Detailed soil gas geochemical surveys were conducted for calibration purposes over two fields, Filo Morado and Loma de La Lata in the Neuquen Basin in Argentina (Figure 1) during November, 1989 as part of a larger regional exploration program. These two fields were chosen for this calibration study because of their differences in both reservoir composition and entrapment mechanisms.
Filo Morado is an anticlinal oil field producing from the Agrio-Huitrin Formations at a depth of 3,000 meters (9,843 feet, Figure 2), while Loma de La Lata consists of two stratigraphically trapped reservoirs formed on a homocline which dips to the northeast (Figures 3 and 4). At the Loma de La Lata field, the shallowest oil pool produces from the Quintuco Formation at 2,000 meters (6,562 feet) and is partially underlain by a separate gas to gas condensate reservoir producing from the Sierras Blancas formation at 3,000 meters (9,843 feet). Both the updip facies change, and downdip water-hydrocarbon contact of this lower reservoir are plotted along with the productive gas condensate wells (Figure 4). The three separate reservoirs from these two fields provide two different oil reservoirs and one deeper gas to gas condensate reservoir for compositional calibration of the soil gas geochemical surveys.
The geochemical data consists of 239 four foot deep soil gas samples collected on 500 to 1,000 meter grids placed directly over these two fields, with 95 sites collected over Filo Morado and 144 sites collected over Loma de La Lata. The free soil gases were analyzed for light hydrocarbons; methane, ethane, ethylene, propane, propylene, iso-butane, and normal butane by flame ionization gas chromatography.
In order to illustrate the distribution and compositions of the light hydrocarbon seepage, color compositional dot maps which combine both the light hydrocarbon magnitudes and compositional information into a single map have been generated as shown in Figure 5 for Filo Morado and in Figure 6 for Loma de La Lata. Each dot is color coded to reflect the composition of soil gases as being indicative of oil (green dot), gas (red dot) or intermediate (yellow dot) based on the C1/C2 ratio. The dots vary in size according to the ethane magnitude, including even those localities with only background magnitudes. The color compositional subdivisions used are derived from the published literature (Nikonov, 1971, and Jones & Drozd, 1983), and are the same as those shown in Figure 7. The color of each of these clustered anomalies suggests the oil versus gas potential of the anomaly according to these empirical divisions alone.
Ratios of methane/ethane, methane/propane and methane/total butanes for all soil gas sites are plotted on Figure 8A and 8B for all sites which exceed the median (the upper 50%) of the data) in order to provide a visual illustration of the compositional differences in the larger magnitude sites. The bimodal nature of the Loma de La Lata soil gas data is clearly shown in Figure 8A by the red (gas) and green (oil) populations. Figure 8B for Filo Morado stands in stark contrast with its unimodal oily (green) population and by the lack of gas type anomalies.
A single oil source is predicted at Filo Morado, in agreement with the known oil field. Much gassier soil gas data is noted over the Loma de La Lata Field where there exists an oil and gas field underlain by a deeper gas condensate reservoir. In addition, a very striking change to fairly large magnitude oil type compositional anomalies occurs directly over the northwestern portion of the Loma de La Lata survey data where the Quintuco oil reservoir is the only known producing horizon. This change in composition from oil to gas condensate type signatures over the Loma de La Lata Field occurs across a permeability pinchout at depth which controls the updip limits of the deeper gas condensate reservoirs. Thus the geochemical soil gas data exhibits clearly defined compositional subpopulations which match the composition of the underlying reservoirs and change in direct response to the major structural and/or stratigraphic features which control the location of these subsurface reservoirs. Predictions of oil versus gas from these soil gas data are in excellent agreement with published soil gas and reservoir data, Jones and Drozd (1983) and Nikonov (1971).