Recognizing Fracture Networks on Seismic and their Contribution to a Working Petroleum System, with Focus on the Mandal High

Abstract

Fracture networks within the crystalline basement are the main contributors toa promising underexplored petroleum play. Advanced technology and elevatedoil prices have triggered an interest for old prospects to be re-opened and newstructures to be researched. The rock matrix of basement has low porosity andpermeability, and is dependent on faults and fractures to create storage space andproduction capability. During exploration and production, the fracture network isin focus, which makes the play more complex than conventional sedimentary plays.A pre- and post-drilling exploration workflow for a basement play is proposed.A three-dimensional broadband seismic data set in combination with integratedwell data were used to reconstruct the geological evolution of the Mandal High andits potential as a reservoir in this study. However, no confident basement well-tieto the seismic has been observed. The Mandal High is an elongated northwestsoutheasttrending horst, with five identified fault trends. Three wells penetratethe basement rock, which have identified fractures within the crystalline basementand hydrocarbon shows. Possible mature source rocks in the Feda- and Tail-End Graben and the Søgne Basin, as well as migration pathway are presented.Inverted structures, anticlines, salt and impermeable sedimentary layers act astraps and seals in the grabens flanking the high. Sediments eroded from the highand deposited on the flanks are also interpreted as potential reservoirs, as well asa weathered zone in the upper part of the crystalline basement. In order to betteridentify the faults and fractures in the study area, attributes were applied to theseismic volume. The Ant-Tracking volume was chosen as giving the best result dueto a more explicit imaging of the complex faults and fracture network. Filteringoptions enables specific strike and dip angles to be chosen. The Ant-Tracking mapcorrelates well with the raw seismic data.The basement rock within the Mandal High is either proposed as a migration barrieror a hydrocarbon pathway/storage space. Through the combination of welldata, seismic attributes and detailed Ant-Tracking map analysis, a partly openfracture network within the basement structure has successfully been detected.Therefore the Mandal High should be considered as a migration pathway or reservoirby itself, thereby opening up future exploration opportunities.