Variations in vitrinite reflectance and clay-mineral composition within vertical sections of Belden Shale, Eaglebasin, Colorado - problems in determining thermal maturity

Conference · Fri May 01 04:00:00 EDT 1987 · AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States)

OSTI ID:5783199

Nuccio, V F; Pollastro, R M; Schenk, C J

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 are (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.

• Research Organization: Geological Survey, Denver, CO
• OSTI ID: 5783199
• Report Number(s): CONF-870606-
• Conference Information: Journal Name: AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States) Journal Volume: 71:5
• Country of Publication: United States
• Language: English

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