Geologic Controls on Remaining Oil in Miocene Transgressive-Barrier, Coastal-Plain, and Mixed-Load Fluvial Systems in the Miocene Norte Area, Lake Maracaibo, Venezuela
Authors: Ambrose W.A. , Wang F.P. , Akhter M.S. , Skolnakorn Jirapa , Mendez Milton , Alvarez Regulo 
Published: December 2007

Abstract
A geologic characterization of the 2,500-ft Miocene section in the 18-mi2 Mioceno Norte Area in northern Lake Maracaibo, using core, wireline log, petrophysical and limited engineering and production data, demonstrates the control of facies architecture and structure on reservoir geometry, watered-out wells, oil depletion, and distribution of remaining oil. Although the Mioceno Norte Area has produced oil since the 1940's, this work found that appreciable volumes of oil (commonly 400 to 1,200 MSTB [thousand stock-tank barrels]) remain at the current 984-ft well spacing. Existing estimates of remaining recoverable reserves were confirmed by decline-curve analysis of production data, as well as by Lagoven's prior reserve estimates. This study documents an additional 161.6 MMSTB (million stock-tank barrels) of current recoverable reserves that can be produced through geologically targeted recompletions, infill wells, horizontal wells, and water-injection wells in all Miocene reservoirs in the Mioceno Norte Area. This study differs from previous approaches to Mioceno Norte Area field development. It integrates detailed facies models with petrophysical analysis to assess and map remaining oil and then targets locations that have the greatest potential for additional oil recovery. We allocated oil production to thin (50- to 150-ft) stratigraphic units using So h values calculated from petrophysical analysis of wireline logs. These So h values were used for calculating original-oil-in-place volumes. The difference between oil in place and the cumulative production of each producing well is the remaining oil volume over a 20-acre area. Remaining-oil maps were compared with lithologic, oil-production, and water-cut-map trends to delineate additional recovery potential.
These methods can be applied to reservoir-development programs in other fields in the final stages of primary recovery. Technical contributions include improved understanding of geologic controls on reservoir performance, a practical method for making remaining-oil maps, and an approach to optimizing recovery through infill drilling.

Introduction
Oil recovery is governed by a variety of geological and engineering factors, including structure, facies architecture, diagenesis, and history of field development. Facies architecture and structure define three-dimensional reservoir architecture, which in turn controls fluid-flow pathways and influences field depletion patterns. The recovery efficiency of many reservoirs varies according to depositional environment. For example, homogeneous barrier-island and wave-dominated deltaic and shoreface reservoirs typically have high oil recovery (commonly &gt;50%), whereas fluvial-deltaic reservoirs, which are heterogeneous, exhibit lower recovery efficiencies of between 20 and 40%.
Volumes of oil remaining in mature reservoirs approaching the end of primary recovery can represent a substantial fraction of the original oil in place (OOIP). For example, previous studies by the Bureau of Economic Geology (BEG) of lower Eocene tidal reservoirs in the LL-652 area in Lagunillas field, Lake Maracaibo, document an additional 20% of OOIP (468 million stock tank barrels [MMSTB]) producible through targeted infill drilling and secondary recovery projects.
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