Gas anomalies

According to Kartsev et. al. (1959), Sokolov (1970), Horvitz (1986) and Fei and Ruan (1991), gas microseepage associated with petroleum has three characteristic styles of surface anomaly (1) apical (2) aimular and (3) linear. One or any combination of these surface expressions may occur over a given oil or gas field. 

An apical anomaly comprises either a continuous zone of elevated gas concentrations or an area with erratic elevated gas concentrations directly over the oil or gas field. An annular or halo anomaly has the form of a doughnut of continuous or discontinuous high gas concentrations surrounding a central zone of lower or background values, the latter overlying the surface projection of the oil or gas field. In a linear or belt anomaly, high gas concentrations are found continuously or intermittently along a line or confined in a belt, usually associated with faults, fracture zones, or matured source beds. 

It is vital, but not necessarily easy, to recognise whether an anomaly is apical, annular or linear, because the interpretation placed upon each of them leads to quite different courses of action. The model used to explain an apical anomaly favours drilling the anomaly peak, whilst the model for an annular anomaly favours the area within the ring of high values. Follow-up of linear anomalies needs to take into account structural or lithological information. 

All types of anomalies can be present over a given oil or gas field depending on the relative depth of the field, its caprock lithology and structural control, the gas species in the microseepage, their modes of occurrence, near-surface lithologies and soil types, climate and even the time of year. This wide range of factors renders gas anomaly interpretation difficult. 

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Experimental measurements have determined the great speed of development of gas anomalies over man-made underground gas reservoirs. 

The time required for diffusion to occur can sometimes be restrictive. Indeed the time required not only often exceeds the age of the hydrocarbon accumulation but also quite often exceeds the age of the host rock. If this were the dominant process for migration, then the appearance of soil-gas anomalies in the near subsurface would indicate only very shallow accumulations. If a non-steady state exists, where the hydrocarbon signal observed represents only 0.001 times the steady-state signal, then diffusion times could be reduced by a factor of 25 compared to that of the steady-state model. Table 5-V... 

Examination and comparison of these ratio plots and dot maps for each of the two fields indicate that the more anomalous magnitude sites (large dots) match the composition of the known underlying reservoirs. The areal groupings and Pixler ratio plots of these specific components with their appropriate reservoirs lends confidence to the deduction that these soil-gas anomalies are the result of migration of petrogenic hydrocarbons from the underlying sedimentary sources. 

Furthermore, the existence of carbonates of different origins may complicate the data. Soils derived from matured source rocks, particularly carbonates, always contain very high concentrations of hydrocarbons with compositions of matured oily sources. If the parent rock of the soil was transported by a surface agency such as water, wind or ice, gas anomalies reflect the source area of the sediments rather than the oil or gas potential of the underlying strata. The western part of the Bayinhaote Basin of west China is characterised by Archaean metamorphic rocks covered by thousands of metres of Cenozoic red clastic sediments with no oil or gas potential. Acid treatment of soils revealed very high C1-C5 concentrations (up to 600 pl/kg of C and 70 pl/kg C2). However, it was found that the anomalous soils were derived from carbonate fragments... 

Subsequently the scientific research well was drilled as planned. When it penetrated Lower Ordovician carbonates it produced 16.3 x lO" m per day of natural gas. The scientific research well thus became the discovery well in this region. Since the well was drilled directly on the surface hydrocarbon gas anomaly, the soil gas survey traverse was thought to cross an apical or linear anomaly. Further work, however, was to reveal that the gas field has an annular anomaly. 

Seismic data have demonstrated three sub-surface southwest-northeast trending faults in the area. The annular gas anomaly partially overlies an uplifted block, a stmcturally-favourable setting for an oil or gas trap. The linear anomaly follows the trend of another of the subsurface faults. 

