Pore pressure, fracture pressure and geomechanical studies require accurately determined overburden stress magnitudes which sum up the combined weight of overlying materials to the depth of interest. Ideally, overburden stress is determined from in-situ density data. Such data is not available for pre-drill studies where offset data or an overburden model must be used. Often, a quick-fix approach is to assume a simple constant gradient (usually 0.9 psi/ft or 1.0 psi/ft) for the overburden or any missing sections of the density log. A simple overburden function or trend is not appropriate in the Central North Sea due to the abrupt changes with depth in lithology and sediment compaction state. In order to minimize this limitation, a new overburden stress model has been developed based on the four distinct, geologically driven, density trends that are seen in the Central North Sea. The sub-divisions are: (1) the over-compacted shallow sediments lying above the mid-Miocene Unconformity (MMU); (2) the progressively compacted sediments between the MMU and the Chalk; (3) the Chalk itself and; (4) the sub-Chalk sediments. The overburden stress (σv [psi]) due to each sub-division is dependent on its thickness (h [ft]) and is given by σv = 0.8858 * h above the MMU; σv = 0.9705 * h + 0.00002 * h2 from MMU to Top Chalk; σv = 1.1254 * h within the Chalk and σv = 1.1155 * h for deeper sediments.
AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015