Abstract
Organic carbon content and thermal maturity are critical to the evaluation of shale gas and shale oil plays, determining the productivity of the formation and type of hydrocarbons that are produced. In this analysis, we demonstrate that these parameters also affect acoustic properties; thus, well logs and potentially seismic data can be inverted to estimate organic richness and maturity. Our study is based on data from two cored wells in the Woodford Formation, Permian Basin, west Texas, that had been previously studied to develop sedimentological and geochemical models for black shale deposition. The KCC 503 well is relatively shallow, and the Woodford Shale here has a maturity of 0.71% Ro, just entering the oil window. The RTC 1 well is relatively deep, and the Woodford in this well has a maturity of 1.48% Ro, in the wet gas window. In our analysis, we link rock composition, including total organic carbon (TOC) content and thermal maturity to data on p-wave (VP) and s-wave (VS) velocities and density and derivative properties including Vp/Vs ratios, impedance, geomechanical parameters including Poisson's Ratio and Young's Modulus and Lamé parameters mu and lambda. We then test whether stratigraphic intervals within the Woodford can be geophysically distinguished and potentially mapped with seismic data.
TOC content is reflected in two well logs: gamma ray, which responds to uranium and is in turn linked to organic carbon content; and neutron porosity log, which responds to hydrogen content. Neutron porosity values are higher than core porosities because of the organic carbon content, much higher in the lower maturity KCC 503 well because of the development of a bitumen phase that is no longer present in the higher maturity RTC 1 well due to cracking to hydrocarbons. Gamma ray and neutron porosity are both inversely related to Vp and Vs. In the KCC 503 well, neutron porosity correlates more strongly to these parameters due to the presence of bitumen; in the RTC 1 well, both gamma ray and neutron porosity provide comparably strong correlations. Correlations to impedance are similar. Both gamma ray and neutron porosity predict Young's Modulus but not Poisson's Ratio. Vp, Vs, and impedance increase as a function of thermal maturity. Vp/Vs decreases as a function of thermal maturity, due to the greater increase in Vs compared to Vp, but is not sensitive to TOC in an individual well.
Stratigraphic intervals of the Woodford Shale have distinctive acoustic parameters. Vp/Vs ratios distinguish the three main subdivisions: the organic-rich Middle Woodford is characterized by low Vp/Vs ratios in comparison to the Upper (intermediate values) and Lower (high values). Poisson's Ratio values follow similar trends. This suggests that internal units within the Woodford Shale can be mapped with seismic data.
