Charsky, Alyssa; Herron, Susan
Total organic carbon (TOC) is an important parameter in the evaluation of kerogen-rich unconventional reservoirs; therefore, a continuous, accurate wireline TOC log is highly desirable. Recent advances in geochemical spectroscopy logging make it possible to obtain an accurate and robust TOC log without the need to calibrate to core. A new tool simultaneously measures carbon and the major elements, calcium and magnesium, that form carbonate minerals, from which TOC is derived as the difference between total carbon and inorganic (mineral) carbon at acceptable open hole logging speeds.
This method differs considerably from the current practice of estimating TOC from conventional log measurements based on empirical approaches. Two well-established and proven techniques are the Schmoker density-log technique and the combination sonic- (or density-) and resistivity-log technique known as Δ log R. Other common methods for TOC estimation include using uranium or gamma ray logs as indicators of organic matter; although they often require a local calibration. The Schmoker density-log technique uses the bulk density log and assumes that the change in density of the formation is due to the presence or absence of low-density organic matter (~1.0 g/cm3). The Δ log R technique relies on separation of the sonic (or density) and resistivity curves to identify areas containing organic matter and the subsequent derivation of TOC from the separation of the two curves based on the level of organic maturity.
Each of these quantitative approaches has been compared with TOC measured by coulometry on hundreds of core samples in field tests covering a variety of formations and wells drilled with both water- and oil-based muds. The organic carbon content of the core samples ranges from 0 to over 15 wt%, and the carbonate mineral content ranges from 0 to almost 100 wt%. TOC from the geochemical spectroscopy log agrees very well with the core data, matching as well as or better than the Schmoker or Δ log R techniques for all datasets. The accuracy and robustness of the TOC derived from spectroscopy logging plays a key role in accounting for organic content when computing density and porosity for unconventional reservoirs to ensure accurate and consistent evaluation across varying rock compositions, depositional environments, and hydrocarbon maturities.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013