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ICA/SPD Organic Thermal Maturity Determinations on Core Samples from the Upper Barnett Shale, Forestburg Limestone and Lower Barnett Shale, Wise County, Texas

Geoffrey S. Bayliss1, Karl W. Schwab2, Michael A. Smith3, and Gordon D. Wood4
1Geochem Laboratories Inc., Houston, Texas;
2EMENU Inc., Houston, Texas;
3Minerals Management Service, New Orleans, Louisiana;
4The IRF Group, Inc., Katy, Texas

Integrated Color Analyses (ICA), derived from the Spectral Power Distribution (SPD) of White Light transmitted through selected organic matter types, is an unconventional, but proven, technique used for determining the thermal maturity of organic constituents. Forty-six (46) core samples were collected from the upper Barnett Shale, Forestburg Limestone and lower Barnett Shale, from the Mitchell T.P. Sims No. 2 and the T.U. Blakely No. 1 wells for visual kerogen typing and thermal maturity analyses.
    More than 5,000 ICA/SPD measurements were made on organic constituents, extracted from the core samples. ICA/SPD data indicates that the organics in the upper Barnett Shale, Forestburg Limestone and lower Barnett Shale is currently at a moderately mature stage of hydrocarbon generation. The maturity level is near, or slightly above “peak” oil generation (i.e., + 0.95 %Ro). Vitrinite reflectance measurements were also made on some of the cored material. The measured values of the vitrinite particles, considered to be in situ, are in close agreement with the equivalent reflectance values (%Roe) derived from ICA/SPD determinations.
    Kerogen extracted from the upper Barnett Shale, Forestburg Limestone and lower Barnett Shale is comprised of high percentages of algal-amorphous debris and degraded herbaceous components. Trilete and non-trilete spores are present, but rare. Visually, vitrinitic (woody) kerogen types and inertinite generally make up less then 15% of the total organic matter suite. The particle size of the organic constituents ranges from fine to moderately coarse. Preservation of the kerogen is interpreted as being fairly good to poor.
    Although the lower Barnett Shale has produced copious amounts of gas in the Mitchell T.P. Sims No. 2 and T. U. Blakely No. 1 wells, there is absolutely no possibility that the organics extracted from the cored samples are within the thermally mature gas condensate and/or wet gas zone. Gas that is currently being generated and produced is interpreted to be a low-temperature immature gas (possibly a combination of biogenic and thermally derived methane). Gas being generated from the lower Barnett Shale may be linked to the unusually high algal-amorphous content of the sediments.
    Geochemical analyses also show that the most favorable production of wet gas and/or gas condensate from the lower Barnett Shale in the Fort Worth basin is not necessarily restricted to areas where sediments have been reported to have average vitrinite reflectance values of 1.20 %Ro or greater (e.g., Montgomery, and others, 2005 edition of the AAPG Bulletin, v. 89, no. 2, p. 155-175). No cored sample material from the lower Barnett Shale analyzed in this study could be regarded as being within the thermally mature wet gas-gas condensate window (i.e., vitrinite reflectance values of greater than 1.20 or 1.25 % Ro). The fact that the lower Barnett Shale is capable of producing vast amounts of “low-temperature” gas suggests an even wider area for exploration than previously thought.


AAPG Search and Discover Article #90065©2007 AAPG Southwest Section Meeting, Wichita Falls, Texas