The Bunter Gas Play and its Seismic Expression in the West Netherlands Basin

Abstract

The Lower Triassic Bunter gas play is a major exploration objective in the West Netherlands Basin with proven gas reserves of about 65 x 10[sup 9] m[sup 3]. The upper and middle Bunter reservoirs consist of continental clastics. Hydrocarbons are trapped mainly in horsts and tilted fault blocks, created during the Late Jurassic. Many structures suffered Late Cretaceous to Early Tertiary inversion due to convergent oblique-slip movements. Top and lateral seals are provided by marls and (anhydritic) claystones of the Muschelkalk and Keuper formations and shales of the Jurassic Altena Group. The main hydrocarbon type is gas originating from the underlying Carboniferous Coal Measures; and oil rim is present in some traps. The most important exploration risks are poor reservoir quality in areas of strong inversion and fault seal integrity. During the last 10 yr, Nederlandse Aardolie Maatschappij drilled almost 40 Bunter exploration wells and acquired about 3300 km[sup 2] of 3-D seismic, both onshore and offshore. Three-dimensional seismic data, although variable in quality due to the different sources used in this urbanized area, has greatly improved the structural definition of the Bunter play. Seismic amplitude anomalies, generally conformable with structural closures, have been identified in the south, the areamore » of best reservoir development. Minor changes in amplitude and wavelet character on seismic correspond with the seismic expression of gas/water contacts from Bunter modeling studies.« less