Abstract The Valhall Field is an Upper Cretaceous chalk reservoir located in the Central Graben area of the North Sea with production coming from the fractured Tor and Hod formations. Well tests and production history indicate that these formations are highly heterogeneous and that significant fluid flow occurs through both the matrix and fracture system. However there remained significant uncertainty about the specific controls and location of the main productivity conduits and how they would influence sweep efficiency during planned water flood. To address these uncertainties a range of possible conceptual fracture models were considered with respect to controls on major flow within the reservoir. Analysis indicated that the reservoir is dominated by a connected series of seismic scale faults acting as major flow conduits with smaller fractures providing a less significant enhancement to matrix permeability. A key input to this study was the examination of over 80 well tests. Simulation of a number of key well tests using a simple discrete fracture network model comprising a connected fault network and pseudo-matrix layer was able to reproduce the majority of the observed pressure derivative shapes. This gave some confidence to the understanding of major reservoir flow paths as well as providing calibrated fault properties for direct inclusion within the simulation model.

Abstract Stratigraphic and subtle combination traps have a well-documented track record as significant producing hydrocarbon resources. The majority of these success cases come from onshore USA where unprecedented drilling densities combined with a long history of hydrocarbon exploration provide a large portfolio of stratigraphic and subtle combination traps. By comparing USA-based examples with other global cases it is evident that numerous basins still have the potential for exploration success associated with these traps. This paper attempts to raise the awareness of the exploration potential of stratigraphic and subtle combination traps through four approaches. Firstly, a summary of key global statistics related to stratigraphic and subtle combination traps is provided with the intention of demonstrating that they have historically represented a key hydrocarbon resource. Secondly, a classification scheme is introduced and acts as a reference for the observations and case studies presented. The third component of this paper is to present a range of case studies, which serve to demonstrate the exploration history behind successful discoveries associated with stratigraphic and subtle combination traps. The final component is to consider the general exploration history and from this long experience highlight the key techniques necessary for the development of a successful exploration strategy.