"Evolution of diagenetic alterations in the Jurassic Lower Safa reservoir sandstones, Obaiyed Field, North Western Desert, Egypt"

The Obaiyed field is one of the largest gas producing fields in the western desert of Egypt. It mainly produces from the Middle Jurassic Lower Safa sandstones at a depth of some 4000 m. Detailed petrological and mineralogical... [ view full abstract ]

The Obaiyed field is one of the largest gas producing fields in the western desert of Egypt. It mainly produces from the Middle Jurassic Lower Safa sandstones at a depth of some 4000 m. Detailed petrological and mineralogical investigations were conducted to study the diagenetic features and their distribution in the Jurassic Lower Safa reservoir bearing sandstones in order to provide a better understanding for the reservoir behavior. Core samples selected from seven wells were qualitatively and semi-quantitatively examined using the petrographic microscope, X-ray diffraction (XRD) analysis and scanning electron microscope (SEM) equipped with backscatter (BSE) and energy-dispersive X-ray (EDX) capabilities.
The study sandstones are mostly of quartz arenite type with subordinate subfeldspathic arenite type, deposited in fluvio-estuarine environment. A complex diagenetic history characterizes these sandstones and resulted in interrelated features of cementation, compaction, replacement and dissolution belonging to both eogenetic and mesogenetic regimes. This diagenetic history is a reflection of an intense eodiagenetic evolution influenced by the fluvio-estuarine setting conditions and the Jurassic humid climate, followed by a progressive burial. The most significant authigenic minerals encountered are quartz overgrowths, kaolin, illite, siderite and pyrite showing complex compositional multiphases and microtextural variations. Eogenesis resulted in the formation of framboidal pyrite, patchy siderite/dolomite/Fe-dolomite, vermicular kaolinite and mechanical compaction. The main factors controlling the eogenetic regime are the texture as well as the detrital composition of the host sediments, climate, sea-level fluctuations and depositional pore-water physico-chemical conditions. On the other hand, mesogenesis resulted in the formation of complex inter-related diagenetic processes being paragenetically formed as; quartz overgrowths followed by chemical compaction and precipitation of euhedral rhombic siderite and Fe-dolomite synchronously with the eogenetic kaolinite recrystallization and transformation into dickite. Formation of illite as well as feldspar and dolomite dissolutions, represent late stage of the mesogenetic processes that followed by fracturing and poikilotopic pyrite cementation.