Page 188 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 165

TABLE 5-VIII

Composition range of soil-gas hydrocarbons over different reservoir types

CI/C n CI/C 2 (C3/C!) x 1000
Dry gas 95-100 20-100 2-20
Gas condensate or oil and gas 75-95 10-20 20-60
Oil 5-50 4-10 60-500

TABLE 5-IX

Average composition ratios of soil-gas hydrocarbons over different reservoirs

Reservoir type Location Date C1/C n CI/C 2 (C3/CI) x 1000
Dry gas Sacramento Basin 1972 95 55 6
1974 95 49 8
1975 94 55 I 1
Oil and gas San Joaquin Basin 1972 82 8 46
1974 84 7 61
1975 82 8 56
Gas condensate Southwest Texas 1975 89 12 33
1976 90 11 30
Western Overthrust 1978 88 12 30
Oil Uvalde, Texas 1975 77 5 77
Permian Basin 1976 75 5 64
Utah Overthrust, Pineview 1976 77 5 83
Appalachians, Rosehiil ! 978 73 4 141
Uinta Basin, Duchesne 1976 68 4 171

The auger hole technique yields excellent compositional information, even though
the magnitudes are influenced slightly by the mechanical disaggregation associated with
the drilling process. Compositional results for auger holes are sufficiently important to
warrant further discussion here. An empirically-determined range of soil-gas data is
shown in Table 5-VIII and a small selection of auger hole survey results is shown in
Table 5-IX. The geochemical distinction between gas-type basins and oil-type basins
was first noted from surveys in the Sacramento and San Joaquin Basins in California.
Initial compositional data were gathered in these two basins in three separate years with
excellent repeatability (Table 5-IX). Additional surveys conducted in southwest Texas
supported the differences noted in California. Final confirmation on the oil versus gas
predictions was obtained when numerous surveys were carried out in all three types of
productive areas: gas, gas-condensate and oil. Soil-gas data from the Sacramento dry-
gas, Alberta gas-condensate, and Permian Basin oil areas were used to establish
statistically-valid populations based on histograms that demonstrate a close association
with reservoir gases and gas shows in drilling fluids.