The use of hydrocarbon compositions in soil gas prospecting requires enough data to allow statistically valid and separate populations to be defined, so that a particular geochemical anomaly can be related to a geologic or geophysical objective or province. In basins having mixed production, prediction of a reservoir gas-to-oil ratio GOR is clearly impossible.
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Where seeps contain gases from more than one reservoir, their compositions may not match those of any of the underlying reservoirs. Mixing of a shallow oil and a deep gas will generally yield an oily but intermediate-type composition. Without some knowledge of the reservoir possibilities, this type of signature cannot be recognized. Nevertheless, the intermediate nature of the seep will indicate some liquid potential at depth.
Hydrocarbon Seal and Migration- Incorporated Research Institutions for Seismology
Thus, dry-gas basins can be distinguished from basins that have at least some liquid oil or condensate potential. As suggested by Bernard , the presence of fairly large ethane-propane-butane anomalies strongly suggests an oil-related source. Pixler found that the gases observed during drilling could distinguish the type of production associated with the hydrocarbon show during mud-logging and published the graph shown in Figure f.
Pixler's data were obtained by monitoring the C1-C5 hydrocarbons collected by steam-still reflux gas sampling during routine mud logging. Individual ratios of the C2-C5 light hydrocarbons with respect to methane provided discrete distributions that reflect the true natural variations of formation hydrocarbons from oil and gas deposits.
Ratios below approximately 2 or above indicated to Pixler that the deposits were non-commercial. The upper range for these ratios for dry-gas deposits has been enlarged by Verbanac and Dunia , who studied more than wells from 10 oil and gas fields. These ratios clearly aid in defining the transition between thermogenic and biogenic gases. Another empirical rule suggested by Pixler is that the slope of the lines defined by these ratios must increase to the right; if they do not, the reservoir will be water-wet and therefore non-productive. Verbanac and Dunia suggested that a negative slope connecting individual ratios may result from fractured reservoir zones of limited permeability.
Auger hole soil gas data for the surveys over the three basins described above are plotted on a Pixler-type diagram of reservoir gases in Fig. Direct comparison of these two independent data sets is very striking and proves the concept of migration of reservoired hydrocarbons to the surface. Thus, in a Pixler-type diagram, soil gas data, like reservoir data, generally plot as line segments of positive slope for the soil gases to represent a typical migrated seep gas. Exceptions to this order have been noted where surface source rocks were drilled, which thus far have yielded ratios with lighter gases depleted in relation to heavier gases.
According to Leythaeuser et al.
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Thus, compositional changes related to diffusion might be expected at or very near a boundary layer where the hydrocarbon gas concentration approaches zero. This behavior has been observed when comparing soil gas probe data measured at very shallow depths 0. The shallow probe data are always "oilier", indicating preferential loss of methane and implying diffusion from the 4 meters 13 feet level to the surface. If diffusion were the dominant migration mechanism, a chromatographic effect would be expected for gas that migrated through the Earth.
The fact that the compositions of the soil gas data from auger holes match the underlying reservoirs confirms that the major migration mechanism to the near-surface must be via faults and fractures, rather than by diffusion. The percent-methane compositions from the auger hole surveys conducted over the Sacramento and San Joaquin basins are plotted in Fig.
These data imply that a soil gas grid would have defined local differences regionally.
Furthermore these geochemical data are repeatable Table 5-X ; the percent-methane values on Fig. Compositional data have remained repeatable throughout our experience with soil gas surveys. Dissolved gas In offshore prospecting "sniffers" have been used to detect anomalous hydrocarbon concentrations in bottom waters. An extensive review of the literature was published by Philp and Crisp Some of the most significant results reported by Williams et al. Gulf Research and Development Company designed and operated several marine seep detectors which were employed aboard various research vessels, such as the RV Hollis Hedberg and its predecessor the RV Gulfrex.
These ships circumnavigated the globe and conducted extensive detailed surveying in areas such as the Gulf of Mexico Mousseau , The RV Hollis Hedberg system employed three separate water inlets which, whilst the ship was underway at normal seismic survey speeds, continuously supplied sample streams from the near surface, intermediate depths to meters feet and a deep towed sample inlet at a depth of nearly meters feet. Each sample stream is analyzed for seven hydrocarbon gases once every three minutes with a sensitivity that depends upon the hydrocarbon and, for example, is about 5 x liters of propane at STP per liter of seawater.
By using multiple depth inlets, surface contamination can be demonstrated to have no effect on seeps observed by the deep inlet. At sea "sniffer" geochemical data from a deep tow inlet were superimposed to scale on a seismic section to aid the explorationist in making real time evaluations of hydrocarbon potential of structurally significant areas. As for surface soil gases, a powerful confirmation of the validity of marine geochemical data can be shown by the very close agreement between the composition of component hydrocarbons in production gases and the composition of seep anomaly gases in the same areas.
