الملخص الإنجليزي
The Late Cambrian Al Bashair Formation of Haima Şupergroup is distributed over north-central Oman subsurface. The extension of this formation is exposed at surface in a relatively small area named Qarn Mahatta Humaid, in the north of Huqf region. This formation was previously considered a seal for the underlying reservoirs of Haima Supergroup. However, recent explorations show successful and commercial production from the sandstone intervals of this formation in blocks 3 and 4 of Oman. Lithologically, Al Bashair Formation can be divided into Upper and Lower members. However, the Lower member comprises interbedded mudstone, sandstones and various types of carbonates whereas the upper member consists predominantly of mudstone interbeds with thin siltstone layers. This project aims to investigate reservoir potentiality and quality distribution of the Lower Al Bashair in the blocks 3 and 4. This study is based on (i) a detail description of two conventional cores for lithofacies determination, clarify the lateral and vertical continuity via correlation and depositional environment interpretation, (ii) the analysis of thirteen wells using Petrel and Techlog Schlumberger's software were used to identify lithology, detailed petrophysical interpretation (iii) textural and compositional analysis on thirty-three thin-sections collected from various lithofacies as well as diagenetic alterations distribution and their subsequent impact on reservoir quality evolution. A detail description of two conventional cores from two drilled wells in block 3 and 4 led into identification of eight distinct lithofacies namely (i) interbeds of sandstone/ siltstone with claystone, (ii) flaser sandstone bedding, (iii) lenticular mudstone bedding, (iv) claystone, (v) oolitic limestone, (vi) hummocky (vii) wavy rippled sandstone and sandstone to siltstone. The presence of oolitic limestones, hummocky cross stratification sandstones, wave rippled sandstones and bioturbations indicate that Lower Al Bashair Formation was deposited in shallow marine system. Conventional wireline logs collected from thirteen wells were used to identify lithology, petrophysical properties, lateral and vertical continuity of different reservoir intervals from the studied fields. Five types of electrofacies were identified, calibrated and compared with the constructed lithofacies from both conventional cores. The reservoir units of formation have been found to be laterally continuous in the three fields. The average limits for Lower Al Bashair Formation of volume of shale av. 34 %, porosity av. 13 %, permeability 3 mD and oil saturation av. 51 % where the highest reservoir quality found in Farah South Fields. The investigation of thin sections under optical microscope has shown that mechanical and chemical compaction of grains together with various types of cementation played major roles in porosity deterioration. Dolomite and calcite and anhydrate cements are significant in porosity reduction. The porosity enhancement is attributed mainly to partial dissolution of dolomite cement and thus development of intergranular secondary porosity. Integration of wireline logging and conventional core data helped in increasing our ability for better understanding of Lower Al Bashair Formation depositional environment and diagenetic alterations, reservoir quality distribution and continuity.
