Adrian Immenhauser¹, Wolfgang Schlager¹

(1) Vrije Universiteit, Amsterdam, Netherlands

ABSTRACT: Polygenic Discontinuity Surfaces In Marine Carbonates - Relevance for Stratigraphy and Reservoir Geology

Discontinuity surfaces that recorded superposition of marine hardground and subaerial exposure stages are rather common in carbonate settings. These surfaces often formed during periods of rapid sea-level drop and subsequent exposure of carbonate seafloors. In terms of sequence stratigraphy, these surfaces may then combine attributes of "sequence boundaries", "maximum flooding surfaces", and "transgressive surfaces". Commonly, the last stage in the history of polygenic discontinuities is dominant in the field. Previous evolutionary stages of these surfaces are often removed or hidden and only revealed by circumstantial petrographic, geochemical, and biological evidence. Due to their complex early diagenesis, porosity in the limestones underlying the discontinuities is rearranged during marine lithification, subaerial exposure, and subsequent burial and hence the permeability of large volumes of limestone is affected at a variety of scales. During burial, these intervals may thus act as either seals or efficient conduits of fluid flow. This study is based mainly on the Middle Cretaceous of Oman where polygenic discontinuities are spaced ten to few tens of meters apart and many of them were traced laterally over distances of 100 km and more. Similar surfaces are known from the Jurassic of the Swiss and French Jura Mountains and from the Mesozoic and Cenozoic of Italy. It seems that polygenic discontinuities play an important, but poorly understood role in compartmentalization of carbonate reservoir rocks.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado