Virtually all porosity is of secondary origin in the productive Lower Cretaceous Sarir Sandstones of the Calanscio area in the southeastern Sirte basin, Libya, where production is obtained from depths of about 8, 000 to 13, 000 ft (2, 438 to 3, 962 m). Principal reservoirs are fluvial sandstones now composed predominantly of quartz, but originally composed of up to 25% mud intraclasts, rock fragments, feldspars, and mica. Even though most of the original porosity was destroyed by compaction and cementation, deep-burial leaching of the non-quartz constituents created considerable porosity. Average porosity is 13%; the maximum is 31%. Most secondary pores are oversized molds of dissolved non-quartz grains. Skeletal feldspars and ragged metamorphic rock fragments are preserved in some layers. Commonly, feldspar and rock fragments are preferentially preserved in finer grained and muddy layers. However, even in some muddy sandstones, rock fragments, feldspars, and matrix were dissolved, creating secondary porosity.
The probable paragenetic sequence of major diagenetic events was: (1) hematite-clay coatings (red-bed units only); (2) quartz-overgrowth; (3) local clay, carbonate, and sulfate cementation; (4) compaction (ductile grains deformed); (5) leaching of non-quartz grains, cement, and matrix; (6) crystallization of authigenic kaolinite and minor illite and halloysite in some secondary pores; (8) minor dolomite cementation and replacement. Hydrocarbons migrated after kaolinite had partly occluded some pores. The products of diagenesis vary according to original composition, porosity, and permeability.