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Hickman, Robert G.1, Mike Hursey2, Paul van Daalen2, Daniel van Staalduinen3, Fred Houtzager4 
(1) Structural Solutions, Sugar Land, TX 
(2) Unocal Thailand Ltd, Bangkok, Thailand 
(3) Unocal Netherlands, The Hague, Netherlands 
(4) Consulting Engineer, Bangkok, Thailand

ABSTRACT: Structural Evolution of an Inversion Anticline and Commercial Development of Horizon Field, Dutch North Sea

Horizon oil field is a northwest-trending anticlinal trap formed by inversion. In the Triassic/Early Jurassic northwest-striking normal faults developed across the region. During the Late Jurassic and Early Cretaceous, additional normal faults developed and some older Mesozoic faults were reactivated. Graben development at the site of Horizon field, locally preserved Jurassic Posidonia Shale source rock and affected the thickness and facies of the Early Cretaceous Vlieland Sandstone reservoir. 
NNW-SSE latest Cretaceous-Early Tertiary compression reactivated the southwestern graben-bounding and graben interior faults as oblique-slip reverse faults. These faults propagated upward as thrusts, producing a north-verging, asymmetrical anticline. At the NW end of the anticline, new thrusts developed that sole into Early Jurassic shales; at the SE end a box fold developed above a detachment in the same shales. 
Original oil in place was 191 MMstb, but the recovery factor and vertical well production rates are low because of the tight nature (porosity ~12.3%, permeability ~0.17 mD) of the Vlieland Sandstone. The field was developed by horizontal wells targeted by 3D seismic to intersect small faults near the top of the Vlieland. Larger faults were avoided to prevent upward water fingering. This approach was successful although some fault zones were calcite cemented. The flanks of the fold have an effective fracture network; the less strained crest does not. Monitoring of mud losses while drilling was successful at identifying permeable fracture zones and estimating the initial productivity of wells. Ultimate production is estimated to be 16.7 MMstb produced largely from the fracture network.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.