CH17 Exploration for Hydrocarbons – General


GamX CH17 1952 Airborne radioactivity survey by Lundgerg over Mungerville Oilfield, Texas

 

For decades, borehole gamma ray logs have aided stratigraphic interpretation within sedimentary basins1, identifying horizontal offsets along faults, and mapping effects of geochemical chimneys2 related to both mineral and hydrocarbon systems.  There are published examples describing relationships between airborne and ground radioactivity patterns and proven hydrocarbon accumulations3. Because the three radioactive elements measured behave differently geochemically (K, U and Th are major, mobile trace and immobile trace elements, respectively) they serve well as indicators of a number of geochemical processes including those related to accumulation, transport and degradation of hydrocarbons.

Many early examples show total radioactivity lows directly overlying the oil field, as shown in Fig 1 from the 2000 meter deep Mungerville oilfield, Texas3, and Fig 2 from the Redwater oilfield, Alberta3.

Modern, calibrated instrumentation improves spatial and anomaly resolution relative to historical surveys, but must be done carefully. Post-survey improvements include measured or empirically derived correction for variations in surficial materials (soil type, moisture, vegetation) 3 or use of radioactive element ratios4 (normalization) to minimize these effects.GamX CH17 1951 Airborne radioactivity survey by Lundgerg over Redwater Oilfield, Alberta

 

 

 

 

 

GamX CH17 Oil and Gas Gechem Exploration Model (Gamma Rays) Sikka and Shives 2002

 

 

 

 

 

 

 

 

 

 

 

 

How does it work?

The conceptual cartoon in Fig 3 illustrates some important principles. Structures play a critical role in the presence and shape of observed surface anomalies related to subsurface hydrocarbon pools. Fractures provide channelways for gases, hydrocarbons (oil, gas), water and salts. Geophysical and geochemical patterns, including radiometric anomalies, are formed at the earth’s surface and throughout the stratigraphic column, in response to complex interactions between various components. Anomalies may appear as enrichment or depletion patterns in a wide range of elements, salts, gases, and hydrocarbons themselves (listed at top of Fig 3) forming apical or annular trends, modified by local conditions. Interpretation is often challenging.

 

Migration occurs by effusion, diffusion to a limited extent, and solution movement, driven at variable but sometimes very fast rates, by pressure and temperature gradients related to diastrophic forces (earthtides, earthquake, seismicity, typhoons, tornadoes, hurricanes, meteoritic impact). Where liquid hydrocarbons or elements are encapsulated and carried on the inner walls of microbubbles of earthgases, migration rates are enhanced.

 

Various elements form compounds with halides and organometallic complexes and migrate in solution or as colloids towards earth surface. Microbiological reactions cause hydrocarbon degradation and mineral precipitation. Electrochemical gradients influence movement of anions and cations.

 

To improve success, modern hydrocarbon exploration techniques should (and most do) incorporate an integrated, multielement, multimedia, multisensor approach. Airborne gamma ray spectrometry contributes high-density, consistent, quantitative geochemical measurements over large areas and in many cases can be used to augment traditional, predominantly geophysical, land-based hydrocarbon exploration.

 

 

 

 

 

 

1. Ehrenberg, S.N. and Svånå, T.A., 2001. Use of spectral gamma-ray signature to interpret stratigraphic surfaces in carbonate strata: An example from the Finnmark carbonate platform (Carboniferous-Permian), Barents Sea. AAPG Bulletin, V. 85, No. 2 (February 2001), P. 295-308.

2. Shives, R.B.K., Wasyluik, K., and Zaluski, G., 2000, Detection of K-enrichment (illite) chimneys using ground gamma ray spectrometry, McArthur River area, northern Saskatchewan, in Summary of Investigations 2000, Volume 2: Saskatchewan Geological Survey, Saskatchewan Energy Mines, Miscellaneous Report 2000-4.2, p. 160-169.

3. Sikka, D. B., and R. B. K. Shives, 2002, Radiometric surveys of the Redwater oil field, Alberta: Early surface exploration case histories suggest mechanisms for the development of hydrocarbon-related geochemical anomalies, in Surface exploration case histories: Applications of geochemistry, magnetics, and remote sensing, D. Schumacher and L. A. LeSchack, eds., AAPG Studies in Geology No. 48 and SEG Geophysical References Series No. 11, p. 243–297. (and many references therein).

4. Saunders, D.F., Burson, K.R., Branch, J.F. and Thompson, C.K., 1993. Relation of thorium-normalized surface and aerial radiometric data to subsurface petroleum accumulations Geophysics, October 1993, v. 58, p. 1417-1427, published online October 1, 1993.