Abstract

The Neptune oil field, ultra-deep water Gulf of Mexico, was discovered in 1995 with the first well identifying oil in Lower to Middle Miocene turbidite sands. Follow-up appraisal wells across the Neptune structure were highly successful and proved the existence of a significant hydrocarbon accumulation in several sand intervals which contained variable quality oils with associated gases. This paper will address the source, charge, and oil quality issues associated with the Neptune Field.

The source of the Neptune Field hydrocarbons are the Upper Jurassic (Tithonian) section which in the Neptune area consist of clastic-enriched marls. By applying modeling techniques such as 2D TemisPack fluid flow models, the modeling results show a peak to post-peak mature oil kitchen surrounding the Neptune structure. Timing of oil generation and expulsion from the Upper Jurassic source rocks occurred from 12-0 Ma BP, and the various Miocene sands at Neptune were charged from about 5-7 Ma BP to present-day depending on the age of the particular sand.

Seals are the interbedded shales within the Miocene section, but these shales only hold a certain amount of oil column before buoyancy pressures exceed the shale capillary pressures, and then the oils vertically leak into the younger sands.

The older reservoir sands were charged at shallow burial depths under conditions conducive for biodegradation. Geochemical methods identified these zones where variable levels of biodegradation exist. Modeling results can explain the biodegradation through the charge and reservoir temperature histories observed within the Lower Miocene sand reservoirs.