....” However, as stated previously, Tables 

1 and 2 show that the 1995 GRI data are within the ranges and very close to the average of other 

data identified. Therefore, these data were determined to be appropriate to use to develop a 

representative gas composition. 

The gas compositions from the 1995 GRI data were then converted to weight percents. 

First, because the average volume percent was not equal to 100, the volume percents were 

normalized for each component. Then the weight of each component present in the gas was 

calculated using the molecular weight (MW) for each component in pounds per pound mole 

(lb/lbmol) and an assumed gas volume of 385 ft3, which represents one mole of gas. Finally, 

relative weight percents for each component were calculated. These weight percents are 

presented in Table 5.

Once the weight percents were calculated for each natural gas component, relative ratios 

were calculated for methane:total organic compounds (TOC), VOC:TOC, HAP:TOC, 

VOC:Methane, HAP:Methane, BTEX:Methane, HAP:VOC, and BTEX:VOC. These relative 

ratios are presented in Table 6.
3

Table 1. Gas Composition (volume %) for Production Sector

Methane VOC HAP

CENRAP* Gas Wells 11 Basins: Louisiana Mississippi Salt, 

Southern Oklahoma, Nemaha Uplift, 

Arkoma, Cambridge Arch Central Kansas 

Uplift, Fort Worth, Cherokee Platform, 

Permian, East TExas, Western Gulf, and 

Anadarko

87.6 3.50 0.019

CENRAP* Oil Wells 11 Basins: Louisiana Mississippi Salt, 

Southern Oklahoma, Nemaha Uplift, 

Arkoma, Cambridge Arch Central Kansas 

Uplift, Fort Worth, Cherokee Platform, 

Permian, East TExas, Western Gulf, and 

Anadarko

46.7 11.62 0.02

GTI Database Gas Wells Nationwide, proven reserves, and 

undiscovered reserves data from 416 US and 

83 Canadian basins/formations

83.1 3.66 n/a

TX Barnett** Unprocessed Natural Gas Barnett Shale 74.0 8.20 0.20

INGAA Unprocessed Natural Gas Unknown 80.0 20.00 n/a

NREL LCA*** Gas Well Worldwide 65.7 5.66 n/a

MARCELLUS† Gas Well Marcellus 97.2 2.02 0.03345

WYOMING DEQ* Gas Well Wyoming 92.4 1.19 0.08

46.7 1.19 0.019

97.2 20.0 0.200

78.3 6.98 0.071

1995 GRI Production Nationwide 82.8 6.98 0.155

n/a = not available

* HAP data contains BTEX and n-Hexane

** HAP Speciation not provided

***Data provided were ranges for each pollutant (min and max). These values represent normalized averages of these

values and may not be valid representations

†HAP data only reported for hexane

Average

Volume %

Data Source Source of Natural Gas Area Covered

Minimum

Maximum
4

Table 2. Gas Composition (volume %) for Transmission Sector

Methane VOC HAP

INGAA Pipeline Gas Unknown 91.9 6.84 n/a

GOADS Offshore* Sales Gas Offshore Gas in the Gulf of Mexico 94.5 1.27 0.099

NREL LCA Pipeline Gas Worldwide 94.4 0.90 n/a

