Goff, Douglas¹
(1) ChevronTexaco Overseas Petroleum, Bellaire, TX

ABSTRACT: Estimating Net: Gross from Visual Inspection of Data Histograms: Examples from Deepwater Turbidites

Pre-drill estimates of net:gross in deep-water turbidite systems is one of the most difficult aspect of the assessment process. Most other parameters vary systematically with depth of burial, however Net:Gross is strictly a function of each individual turbidite system.
A work flow has been developed which places a large window on the possible Net:Gross range, but is useful when no other calibration is available. The workflow consists of 4 steps: 1) Map and interpolate the channel in 3-D in sufficient detail to outline channel edges, top and base. 2) Sculpt the seismic volume to the channel morphology. 3) Re-histogram the seismic volume data only including voxels (samples) in the mapped channel. 4) Visually examine the histogram curve to identify the inflection points of the data distribution.
Implicit in this workflow is that the seismic data set being utilized can be used to distinguish between sand and shale, such as an inversion or attribute volume (e.g., Geoprobe “Sweetness”), in which sand occupies one polarity and shale the opposite.
The inflection points of the distribution generally represent the limit of noise close to the zero crossing (maximum net:gross) and the point at which amplitudes are nearing saturation (minimum net:gross). Net:gross can then be calculated by dividing the number of voxels which lie above the minimum and maximum amplitude cutoffs by the total number of voxels in the channel.
Visual ground proofing of these ranges is simple in a volume visualization package, and can refine the ranges calculated.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.