Sedimentological analysis and hydrocarbon potential of the upper devonian-lower carboniferous Tahara sandstones, ghadames basin, western Libya

Abstract

The Palaeozoic (Upper Devonian-Lower Carboniferous) Tahara Formation, western Libya, was deposited in shallow-water, marginal and nearshore marine environments influenced by waves and storms. These are interpreted to represent interactive shelf to nearshore, shoreface and fluvio-deltaic environments. The analysis and interpretation of the subsurface data of this formation allow for the recognition of seven fades on the basis of core and electric log data obtained from each of the study wells. Each facies is identified on the basis of lithology, sedimentary structures and biogenic features, and four of these facies have been grouped into a shelf-nearshore facies association. Vertical and lateral facies relations enable the Tahara Formation to be divided into upper and lower sandstones. The upper sandstone shows an increase in shale content accompanied by increased bioturbation, towards the southwest. Sequence stratigraphic analysis indicates that the Tahara Formation is characterised by transgressive and regressive phases of deposition in response to changes in relative sea level. The overall succession stacks into two partial sequences separated by a Type 1 sequence boundary. The lower sequence below the sequence boundary represents a coarsening-upward prograding shoreline comprising sediments deposited as part of a highstand systems tract. Above the sequence boundary, the incomplete sequence 2, includes the Tahara sandstones. These are believed to have been deposited as part of a transgressive systems tract bounded at the top by a maximum marine flooding surface. The succession includes marine flooding surfaces of different hierarchical level which record a deepening of the depositional environment in response to sea-level rise. Both preserved sequences form part of a third-order cycle which in the upper part includes fourth-order higher frequency cycles. The deposition and distribution of facies within the sequences are controlled by different factors including rate of relative sea-level change, rate of subsidence and sediment supply. Gamma ray log shapes in the Tahara sandstones are variable from funnel to roughly cylindrical shape. The lower Tahara sandstone shows a serrate cylindrical gamma ray log shape in well C1-49 and AST2, and a funnel gamma ray log shape in well K1-1, A1-NC151 and AST1. The upper Tahara sandstone shows a funnel gamma ray log shape in well CI-49, a complex gamma ray log shape in well K1-1 trend, and a serrate cylindrical gamma ray log shape in wells A1-NC151 and AST2.The Tahara sandstones can be classified as quartz arenites and arkoses in terms of their modal composition. The detrital quartz grain size of the Tahara sandstones is generally fine to very fine-grained, and well sorted with rounded to subrounded grains. The overall grain-size trends are slightly variable in both the upper and the lower sandstone with the coarser grains concentrated in the upper part of the lower sandstone. Three types of porosity are recognized in the Tahara sandstones: (1) primary intergranular porosity which occurs as isolated pores; (2) secondary porosity, which is the most common porosity, originates from dissolution of unstable minerals or authigenc grains during fluid movement; and (3) microporosity which occurs within kaolinite clay minerals. The upper part of the lower sandstone has a porosity of up to 27.6%. This reflects the secondary dissolution of feldspar and the development of secondary porosity and pore-filling kaolinite clays. Palaeocurrent evidence from previous workers suggested that the sediments were derived from the uplifted Al Gargaf area to the southeast of the basin. Four sequential depositional models are proposed for the evolution of the Tahara Formation in response to relative sea-level changes:1. Marine shelf sediment was deposited during a relative sea-level rise. This is interpreted to form part of an early transgressive systems tract before deposition of the lower Tahara sandstone.2. The lower Tahara sandstone was deposited in a shoreface environment, and was gradually overlain by fluvio-deltaic deposits as the depositional system gradually prograded basinwards across the shelf. This sandbody overlies marine shelf deposits and records shoreline progradation and increased fluvial sediment supply from a nearby uplifted source area.3. The unit represents a short-lived marine transgressive event which terminated lower Tahara sandstone progradation as the depositional system temporarily retreated landwards.4. The final stage in the evolution of the depositional system was shoreline progradation during which the well-sorted, fine to very fine-grained upper Tahara sandstone was deposited in a shoreface environment. This sandbody once again overlies marine shelf deposits and records another phase of shoreline progradation, and the development of another a coarsening-upward package.