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

 

MEMORANDUM

DATE: July 28, 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.1This 1995 GRI data represents gas samples from across the United 

States. 

Gas Composition for Pneumatics, Equipment Leaks, and Compressors

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

For production, the 1995 GRI data is well within the ranges of the other data sources which 

range from 1.19 to 11.6 percent for VOC by volume. The 1995 GRI data is also within the 95 

percent confidence interval of the production data which range from 2.81 to 7.82 percent volume 

for VOC. 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. In addition, it can be expected that the gas 

composition for pneumatic controllers, equipment leaks, and compressors associated with these 

emissions units are associated with gas from oil wells and gas wells making the range of VOC 

composition widely varied. Therefore, it was determined that the 1995 GRI data was appropriate 

to use to develop a representative gas composition for pneumatic controllers, equipment leaks, 

and compressors.

For the transmission sector, the average 1995 GRI VOC concentration of 0.89 percent

volume was compared to other data sources and was found to be in the range of the VOC 

composition, which ranged from 0.29 to 6.84 percent VOC by volume. It was determined that 

the 1995 GRI gas composition would be used to represent the average composition of natural gas 

in the transmission sector, because the other data sources represented natural gas compositions 

outside the U.S.

i

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 cubic feet (ft3), which represents one pound mole 

of gas. Finally, relative weight percents for each component were calculated. These weight 

percents are presented in Table 5.

 

i

It should be noted that the GRI data contains a statement that the BTEX data are “skewed toward high BTEX and 

VOC content gases....” However, the 1995 GRI data are within the ranges of the other data 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 for pneumatics, equipment leaks and compressors.
3

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

Data SourceaSource of Natural Gas Area Covered

Volume %

Methane VOC HAP

CENRAP b Conventional 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

87.8 3.50 0.019

GTI Databasec Gas Wells Nationwide, proven reserves, and 

undiscovered reserves data from 462 

basins/formations

82.8 3.61 n/a

INGAA Unprocessed Natural 

Gas

Unknown 80.0 5.00 n/a

NREL LCAd Gas Well Worldwide 65.7 5.66 n/a

MARCELLUSe Gas Well Marcellus 97.2 2.02 0.03345

WYOMING 

DEQb

Gas Well Wyoming 92.4 1.19 0.08

Minimum 65.7 1.2 0.0

Maximum 97.2 5.7 0.1

Average 84.3 3.50 0.0

Gas 

Composition

Production Nationwide 83.1 3.66 0.164

n/a = not available

a Data from the Barnett Shale database was not speciated and therefore not included in this analysis.

b HAP data contains BTEX and n-Hexane

c HAP Speciation not provided; hexanes reported as Hexanes Plus

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

values and may not be valid representations

eHAP data only reported for hexane
4

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

Data Source Source of Natural Gas Area Covered

Volume %

Methane VOC HAP

INGAA Pipeline Gas Unknown 91.9 6.84 n/a

GOADS 

OffshoreaSales 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

Minimum 91.9 0.3 0.099

Maximum 95.2 6.8 0.099

Average 94.0 2.3 0.099

GRI-MACT Transmission/Unknown Nationwide 92.7 0.89 0.014

n/a = not available

a HAP data contains BTEX and n-Hexane
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 UnknownaTransmission Unknown aTransmission

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

a 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 a

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

a 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

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.

Natural Gas Composition for Completions and Recompletions

The gas composition for completions and recompletions from gas wells were determined 

by performing a sensitivity analysis on the compositions of the gas well data using a larger 

sample size which included data from hydraulically fractured wells. The results of this analysis 

are shown in Table 7. A mean of 3.63 percent VOC with a 95 percent confidence interval that 

ranges from 3.30 to 3.96 percent VOC by volume was determined. Based on the summary 

statistics, these data appear to be reasonable for use in developing an average natural gas 

composition to use for completions and recompletions of gas wells. 

Once it was determined that this data was appropriate, the average gas composition was 

calculated and then normalized so that the total volume percent equaled 100. This average gas 

composition is presented in Table 8. The gas composition data was then converted to weight 

percent by normalizing the volume percent for each component, then calculating the weight of 

each component using the MW for each component in lb/lbmol and a standard gas volume of 

385 ft3. Finally, relative weight percents for each component were calculated. 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 9.

A similar analysis was performed for completions and recompletions from oil wells. The 

results of this analysis are presented in Table 10. The average VOC composition was 

11.62 percent by volume, with a 95 percent confidence interval that ranges from 6.73 to 

16.5 percent VOC by volume. As was done for gas wells, the average composition was 

normalized. The gas composition used for completions and recompletions for oil wells is 

presented in Table 8. The gas composition data was converted to weight percent using the same 

approach detailed for gas wells and are presented in Table 9.
10

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.

