Abstract
Siliciclastic Ghariff reservoirs in Southern Oman fields are prolific reservoirs of fluvio-deltaic origin. These reservoirs are classified into Upper, Middle and Lower Ghariff sands depending on the geological environment of deposition. These reservoirs are developed via an initial primary depletion phase followed by secondary recovery techniques of water flood (Al Kalbani, Al Barwani, et al., 2023). One aspect of the field's development history is that the most promising reservoirs; the middle and upper parts of Lower Ghariff were given the highest drilling and development priority. Over the years, focus has shifted towards poorer quality reservoirs. These are thin, laminated and low permeability reservoirs. The lower most sub-units (LGB) of Lower Ghariff sands are an example of these poor-quality reservoirs which require stimulation to achieve economical production targets
The field discussed in this paper is an erosional truncation on one side of the Haima turtleback anticline. The Nahr Umr (Wasia) Unconformity erodes much of the top of the Gharif, generally leaving the Middle and Lower Gharif reservoirs and occasionally the lower parts of the Upper Gharif. The Gharif Formation has undergone major changes in deposition styles through time. During the Late Carboniferous to Early Permian, Oman was situated around 50 degrees South Latitude and gradually moved north (Sharland, P. R. et. al 2001). This period was cold over much of Gondwanaland with deposition of the glacio-lacustrine Al Khlata Formation in Oman. The Rahab Shale represents the final melt phase of the Al Khlata and was deposited as localized ponded deglaciation features (Osterloff, 2000) over a relatively widespread area of Southern Oman. The base of the Lower Gharif is a transgressive marine flooding surface due to relative sea level rise at the end of the Permo-Carboniferous glacial period. In the south, the LG B unit consists of marginal marine deltaic sediments while further to the north it grades laterally to wave dominated shoreface and deltaic environments. The LG B unit varies greatly in thickness even within a field and is likely to represent the "topographic healing" phase post glaciations. Above this, the LGA unit is relatively uniform in thickness. The LGA unit is dominated in the north by the Haushi Limestone and in the south by shoreface and deltaic deposition so the main channels recognized are distributary channels. The Middle Gharif unit shows a major basin ward shift in facies across the Lower Gharif Unit boundary. In the south of Oman the shift is marked by a change from marine shoreface deposition to terrestrial semi-arid palaeosols. Thicker sandstone sections within the Middle Gharif are often channels sandstones or braided river deposits. The upper part of the Middle Gharif is a widespread palaeosol. The top of the palaeosol (HSGHM) marks the boundary between the Middle Gharif and the Upper Gharif. This surface represents the breakup unconformity of Gondwanaland with a 5-10 million years break in deposition. During this depositional hiatus, Oman moved further northwards causing a climatic change to humid tropical conditions. The main reservoirs in the Upper Gharif are fluvial to braided channel deposits as well as small lacustrine deltas. The top of the Upper Gharif reservoir section is identified by the top sandstone. Above the upper sandstone there is a transgressive shale transforming upwards into the Khuff Carbonate. The shale is thick in the south and inter-fingers with the carbonate in the north suggesting lateral facies change. (Osterloff, P. 2004; Alsop, D. 2015).
