Predicting Overpressures in Zechstein Stringers Based on Their Seismic Expression (2019)
Deze MSc scriptie is geschreven door oud-stagiair Peter Schilder.
Late Permian Zechstein intra-salt bodies of the North Sea Basin represent a drilling risk, as they can be highly overpressured. Folded and boudinaged clay–carbonate–anhydrite bodies (termed ‘Z3 stringers’) isolated in halite are present throughout the Southern Permian Basin and can often not be avoided when targeting deeper reservoirs. This study assessed the predictability of overpressures in Z3 stringers based on their seismic expression. Energie Beheer Nederland, the Dutch state oil company, set up a database that currently contains information of about 960 Dutch wells that have been analysed for drilling events with a significant geological component in the cause. Included were 40 cases of Z3 stringer penetrations that showed pore pressures close to lithostatic, the majority of which were related to carbonates. Seismic expressions of Z3 stringers that are correlated with an increased chance of encountering overpressures include a small size (<1 km2), areas of maximum curvature and areas within 300 m of a stringer’s edge. These are areas of enhanced fracture porosity and permeability, particularly in the carbonates. Rock mechanics dictate that in a small volume of fractured material surrounded by an impermeable boundary, most of the stresses are applied to the pores, resulting in pore pressures up to lithostatic. The two proposed mechanisms of overpressure generation in the Zechstein are disequilibrium compaction through effective sealing of deformed Z3 stringers by halite and fluid expansion as a result of gas generation.