ABSTRACT: Old Log Interpretation Using a Computer Database Program

Ted Campen, Betsy Campen

"Old logs" make up approximately 45% of all existing well logs. The Northern Rockies compose a region especially abundant in old log suites, and regional interpretation using Schlumberger charts and hand-held calculators is a tedious process that may result in passed-over pay zones.

The authors have co-written a database software program whereby Tixier's Rocky Mountain Method is used in the analysis of old electric logs, aided when possible by micrologs, neutron logs, and microlaterologs. Equations were derived from Schlumberger charts so that porosity, water resistivity, and water saturation can be obtained from old log measurements such as formation temperatures and mud resistivities. Subsurface data including formation tops, thicknesses, and log shapes are incorporated in the data base.

Modern logs may be entered and interpreted in conjunction with the old logs. The same solutions are achieved by overriding the Rocky Mountain System with a variety of water saturation equations such as "Archie" or "Humble." Thus, vast amounts of data can be quickly recorded and calculated in a complete log database.

The model area in central Montana is an example of interpretation pitfalls in areas of shaly gas sands where vastly different types of water have been used in drilling muds. Resulting flat spontaneous potential curves mask commercial wells whereas some minerals associated with fresh water can make a tight sand look porous. Old log computer calculations expose imposters and highlight potential producers.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990