FORMATION EVALUATIONPETE 321

Spring 2005

Lab 2_______

GR

Sections 503, 504Dr. James RussellLab: Kalwant Singh

Sections 501, 502Dr. David SchechterLab: Charles Ozobeme

Lab 3 objectives

o More information about GRo Calculation of Vsho 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

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.

Gamma Ray Log

Figure: Scintillation Gamma Ray Detector

Gamma Ray Log

Gamma Ray Log

Sand

Shale

Shale

Gr min

Gr max

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

Spectral Gamma Ray

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.

Spectral Gamma Ray Log Gamma Ray Log

Gamma Ray Log

Gamma Ray Log

Gamma Ray Log

Shale Content from GR Log

Shale Content from GR Log

MINMAX

MINSH GRGR

GRGRI