Palaeomagnetic reorientation of cores and the magnetic fabric of hydrocarbon reservoir sands

Abstract

Abstract The palaeomagnetic method is established as an important complement to more traditional methods for reorienting cores drilled in hydrocarbon exploration and production programmes. The method is based on the isolation of one or more components of remanent magnetization within the core. These provide records of the direction of magnetic north, either for recent times (carried by magnetic overprints in the recent geomagnetic field at the drill site) or at the time of formation of the sediment (carried by the primary magnetization). Provided that any spurious components of magnetization associated with drilling are removed by appropriate analysis in the palaeomagnetism laboratory, then the magnetic components of interest can be separately isolated. The mean direction of stable remanent magnetization, averaged over several tens of centimetres of core, defines the direction of geographical north in that core section. For a recent component, the direction of north is specified directly, whereas for an ancient component this is given by reference to the direction predicted from published apparent polar wander curves for that plate or geographical region. An important attribute of the palaeomagnetic core reorientation method is that the measurements are made on samples from the cores themselves, rather than by downhole instruments such as multishot orientation tools, dipmeters or imaging tools. This makes the technique particularly cost-effective and frees it from reliance on the proper performance of downhole tools or the correlation of planar features observed in the cores with those in the well. Consequently, there is no requirement for the well to be deviated or for there to be a significant natural dip to the formation (although this does help in fitting adjacent core pieces together to allow extrapolation of the reorientation data to longer intervals of core). Furthermore, as the determinations are made directly on the cores, the precision of palaeomagnetic reorientation is inherently greater than for methods based on measurements made in the well. It is possible to apply the method to cores for which the need for orientation was not apparent at the time of drilling, but for which this need arises as a result of later developments. The technique can be applied equally well to recently drilled cores or cores which have been stored for many years. An account is given of the palaeomagnetic reorientation method and its applications to (1) defining fracture orientations and (2) establishing sediment transport directions in a channel sand body. The transport directions are inferred from magnetic fabric measurements and the principles of the anisotropy of magnetic susceptibility technique are briefly reviewed.