Middle to Upper Khuff (Sequences KS1 to KS4) outcrop-equivalents in the Oman Mountains:Grainstone architecture on a subregional scale

Abstract

ABSTRACT The upper part of the Saiq and lower part of the Mahil formations in the Oman Mountains represent outcrop time-equivalents to the highly prolific, hydrocarbon-bearing subsurface Middle and Upper members of the Khuff Formation (K4-K1 reservoir intervals). In this study, four outcrops sections on the northern flank of the Oman Mountains (Al Jabal al-Akhdar region) are sedimentologically documented and integrated with the sequence-stratigraphic scheme initially developed at the Saiq Plateau reference section. The focus of this study is the description of the distribution and textural variation of grainstones as potential reservoir facies on a subregional (ca. 60 x 40 km) scale. Stratigraphic cross-sections are constructed based on two sequence-stratigraphic orders: (1) one second-order supersequence (DS2 18) that provides a well-constrained general framework and (2) four third-order depositional sequences (KS1-KS4), within which subtle temporal and spatial variations of grainstones occur. From these correlations predictive rules and correlation lengths of shoal bodies are extracted. The presence of Khuff grainstones is strongly governed by stratigraphic position. Thicker and more abundant grainstones are present during the early transgressive (KS4) and late regressive (KS1) portions of the supersequence. Thinner and less abundant grainstones are present during the late transgression (KS3 and lower KS2) and early regression (upper KS2). They are absent around the second-order zone of maximum flooding (middle KS2). High lateral continuity of correlated cycle sets is observed, suggesting the absence of significant tectonic activity of the area during the Late Permian and Early Triassic. Integrated litho-, bio-and sequence stratigraphy provides a robust framework for correlation on a regional scale (ca. 700 km). The Oman Mountains area shows a more distal facies pattern on the Khuff platform compared with other Khuff reservoir sections in the region. This is especially evident around KS2 maximum flooding with muddy foreshoal and offshoal deposits in contrast to mainly oolitic shoal deposits in the Musandam (UAE) and offshore Fars (Iran) area.