Union Gas Pipeline Gas United States, Western Canada, and Ontario 95.2 0.29 n/a

91.9 0.29 0.099

95.2 6.84 0.099

94.0 2.32 0.099

1995 GRI Transmission/Unknown Nationwide 92.7 0.89 0.014

n/a = not available

* HAP data contains BTEX and n-Hexane

Area Covered

Average

Data Source Source of Natural Gas

Volume %

Minimum

Maximum
5

Table 3. 1995 MACT Correspondence with GRI & EC/R- Production Data

Sector Production

Site GRI1 GRI2 GRI3 GRI4 GRI5 GRI6 GRI7 GRI8 GRI9 GRI10 GRI11 GRI12

Mole %

Nitrogen 2.72 0.44 0.78 0.46 0.79 1.52 1.18 1.74 1.90 1.30 0.52 6.81

Carbon Dioxide 0.04 0.90 0.29 3.37 1.00 0.38 1.67 0.68 0.00 0.47 0.54 8.12

Methane 95.60 93.26 90.62 56.62 80.40 78.38 79.55 74.67 83.90 91.93 88.40 79.83

Ethane 1.04 3.16 4.31 10.87 10.41 10.88 10.40 12.57 7.90 3.80 7.25 2.89

Propane 0.33 1.14 1.90 13.90 4.25 5.41 4.15 5.98 3.86 1.23 1.53 0.94

Butanes 0.16 0.64 1.15 8.59 1.65 2.10 1.74 2.55 1.70 0.70 0.90 0.54

Pentanes 0.07 0.22 0.51 3.61 0.65 0.77 0.69 1.21 0.49 0.24 0.36 0.30

Hexanes+ 0.03 0.20 0.37 2.03 0.60 0.36 0.43 0.35 0.17 0.24 0.42 0.52

ppmv

n-Hexane 88.7 277 664 2783 965 1173 937 2125 517 307 510 681

Isooctane 8.0 31.5 63.5 1552 151 145 112 103 52.0 49.6 32.0 87.0

Benzene 4.9 257 218 328 294 74.4 294 102 57.9 143 617 196

Toluene 2.9 108 117 251 468 92.4 263 31.4 45.6 142 222 213

Ethylbenzene 0 19.7 6.7 27.3 14.5 4.3 3.3 0.8 1.2 11.2 9.0 10.4

m,p-Xylenes 0 34.0 26.6 26.0 87.9 21.7 16.7 1.7 7.3 56.6 45.0 66.0

o-Xylene 0 19.9 5.0 6.2 16.1 3.2 2.4 0.3 0.6 16.9 10.0 16.4

NR = Not Reported
6

Table 4. 1995 MACT Correspondence with GRI & EC/R (Transmission Data)

Sector Transmission Unknown* Transmission Unknown* Transmission

Site GRI13 GRI14 GRI15 GRI16 GRI17 GRI18 GRI19 GRI20 GRI21 GRI22 GRI23 GRI24

Mole %

Nitrogen 9.89 8.68 2.96 2.55 0.22 1.25 1.16 1.1 1.15 1.12 0.3 1.85

Carbon Dioxide 0.28 0.40 0.58 0.54 0.35 2.62 0.15 0.12 0.07 1.06 1.36 0.66

Methane 81.97 82.61 91.8 92.7 97.4 95.4 98.5 88.2 81.1 94.6 95.8 93

Ethane 6.84 7.06 3.68 3.35 1.94 0.31 0.09 9.69 11.8 2.81 2.03 3.13

Propane 0.78 0.99 0.59 0.52 0.042 0.075 0.005 0.67 3.95 0.155 0.4 0.8

Butanes 0.14 0.17 0.159 0.148 <0.006 0.059 <0.006 0.035 1.189 0.116 0.075 0.314

Pentanes 0.04 0.05 0.045 0.042 <0.003 0.039 <0.003 <0.003 0.341 0.039 0.014 0.132

Hexanes+ 0.04 0.03 0.042 0.042 0.004 0.202 <0.002 <0.002 0.226 0.129 0.015 0.103

ppmv

n-Hexane 63.2 66.9 NR NR NR NR NR NR NR NR NR NR

Isooctane 17.5 14.5 NR NR NR NR NR NR NR NR NR NR

Benzene 5.0 7.9 51 36 <0.2 471 <0.2 <0.2 10 <0.2 4.5 15

Toluene 5.1 8.1 16 13 <0.1 100 <0.1 <0.1 13 <0.1 3.7 14

Ethylbenzene 0.5 0.6 3 3 <0.1 15 <0.1 <0.1 9 <0.1 0.1 1

m,p-Xylenes [1] 1.4 2.2 12 7 <0.1 11 <0.1 <0.1 1 <0.1 0.6 3

o-Xylene [1] 0.4 0.4

[1] Sites 15-36 reported only a total xylene result that includes all xylene isomers.

