ABSTRACT: Interpretation of Well Log Response in the Austin Chalk

Gregory S. Hinds

The Austin Chalk in south-central Texas is characterized as both a source and reservoir for hydrocarbons. Deposited on a gently sloping ramp during the Late Cretaceous, the Austin Chalk is a dense carbonate chalk containing substantial amounts of organic matter (3.5%). The reservoir trend extends from Zavala to Brazos counties and roughly parallels the present Gulf Coast shoreline. Production is mainly from fractures but is also dependent on organic matter abundance and maturity.

Log signatures and core analyses from wells scattered throughout the trend and comprising a complete vertical sequence were compared with published geochemistry using the ES-LOG log analysis system. Three distinct zones of organic matter maturity were identified: a shallow (0-6000 ft) immature zone, an intermediate (6000-7000 ft) peak generation zone, and a deep (7000-9500 ft) migration and potential reservoir zone. An increase in resistivity, gamma, and density porosity accompanied by a decrease in neutron porosity mark the threshold of oil generation at approximately 6000 ft and a temperature of 185°F. Below 7000 ft, a decrease in resistivity and gamma values along with crossover of density and neutron porosity curves mark the end of peak generation and the beginning of migrati n and potential production. Regression analysis of averaged log values shows linear trends directly relating resistivity to core-derived water saturation in the three maturation zones. Intervals with average resistivities between 9 and 40 ohm-m will have water saturations in the range of 45-75% and are considered potential reservoirs. Resistivities averaging below 9 ohm-m and above 40 ohm-m fall into the immature and peak generation zones, respectively, and are not considered potential reservoirs.

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