Emanation seismotomography in oil exploration

V. I. DIVAKOV, L. S. BORISENKO, V. V. NEVEDROV, S. T. YAKIMENKO
Peoples' Friendship University of Russia

The Scientific Group of Peoples' Friendship University of Russia (hereinafter Scientific Group) has developed and successfully applying "Emanation Seismotomography" technique which is an advanced combination of soil gas geochemistry, passive seismic and electromagnetic methods. One of the most important features of the method is an original software package for the field data processing. Emanation Seismotomography has been used by Scientific Group since 1989 in more than 114 projects such as "Caspian Pipeline Consortium", "Sakhalin-2", "Blue Stream" and others in Russia, Kazakhstan, Belourussia, USA, Bolivia.

The Scientific Group is consistent with an "Integrated Evaluation Approach" concept, meaning that any site evaluation should be based on several independent expeitises. Emanation Seismotomography is ensuring services that reduce exploration risks for oil production & exploration, site engineering assessment- safety & environmental studies. Such variety of expertise provides us with opportunities of combining soil gas survey with other methods while working on different projects worldwide.

Emanation Seismotomography is based on the analysis of soil gases emission activity triggered by dynamic interaction of natural elastic and electromagnetic waves with existing geological structures (i.e. faults, folds, oil & gas reservoirs). Such interaction results in origin of local stress fields which, depending on their type, vary in shape and intensity. These fields can be traced on the earth' surface by anomalous concentrations of radioactive and other gases in the soil air as the evidence of tension effect created by waves generating structures. Therefore Emanation Seismotomography can be considered as a passive seismic technique dealing exclusively with natural response reaction of geological medium in contrast to standard methods which are using artificially initiated seismic or electromagnetic signals.

It is understood that radioactive, hydrocarbon and some other gases1 anomalies in the soil air are subject to their migration and accumulation behavior. Short lifetime of radioactive gases, in contrast to others volatile components, proves that they are being mainly released in situ from radioactive elements by local stress caused by elastic waves dynamic proliferation. The source area and local seismic response effect can be evaluated by measuring the amount and distribution of these gases in the soil air. Furthermore, it is possible to convert the data received into digital images of geological structures, sponsored such waves and gases distribution.

The fieldwork is being done in a classic way on a grid of survey lines with soil gas samples to be taken from holes of 0.5m deep and about 4-5 cm in diameter, made in the soil by a crow-bar. The line length is to be twice as long of the desired depth of penetration. The distance between the lines is chosen depending on the scale of the survey (100 m-2500 m). The sample spacing is chosen depending on the objectives of the survey and usually varies from 25m to 50m.

The Scientific Group uses new generation of statistical processing technique and continually upgrades its instrumentation. Field emanometers analyze the radioactive gases values (accuracy 3%). Gas analyzers measure carbonic acid and other gases, methane and total value of methane homologies (accuracy 0.0001%). Portable gas chromatographs (accuracy 0.00001%) are applied over the suggested oil pools for homologies series (methane-hexane) measurement.

Multivariate statistical methods and harmonic analysis are used for calculation of natural elastic waves length, their conditional amplitudes and energy coefficients according to the sophisticated processing. Waves1 origin depths, length and amplitudes-dependent model for each site is determined and used in conjunction with available geological information. The data obtained are the basis for the compilation of seismotomographic sections, geochemical and structural slice maps at any desired depth showing rocks1 tomographic (petrophysical) boundaries and tectonic dislocations of various types (compression, extension, and shear). Identification of small stratigraphic horizons is difficult due to the technical and processing limitations.

Structural maps of any proposed surfaces can be built using only soil gas distribution data and a few marker points.

The Scientific Group has been using Emanation Seismotomography for frontier evaluation of hydrocarbon potential, oil prospecting and exploration for more than 15 years. Study of soil gases evidences of hydrocarbons accumulation can be hardly underestimated in the site evaluation. Volatile components of oil pool can migrate for significant distances keeping information about their source and/or accumulation area, its dimensions and structure. The latest statistics shows that 93%-95% of wells drilled beyond of geochemical anomalies are dry, 89%-92% of wells drilled in positive anomalies discovered petroleum (Potter et a.l. 1996; W.L. Gore & Associates, 2003).

The high level of Emanation Seismotomography prognosis accuracy is based on integrated site evaluation covering as much evidences of hydrocarbons migration and accumulation as possible including:
- geochemical component, allowing to establish which sites have petroleum presence, type and dimensions of anomalies;
- passive seismic component- helping to determine 3D structure of oil traps;
- electromagnetic component, allowing to evaluate the reservoir's dimensions and intensity of rocks1 alteration which is tightly connected with petroleum presence.

High concentrations of volatile hydrocarbons1 components in the soil gas witness the process of active migration towards the earth's surface from existing petroleum traps. It is supposed that subvertical upward gases microseepage occurs in most cases from the whole surface of oil table with limited lateral shift. Such mechanism ensures the appearance of oil pool's hydrocarbons at the surface. A relative match between oil reservoir structure and surface hydrocarbon distribution is often observed. Reservoir's geochemical signature expression on the surface depends mainly on the intensity of gas emanations and sampling conditions irrespectively of the depth and permeability of overlaying rocks. Successful results were received for oil reservoirs, situated up to 4 500 meters deep.

Surface distribution of volatile components from the oil pool under favorable conditions is dominated by concentric zoning subject to internal oil reservoir structure. Circular multicomponent anomalies of radioactive and hydrocarbon gases represent strong evidences of petroleum charge. Preliminary evaluation of pay zones and estimated oil reserves within validated structures can be effected under favorable coincidence of the following elements:
- presence of separate oil pool with relatively high content of dissolved gas, ensuring appreciable concentrations of hydrocarbon compounds in soil air;
- surface expression of geochemical concentric zones, reflecting the internal structure of oil pool;
- equal sampling conditions within investigated area.

The final results of Emanation Seismotomography are presented by the set of seismotomographic sections and models, distribution maps of methane, ethane, propane, butane, pentane, hexane, radioactive and other gases, structural slice maps at any desired depths showing delineated structural and lithological oil traps, hydrocarbon perspective maps based on geochemical, structural and electromagnetic evidences of oil migration and accumulation.

The Scientific Group is applying Emanation Seismotomography at oil fields of Kazakhstan, Russia, the USA and Bolivia for the solution of the following targets:
1) frontier evaluation of hydrocarbon potential of unexplored areas for precise focusing of further seismic efforts.
2) delineation of structural and lythological petroleum haps, their evaluation based on geochemical and electromagnetic evidences of hydrocarbons presence.
3) identification of oil pools and hydrocarbon potential estimation.
4) characteristics of oil reservoirs internal structure and detection of highly permeable zones with maximum oil and gas flow (special program for deviated wells).
5) location of secondary recovery areas within exhausted oil fields.

There is also an emergency support team. Sometimes one has to make vital decision where to locate new well at a problematic site. This happens quite often in practice due to the lack of information required. In such case several seismothomographic profiles earned out by our emergency support team can provide you with additional independent geochemical, passive seismic and electromagnetic expertises. This team for express investigation of problematic sites can minimize the uncertainty of decisions to be taken urgently for further investigation of the site, evaluation of seismic leads and stratigraphic traps already surveyed by seismic. In such cases precisely defined objectives of the survey are crucial for obtaining the information required due to timing and sampling limits. Usually, the complete set of field survey, data interpretation and final reporting takes one-three weeks depending on the site size.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004

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