Petrography of Abushyba Formation columnar-jointed sandstones (Triassic-Jurassic) from Jabal Nafusa- Gharian, NW-Libya
الخلاصة
A petrographic study is done on (15) samples cut from Abushyba Formation columnar sandstones deposited during the upper part of Late Triassic and the lower part of Early Jurassic periods from Jabal Nafusa that situated in Gharian Region, NW-Libya. About (20) thin sections were prepared at the Libyan Oil Research Institute in Tripoli. Ten (10) thin-sections were selected to characterize the columnar-jointed sandstones in the area of study which are subjected to petrographic studies using a polarized microscope at the Geology Department Laboratory in Al-Khoms City. Medium-grained, texturally mature, but chemically immature an arkosic arenite sandstone facies is found to characterize the studied samples which are thought to be deposited in riverine environments in which the sand grains were originated from granitic and gneissic outcrops. Clay and iron-oxide (and probably pyrite) rims and coatings around particles were evident and may form part of the cement. Euhedral quartz grains sometimes appeared with quartz overgrowth. Intergranular primary porosity as well Intragranular, fracture, stylolite and vug porosities were encountered. Deposition at increased depth of burial for the studied samples is proposed which was inferred from the presence of sutured, concavo-convex and elongated grain contact as well as the appearance of broken, over stacked and twisted or embayed and deformed grains where the porosity become reduced. In general, four (4) late diagenetic stages took place after deposition were found to characterize the studied samples: this include the "Redoxomorphic Stage": "at shallower depths remarked by iron-oxide, clay mineral and pyrite rims around grains as well as grain coatings, the " Locomorphic Stage" as depth of burial increased remarked by cement precipitation between grains which included clay material, iron-oxide and/or pyrite and silica (in the form of quartz overgrowth) cements, the " phyllomorphic Stage" at greater and increased depth of burial remarked by formation of mineralized material around grains (probably chlorite and micas)", the "Epi-diagenesis Stage" took place after the uplifting of the rock to shallower depths towards the surface and weathering agents activated lead to formation of many types of secondary porosities such as solution porosity, vuggy and moldic porosities as well fracture and stylolite porosities. The total porosity of Abu-Shaybah Formation columnar-jointed sandstones in Gharian area is enhanced through the last stage of diagenesis signifying the studied samples as having good reservoir quality for ground water (good water aquifer).