Hydrocarbon Identification Using Mud-Log Chromatography in Low-Resistivity Simpson Sandstones

Raymond P. Sorenson

Oil-productive Simpson sandstones often exhibit very low resistivities, with little contrast to underlying aquifers. Modern log suites have improved interpretation, but wells continue to be drilled that defy accurate S_w calculation. Traditional methods, using DSTs based on sample shows and mud-log gas increases, continue to be the rule; but they are expensive and allow only a crude estimation of fluid contacts dependent on the subjective judgment of the well-site geologist. At little or no additional cost, interpretation techniques incorporating mud-log chromatograph ratios can add considerably to reservoir knowledge, although better data quality than typically provided by Mid-Continent mud logging companies is required. Published methodologies were modified in this study to eliminate C⁵ (pentane) data because of measurement difficulties and to improve vertical resolution by shortening chromatograph cycle times.

Mud-log ratio methods have been applied to a well in the Noble Northwest field of Cleveland County, Oklahoma, in which only one of three commercial oil reservoirs was identifiable with open-hole wireline logs. In all three cases, mud-log ratios remained very stable within the oil column, became somewhat erratic in the transition zone, then stabilized at different values in the water column. In the subject well, only the ratio methods predicted oil-water contacts consistent with structural relief and test results. One potential difficulty was the inability to recognize depletion; a watered-out interval displayed ratios characteristic of productive reservoirs.

AAPG Search and Discovery Article #91025©1989 AAPG Midcontinent, Sept. 24-26, 1989, Oklahoma City, Oklahoma.