Computer Applications and Use of Old Logs for Hydrocarbon Exploration

Ted Campen, Betsy Campen

Old logs make up approximately 45% of all existing well logs. Old log suites are especially abundant for the northern Rockies. Therefore, regional interpretation using charts and hand-held calculators is a tedious process that may result in passing over pay zones.

Initial log analysis consisted of evaluating porosity from the spontaneous potential (SP) from an electric log. Water resistivities and saturations were not possible to calculate. However, in 1941, G. E. Archie developed basic equations for the determination of these properties and of porosity. Porosity evaluations, however, could be done only from core samples rather than from log analysis. In 1949, M. P. Tixier used a ratio approach to develop "the Rocky Mountain method." Schlumberger collaborated by developing log interpretation charts to be used with Tixier's theories. It then became possible to determine water saturation, resistivity, and porosity from old electric logs, aided when possible by Micrologs, neutron logs, and Microlaterologs.

The personal computer has enhanced old log interpretation enormously. Both spreadsheets and databases lend themselves to analysis, and vast amounts of data can be gathered and subsequently interpreted. By using the old Schlumberger charts, equations can be created so that porosity, water resistivity, and saturation can be derived from old log measurements. Subsurface data including formation tops, thicknesses, and log shapes can be incorporated so that the explorationist has a complete log history in a minimal amount of time.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988