Depositional Environment, Geochemistry, and Diagenesis of Paleocene and Early Eocene Carbonates of Agdabia Trough, Libya

Ahmed, Sharif S (1992) Depositional Environment, Geochemistry, and Diagenesis of Paleocene and Early Eocene Carbonates of Agdabia Trough, Libya. MSc(R) thesis, University of Glasgow.

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The study of the Paleocene and Early Eocene rocks of the Agdabia Trough, on the eastern edge of the Sirte basin of Libya, was undertaken to assess the depositional environment, the geochemistry, the succeeding diagenetic processes, and the resultant porosity of the sequence, and to investigate the causes of low permeability. The results indicate predominantly slope deposition. The investigated sequence has been divided into seven lithofacies. These start with lithoclast bearing mudstones which are interpreted as debris flow deposits, associated with hemipelgic muds deposited at the base of the slope. These are overlain locally by thin pelletal packstones which are followed by chalks. The sequence is capped by lithofacies with large nummulites and finally by lithofacies with small nummulites. In the northern part of the area dolomite dominates sometimes co-existing with diagenetic evaporites. The slope stretched more than 30Km from the Cyrenaica platform to the basin centre where relatively deep water sediments dominated. The early Eocene interval documents a dramatic change in deposition and transition to shallow water sediments, represented by nummulitic limestones. The sedimentary pattern in the Eocene was dominated by shallowing upward cycles in the nummulitic limestone. These are belived to be due to falls in sea level which caused progradation of shallow water nummulitic banks over deep water slope deposit. Many diagenetic alterations have been revealed. Diagenetic events began with dissolution and compaction, locally succeeded in the northern area by dolomitization. A second stage of dissolution followed by anhydrite deposition post dated the dolomitization. Equant calcite is the dominant cement and it is present mainly filling intraskeletal chambers of foraminifera. Dolomite dominates the Paleocene section in the northern part of the area. The geological, petrographical and geochemical evidence implies that the dolomitization process was accomplished by penetration of highly saline fluids derived from the Cyrenaica platform and took place under more than 1Km of burial. Cathodoluminescence petrography reveals four types of correctable luminance within the studied wells. These include bright matrix and cement, dull luminescence, non luminescence and finally dull matrix with two cement stages. The distribution of these types in the area implies that oxic water driven from the Cyrenaica platform which gradually changed to a more reduced state down slope. The lateral changes in the luminescence types are paralleled by similar changes up section which are interpreted as due to progressive subsidence or burial. Geochemical studies show moderate strontium (Sr) values, in the limestones rangeing between 322 to 1600ppm, while in the dolomite Sr is relatively low. Other trace elements , such as iron (Fe) and manganese (Mn) are generally low, while sodium values are quite high in the dolomite reaching 7000ppm. The isotope analyses carried out chiefly on the dolomites, show depletion in delta18O, with quite negative values ranging between -2.4 to -8.3‰ (PDB). The equilibrium relationship calculation between delta18O of the dolomite, the delta18O of the precipitating water and the temperature reveals that the temperature of the precipitating water lay between 50-80°C and that the water was enriched by +5‰ in delta18O with respect to the sea water. This implies that the water which was responsible for dolomitization was an evaporated sea water. The delta13C values of the dolomites range between 0.6 to 2.5‰ (PDB), reflecting formation in water in equilibrium with marine sediments. The Sulphur isotope (delta34S) results from the anhydrite average 21.37‰ (CDT). This figure is consistent with the range of delta34S isotopic values known for marine evaporites of Tertiary age. Fluid inclusions in the dolomites reveal an average recorded temperature of 105°C and salinity ranges between 162-210ppt NaCl equivalent. Inspection of porosity in thin-sections and sonic logs suggests that the large nummulite lithofacies which apparently has good porosity might serve as a good reservoir rock, but more detailed study is required. Porosity and permeability are enormously influenced by calcium sulphate precipitation which occluded some porespaces and narrowed porethroats, resulting in a reduction of the permeability in the area.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Keywords: Geology, Geochemistry, Sedimentary geology
Date of Award: 1992
Depositing User: Enlighten Team
Unique ID: glathesis:1992-78398
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:28
Last Modified: 30 Jan 2020 15:28

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