Fault Seal in the Upper Slochteren (Rotliegend), Case Study from the Gillian Gas Field, Block L11c, Netherlands Offshore

In 2015 Oranje-Nassau Energie (ONE), operator of the Gillian Unit joint venture, drilled the L11-14 exploration well targetting the Gillian prospect. This well found the Permian-aged Rotliegend reservoir sands (the Upper Slochteren) gas bearing with virgin pressure. The Gillian gas field is a downthrown fault-closed tilted fault block, vertically sealed by Zechstein evaporites, in a location adjacent to the depleted L11b-A gas field. Pre-drill the critical risk for the Gillian prospect was the fault seal required to effectively trap gas in the Upper Slochteren sands. The present paper focusses on the pre-drill evaluation of the Gillian gas field, with a particular emphasis on the seismic reservoir characterisation using AVO-stacks and rock physics models, structural modelling and fault seal analysis. This case study demonstrates how a Rotliegend downthrown fault closure was derisked by a combination of conventional seismic interpretation complemented by seismic reservoir characterisation and structural modelling.