Variations in Vitrinite Reflectance and Clay-Mineral Composition Within Vertical Sections of Belden Shale, Eagle Basin, Colorado--Problems in Determining Thermal Maturity: ABSTRACT

Abstract

A major problem in defining true thermal maturity of shales by mean random vitrinite reflectance (R/sub m/) and clay-mineral geothermometry is determining the correct population of organic matter and/or interstratified illite/smectite (I/S) clay, respectively. Shales commonly contain a variety of both organic matter and I/S clay; the types of both the organic matter and the I/S clays vary widely, as do the interpreted values, reflecting the heterogeneous sources. The organic-rich, carbonate-bearing Belden Shale (Early Pennsylvanian) contains three distinct populations of organic matter. The vitrinite with the lowest reflectance is weathered-looking or porous, and is generally in the 0.90-1.30% R/sub m/ range. The next higher population of vitrinite R/sub m/ values is consistently in the 2.0-3.0% range. The highest R/sub m/ population is fusinite, measuring in the 4.0-5.0% range. Other indicators of thermal maturity (T/sub max/ and time-temperature index reconstructions, which show that these rocks have been subjected to relatively high temperatures, are in best agreement with the intermediate R/sub m/ values 2.0-3.0%). Although the ordered and highly illitic I/S is in good agreement with the other thermal indicators, randomly interstratified I/S typically does not exist in sedimentary rocks with R/sub m/ > 0.80%. Possible explanations for the problems stated aremore » (1) that weathering of precursor organic matter and minerals can produce lower values in R/sub m/ and highly expandable I/S, respectively; (2) suppression effects reported for vitrinite in other organic-rich marine shales and for the smectite-to-illite reaction in other calcium-bearing rocks; and (3) very late diagenesis at near-surface conditions producing highly expandable I/S.« less