Lithology Log in well logging techniques
- 1. Lithology Logs Two lithology logs are commonly used in formation evaluation, both are recordings of naturally occurring phenomena in the formation. 1. Spontaneous Potential (SP) log 2. Gamma Ray (GR) log. 1. Spontaneous Potential Log (SP) The SP curve records the electrical potential produced by the interaction of formation water, conductive drilling mud, and certain ion selective rocks such as shale. • It is a recording versus depth of the difference between the electrical potential of a moveable electrode in the borehole and the electrical potential of a fixed surface electrode. • Opposite shales, the SP curve usually defines a more or less straight line on the log, called the shale baseline.
- 2. • Opposite permeable formations, the curve shows deflections from the shale baseline. • In thick beds, these deflections tend to reach an essentially constant deflection defining a sand line. • The deflection may be to the left (negative) or to the right (positive), depending primarily on the salinities of the formation water and of the mud filtrate. • If the formation water is more saline than the mud filtrate, the deflection is to the left. If it is less saline than the mud filtrate, the deflection is to the right.
- 3. • An SP curve cannot be recorded in boreholes filled with nonconductive muds, such as oil muds or air, because such muds do not provide electrical continuity between the SP electrode and the formation. • Also, if the resistivity of the mud filtrate and formation water are about equal, the SP deflections will be small and the curve will be rather featureless and useless.
- 5. Figure A. shows the presentation of an SP curve. • In general, shale-free permeable beds of moderate to low resistivity are sharply defined by the SP curve. • High resistivity beds distort the SP currents, causing a change in the shape of the SP curve at bed boundaries and thus poor boundary definitions. • Also, the SP curve is depressed in permeable zones that contain shale or hydrocarbon. The shape of the SP curve is influenced by (1) the thickness (h) and resistivity (Rt) of the permeable bed, (2) the resistivity (Ri) and the diameter (di) of the invaded zone, (3) the resistivity (Rs) of the surrounding formation, and (4) The resistivity of the mud (Rm) and the diameter (d) of the borehole.
- 6. Figure A. Presentation of an SP curve in a sand-shale sequence
- 7. Figure shows a log of spontaneous potential and formation resistivity made prior to 1935. The notations on the figure make clear which zones are oil- bearing and which are water-bearing. It seems possible, noting the higher resistivity, that zone a-A contains more oil (has a lower Sw) than zone B-b. Fig. An early resistivity-SP log. The scale “Ohms m3” presumably refers to ohm-m. From Martin et al.
- 8. Static Spontaneous Potential (SSP) The theoretical maximum deflection of the SP opposite permeable beds is called the static SP or SSP. It represents the SP value that would be measured in an ideal case with the permeable bed isolated electrically. It is the maximum possible SP opposite a permeable, water-bearing formation with no shale.
- 9. Static Spontaneous Potential (SSP) The SSP is used to calculate formation-water resistivity (Rw).
- 10. Shale Volume
- 11. 2. The Gamma Ray Log (GR) The GR log is a measurement of the natural radioactivity of the formation. • In sedimentary formations, the log normally reflects the shale content of the formation because the radioactive elements tend to concentrate in clays and shales. • Clean formations usually have a very low level of radioactivity, unless a radioactive contaminant such as volcanic ash or granite wash is present or formation waters contain dissolved radioactive salts. • The GR log can be recorded in cased holes, which makes it very • useful as a correlation curve in completion and workover operations. The Gamma Ray Curve: Measures the naturally occurring radiation from the rocks surrounding the borehole. It’s a passive recording. Shales usually exhibit higher levels of radiation than non shale formations.
- 12. Figure B, shows the energies of the emitted gamma rays. Potassium emits gamma rays of a single energy at 1.46 MeV, whereas the uranium and thorium series emit gamma rays of various energies. Figure B. Gamma ray emission spectra of radioactive minerals
- 13. • The gamma ray reflects the proportion of shale and, in many regions, can be used to quantitatively as a shale indicator. • It is also used for the detection and evaluation of radioactive minerals, such as potash and uranium ore. • The GR log is part of most logging programs in bothopen hole and cased hole.
- 14. Uses of Gamma ray Measurement The GR log has traditionally been used for: correlating zones from well to well, crude identification of lithology, and rough estimation of the volume of shale (Vcl ) present in the formation. Continuous shale beds can be readily identified in wells separated by large distances from their characteristic gamma ray “signature.” Due to the simplicity of the GR tool, it is present as an auxiliary sensor on most other logging services to provide routine depth control. With the current state of knowledge of clay composition and with other more refined lithology determinations available, it seems likely that the GR log will be used in the future only for correlation, depth control, and in low-cost development wells for an estimate of Vcl .
- 15. Q & A