A critical review of currently available pore pressure methods and their input parameters: glaciations and compaction of north sea sediments.

Abstract

Historically pore pressure evaluation in exploration areas was based on empirical relationships between drilling parameters, wireline logs and the mud weight. Examples include Eaton's Ratio and the Hottman & Johnson Methods, which were based on data from the Gulf of Mexico. These methods are not readily transported to other areas, such as the North Sea Basin, where the sediments are different in character and where burial and temperature histories are distinctly different. Data from several offshore North Sea wells, with high quality wireline and associated data have been analysed to determine the most appropriate method to estimate pore pressure in mudrocks. The data have led to an understanding of the key parameters for successful pore pressure estimation. The most effective method is shown to be the Equivalent Depth Method, but only where disequilibrium compaction is the source of the overpressure in the mudrocks. Core samples from 576 British Geological Survey sites in the offshore area of the British Islands were compared with > 10,000 porosities collected from the deep oceans (DSDP/ODP sites), which show that the porosities in the shallow section in the North Sea are anomalously low. The shallow section of the North Sea includes large volumes of Pleistocene-Recent sediments deposited as glacial and inter-glacial deposits. Frequency analysis (Cyclolog) of the wireline data covering this interval in several North Sea wells revealed a pattern in the relative featureless original data. Comparison with the global signature for oxygen isotopes for the same time period suggests that there have been ten cycles of ice sheet build up (Glacial period) followed by melting (Interglacial period) during the last one million years. Glacial deposits from 10 individual glacial cycles have therefore been identified in several exploration wells in the North Sea. Implications of loading/unloading of ice for the migration and trapping of hydrocarbons in the North Sea Basin are assessed.