The Hercynian Unconformity Identification at the Continental Permo-Carboniferous Transition (Dutch Offshore - North Sea): Its Importance in Oil and Gas Exploration

Ph. Renaud

In the Netherlands, the major part of exploration activity concerns the aeolian and fluviatile sand bodies of the Rotliegend series (mid Permian). These sands of decreasing thickness from the South to the North, lie unconformably (3 to 5°) on fluviatile to coastal plain carboniferous series (Namurian to Stephanien). The precise identification of the Hercynian Unconformity, consequence of a major tectonic event in the late Paleozoic, is one of the key points of the gas in place calculation. The most reliable investigations are given in the core studies:

Sedimentological analysis show, in most cases, the presence of a thin brecciated or conglomeratic layer at the bottom of the Permian and significant changes in depositional environment on each side of the unconformity. These changes are also evidenced thanks to inorganic geochemistry by studying within the shaly units the mineralogy of clays and the quantitative determination of trace elements (Bo, Sr, Ga, Li,...).

Electrical logs (NGS, DT, Resistivities...) remain the main analytical technique when the Permian-Carboniferous transition is uncored. The biostratigraphy based on pollens and spores analysis is often of no use due to barren series (oxidizing depositional environment or strong diagenesis). Finally, validation of the Hercynian Unconformity by magnetostratigraphy or K/Ar dating method is far too imprecise. In most cases, a single analytical technique does not help to locate the Hercynian Unconformity. The use of two or more techniques is often necessary.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995