Figure shows the well database used for this confirmation in the Gulf of Mexico Rice, For each of the 32 fields shown on this figure, the USGS has published the composition of gases produced from predominantly gas fields, oil fields and combined oil and gas fields or condensate fields. A crossplot of the compositions of gases from all field types is shown in Fig. The underlying color code on this figure was chosen to distinguish oil, oil-condensate, gas-condensate and gas production using the Rice well analysis data as a standard.. The log of the ratio of ethane to propane and butane is plotted against the log of the ratio of methane to ethane plus propane.
A distinctive compositional clustering of gas anomalies signifies different kinds of production: oil anomalies occur near the origin and become gassier as the points move up and to the right in Fig. A comparison of sniffer geochemical anomalies from the same part of the Gulf of Mexico is plotted in Fig.
As shown, the overall distribution is very similar to the well data. Figures d and e illustrate the contrast in composition of dissolved hydrocarbon anomalies from a gas area and an oil area in the Gulf of Mexico. This type of regional separation was found to be typical of surveys conducted throughout the world. The fact that production and surface anomaly gases correspond both onshore and offshore is significant.
It proves that the observational techniques are valid despite the great variation in these surface environments. Headspace gas A headspace sampling technique is commonly employed for the analysis of canned samples from drilling returns and from shallow sediments.
In this technique a controlled volume of sediment is placed in a can or jar filled with a measured volume of degassed brine. The can is sealed and a measured volume of brine is displaced with nitrogen to create a known volume headspace. The can is then allowed to come to equilibrium. The concentration of light gases can then be measured by syringe injection of a headspace sample into a gas chromatograph equipped with an Flame Ionization detector.
In order to maintain reproducibility it is important to measure all volumes accurately. In a typical operation using ml one pint cans, the procedure is to place ml of degassed salt water brine into the ml can and add sediment until the can is filled to the brim, giving ml of sediment and ml of brine. The can is sealed and then zero-grade nitrogen is injected through a prepared septum to displace ml of brine and leaving the can with a mixture of ml brine, ml sediment, and ml headspace. Experiments have shown that a fairly long time is required for the adsorbed sediment gases to completely equilibrate with the headspace.
This equilibrium time is shortened by heating and shaking the cans before analysis. A generally accepted procedure is to heat the cans for about 12 hours at 60oC to 70oC, followed by shaking in a paint mixer for five minutes. After heating and shaking, the cans are allowed to stand for at least five further minutes to ensure that dissolved gases return to the headspace.
One of the drawbacks to using this technique is the need to freeze the canned samples if they cannot be analyzed within one or two weeks of their collection. Failure to follow this procedure can create problems because of the generation of biogenic gas in the cans or the bacteria oxidation of the hydrocarbon gases to carbon dioxide. Hydrocarbon concentration values are reported in terms of ppm by volume in the nitrogen headspace or as ppm or ppb by weight, normalized to the weight of sediment. Gases concentrations reported by weight are not truly representative of the actual gas migrating from depth because some of the free gas has been allowed to escape during collection and sample preparation.
Furthermore, the sorbed gas is never completely extracted into the headspace, and may not always reflect the true gas content of the soil. The headspace sampling technique can yield useful results if sufficient numbers of samples can be collected to use statistical populations to suggest anomalous areas. One should always exercise caution, however, with respect to characterization of gas composition, since evaporation during the collection stage always occurs, resulting in the relative depletion of the lighter gases.
This suggests that it would be advantageous to employ a soil core disaggregation technique which would closely mirror the effect of auger hole drilling. A device developed at Citco and commonly used in both industry and academia for analyzing well cuttings appears suitable for accomplishing this objective Whelan, ; Hunt and Whelan, ; Whelan et al.
In fact, Richers has demonstrated successfully that in some instances, such as at Rose Hill, Virginia, and in the Western Overthrust Belt, the results obtained by this technique are in very good agreement with data from auger holes. The device used in this technique is a small stainless steel ball-mill containing two stainless steel or ceramic balls which crush and disaggregate the sample when the ball-mill is shaken Fig.
Condensation Mechanism of Hydrocarbon Field Formation
This approach concentrates the loosely bound adsorbed gases into the headspace of the ball-mill. Because of the equilibrium problem mentioned above under headspace techniques, this sampler was adapted by Whelan and Whelan et al. Basically, the technique is as follows. A small but constant volume of sediment, soil or cuttings is placed into the mixer cell along with two ceramic or stainless steel ball-bearings, and water is added to bring the remaining headspace to 10 cc.
The cell is then immersed into a hot-water bath at 90oC for three minutes.