Structurally complex, heterogeneous, tide-dominated deltaic reservoirs in the Lower Misoa Formation (lower Eocene C members) in the LL-652 area of Lagunillas field in the Maracaibo Basin, Venezuela, have produced 166 million stock-tank barrels (MMSTB) of oil but have a low recovery efficiency of 22 percent. These reservoirs will contain more than 900 MMSTB of unrecovered mobile oil when primary recovery operations at the current 80-acre well spacing end. In this study we characterized lower Eocene reservoirs in the LL-652 area, Lake Maracaibo, to improve estimates of hydrocarbon reserves, to identify potential areas for secondary-recovery projects, and to establish a field-depletion plan to evaluate advanced recovery opportunities and extended development.A significant remaining oil resource lies in poorly drained or undrained reservoir compartments confined by a combination of complex structure and depositional heterogeneity. Sealing and partly sealing faults, including northwest-trending normal faults and younger north-northeast-trending reverse faults, bound large-scale structural compartments. Stratigraphic heterogeneity is controlled by dip-elongate, distributary-channel, and tidal-ridge sandstones that commonly pinch out over distances of less than 2,000 ft (>610 m). Because these two facies compose most of the reservoir sandstones, they contain most of the remaining oil.The main control on porosity and permeability distribution in the C members in the LL-652 area is depth. Volume of quartz cement in particular influences reservoir quality, and because volume of quartz cement increases significantly with depth, reservoir quality decreases with depth. Within each reservoir interval, however, depositional architecture controls porosity and permeability distribution. For example, significant permeability contrasts (as much as three orders of magnitude) exist locally between distributary-channel and tidal-flat, fluvial-estuarine channel and distal deltafront, and distributary-channel and delta-front facies, where clay clasts at the base of the distributarychannel facies may retard vertical fluid flow.Cumulative production varies greatly in each reservoir interval as a result of differences in ages of producing wells, greater-than-average production in areas of repeat section and inferred increased fracture permeability in zones of reverse faults, and differences in net thickness of perforated intervals. The tide-dominated deltaic depositional fabric, however, controls net sandstone and net pay thickness of each reservoir interval and therefore must affect cumulative production.We made maps of hydrocarbon pore volume (SoPhiH) and remaining oil on the basis of improved petrophysical characterization and production apportioning to specific reservoir horizons by permeability feet (kh). These maps indicate that most remaining oil lies in the poorly developed and structurally complicated north part of the field and where narrow (less than 2,000 ft [<610 m wide), high-SoPhiH belts are intersected by sealing and partly sealing reverse faults. The original oil-in-place resource base of the C members in the LL-652 area increased by 867 MMSTB (60 percent) to 2,318.2 MMSTB, mainly in the C-3-X and C-4-X members, by our identifying additional reservoir areas and improving quantifiquantification oi porosity and other petrophysical parameters. Extended development on the current 80-acre (1,968-ft [600-m) well pattern that has 97 new wells will increase reserves from 127 to 302 MMSTB. However, 116 MMSTB, in addition to the 302 MMSTB, can be produced from 102 geologically based infill wells strategically targeted to tap areas of high remaining oil saturation in narrow sandstone bodies that pinch out over distances of less than the current well spacing. Horizontal and inclined wells in steeply dipping strata in the C-3-X and C-4-X members can capture additional volumes of poorly contacted mobile oil. An integrated reservoir-characterization program that includes structural, stratigraphic, petrophysical, petrographic, production-engineering, and volumetric analyses can be used to improve oil-recovery operations in other mature, petroleum-producing provinces of Venezuela where many fields are nearing the first stages of primary depletion. The LL-652 area can serve as a model to demonstrate the most efficient means for recovering the remaining oil resource--a strategy targeting reservoir compartments defined by depositional-facies geometry and structure
Ambrose, W. A., Ferrer, E. R., Dutton, S. P., and others, 1995, Production Optimization of Tide-Dominated Deltaic Reservoirs of the Lower Misoa Formation (Lower Eocene) LL-652 Area, Lagunillas Field, Lake Maracaibo, Venezuela: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 226, 46 p.