Facies analysis of early tertiary carbonates of northeast Libya : results and discussions

Elwerfalli, Hamed Omar (1997) Facies analysis of early tertiary carbonates of northeast Libya : results and discussions. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Early Tertiary carbonates are known from both surface and subsurface exploration in NE Libya but they had not previously been the subject of detailed sedimentological investigation. The principal aim of this study, therefore was to undertake a detailed examination of the Palaeogene sediment facies, their distribution and environments of deposition, their mineralogy, geochemistry and diagenesis, and, as a consequence, the hydrocarbon potential of the area. The study is based on the analysis of many surface samples from five outcrops in the coastal area of Al-Jabal Al-Akhdar and a large number of core samples and cuttings from 14 wells ranging from depths between approximately 320 and 3900m.

The combined results of both macro- and micro-petrographic investigation indicate that the Lower Tertiary section in the study area comprises three main facies associations. These are: (1) slope facies association, including lithoclastic packstone, nummulitic packstone - grainstone, nummulitic wackestone - packstone, molluscan bioclastic wackestone, algal packstone, miliolid packstone and dolostone with anhydrite inclusions; and (3) lagoonal facies, including dolostone with anhydrite interbeds. The slope facies association occurs in the southwestern, northern and western parts of the area. The shallow marine facies association occurs mainly in central parts whereas the lagoonal facies association is found in the eastern part of the area behind the shallow marine bank. The principal control on development and distribution of these facies was sea-level variation.

The major minerals present in all facies are calcite, dolomite, anhydrite and quartz. Trace minerals such as celestite, apatite and pyrite are also present. The clay mineral species identified in the area include smectite, kaolinite, illite and mixed layers of illite/smectite and chlorite/smectite. The landmass in the southern part of Libya and adjacent African mainland was the main source of detrital clay mineral species supplied to the area. Geochemical results indicate that the SiO₂, Al₂O₃, K₂O, Fe₂O₃, Na₂O, Rb, Zr, Th, Ga, MnO and TiO₂ are the main components associated with the clay fraction, whereas Ni, Zn, V, Cr, P₂O₅ and U are closely associated with total organic carbon (TOC). The dolostone facies has a low content of Al₂O₃, K₂O, S_iO₂ and Fe₂O₃ compared with the limestone facies due to sparsity of clay content. The Sr content in the dolostone facies is low compared with the calcite-dominated facies due to the recrystallization process. TOC is more abundant in the chalk and shale than in the nummulitic limestone and dolostone facies.