Diagenetic Alterations of the Outcropped Lower Triassic Mahil Formation (KS-1 Khuff-Equivalent) in the Oman Mountains, North Oman

Abstract

The Lower Triassic Mahil Formation in North Oman is the outcrop time-equivalents to the hydrocarbon-bearing reservoir, Upper Khuff Formation (referred to as KS-1 cycle). The studied sections of the KS-1 cycle in the Saiq Plateau and Wadi Sahtan of North Oman are ca. 50 m thick. They are dominated by dolomitized grainstones, less commonly dolomitized mudstones, and brecciated dolostone. The detailed petrographic analyses revealed various diagenetic alterations affecting pore types, pore throats, and pore size distributions. The identified diagenetic alterations include (i) near-surface to shallow burial (eodiagenesis), fine- to medium-crystalline dolomite, mostly with non-planer texture, through prevalent dolomitization of carbonates (allochems and matrix) via modified marine pore waters (δ18OVPDB = −4.2 to + 0.2‰), (ii) shallow burial (eodiagenesis), mechanical compaction as indicated by closely packed carbonate allochems grains, (iii) chemical compaction and stylolite development at the onset of deep burial (mesodiagenesis), (iv) precipitation of deep burial (mesodiagenesis), various types of coarse-crystalline dolomite cement, e.g., saddle texture cement (typically for deep burial) and rarely calcite cement via evolved marine pore waters, (v) fracture-filled calcite and dolomite cement with saddle texture, which is typical for deep burial (mesodiagenesis) and/or with equant drusy texture, which is typical for meteoric water diagenesis indicating its formation during telodiagenesis, and (vi) carbonate allochems grains and cement dissolution during progressive burial (eo- to mesodiagenesis) and/or during telodiagenesis. This study revealed that the Lower Triassic Mahil Formation in North Oman is deeply buried. Consequently, the porosity is destructed by extensive cementation and compaction through near-surface and progressive burial. However, on the other hand, porosity is enhanced by fracturing and dissolution of allochems grains during progressive burial and subsequent uplifting.