FORMATION EVALUATION PETE 321 Lab 2 _______ GR Spring 2005 Sections 501, 502 Dr. David Schechter Lab: Charles Ozobeme Sections 503, 504 Dr. James Russell Lab: Kalwant Singh

Lab 3 objectives More information about GR Calculation of Vsh Geographix – PRIZM tutorial

Gamma Ray Log Natural gamma-ray log is based on the measurement of natural radioactivity of formation rocks, which depends on the content of radioactive isotopes of uranium, potassium and thorium and, also, on the lithological composition of formation rocks. Types: Natural Spectral Thorium, Uranium, Potassium Applications: Correlation. Lithology indicator Determining the shale content of rocks. Introducing depth corrections to other well logs. Detection of radioactive minerals. Paleoenvironmental indicator 3

Gamma Ray Log Interpretation The interpretation of gamma ray logs can be summarized as follows: Clean sands, sandstones, limestones, and dolomites have low radioactivity. Anhydrite, salt, lignite and coal have also a low radioactivity. Ordinary shales have a much higher radioactivity than the rocks listed above. Shales are sufficiently high in radioactivity and can generally be easily distinguished from the other rocks on a gamma ray log. 4

Gamma Ray Log Figure: Scintillation Gamma Ray Detector A SNG probe is a tool (an instrument) that can be lowered into a borehole where it measures the amount and composition of the gamma-rays that enters the borehole from the surrounding rocks. The SNG probe contains a gamma-ray detector that often is a large crystal of NaI. When the crystal is hit by a gamma photon a tiny amount of light is generated in the crystal. A photomultiplier tube is used to convert the light to an electrical pulse the amplitude of which is proportional to the amount of gamma energy deposited in the crystal. Included in the SNG probe also is an analyser that sorts the electrical pulses from the photomultiplier tube. The distribution after amplitudes of the electrical pulses is termed a gamma spectrum. Once per two to ten seconds a new gamma spectrum is sent from the SNG probe to a computer at the surface of the ground. Inside the steel cable that carries the SNG probe there are electrical conductors that carry the signals from the probe to the surface computer - and also carry "messages" the other way. Modern SNG probes also contain electronics and a tiny computer that perform the signal processing in the down-hole probe and transmits digital results to the surface. This technique - the down-hole processing - is needed in order to obtain correct gamma spectra. Figure: Scintillation Gamma Ray Detector 5

Gamma Ray Log 6

Gamma Ray Log Gr max Shale Sand Gr min Shale 7

Spectral Gamma Ray Log It is a borehole log indicating the concentration of thorium (Th), uranium (U), and  potassium (K) in the rocks surrounding a borehole. Applications: Estimate volume and type of clay minerals Identify fractures Differentiate radioactive carbonates from shales and clays 8

Spectral Gamma Ray 9

Spectral Gamma Ray Log Vs Gamma Ray Log It is not able to discern between the gamma-rays from Th, U, and K respectively; it just "counts" the total number of gamma-rays that hits the detector per second. The "output" is counts per second (cps) that also may be converted to API units for gamma radiation. It is able to detect and identify the radioactive source (gamma-rays from Th, U, and K respectively.) It is displayed as three curves from all sources of thorium, uranium and potassium. 10

Gamma Ray Log 11

Gamma Ray Log 12

Gamma Ray Log 13

Shale Content from GR Log

Shale Content from GR Log