Source Rock Evaluation From Well Logs – Four Decades of Technical Tipping Points
The evaluation of geologic formations and their fluids has evolved from the 1920’s when downhole resistivity logs were first pioneered by the Schlumberger brothers. In 1942, Gus Archie conducted laboratory experiments on various sandstones and determined that the resistivity of a rock depends on the water saturation, water salinity, porosity, and a factor (tortuosity) related to how the pores are connected. After World War 2, new nuclear‐based technology began to be incorporated in downhole well‐logging tools, and the natural gamma‐ray, gamma‐gamma density tool, neutron porosity, and Photoelectric (PE) logs became part of the arsenal of downhole measurements, along with a variety of acoustic‐based sonic logs. Initially, combinations of these logging instruments were used to differentiate and quantify lithology, mineralogy, porosity, and fluid type (oil, gas, water). In the 1950’s‐60’s, scientists applied the natural gamma‐ray well log to explore for uranium in Appalachian Devonian‐age organic‐rich rocks. In the late 1960’s, research scientists (Waxman & Smits), determined that additional electrical conductivity due to the presence of dispersed and/or laminated clay required corrections to the earlier Archie equation. Various combinations of well‐logging technologies were applied to source rocks during the 1970’s‐80’s and it was recognized that source rock maturity was related to resistivity log response in the Bakken formation (Meissner), and that the organic matter had anomalously low density and slow velocity (long transit time). Early studies in the Bakken and Mowry shales indicated that well logs could easily identify potential source rock intervals. In the 1980’s, the research organizations of several major oil companies developed and published a variety of techniques to evaluate organic richness based on combination of sonic/resistivity or density/resistivity well‐log crossplots. Also, in the late 1980’s, new logging tools evolved for the direct determination of carbon. In 1989‐90, the logR technique (Passey et al.) was published after details of the previously internal proprietary approach began leaking out into industry; this approach utilized previous approaches but also incorporated source rock maturity as an additional factor in determining organic richness (or TOC – total organic carbon) from well logs; moreover, in addition to commonly used crossplot methods, the logR approach utilized a well‐log overlay technique that allowed for “normalizing” log responses to address variable porosity, lithology, and fluid salinity (all of which were previously problematic to determine in organic‐rich mudstones). The well‐log overlay plots allowed for the determination of the stratigraphic distribution of TOC, and demonstrated the role of sequence stratigraphy on source rock occurrence (Creaney & Passey, Bohacs) ‐‐ a key input to today’s placement of horizontal wells in unconventional reservoirs. A major tipping point in the evaluation of organic richness came about with the onset of the shale‐gas and later shale‐oil unconventional reservoirs, started by the Barnett Shale work in central Texas in the 1990’s‐2000’s. The expansion to explore dozens of “source‐rock” formation as reservoirs worldwide provided abundant fresh mudstone cores, development of new core analysis techniques, and the application and revision of well‐log evaluation techniques. Among the key recent learnings include: 1) utilization of ion‐milled samples which allowed for recognition of nano‐meter scale pores in the organic matter (Loucks & Reed), 2) identification of different habits for kerogen and bitumen, 3) ability to make accurate nano‐Darcy permeability measurements on core plugs (Sinha et al.), 4) recognition of the presence of early graphite at very high thermal maturities (Vro>2) (Walters et al.) resulting in additional electrical conductivity paths and, often, extremely low resistivity values, and 5) modification of previous well‐log interpretation methods (Passey et al., 2010). Work continues on determining the presence of organic porosity and its role in the production of shale‐oil reservoirs (Eagleford, Bakken, Marcellus, and others). Currently, the knowledge of source rocks and their ultimate transformation to unconventional reservoirs is high; with this knowledge we are able to work with our engineering teams to optimize the production of hydrocarbons for the future.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019