SUPPRESSION AND RETARDATION OF VITRINITE REFLECTANCE, PART 1. FORMATION AND SIGNIFICANCE FOR HYDROCARBON GENERATION

Abstract

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.