Thermal Maturity-Adjusted Log Interpretation in Organic Shales


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SPWLA Distinguished Speaker Series: 2019-2020. The petrophysical interpretation of downhole logs requires knowledge of accurate matrix properties. In organic-rich mudrocks (shale), the presence of abundant kerogen (solid, insoluble organic matter) has a particularly large and variable impact on matrix properties. Matrix properties are highly sensitive to kerogen properties because kerogen is compositionally distinct from minerals that comprise the remainder of the matrix. In practice, kerogen properties must be accurately known to separate tool responses to kerogen (in the matrix volume) and fluids (in the pore volume), to arrive at accurate volumetric interpretations. Unfortunately, relevant petrophysical properties of kerogen are nearly always unknown in the formation of interest, and otherwise impractical or impossible to measure.

In this talk, I will present the petrophysical properties of kerogen from the study of more than 50 global shale samples. The determined kerogen properties include measured chemical composition and absolute density, as well as calculated nuclear properties such as apparent log density, hydrogen index, thermal and epithermal neutron porosities, photoelectric factor, macroscopic capture cross-section (Sigma), and fast neutron cross-section. For kerogen samples relevant to the petroleum industry (predominantly type II with thermal maturity ranging from immature to dry gas), it is found that petrophysical properties are controlled mainly by thermal maturity, with differences between basins/formations having relatively little effect on kerogen properties. As a result, universal curves can be established that relate kerogen properties to thermal maturity. This thermal maturity-adjusted log interpretation (TMALI) establishes a consistent framework to evaluate organic shales globally, requiring only knowledge of the thermal maturity of the play of interest, and provides more accurate, robust, and confident estimates of critical formation parameters including porosity, saturation, and hydrocarbon-in-place.

Your Instructor

Paul Craddock, PhD, SPWLA
Paul Craddock, PhD, SPWLA

Paul Craddock is a geochemist and Senior Research Scientist in the Applied Math & Data Analytics Department at Schlumberger-Doll Research Center in Cambridge, Massachusetts. His research provides solutions for reservoir characterization using nuclear, X-ray, and infrared spectroscopy methods, such as to derive saturation in low-resistivity pay in conventional reservoirs using nuclear spectroscopy; identify favorable pay for well placement and production in unconventional resources (“RPI”); integrate cuttings and logs for enhanced petrophysics in shale (“DRIFTS”); and optimize kerogen endpoints for global shale interpretations (“TMALI”). Paul received a PhD in chemical oceanography from Massachusetts Institute of Technology/Woods Hole Oceanographic Institution in 2009 and has co-authored more than 40 technical journal and conference publications. Paul is twice a SPWLA Distinguished Speaker and his paper “Thermal maturity-adjusted log interpretation (TMALI) in organic shales” was awarded Best Oral Presentation at the 2019 SPWLA 60 th Annual Logging Symposium.