Gamma ray log

If a sand line (0% shale) and a shale line (100% shale) are defined on the gamma ray log, a cut-off limit of 50% shale can be used to differentiate the reservoir from non-reservoir intervals. This type of cut-off is often used in preliminary log evaluations and is based on the assumption that reservoir permeability is destroyed once a rock contains more than 50% shale. 

Total Gamma Rays. Total gamma ray logs have been run on electric wireline since 1940. The sondes are rather small in diameter (1.5 to 4 in. or 37 to 100 mm). 

In MWD, the recording speed is the rate of penetration which rarely exceeds 120 to 150 ft/hr or 2 to 2.5 ft/min, two orders of magnitude less than the logging speed. Counters can be made shorter and time constant longer (up to 30 s or more). This results in a better accuracy and a better bed definition. Figure 4-269 shows an example of comparison between an MWD gamma ray log and the wireline log ran later. 

Radioactive events occur in the measured depth interval (8,100-8,200 ft) with no displacement of the low/high side gamma ray logs. The radioactive events must be perpendicular to the gamma detector and could be indications of vertical natural fractures in the formation. 

Figure 4-269. Example of good similarity displayed between the MWD gamma ray log and the wireline log.

Spectral Gamma Ray Log. This log makes use of a very efficient tool that records the individual response to the different radioactive minerals. These minerals include potassium-40 and the elements in the uranium family as well as those in the thorium family. The GR spectrum emitted by each element is made up of easily identifiable lines. As the result of the Compton effect, the counter records a continuous spectrum. The presence of potassium, uranium and thorium can be quantitatively evaluated only with the help of a computer that calculates in real time the amounts present. The counter consists of a crystal optically coupled to a photomultiplier. The radiation level is measured in several energy windows. 

Figure 4-294 shows a set of resistivity logs run in a sand-shale sequence of the Gulf Coast. We have one wireline dual induction log, one MWD resistivity log, a wiper-MWD resistivity log and one gamma ray log. 

Log Samples. Figure 4-300 shows samples of gamma ray and spectral gamma ray logs. The boundaries of the clean (not shaly) zone can be seen very clearly ... 

Gamma-ray log This is a log of the natural formation radioactivity level. Gamma radiation is measured along the borehole to determine the rock and sediment formations... 

Caliper and gamma ray logs were run to detect caving and borehole irregularities and to correlate the oil shale formations. Airflow tests were made to measure initial communication between injection well 6 and the remaining wells in the pattern. 

Holmes M. A. (1988) Evidence for continuous and discontinuous alteration in DSDP hole 418A basalts and its significance to natural gamma-ray log readings. In Proceedings of Ocean Drilling Project Scientific Results (eds. M. H. Salisbury and J. H. Scott, et al.), vol. 102, pp. 135-149. 

Fig. 3. Example of the different depositional environments and progradational and retrogradational cycles of the Tirrawarra Sandstone in the Moorari and Fly Lake Fields. The gamma ray log trace is derived from the Moorari 9 well.

Logging of exploration drill-holes is itself an extremely useful exploration technique.Borehole logging should include, at a minimum, systematic lithologic and alteration logging, gamma-ray logging and spontaneous potential-resistivity logging. 

Referring to the occurrence and mechanism of thorium, uranium and potassium presence in clay minerals and shale. Rider (1996) stated To summarize as shale indicators, thorium may be used in most cases, potassium may be used in many cases, but uranium should not be used at all. This obviously has implications for the simple gamma ray log it is not necessarily a good shale indicator". 

Gegenhuber (2011a,b) developed an improved method to determine heat production from gamma-ray logs implementing mean ratios of K, U and Th content. 

Doering, M.A., Smith, D.P., 1974. Locating extraneous water sources with the gamma ray log. In SPE Fall Meeting. Paper SPE 5116. 

King, G.E., 2010. Thirty Years of Gas Shale Fracturing What Have We Learned SPE 133456. King, R.L., Bradley, R.W., 1977. Gamma ray log finds bypassed oil zones in six Texas oil fields. Oil Gas J. 4, 92-99. April. 

Kopf, M., Carl, H., Giickelhom, M.-M., 1981. The determination of a continuous susceptibility depth section of clastic rocks by means of the gamma-ray log and a bed table. In Trans. 26-th Geophysical Symposium, Leipzig. 

Rowan, E.L., Hayba, D.O., Nelson, P.H., Bums, W.M., Houseknecht, D.W., 2003. Sandstone and Shale compaction curves derived from sonic and gamma ray logs in offshore wells. North Slope, Alaska—parameters for basin modeling. U.S. Geological Survey, Open-Eile Report 03-329. 

Fig. 16. [Reproduced in colour in Plate 26 on page 440.] Stratal slice of the seismic facies model 8 ms above the base of the Grane reservoir. The dark blue and light blue facies represent flat continuous seismic facies and are caiibrated as sands. The yellow and red facies represent discontinuous and dipping seismic facies and are calibrated as shale prone deformation featmes as indicated by the Gamma Ray log along a horizontal well.

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