ABSTRACT: Interactive Geological Subsurface Interpretation Using Well Logs on a Geophysical Workstation--A Case History

Gary L. Jones, Ken Mallon

This project involves loading well log data from a major oil and gas field into a geophysical workstation, interpreting the data, and generating geological displays using procedures similar to those used when interpreting two-dimensional and three-dimensional seismic. The well data includes four log traces per well from 261, as well as the depths of geological tops for each well. The well log curves were received in ASCII file format and loaded into the system along pseudoseismic lines created to fit the location information.

The data consists of latitude and longitude information, reference elevations for well logs, gamma ray, acoustic, calculated porosity, and calculated permeability curves, as well as 13 tops for each of the 261 wells. The relationship of one well to another is shown realistically.

The interpretation process started once location information was converted to x and ys and appropriately disguised. The operating procedure involved converting well locations to theoretical shot point locations on a 330-ft grid and loading log curves as seismic traces, forming a sparsely sampled three-dimensional grid.

Lines of section were created, and well-to-well correlations were drawn on the sections. Strike and dip cross sections were constructed on the workstation screen. Formation and zone correlations were edited interactively.

A series of color schemes were used for various log curve values, lithology, and porosity heterogeneities. The Vairfill feature caused correlative zones to be highlighted dramatically.

Structural and isopach maps, with triangulated contours, were computed from input tops. These triangulated surfaces were written into a grid and smoothed by the geologist to provide structural contour maps on each of the 13 horizons. Inputting gas-oil contact depths for the field permitted calculation of closure for several formations with an easily drawn gas thickness isopach map.

Depth slices were created and picture files of these depth slices can be shown in sequence to give the impression of moving up or down through the section. Each horizon has a characteristic color, which allows the analyst to follow the individual zones.

Animation of these slices provided some insight about depositional environments and also indicated locations of key producing sands. Animation is produced by making picture fields of depth slices created every 5 or 10 feet and then showing the slices in rapid succession on the screen.

A three-dimensional cube representation of one horizon top can be rotated by the interpreter to allow viewing of the surface from any direction. Several interpreted horizons may be viewed simultaneously. Well locations may also be shown.

Interpretations by a geologist on an interactive workstation can be produced more efficiently and with more accuracy than those made in a paper environment. The interpretation process if freed form the bonds created by reluctance to make changes because of the work resulting in redrawing maps and sections.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990