Org. Geochem. Vol. 20, No. 2, pp. 187-196, 1993 0146-6380/93$6.00+ 0.00 Printed in Great Britain.All rights reserved Copyright © 1993Pergamon Press Ltd Rotational reflectance properties of Arkoma Basin dispersed vitrinite: insights for understanding reflectance populations in high thermal maturity regions DAVID W. HOUSEKNECHT, t DAVID F. BENSLEY, 2 LORI A. HATHON 3 and PAUL H. KASTENS 3 IU.S. Geological Survey, 915 National Center, 12201 Sunrise Valley Drive, Reston, VA 22092, 2Department of Geology, Southern Illinois University, Carbondale, IL 6290! and 3Department of Geological Sciences, University of Missouri, Columbia, MO 6521l, U.S.A. A~traet--Analysis and interpretation of dispersed vitrinite reflectance data in regions of high thermal maturity (>2% vitrinite reflectance) have been equivocal partly because of an increase in width and complexity of reflectance histograms with increasing mean reflectance. Such complexity is illustrated by random reflectance(R~n) data from the Arkoma Basin that display a linear increase in standard deviation of Rra n with an increase in mean Run from 1 to 5%. Evaluating how much of the dispersion in these data is the result of vitrinite anisotropy and how much is the result of mixing of kerogen populations by sedimentary processes and/or sampling procedures has been problematic. Automated collection of reflectance data during polarizer rotation provides preliminary data for solution of this problem. Rotational reflectance data collected from a subset of Arkoma Basin samples reveal positive, linear relationships among maximum (R~x), random (R~n), rotational (Rro,), and minimum (Rmin) reflectance, as well as a systematic increase in bireflectance (R~nax-R~nin) with increasing reflectance. R~x and Rro t display lower standard deviations and narrower, more nearly unimodal histograms than R~n and Rmin, suggesting that R~x and Rro t are superior (less ambiguous) indices of thermal maturity. These data patterns are inferred to be mostly an indication of increasing vitrinite anisotropy with increasing thermal maturity, suggestingthat the linear covariance observed between mean Rra n and standard deviation in dispersed organic data sets from regions of high thermal maturity may be explained mostly as the result of increasing vitrinite anisotropy with increasing thermal maturity. Key words--rotational reflectance, reflectanceanisotropy, bireflectance,Arkoma Basin, Atoka formation INTRODUCTION Vitrinite reflectance principles, developed by coal petrographers and widely applied to dispersed or- ganic matter, provide the most universally applicable methods for characterizing thermal maturity of sedi- mentary rocks, and for inferring their thermal his- tory. However, analytical techniques and modes of data presentation differ between coal and dispersed organic matter because of differences in sample material and objectives of petrographic analyses. Moreover, a limited amount of research has been conducted on dispersed organic matter of high ther- mal maturity (vitrinite reflectance >2%) because most studies have been conducted by the petroleum industry, which has little interest in strata that are "supramature". As a consequence, analysis and in- terpretation of dispersed vitrinite reflectance data collected from samples of high thermal maturity have been enigmatic, mostly because reflectance histo- grams are wide and multimodal (see later). The objectives of this paper are: (1) to illustrate some of the difficulties of applying v~trinite reflec- tance techniques to dispersed organic matter from high thermal maturity samples; and (2) to present preliminary data collected using a new instrument that facilitates measurement of rotational reflectance data from dispersed organic particles and demon- strate the potential of this new technique for resolving some of these difficulties. VITRINITE REFLECTANCE Optical Characteristics The reflectance of vitrinite generally increases with maximum temperature to which host strata have been exposed, although many additional variables may contribute to either suppression or increase of reflec- tance (see Teichmfiller, 1987 for recent summary). When measured in polarized light, vitrinite reflec- tance is characterized by anisotropy that can be represented by an indicating surface [called "reflec- tance indicating surface" (RIS) by Kilby, 1988, and "vitrinite reflectance indicating surface" (VRI) by Levine and Davis, 1989a, b], whose radius in any direction is proportional to the reflectance in that direction (Kilby, 1988; Levine and Davis, 1989a, b). In coals that have attained thermal maturity during stratigraphic burial in the absence of tectonic stress, vitrinite indicating surfaces typically have uniaxial negative properties with Rm~n oriented vertically (Hower and Davis, 1981a, b; Levine and Davis, 1984, 1989a, b; Kilby, 1988). Thus, a polished surface of vitrinite oriented normal to bedding and rotated under plane polarized light will display reflectance 187