http://k2sistemas.com.br/estratigrafiaquimica/artigos/2004/paleoredox.pdf

Geochemical paleoredox indicators in Devonian–Mississippian black shales, Central Appalachian Basin (USA)

Multiple controls on the preservation of organic matter in Devonian¿Mississippian marine black shales: geochemical and petrographic evidence

Review and update of the applications of organic petrology: Part 1, geological applications

Mobility of rhenium, platinum group elements and organic carbon during black shale weathering

Vitrinite consists of a number of macerals with varying morphology and reflectance. Although vitrinite reflectance (VR) generally increases with increasing depth of burial, a spread of values often occurs due to the presence of the different macerals. However, reports of suppressed and/or retarded VR values introduce a potential problem for the geologist attempting to assess the thermal maturity of a sedimentary basin.

Suppression is defined as the reduction in reflectance resulting from:


•high liptinite contents


•the presence of aliphatic lipids and bitumens derived from associated liptinites;


•the generation of perhydrous (hydrogen/aliphatic-rich) vitrinite in anaerobic and/or alkaline depositional environments;


•the occurrence of hydrogen- and aliphatic-rich vitrinite originating from a particular flora.



Suppression occurs immediately after a sediment has been deposited, and continues to reduce the reflectance until the hydrogen/bitumen is removed from the structure. Although suppression has been recognised in many basins, it probably occurs more frequently than is currently recognised.

Retardation, by contrast, refers to a thermochemical reduction in reaction rate resulting from the effects of overpressure in a sedimentary basin. Unlike suppression, retardation only occurs at depth in a basin due to the development of a pressure seal; retardation continues to occur as long as the overpressure remains. The presence of overpressured sediments in many sedimentary basins means that VR retardation probably occurs more frequently than is commonIy recognised.

In this paper, methods for recognising suppression and retardation of VR together with examples of both phenomena are described, together with the implications for petroleum systems analysis resulting from the interpretation of suppression when the vitrinite reflectance is retarded. Suppression and retardation are not restricted to coals, and the recognition of either or both of these phenomena will add value to a petroleum systems analysis.

D.J. Cantrell

Papers

Rock-eval pyrolysis and vitrinite reflectance trends in the Cleveland Shale Member of the Ohio Shale, eastern Kentucky