Estimating Net:Gross from Data Histograms: Examples from Deepwater Turbidites

Douglas Goff
Southern Africa Business Unit, ChevronTexaco Overseas Petroleum, P.O. Box 430, Bellaire, TX 77402-0430
phone: (713) 432-3189, DougGoff@ChevronTexaco.com

Pre-drill estimates of Net Gross (N:G) in deep-water turbidite systems is one of the most difficult aspects of the assessment process. Most other parameters vary systematically with depth of burial (porosity, API, Bo, etc...), however N:G is strictly a function of each individual turbidite system.

A work flow has been developed which places a large window on the possible N:G range, but is useful when no other calibration is available. The workflow consists of 5 steps:

1. Generate a seismic data set that can be used to distinguish between sand and shale, such as a phase rotated (90°) seismic volume, an inversion or attribute volume (e.g., Geoprobe "Sweetness"), in which sand occupies one polarity and shale the opposite.
2. Map and interpolate the channel in 3-D in sufficient detail to outline channel edges, top and base.
3. Sculpt the seismic volume to the channel morphology. The tightness of the mapped surface to the channel sands will ultimately be reflected in the N:G calculated.
4. Re-histogram the seismic volume data only including voxels (samples) in the mapped channel.
5. Visually examine the histogram curve to identify key points of the data distribution.

The median of the amplitude distribution for the turbidite will generally be skewed to either side of the "Zero Crossing": high N:G systems will show a shift toward the sand amplitudes, and low N:G systems will show a shift toward the shale amplitudes.

Visual ground proofing of these ranges is simple in a volume visualization package, and can refine the ranges calculated.