This study in the well-documented Pliocene-Pleistocene South Eugene Island Block 330 (SEI330) field, offshore Louisiana, unravels a complex petroleum system by evaluating both the inorganic and organic geochemical characteristics of reservoir fluids. The brines at SEI330 dissolved halite prior to entering the reservoirs and equilibrated with reservoir sediments, exchanging sodium for calcium, magnesium, and other cations. The systematically varying extent of brine sodium depletion in two reservoirs defines south to north flow in those sands. These sands were filled from a fault that bounds the reservoirs on the south. Oil compositional parameters also show north-south variation across these reservoirs. The SEI330 oils and gases each had different sources. In contrast to published Jurassic sources for oil, carbon isotope data indicate that SEI330 hydrocarbon gases probably sourced from early Tertiary or Cretaceous sediments, after oil had migrated through them. Distribution of biodegraded vs. unbiodegraded oils indicates the reservoirs filled much more recently than formation of the salt weld beneath the field. Oil compositions indicate that some SEI330 oils were partially stripped of low-molecular weight compounds by their dissolution in a mobile vapor phase (gas washed) by large volumes of gas several hundred meters below the deepest reservoir. Modeling of this gas-oil interaction aids in identifying deep potential targets in which gas washing occurred.

Hydrocarbon distribution, combined with oil chemistry and reservoir pressures, indicate reservoirs filled from fault systems on both the north and south sides of the field. The fault feeders are wide (>100 m), structurally complex zones that can direct different types of fluids into different reservoirs.

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