Table 7. Summary Statistics of Sensitivity Analysis on Gas Composition for Gas Well and 

Hydraulically Fractured Wells

Methane VOC

Mean 83.238 Mean 3.630

Standard Error 0.709 Standard Error 0.170

Median 86.581 Median 3.104

Mode 0 Mode 0.000

Standard Deviation 15.207 Standard Deviation 3.626

Sample Variance 231.244 Sample Variance 13.149

Kurtosis 12.943 Kurtosis 9.258

Skewness -3.08 Skewness 2.262

Range 99.75 Range 29.560

Minimum 0 Minimum 0.000

Maximum 99.748 Maximum 29.560

Sum 38289.387 Sum 1655.427

Count 460 Count 456.000

Confidence Level(95.0%) 1.393 Confidence Level(95.0%) 0.334

Volume 

Percent

Volume 

Percent

(Lower of 95% conf interval) 81.844 (Lower of 95% conf interval) 3.297

Methane 83.238 VOC 3.630

(Higher of 95% conf interval) 84.631 (Higher of 95% conf interval) 3.964
11

Table 8. Average Gas Composition for Completions and Recompletions of Gas and Oil 

Wells

Average Volume Percent

Pollutant Gas Wells Oil Wells

Carbon dioxide (CO2) 1.631 1.00162

Nitrogen (N2) 4.455 29.19

Methane (C1) 83.081 46.73

Ethane (C2) 4.924 10.17

Propane (C3) 2.144 6.62

i-Butane (i-C4) 0.348 1.067004

n-Butane (n-C4) 0.643 2.136346

i-Pentane (iC5) 0.095 0.550849

n-Pentane (nC5) 0.119 0.515798

Cyclopentane 0.005 0.001091

n-Hexane (n-C6) 0.155 0.005182

Hexanes (C6) 0.000 -

Cyclohexane 0.001 0.001455

Other Hexanes 0.010 0.007636

Methylcyclohexane 0.002 0.001818

C6+ Heavies 0.114 -

Heptanes (C7) 0.009 0.697080

n- Heptanes (C7) 0.000 0.001909

C8+ Heavies 0.004 0.005182

Benzene 0.005 0.006182

Toluene 0.003 0.000223

Ethylbenzene 0.000 0.000445

Xylenes 0.001 -

2,2,4-Trimethylpentane 0.000 0.000223

Helium 0.140 -

Oxygen 0.084 -

Hydrogen 0.001 0.575909

Hydrogen disulfide (H2S) 2.027 0.709092

Total 100 100

VOC 3.66 11.62
12

Table 9. Weight Ratios to Use in Estimating Emissions for Completion and Recompletions

Gas Wells Oil Wells

Methane:TOCa0.796 0.4453

VOCb:TOCa0.116 0.3729

HAP:TOCa0.0084 0.0006

VOCb:Methane 0.146 0.8374

HAP:Methane 0.0106 0.0001

BTEX:Methane 0.0006 0.0007

HAP:VOCb0.0726 0.0016

BTEX:VOCb0.0040 0.0009

a TOC = all organic compounds listed in Table 3.

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

Table 10. Summary Statistics of Sensitivity Analysis on Gas Composition for Oil Wells

Methane VOC

Mean 46.73157 Mean 11.61755

Standard Error 4.196101 Standard Error 2.193276

Median 49.63115 Median 9.697621

Mode 49.63115 Mode #N/A

Standard Deviation 19.68146 Standard Deviation 7.274275

Sample Variance 387.3598 Sample Variance 52.91508

Kurtosis 1.385922 Kurtosis 1.438744

Skewness -1.15094 Skewness 1.127773

Range 71.93094 Range 25.91599

Minimum 0.156 Minimum 1.381007

Maximum 72.08694 Maximum 27.297

Sum 1028.095 Sum 127.793

Count 22 Count 11

Confidence Level(95.0%) 8.72627 Confidence Level(95.0%) 4.886924

(Lower of 95% Conf interval) 38.0053 (Lower of 95% Conf interval) 6.730621

Methane 46.73157 VOC 11.61755

(Higher of 95% Conf. Interval) 55.45784 (Higher of 95% Conf. Interval) 16.50447
13

REFERENCES

1. Letter and Attachments from Evans, J. M., Gas Research Institute, to G. Viconovic, EC/R 

Incorporated. Natural Gas BTEX Content. April 19, 1005. Legacy Docket Number A94-04, Item II-D-35.