Surface microlayer samples have shown sulphur-gas anomaly patterns that are more closely related to known mineralisation than sulphur-gas patterns from deeper soil samples (Lovell, 1979). At Johnson Camp, Arizona, mineralisation is best expressed by sulphur compounds from the surface microlayer, while sulphur compounds from 0-5 cm reflect the same mineralisation to a lesser extent, and sulphur compounds from 30-40 cm show the least expression of the mineralisation. A comparison of concentrations of COS, CS2, and SO2 degassed from soils collected at depths of 0.5-2 cm and 30-40 cm at the same sites near Casa Grande, Arizona, showed almost identical patterns of sulphur-gas concentrations over a 150 km (58 square miles) area. Average concentrations of COS, CS2 and SO2 were slightly higher in the shallow samples than in the deeper samples (Hinkle, unpublished data, 1981). These data indicate that, at least in arid areas, surficial soil and microlayer samples are superior to deeper augered samples. 

Sulphur-gas geochemistry is not, ideally, a "stand alone" technique. It should be used in conjunction with other geochemical, geophysical and geologic studies to understand the processes that cause and influence sulphur-gas anomalies around and over mineral deposits. When the processes become more fully understood, predictive models may be developed for the occurrence of volatile sulphur compounds over different types of mineralisation and over buried mineralisation. Future work will then expand the use of sulphur-gas anomalies from studies of specific mineral deposits to regional and reconnaissance studies. 

In contrast to the very restricted size of the soil-gas anomaly at Jabiluka, Zonghua and Yangfen (1981) describe broad anomalies associated with a buried skam Cu deposit near Shanghai, China. The ore body is confined to the contact zone of granodiorite and Palaeozoic and Mesozoic sedimentary rocks. The major sulphide minerals in the ore body are chalcopyrite, pyrite and molybdenite. Part of the deposit has been oxidised to limonite. The area is covered by 140-180 m of alluvium. A soil-gas survey of the area was complemented by a multielement study of soils including the determination of Hg. Significant anomalies of Hg in soil and soil gas occurred above the buried sulphide... 

In four case histories from the CSIRO study, soil gas was collected in addition to soil. In the example of the Currawang deposit, a soil-gas Hg anomaly of about twice background occurs in association with a soil anomaly of about 100 times background (Fig. 12-34). The detection limit of Hg in soil gas was 0.07 ng/L and the background levels were in the range 0.1-0.2 ng/L. In two other examples, the Woodcutters Zn-Pb deposit (Roberts, 1973) and the Ranger IV U deposit, both in the Northern Territory, Hg soil anomalies, but not soil-gas anomalies, were detected. In the fourth example, the Jabiluka U-Au deposit, a soil-gas anomaly was present but not a soil anomaly (Fig. 12-22). 

McNerney and Buseck (1973) analysed Hg in soil and soil gas over the Vekol Ag-Pb mine in Arizona. The deposit was defined by both a soil-gas anomaly of 6 ng/L over a background of about 1 ng/L and by a soil anomaly of 300 ppb over a background of about 30 ppb. 

Gole, M.J. and Butt, C.R.M., 1985. Biogenic-thermogenic near-surface gas anomaly over Gingin and Bootine gas fields. Western Australia. Bull. Amer.Assoc. Pet. Geol., 69 2110-2119. 

Bogard DD, Rowe MW, Manuel OK, Kuroda PK (1965) Noble gas anomalies in the mineral thucholite. J Geophys Res 70 703-708... 

Top Z, Martin S, and Becker P (1988) A laboratory study of dissolved noble gas anomaly due to ice formation. Geophysical Research Letters 15 796-799. 

The Telecommunications responses were low and out of character in the survey findings and this may be a function of the fact that the survey covered both older more established industry players like Advent, Cannon, Orange and Motorola and also some of the new and emerging start-up companies in this sector. For some of these firms there may be lower strategic risks than for others and this may be the explanation of the anomaly in the findings expected. The Oil Gas anomaly is relatively more easily explained and appears to be a function of the fact that many of the respondent companies were located in the upstream or exploration and production side of the Oil Gas sector. 

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