Christopher D. Laughrey1, Tim E. Ruble2, and Mark H. Tobey3

Generation and Modification of Natural Gases in the Marcellus Formation, Central Appalachian Basin, USA: Evidence from Stable Isotope Geochemistry

Christopher D. Laughrey1, Tim E. Ruble2, and Mark H. Tobey3

1Weatherford Laboratories, Golden, Colorado 80403 2Weatherford Laboratories, Houston, Texas 77064 3Encana Natural Gas, Denver, Colorado 80202

Exploration and production in the Marcellus Formation of the central Appalachian basin are focused in two core areas: southwestern Pennsylvania/northern West Virginia and north central to northeastern Pennsylvania. Both associated gases and non-associated gases are produced in the southwestern core region. Only non-associated gases are produced in the northeastern core region. BTU values vary throughout the play.

Gases produced in the southwest core discovery area have wetness values ranging from 19 to 30%. Methane δ13C and methane δD range from -42.19 to -45.23‰ and -184.4 to -212.3‰, respectively. Corresponding ethane and propane δ13C values range from -33.15 to -35.84‰ and -28.46 to -31.22‰. The isotope compositions of the various C1 to C5 homologs in these gases exhibit normal distributions on natural gas plots with δ13C1 < δ13C2 < δ13C3 < δ13C4 < δ13C5. These plots intersect the distribution of δ13C in Marcellus kerogens in the Appalachian basin. We interpret these data to indicate a thermogenic origin from primary cracking of kerogen at thermal maturities equivalent to the peak oil-late oil window transition (equivalent vitrinite reflectance, %VRo, approximately 0.92 to 1.07%). Nonhydrocarbon gases in these samples include 0.47 to 0.57% N2 and traces of CO2. The δ15N2 values of these gases range from -12.7 to -13.9‰ and support the interpretation of gas generation from thermally mature organic matter.

Deeper Marcellus reservoirs in the southwest core region produce associated and nonassociated gases, with wetness ranging from 2.1 to 22%. Methane δ13C ranges from -37.12 to -44.8‰ and methane δD ranges from -165.8 to -188‰. Ethane and propane δ13C values range from -27.5 to -41.91‰ and -27.2 to -38.86‰, respectively. Natural gas plots reveal normal carbon isotope distributions, but the deepest and isotopically heavier samples show some deviation from predicted straight line trends suggesting some mixing and/or alteration of the gases. We interpret these gases as products of both primary cracking of kerogen and secondary cracking of residual oil at thermal maturities equivalent to the late oil-wet gas/condensate window transition (equivalent to %VRo of 1.2 to ≥1.3).

Marcellus gases produced from reservoirs in the northeast core region are dry and isotopically heavy. Free gases from productive wells in this region have methane δ13C ranging from -28.2 to -39.87‰ and methane δD ranging from -147.4 to -170.2‰. Headspace gases from well cuttings are sometimes heavier. Natural gas plots reveal carbon isotopic reversals with δ13C1 > δ13C2 > δ13C3. N2 ranges from 0.5 to 9.8%; BTU values vary with the N2 content. The δ15N2 composition ranges from -1.3 to -0.5‰. CO2 concentrations are low, but δ13CO2 is variable ranging from -22.2 to -5.5‰ implicating both organic and carbonate sources. We interpret these data to indicate a metagenetic origin from secondary cracking of pyrobitumen, residual oil, and wet gas at thermal maturities equivalent to %VRo > 2.0.

Marcellus gases recovered from unproductive wells southeast of the productive northeast core region are dry and isotopically heavy. Methane δ13C and δD range from -18.9 to -23.8‰ and -166 to -190‰, respectively. Methane δ13C is heavier than that reported for Marcellus kerogen anywhere in the basin. Ethane δ13C ranges from -21.5 to -30.0‰ and all samples exhibit reversals with δ13C1 > δ13C2. The δ13CO2 values range from -11.8 to -15.0‰ indicating an organic origin. We interpret these gases as originating mostly through cracking of residual wet gases and light oils at thermal maturities approaching %VRo ± 3.0. The unique chemistry of these gases might be diagnostic of commercial limits to the Marcellus play imposed by high thermal maturity in the deepest portions of the basin.