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Earth and Space Sciences · Glossary

What is Net-to-gross ratio?

Definition 17.12 University Earth and Space Sciences — Year 3 · Chapter 17 — Facies Models

The net-to-gross ratio of an interval is the thickness of its reservoir-quality rock, usually sand picked on the gamma-ray log below a cutoff, divided by the total thickness of the interval.

Synthetic logs of the well Varden-2 from 700\, m to the basement: gamma ray (the dashed line is the sand cutoff of 60\, API in the sandstone), bulk density and sonic transit time (time increasing to the left, as logs are drawn); right, the gamma ray of the sandstone enlarged. The chalk densifies with depth; the shale reads hot, slow and of intermediate density; the limestone is cold, dense and fast; the sandstone is a stack of channel fills, each a clean base passing upward into mud: bell shapes.
Synthetic logs of the well Varden-2 from 700 m700\,\mathrm{m} to the basement: gamma ray (the dashed line is the sand cutoff of 60 API60\,\mathrm{API} in the sandstone), bulk density and sonic transit time (time increasing to the left, as logs are drawn); right, the gamma ray of the sandstone enlarged. The chalk densifies with depth; the shale reads hot, slow and of intermediate density; the limestone is cold, dense and fast; the sandstone is a stack of channel fills, each a clean base passing upward into mud: bell shapes.
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