--> Shale Velocity and Density as Functions of TOC and Thermal Maturity: Upper Devonian Woodford Shale, Permian Basin, Texas

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Shale Velocity and Density as Functions of TOC and Thermal Maturity: Upper Devonian Woodford Shale, Permian Basin, Texas

Abstract

The Woodford Shale is an Upper Devonian organic-rich black shale and potential gas and liquid reservoir in the Permian Basin, west Texas. It is widespread and thick (up to 200 meters) and represents the longest continuous record of black shale deposition in North America. Detailed sedimentological and geochemical studies have been carried out on two Woodford cores, from 8300 feet and 12900 feet, equivalent to 0.71% and 1.48% Ro, based on Rockeval Tmax values. The geochemical data, combined extended modern log suites, provides an opportunity to examine the impact of varying total organic carbon (TOC) content and thermal maturity on acoustic velocities and density. By correlating the gamma log to core TOC, a complete record of organic carbon content through the well is obtained; the log-estimated TOC is then compared to Vp, Vs and density logs. Vp and Vs decrease systematically with increasing TOC, by 20 to 25% as TOC increases from 0% to 10%. Because density also decreases with increasing TOC, the effect on acoustic impedance is substantial, as much as 30% in the range of 0 to 10% TOC. Velocity is affected because organic matter is relatively soft material, while density is affected because organic matter is composed primarily of the light elements carbon and hydrogen. Although organic matter is relatively ductile, particularly in lower maturity rocks, and would be expected to transmit shear waves relatively poorly, a decrease in Vp/Vs ratios is evident with increasing TOC; this may be because high TOC is accompanied by high volumes of quartz cement, which would provide a stiff framework to the shale. The effect of thermal maturity is significant. The velocity of both P waves and S waves is approximately 20% higher in the higher maturity well than in the lower maturity well. Acoustic impedance is also substantially higher. This may result from loss of organic carbon during oil and gas generation and expulsion, or it may result from changes in the physical properties of kerogen related to thermal maturation. These results suggest that it may be possible to map isomaturity lines in a source rock formation based on acoustic velocities, and where seismic resolution is sufficient, it may be possible to identify intervals of high TOC based on impedance maps. However prediction of shale TOC may have to be accompanied by an independent interpretation of thermal maturity such as basin modeling.