The Importance of Recognising Hydrodynamics for Understanding Tilted Fluid Contacts

Abstract

By recognising and quantifying the regional distribution of overpressure a better understanding of hydrocarbon distribution can be built. Changes in aquifer overpressure represent fluid potential driving hydrodynamic flow. When hydrocarbons are trapped above a dynamic aquifer the hydrocarbon-water contact becomes tilted; the magnitude of the tilt is controlled by the differences in overpressure and the relative fluid densities. Structural closure may no longer be the key control on fluid distribution which is now being controlled by the hydrodynamic spill point. If the distribution of hydrocarbons is no longer controlled by structure then the placement of exploration or appraisal wells requires careful consideration. If a fluid contact can be shown to tilt down to the West then a well drilled on the East will encounter a shallower contact, or possibly water if the tilt is significant, and the decision may be made to abandon a prospect. However, a well drilled on the West may have a deeper than expected contact leading to an interpretation of a significant discovery. Furthermore, if the fluid contact is tilted then the volumetrics of the trapped hydrocarbons changes, either positively or negatively, but must be assessed. In all cases the impact for future exploration, development and appraisal are important. The Miocene reservoir system has been shown to be laterally drained (Hauser et al, 2013), i.e. low overpressure reservoirs sandwiched between high overpressure shales in the K2 (Sanford et al, 2006) and Knotty Head (Williams et al, 2008) Fields. Systematic changes in reservoir overpressure have been identified in the location of the Mad Dog Field as well as a tilted hydrocarbon-water contact (Dias et al, 2009). The work presented in this discusses in more detail the likely impact on hydrocarbon distribution for the Miocene reservoirs. Furthermore, the paper will extend the understanding of the geological-pressure controls on fluid distribution to the Lower Tertiary Wilcox play and comment on the likely impact for that system. The major implications of hydrodynamics are; a) Identification of regional overpressure gradients dictating natural aquifer flow and regional extent of sub-salt hydrodynamic reservoirs (natural pressure drive/support to a field), b) Identification of tilted fluid contacts and the impact on hydrocarbon distributions and volumetrics and c) Description of the impact for other reservoir systems in the Gulf of Mexico, e.g. the Wilcox.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014