Stratigraphy
The Sarir stratigraphic column generally resembles succession patterns throughout the Sirte Basin, with some variations. In the early regressive phase, basal sandstones were deposited on a Precambrian basement of igneous and metamorphic rocks. Sandstones are dated on angiosperm pollen as younger than Albian, probably from the Late Cretaceous. After a lengthy hiatus, represented by unconformity and sandstone erosion, a transgressive sequence of red, green, and purple Anhydrite shales was laid. Variegated bed remnants occur in crestal sections of many northern structures, such as in wells B-1-65 and C-1-80.
Above the transgressive sequence are Late Cretaceous shales with tight, micritic carbonate, marking the top of the Mesozoic. These shales thicken into troughs, providing the field's sole source rock. The youngest fauna are Maastrichtian, with an apparent disconformity between the Late Cretaceous and Paleocene, marked by high levels of gamma radiation on logs.
The Paleocene is also a carbonate-shale succession with carbonate build-ups over structural highs, forming some of the basin's best reservoirs. Although thick carbonates exist over the field and other highs in C-65 and surrounding areas, no hydrocarbons were found. Reservoir water is usually fresh or brackish, suggesting flushing. Limestones were extensively dolomitized, resulting in large cavities and lost circulation while drilling.
In the Lower Eocene, Basin conditions became restricted, producing an alternating dolomite and anhydrite sequence with a consistent thickness. The mid-Eocene saw development of a wide carbonate platform, richly nummulitic, also constant in thickness. There are argillaceous limestone interbeds, marl, and occasional calcareous sandstones. The Late Eocene reflects more frequent lateral variations of interbedded limestones, dolomites, marls, and shales.
Recent to Oligocene succession consists of; (1) lower zone of fine to coarse sands with some clay partings and dolomite beds; (2) middle zone of gray-green, red-brown shales and clays; and (3) an upper zone of largely unconsolidated, slightly feldspathic sands. This is typical of the area where total thickness is 914 m, almost equally divided into the three main components.
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