Abstract
Available cores cut in the upper pool of Lower Nubian Sandstone Formation of 001/82 well drilled in OO structure, located in the southeast Sirt Basin, are analyzed according to their sedimentology, petrography, and diagenetic products. The analyses are correlated with the results obtained from wire-line log interpretation. Analysis of these logs on the basis of different cross plots results, with the support of petrographic and diagenetic investigation, allows analysis of the mineralogical composition, particularly clay minerals, and estimation of the depositional environment. Petrography examination shows that the sands are mineralogically and texturally mature. Diagenetic processes resulted in different cements such as authigenic quartz overgrowth, authigenic kaolinite with subordinate minor amounts of illite and chlorite. High readings of neutron and density logs plus low sonic and fair to medium gamma-ray reading characterize the very fine to silty sandstone while the opposite, with low gamma-ray readings, is true for medium to coarse sands. These values reflect quartz/kaolinite composition of the latter and presence of iron carbonates (e.g. siderite and/or ankerite) and pyrite in the former. These characteristics are confirmed by cross plots of ROMA (matrix density) versus UMA (absorption photoelectric cross section of matrix). Plotting of PEF (photoelectrical absorption index) against log Th/K and/or K confirm kaolinite as the most common clay mineral. Various cross plots obtained from analysis of spectral gamma-ray logs show that the sandstone are characterized by abundant thorium, minor amounts of potassium and locally uranium, interpreted as indicating fluvial continental environment. Higher amounts of potassium and uranium in the middle shale define a lacustrine environment.