The Balize delta in Plaquemines Parish, Louisiana, and the six preceding Holocene deltas offer models for subsurface interpretation. Nannoplankton counts were made from 65 bottom samples from the shelf area of the Gulf of Mexico off Louisiana. This work indicated a correlation between surface salinity and nannoplankton counts. In the subsurface, an ecology of outer middle neritic (water depth approximately 120 ft; 37 m) or deeper, accompanied by low (less than 5,000 per slide) nannofossil counts, indicates a deltaic environment. The Miocene Cristellaria "I" Hollywood and Krumbhaar sands, which were deposited by prograding deltas, are examples of nannofossil

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counts applied to subsurface deltaic interpretation. The major hydrocarbon reserves at the Hollywood and Houma fields are attributed to thick prodelta shale (low nannofossil counts) prior to sand deposition. This thick shale triggered faults and diapiric structures that were timed perfectly for receiving the early-migrating hydrocarbons.

The Hollywood and Krumbhaar deltas prograded over the thick shale depositing distributary-mouth bar sands. Accumulation in the Krumbhaar sand at the Hollywood and Houma fields was controlled by the lenticular nature of the distributary-mouth bar sand. Distributary-mouth bar sand "E" contains most of the reserves discovered to date in the Krumbhaar sand. Perfectly timed structure, faulting, and lenticular deltaic sand are responsible for this geographic concentration of hydrocarbons in a sand covering an area of 50 by 100 m (80 by 160 km).

The Krumbhaar sand was deposited by two distinct and separate deltas. The deltaic environmental setting for hydrocarbon accumulation in the Houma embayment area compares favorably with a similar Oligocene Vicksburg-Frio delta in Jefferson County, Texas. Deltaic information obtained from this study may serve as a subsurface model for discovering large reserves hidden by subtle deltaic traps.

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