AAPG Middle East Region Geoscience Technology Workshop

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Reservoir Quality Prediction and Diagenetic Controls of Gharif Reservoir (a Case Study from RSF, South Oman)

Abstract

The distribution of Gharif sediments is widespread across Oman and is equivalent to the Unayzah A and Basal Khuff clastics in Saudi Arabia. In South Oman, Gharif is considered as one of the most prolific oil-bearing sandstone reservoirs. In this study, a total of 69 SWC samples from 7 different wells located in 7 different oil fields of RSF with a detailed applied routine core analyses (porosity and permeability measurement) and petrographical analyses (include thin sections, scanning electron microscope “SEM” and X-Ray diffraction analyses” XRD”) were used to evaluate the mineralogical and diagenetic effects that control Gharif Reservoir. Gharif reservoir is very heterogeneous in terms of sand quality (grain size, sorting, mineralogy, clay content, effects of diagenesis) with variable geometries and connectivity. The analyzed samples are classified (after Dott, 1964) as Subfeldspathic Arenite / Wacke, Feldspathic Arenite / Wacke, Sublithic Arenite and Lithic Wacke. Gharif formation subdivided into three major reservoir units (upper, middle and lower) in which their quality varies and ranges from very poor to good because of various diagenetic processes (include two regimes eogenesis and mesogenesis), which have influenced the porosity and permeability. The key minerals that were recorded in the analysed sandstone samples and reflect the two diagenetic processes as follows: (i) pore-filling detrital clays matrix (early eogenesis); (ii) eogenetic pore-filing kaolinite has a vermicular and book-like habit; and forms under humid climatic conditions in continental sediments by the action of low-pH groundwaters on detrital aluminosilicate minerals such as feldspars, mica, rock fragments (mainly igneous-granite fragments as they are rich in feldspars); (iii) pore-filling and grain-coating chlorite occurred early in eogenesis and continued till middle stage of mesogenesis; and (iv) carbonate cements include mainly calcite and dolomite; and both occur as ferroan and non-ferroan compositions. This is clearly revealed by the sensitivity of the mineral to Fe-detecting. All main carbonate mineral cements that are found in clastic rocks can develop during eogenesis as well as mesogenesis. Eogenetic and burial diagenetic carbonate cements frequently have an exaggerated effect upon the flow properties of sandstone because of their patchy fill pores or preferentially block pore throats. In general, the identified pore types in the sandstones of Gharif include: (i) secondary inter-and intraparticle porosity formed due to partial to near complete dissolution of interparticle carbonate cements and labile grains (feldspars and lithic fragments); (ii) fracture porosity within detrital grains due to shattering of grains as a result of tectonic activity or overburden pressure; and (iii) microporosity within detrital clays, kaolinite booklets and chlorite plates identified by SEM.