NR = Not Reported

* Based on the high methane content (greater than 90 percent) of this datapoint, it was assumed that they were samples from the transmission 

segment.
7

Table 4. 1995 MACT Correspondence with GRI & EC/R - Transmission Data

(Continued)

Sector Transmission Unknown*

Site GRI25 GRI26 GRI27 GRI28 GRI29 GRI30 GRI31

Mole %

Nitrogen 1.24 1.75 1.02 1.04 0.49 0.42 0.54

Carbon Dioxide 0.3 0.13 0.44 0.65 1.76 0.87 0.92

Methane 90.2 97.8 96.6 96.1 95.5 96 95.7

Ethane 7.02 0.26 1.78 1.86 1.74 2 2.12

Propane 1 0.014 0.091 0.213 0.351 0.413 0.414

Butanes 0.146 <0.006 0.025 0.06 0.093 0.181 0.175

Pentanes 0.03 0.0015 0.0089 0.0218 0.0354 0.0675 0.0665

Hexanes+ 0.021 0.0037 0.0052 0.0219 0.0322 0.073 0.069

ppmv

n-Hexane NR NR NR NR NR NR NR

Isooctane NR NR NR NR NR NR NR

Benzene 9 1.2 0.8 6 7 59 58

Toluene 13 0.4 <0.4 6 6 23 26

Ethylbenzene <0.3 0.3 <0.1 0.3 0.5 1.8 2

m,p-Xylenes [1] 4 0.2 <0.1 1 1.5 7 5

o-Xylene [1]

[1] Sites 15-36 reported only a total xylene result that includes all xylene isomers.

NR = Not Reported

* Based on the high methane content (greater than 90 percent) of this datapoint, it was 

assumed that they were samples from the transmission segment.
8

Table 5. Gas Composition Conversion to Weight Percent 

Component

MW 

(lb/lbmol)

Production Transmission

Avg 

Vol

%

b

Normalized 

Vol %

Weight per 

385 ft3 Gas 

(lbs) Weight %

Avg 

Vol

%

b

Normalized 

Vol %

Weight per 

385 ft3 Gas 

(lbs)

Weight 

%

Carbon Dioxide 44.01 1.46 1.5% 0.002 3.2% 0.70 0.70% 0.001 1.8%

Nitrogen 28.02 1.68 1.7% 0.001 2.3% 2.04 2.0% 0.001 3.3%

Methane 16.04 82.76 82.9% 0.035 65.7% 92.68 92.8% 0.039 86.2%

Ethane 30.07 7.12 7.1% 0.006 10.6% 3.66 3.7% 0.003 6.4%

Propane 44.09 3.72 3.7% 0.004 8.1% 0.60 0.60% 0.001 1.5%

Butane 58.12 1.87 1.9% 0.003 5.4% 0.16 0.16% 0.000 0.55%

Pentane 72.15 0.76 0.76% 0.001 2.7% 0.05 0.052% 0.000 0.22%

n-Hexane 86.17 0.09 0.092% 0.000 0.39% 0.01 0.0065% 0.000 0.032%

Other hexanes 86.17 0.32 0.32% 0.001 1.4% 0.001 0.00086% 0.000 0.0043%

Isooctane-a 114.23 0.02 0.020% 0.000 0.11% 0.002 0.0016% 0.000 0.011%

Benzene 78.11 0.02 0.022% 0.000 0.083% 0.004 0.0039% 0.000 0.018%

Toluene 92.14 0.02 0.016% 0.000 0.074% 0.001 0.0013% 0.000 0.0070%

Ethylbenzene 106.17 0.001 0.00090% 0.000 0.0047% 0.0002 0.00020% 0.000 0.0012%

Xylene 106.17 0.004 0.0041% 0.000 0.021% 0.0003 0.00030% 0.000 0.0019%

Total 99.85 100.0% 0.053 100.0% 99.91 100.0% 0.045 100.0%

a- Isooctane = 2,2,4, Trimethylpentane

b- Average of all gas compositions presented in Tables 1 and 2 for production and transmission, respectively.
9

Table 6. Weight Ratios to Use in Estimating Emissions

Production Transmission

Methane:TOCa0.695 0.908

VOCb:TOCa0.193 0.0251

HAP:TOCa0.00728 0.000746

VOCb:Methane 0.278 0.0277

HAP:Methane 0.0105 0.000822

BTEX:Methane 0.00280 0.000322

HAP:VOCb0.0377 0.0297

BTEX:VOCb0.0101 0.0116

a TOC = all organic compounds listed in Table 3.

b VOC = all organic compounds listed in Table 3, except ethane and methane.
ATTACHMENT 1

Original data submitted by GRI