The effects of organic carbon content and thermal maturity on acoustic parameters in a black shale: Woodford Shale, Permian Basin, West Texas

Abstract

We have derived relationships between organic carbon and acoustic properties in the Woodford Shale, an Upper Devonian organic-rich shale in the Permian Basin, Texas. Extensive core data enable us to relate measured total organic carbon (TOC) on core samples to gamma log response; in turn, gamma log-calculated TOC is compared with P-wave [Formula: see text] and S-wave [Formula: see text] velocities, density, and derivative properties including [Formula: see text], Young’s modulus, Poisson’s ratio, [Formula: see text], and [Formula: see text]. Because we sampled two wells at very different thermal maturity (early oil window and wet gas window), we can examine the effect of thermal maturity on these parameters. In both wells, [Formula: see text] and [Formula: see text] decrease with increasing TOC, exhibiting a rapid decrease from 0% to approximately 1.5% TOC, and a slower decrease above 1.5% TOC, similar to results from ultrasonic experiments on core samples, that we attribute to a shift from a mineral grain load-bearing to an organic matter load-bearing fabric. The [Formula: see text] decreases by 28% and [Formula: see text] by 20% over the range of 0.5%–12% TOC. [Formula: see text] and [Formula: see text] correlate more strongly to neutron porosity than to TOC in the less mature well, related to the development of a bitumen phase that is relatively soft and less dense and has greater impact on rock physical properties than kerogen; this effect disappears in the high-maturity well in which bitumen was further cracked to petroleum and physical properties of bitumen changed with increased aromatization. The effect of increased thermal maturity is to increase [Formula: see text] by approximately 12%, [Formula: see text] by 25%, and P-wave impedance by 12%. [Formula: see text] ratios decrease with increasing TOC, contrary to the behavior expected for the incorporation of a soft material into a rock. We suggest that the rock develops a horizontal planar fabric at lower TOC values that absorbs S-wave energy, counteracting the effect of the organic matter.