The sedimentology and reservoir characteristics of the Lower Acacus formation, NC2 concession, Hamada Basin, NW Libya

Elfigih, Omar Bouzid (1991) The sedimentology and reservoir characteristics of the Lower Acacus formation, NC2 concession, Hamada Basin, NW Libya. Masters thesis, Memorial University of Newfoundland.

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Abstract

Analyses of core and wireline logs from twenty one (21) wells distributed inside and outside the study area demonstrate that the Lower Acacus Formation consists regionally (across the Hamada Basin) of at least fourteen progradational sandstone units (A1-A14) from south to north. Locally, in the study area, at least nine progradational sandstone units (A6-A14) exist. On the basis of a best-facies-fit approach the stratigraphic framework indicates deltaic packages change laterally northward, in a progradational fashion, from fluvial-channel sands to coastal-deltaic sands and silts and eventually to offshore-marine reworked sands and shales. The depositional patterns of some sandstone units in the Lower Acacus Formation are lobate. Sand axes in each unit are offset from those in the older units due to sediment loading and differential compaction within the delta complex. From core data and log responses the Lower Acacus sandstones are interpreted to been have deposited as deltaic lobes during shoreline progradation, where each lobe consists of two lithofacies; (1) a horizontal cross-laminated, clean sandstone or proximal delta front lithofacies which has the best reservoir quality, low clay content, and (2) a bioturbated silty-sandstone or distal delta front lithofacies (marginal), with poor reservoir quality and high clay content. -- Secondary porosity, found to be the dominant porosity type in the Lower Acacus Sandstones, was derived mainly from partial dissolution of carbonate cements, some detrital grains, and rock fragments. This porosity is characterized by oversized, elongate, corroded and irregular pore spaces. Primary pores are minor, but where present are small, concavo-convex and trianglular in thin-section cross-sectional shape and are usually associated with quartz-supported clean sandstones. -- Core plug porosity quantifies total porosity but does not distinguish micro-porosity from macroporosity (effective porosity), and shows an insignificant relationship with corresponding permeability values due to microporosity associated with clay-rich sandstones. Thin-section porosity on the other hand most significantly reflects corresponding permeability values and identifies meaningful correlation after the elimination of clay-rich sandstones. -- Oil-bearing sands with high reservoir quality are found to be closely related to sedimentary environmental facies. The best reservoir quality sandstone units are found in the proximal delta front facies at the top of deltaic lobes in wells which occur in the central and southern part of the study area. They are characterized by an average porosity of 20.3%, an average permeability 921 md., an average hydrocarbon saturation of 62% and a cutoff point of 48% for formation water saturation. The cumulative estimation (total hydrocarbon estimation) of hydrocarbons from multiple pay zones in the Lower Acacus Sandstones on a per well basis is 803 BOPD with 0.931 MMCFGPD, while the estimated oil-in-place for all structures producing from the Lower Acacus Sandstones in the study area is calculated to be about 190.12 MMSTBOIP and 117.19 MMMSCFGIP. -- Information from wells in the study area, with specific reference to GR log shapes, and integrated with sand isopach maps has resulted in paleogeographic maps which illustrate the depositional history of the Lower Acacus Formation and provides the basis for future exploration prospects.

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