الملخص الإنجليزي
The Late Carboniferous-Early Permian glacial deposits of the Al Khlata Formation are an important target for oil companies in Oman. However there are a number of uncertainties due to vertical and lateral variations in the glacial deposits. The complexity increases on the Eastern Flank of the South Oman Salt Basin (EFSOSB) as Cambrian to Recent sedimentary rock sequences are highly affected by the withdrawal and dissolution of underlying Infracambrian salt.
A regional 3D PETREL model of Al Khlata Formation has been created for first time in the Rima area using correlated geological surfaces constrained within surfaces interpreted from 3D seismic. Four distinct lithofacies have been identified from wireline logs and their distribution assessed within the model. The Al Khlata, which reaches thicknesses of up to 800 m in the area, is dominated by sandy diamictite (40%), with lesser proportions of shale (30%), silty diamictite (20%) and sandstone (10%). These deposits are interpreted as being deposited in glacio-lacustrine, glacio-deltaic and glacio-fluvial settings. The interval containing the highest proportion of reservoir sand is the P9 (15%), whereas the Rahab Member comprises mainly shale (70%) resulting in an excellent top seal to many hydrocarbon accumulations. Other shales or thick silty diamietites form more localized intra formational seals and increase the potential for further discoveries of oil to be made in areas of thick Al Khlata.
A regional 3D Structural Model has also been created for the post-Ara to pre-Al Khlata interval (Hugf. Nimr, Haima and Misfar Supergroup/Groups) and for the Gharif Formation to track the periodic removal of the Infracambrian Ara Salt and its impact on the overlying sequence. A detailed description of the depositional history and structural trap formation of the Rima fields has been achieved, as the former peripheral synclines were progressively inverted to form turtle-back anticlines. The inversion of pods and the foundering and draping of strata has resulted in localized faulting and fracturing of the sediments. Large volumes of the hydrocarbon have been trapped within turtle-back and draped anticlines.
Thick deposits of the Nimr Group in Ara Salt depressions resulted in the formation of the Ghubara, Rasha, Qadam, Faras, Tannum, Maha, Rukh, and Raihan anticlinal trap structures. The two largest and thickest depositional bodies of the Haima Supergroup formed in adjacent Nimr mini-basins where they led to the formation of the major Rima main, and smaller Jalmud North, structures. Thick deposits of the Misfar Group resulted in the formation of the Thurayah, Tarish, Wifaq, Nawal and Ghanimah field trap structures. Thick pods of Al Khlata deposits, particularly those of the P9 interval, are responsible for the formation of the Jawdah, Jalmud, Shahin, Tabrir, Saham and Nuham field structures.
Continuing salt movement during the deposition of Gharif resulted in thinner deposits over each of the pre-Gharif pods and thicker deposits elsewhere. This is contrary to the traditionally held layer cake model for the Gharif interval.
A consistent regional correlation of the Rahab interval is problematic due to lateral variations in thickness and lithofacies architecture. The interval comprises one thick lacustrine shale in the south, two shales separated by a thin diamictite in the central area, and only one thin upper shale and a thick diamictite in the northern area. There is the potential to reconstruct the paleogeography of ice margins (indicated by diamictites) and lakes for this last major Al Khlata deglaciation event.
There is conflicting evidence for the occurrence of the Misfar Group deposits in the Rima area. Glacial facies evident in core imply the so-called Misfar deposits belong to the Al Khlata Formation and contain reworked Mid-Devonian palynomorphs. The interval is dominated by sand and, to a lesser extent sandy diamictite, and its depositional trends are similar to those of the overlying Al Khlata P9.
The accuracy and reliability of this regional 3D model can be improved by updating it periodically with new data, new interpretations and by adding further detail. It is recommended that reviews are made of the character and occurrence of the Misfar Group across PDO's concession area, and also of the palynological subdivision of the Rahab interval in the Rima Satellites Small Fields area.
