Identification of magnetic enhancement at hydrocarbon/water contacts

Pyrolysis experiments and calculated thermostability diagrams show that iron bearing minerals (< 60nm) can be produced inorganically during oil formation in the ‘oil-kitchen’ or be precipitated in the reservoir via alteration or replacement of existing minerals. Here we use this observation to find a magnetic proxy that can be used to identify hydrocarbon fluid contacts by determining the morphology, abundance, mineralogy and size of the magnetic minerals present in reservoirs. We address this by examining core samples from the Tay Sandstone Member in the Western Central Graben in the North Sea. The magnetic properties of core samples from the study area were determined using room-temperature measurements on a Vibrating Sample Magnetometer (VSM), low-temperature (0-300K) measurements on aMagnetic Property Measurement System (MPMS) and high-temperature (300-973K) measurements on a Kappabridge susceptibility meter while hydrocarbon fluid contacts were determined using wireline logs. We observed magnetic enhancements at both gas-oil and oil-water contacts that are detectable both through magnetic susceptibility measurements and magnetic hysteresis measurements. This magnetic enhancement is due to the precipitation of new nanometric iron oxide (magnetite) and iron sulphide (greigite) phases. The magnetic enhancement may be caused by diagenetic changes or preferential biodegradation at the top of the oil column during early filling and at the oil water contact.