Natural Gas Compositions from Large Tight-Gas-Sand Fields

in the Rocky Mountains: A Clue to How these Reservoirs Fill

Nicholas B. Harris¹, Ting-Wei Ko¹, R. Paul Philp²,

Trevor Stroker¹, and Andrew Govert¹

¹Department of Geology, and Geological Engineering, Colorado School of Mines,

1516 Illinois St., Golden, Colorado 80401

²School of Geology and Geophysics, University of Oklahoma,

100 Boyd St., Norman, Oklahoma 73069

Extended ABSTRACT

The tight-gas-sand reservoirs of the Rocky Mountains comprise a significant natural gas resource for the United States. These fields include: Pinedale Field (>44 TCF) and Jonah Field (14 TCF) in the greater Green River Basin, Wyoming; the Mamm Creek–Rulison–Parachute–Grand Valley cluster of fields (>8 TCF) in the Piceance Basin, Colorado; and Greater Natural Buttes (5 TCF) in the Uinta Basin, Utah (Fig. 1). Many of these reservoirs, including all of those in our study, are sequences of fluvial sandstones, isolated in finer-grained overbank deposits.

The origin of low porosity and permeability in these sandstone reservoirs is a function of their initial composition. Our studies of two tight-gas-sand reservoirs, Pinedale Field and Rulison Field, show that the sandstones are rich in lithic grains, including volcanic rock fragments and chert, and pseudo-matrix. As a consequence, these rocks display a high degree of compaction. They also locally show large amounts of quartz cement, which we attribute to feldspar dissolution and the conversion of smectite to illite. Mixed layer illite/smectite and fibrous illite are commonly present in pore space. Secondary porosity from dissolution of feldspar and lithic grains is a major component of porosity. At Rulison Field, isolated reservoir intervals have somewhat better reservoir quality due to dissolution of carbonate cement or porosity preservation by chlorite grain coats, but these appear to be localized effects. The chlorite grains coats may be associated with short-lived transgressive episodes.

While the degree to which the tight-gas-sand reservoirs are analogous to more conventional reservoirs (for example, the debate over “basin-centered gas”) is controversial, it is clear that these reservoirs differ in some important ways from conventional reservoirs:

(1) Low permeability – generally less than 1 mD and commonly less than 0.1 mD (Shanley et al., 2004; Cluff and Cluff, 2004).

(2) Lack of a normal seal, where a lower permeability rock overlies a higher permeability reservoir rock. In fact, in at least some cases, the reservoir is overlain by a higher permeability rock.

(3) Abnormal pressure, either above or below hydrostatic pressure.

Shanley et al. (2004) discuss at length evidence for extremely small pore throat sizes in these reservoirs and the impact this has on water and gas saturation and relative permeability to gas.

These relationships suggest that gas fills tight-gas-sand reservoirs through a mechanism substantially different from conventional reservoirs. Three mechanisms appear to be possible (Fig. 2): (1) gas forces its way upward by fracturing through the intermediate seals (Cumella and Scheeval, 2008); (2) gas diffuses upward through a series of low permeability, intermediate seals; and (3) gas migrates vertically through permeability pathways such as faults or fracture systems and then diffuses laterally.

We are testing models for gas migration into these reservoirs with an extensive study of gas compositions. The premise of our study is that each of the three models described above should leave a distinctive record in the gas composition. Consider the second model, which is vertical diffusion through a series of semi-permeable seals. This process should lead to considerable fractionation of gas species; this is, in essence, chromatography on a large scale. It should be expressed in terms of ¹³C and deuterium-hydrogen (D-H) isotopes, with lighter compositions in shallower horizons as ¹²C and H diffuse more rapidly through seals than ¹³C and D and possibly small atomic-radii gases such as He in preference to larger molecules such as the hydrocarbon gases and CO₂. Alternatively, if gas fills the reservoirs through successive natural fractures and rapid filling of successive compartments, the fractionation should be much less pronounced. Finally, if gas is channeled along faults and fractures, then migrates laterally, we should see fractionation laterally away from the channels and probably substantial compositional differences between fault blocks.

We are using the full range of natural gas composition to develop models for how these reservoirs fill and for the extent of lateral communication and compartmentalization, including: (1) bulk hydrocarbon gases (methane, ethane, etc., and CO₂); (2) the ¹³C and D-H isotopic composition of compound-specific gases, i.e., of methane, ethane propane, etc.; (3) the trace gas composition, including N and He; and (4) the radiogenic noble gases such as Ar and Ne. Bulk hydrocarbon gas composition has been studied in the tight-gas-sand reservoirs (although published data are limited) as have compound-specific isotopes to a very limited degree. The database will be developed in three fields: Jonah; Rulison–Parachute–Grand Valley; and Greater Natural Buttes. The extensive analytical database will be combined with hydrous pyrolysis experiments on likely source rocks to determine the composition of gases entering the reservoir and with modeling of gas compositions with the migration simulator MPath.

REFERENCES cited

Cluff, S. G., and R. M. Cluff, 2004, Petrophysics of the Lance Sandstone reservoirs in Jonah Field, Sublette County, Wyoming, in J. W. Robinson and K. W. Shanley, eds., Jonah Field; case study of a tight-gas fluvial reservoir: American Association of Petroleum Geologists Studies in Geology 52, Tulsa, Oklahoma, p. 215-241.

Cumella, S., and J. Scheevel, 2008, The influence of stratigraphy and rock mechanics on Mesaverde gas distribution, Piceance Basin, Colorado, in S. P. Cumella, K. W. Shanley, and W. K. Camp, eds., Understanding, exploring, and developing tight-gas sands—2005 Vail Hedberg Conference: American Association of Petroleum Geologists Hedberg Series 3, Tulsa, Oklahoma p. 137-155.

Shanley, K. W., R. M. Cluff, and J. W. Robinson, 2004, Factors controlling prolific gas production from low-permeability sandstone reservoirs: Implications for resource assessment, prospect development, and risk analysis: American Association of Petroleum Geologists Bulletin, v. 88, p. 1083-1121.

Harris, N. B., T.-W. Ko, R. P. Philp ,T. Stroker, and A. Govert, 2009, Natural gas compositions from large tight-gas-sand fields in the Rocky Mountains: A clue to how these reservoirs fill: Gulf Coast Association of Geological Societies Transactions, v. 59, p. 347-350.