Drilling for oil in Sirt Basin as well as in other basins in Libya and elsewhere may encounter rocks and structures that were not initially anticipated. Intraformational igneous rocks in a core or in cuttings that look fresh can be misinterpreted as basement, and if a decision is made on such an interpretation, it can be costly inasmuch as it could miss an oil-bearing horizon below. Severely altered intrformational igneous rocks of granitic and related composition can be misinterpreted as sandstone causing confused interpretation no matter how good the seismic work was. On the other hand, poor knowledge of what the basement is made of can lead to errors in evaluation of the drilled section inasmuch as igneous rocks or even some metamorphic rocks such as hornfels can be mistaken for real basement when in fact they might not be. The Arabian Gulf Oil Company (AGOCO) had the conviction that there was a need to determine what could be considered basement-like transient/intraformational horizons and what is most likely a persistent basement. Eighty-one (assumingly basement) wells cored at bottom depths of 798-3,876 m within three large AGOCO concessions in in southeast Sirt Basin covering an area of approximately 28,000 sq km constituted the bulk of a study we carried out almost two decades ago.

Rocks encountered in the subsurface include both igneous and metamorphic rock suites, although the frequency of wells where a metamorphic basement was encountered is much greater. The igneous rock suite includes granite, granodiorite and minor diorite and gabbro. Andesite and altered aphanites of apparently an intermediate composition are encountered in a few wells and there are instances where altered aphanites underlie granite or intrude into metagreywacke and  schist.  On the other hand, the metamorphic rock suite, which is largely of greenschist to amphibolite facies, consists mainly of metagreywacke and quartz-mica schist, minor argillite, quartzite, gneiss, amphibolite and migmatite. Although psammitic and pelitic lineage is indicated, some of these rocks, but especially the latter types appear to have had igneous precursors. Both the igneous and metamorphic rock suites, but the latter in particular, show a variety of structures. These include layering (mineral banding/segregation), foliation, folding, fracturing, faulting, brecciation,  cataclasis, and mylonitization. Pyrite and associated minerals are evident in several igneous and metamorphic rocks, but are more widespread in the meta-pelites. Pyritization is developed in association with mobilized and/or segregated quartz and occurs independent of it as well.

It is concluded that the subsurface igneous and metamorphic rock assemblages of southeastern Sirt Basin are not much different from some of the surface assemblages of the Pan African Tibestian basement; and although both can be partly correlated, each is distinguished by certain features. Some of the common features of both, however, is the occurrence of suites of metagreywacke, quartz mica schist, quartzite, amphibolite and granite and related rocks and gabbro, although it cannot be said that the granites in the subsurface and in the Tibestian basement, are very similar. The prevalent metagreywackes and mica schist in the subsurface compare favorably with parts of Western Tibesti. However, no counterpart of the high-grade gneiss, schist and granoblastite of Eastern Tibesti has been found in the subsurface. Moreover, none of the syn-tectonic diorites which characterize the higher-grade parts of the Tibestian basement appear to have been emplaced in southeastern Sirt Basin. In fact, neither has there been an encounter in the subsurface of any of the very coarse-grained to pegmatitic granite and granodiorite that are very common in Western Tibesti.

Published geochronological data on the subsurface basement indicate a Pan African age, which is also the age of all surface basement occurrences in Libya with the exception of Al'Oweinat Basement Province.