TOC Prediction Analysis of Utica-Point Pleasant Formations in the Appalachian Basin

Abstract

Total organic carbon (TOC) content is one of the most important parameters in shale gas and oil exploration and development. Core and cutting test data, well logs, and seismic data all can provide a certain information of TOC content. However, the most feasible method to analyze the vertical and lateral distribution of TOC is to calibrate well log interpretation with core analysis data. The TOC prediction from well log data is not a trivial task. Uranium from spectral gamma ray log or normal gamma ray (GR) log has been used as an important proxy of TOC content in many shale plays, for instance the Marcellus Shale in the Appalachian Basin. But, as we move to the Utica-Point Pleasant formations in the same basin, Uranium and GR log lose their power. A potential reason could be that organic-rich shale in Utica-Point Pleasant formations was deposited in a relatively shallow (<100ft), well oxygenated water body with possible seasonal anoxia. As a result, most uranium was dissolved in the water and did not deposit into the rock with organic matters Therefore, we should utilize other logs such as density. There are some problems with using density for TOC interpretation. First, it is hard to determine the density of shale matrix: density varies in different minerals from 2.5 to 3.0 g/cm³. This problem becomes more serious when pyrite, siderite, and barite exist in shale. The physical density of organic matter varies also from 0.95 to 1.6 g/cm³ with different thermal maturity, burial depth, and organic matter type. Given that different types of fluids in the pores with a range of densities from 0.3 to 1.1 g/cm³, TOC content can look like porosity in the density log. Thus, for TOC prediction, we should combine density log with other types of logs such as GR, neutron and PE curve to evaluate the mineral composition for matrix density and porosity in shale. For example, we grouped all the minerals in shale to silica minerals, clay minerals, carbonate minerals, and trace heavy minerals (e.g., pyrite) to estimate the matrix density using density, PE, and GR logs. Thermal maturity serves as an indicator of organic matter density and hydrogen types in the pores. With careful evaluation of matrix density, organic matter, and fluids, the TOC prediction from density logs becomes more reliable.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016