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Evidence for a Hydrodynamic Aquifer in the Lower Miocene Sands of the Mad Dog Field, Gulf of Mexico

Dias, Troy 1; Tett, David 1; Croasdaile, Michelle 1
1 Petroleum, BHP Billiton, Houston, TX.

Hydrodynamic aquifers, which are associated with tilted hydrocarbon fluid contacts, have been observed in various basins around the world. This paper presents evidence for a regionally extensive hydrodynamic aquifer in the Lower Miocene sands of the Atwater Fold Belt in the deepwater Gulf of Mexico, believed to be the first of its kind in such a deep sub-salt structure. The evidence for a hydrodynamic aquifer comprises formation MDT pressure data from various drilled structures in the Atwater Fold Belt area, as well as mapped wide-azimuth seismic data, drilled oil-water contacts and production data from the Mad Dog field.

The inferred mechanism for creating the hydrodynamic effect is believed to be mechanical compaction and dewatering of the large column of sediments in the deepwater Gulf of Mexico. Due to increasing overburden pressure, the aquifer gradually becomes overpressured, and expulsion of aquifer brine occurs after the fracture gradient is exceeded at a weak point in the regionally connected sand system, creating an escape valve to shallower formations. It is theorized that the geographic locations of the areas of highest compaction and of the “escape valve” control the direction of flow in the aquifer.

The observed hydrodynamic aquifer has a pronounced impact on the oil-in-place distribution for the Lower Miocene sands of the Mad Dog field, where a tilted contact caused by a hydrodynamic aquifer is required to explain the oil-water contacts observed from drilling results and seismically-mapped spill points. The presence of a hydrodynamic system in the Lower Miocene of the Gulf of Mexico may also impact the understanding of the charge history and fluid contact distribution within other reservoirs and exploration play types in this deepwater region.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009