Spectral Gamma-Ray Logging in the Texas Austin Chalk Trend

Abstract

This paper reviews properties of Austin chalk and Buda limestone formations of the Cretaceous carbonate trend in Texas and highlights interpretive well logging concepts. Emphasis is on initial field experience with Spectralog, a spectral gamma-ray logging device that has found continuous and widespread application in old and new wells in the Austin chalk formation since Nov. 1977. Introduction Early oil production in the 1920's was developed in the Austin chalk, dolomitic limestone streaks in the Eagle Ford shale, and the Buda limestone of the Luling, Darst Creek, Branyon, and Salt Flat fields in Caldwell and Guadalupe counties of south Texas. Subsequent exploration drilling for the Edwards limestone established incidental production in the Austin chalk in Bexar, Frio, and Wilson counties, with occasional completions being made in the Austin chalk and/or Buda limestone in Atascosa County. Interest in the deeper trend of the Austin/Eagle Ford/Buda has been rekindled, reaching, in fact, into a dozen south Texas counties as well as into east Texas due to indicated successes based on high initial productivity indices (PI's) of new wells in this trend. Long-term performance of some of these wells, however, has been disappointing. Most participants exploring in this Upper Cretaceous trend state that an oilwell completion can be made anywhere in the trend. Hence, the key problem is to distinguish the economical wells, prior to the expensive completion operation. Characteristics of solution-drive oil production in the Austin chalk and Buda limestone are initial PI's indicative of high flow rates followed by rapid pressure decline and eventual stabilized low-level production. The conjecture is that some of these completions are in pay zones that have insufficient porosity, low permeability, and only limited development of macro- and microfracture systems. Thus, after stimulation by acidizing and/or fracturing, the artificially created fracture system delivers high initial production rates that will decline rapidly to low production rates upon stabilization. Where the macro- and microfracture system is extensive and supported by some matrix porosity, the wells behave similarly but stabilize at a higher, more economically attractive production rate. Exceptions to the general patterns of oil accumulation in natural fracture systems are the shallow, older fields of Caldwell and Guadalupe counties. In these counties, formation porosity and permeability, along with natural fracture planes, have been adequate to allow migration of oil into fault-controlled anticlinal traps. Hence, the problem is to define and evaluate the better-developed of more numerous fracture systems by geophysical methods before setting the production string. Several formation evaluation approaches have been attempted in the past.1,2 Application of gamma-ray spectral logging data to locate uranium salts precipitated over geologic time on the fracture planes, along with other geophysical fracture detection methods, appears to offer a reliable evaluation system.