Musandam peninsula evolution, structural styles, and petroleum implications: new insights from surface and subsurface data from northern Oman Mountains (UAE, Ras Al Khaimah)

Abstract

The Musandam peninsula (northern part of the Oman thrust-and-fold belt), although flanked by a foreland basin rich in petroleum resources, has remained so far without a proven play. As part of an exploration project that covered partly the exposed external part of the peninsula made up of mainly Mesozoic carbonate platform, i.e., Arabian (including the equivalents of the producing reservoirs), the study of its outcrops, easily accessible along existing valleys, became the best opportunity to record details far beyond the resolution allowed by seismic surveys or wells and thus, to refine the structural model. The purpose of the paper is to update the general tectonic framework based on novel observations presented as series of outcrop pictures, supplemented with two interpreted seismic lines imaging the subsurface structure. These observations were made in around two hundred points where bed and/or fault/fold axis positions were measured or estimated, eventually modifying an existing geological map. Although it has been generally considered that in Musandam peninsula the fault systems affecting the Arabian platform were related in essence only to the continuing shortening since Late Cretaceous (after the obduction of the Semail ophiolite), the first outcomes of the fieldwork refer to those tectonic events pre-dating the contraction in the foreland plate which have not been previously noticed, namely a widespread Upper Triassic extension followed by inversion and development of a major angular unconformity at the Jurassic base (an equivalent of the “Old Cimmerian phase” encountered elsewhere along former Neo-Tethys margins). Within the newly installed carbonate platform, a second but milder angular unconformity occurs at the Dogger/Liassic boundary. Surprisingly, two ubiquitous ~ 20-m-thick reservoir beds—Upper Triassic and Lower Jurassic—leaked (everywhere accessible) hydrocarbons or hydrogen sulfide odor upon hammering. Other new outcomes are related to the structures and tectonic displacements induced during the shortening period, which appear far more complex and significantly larger than usually interpreted, as documented by series of thrusts making a duplex system, farther coupled within the foredeep to a triangle zone as supported by seismic data; these data also allowed to depict an offshore canyon initiated roughly at the Oligocene base which lies in continuity with a few small Paleogene clastic units cropping out just downstream the northern Musandam valleys (in UAE). The last (Late Miocene) major outcropping structure is a NW–SE-trending long and steep reverse fault running from near the confluence of valleys Shah and Bih in the south, up to at least the western parts of the Tibat and Bukha offshore hydrocarbon fields (Sultanate of Oman) in the north (called herein Bukha-Tibat-Ghalilah-Rahbah fault), which cuts obliquely the former nappe pile. As it appears to be synchronous with the orthogonal, also major, Dibba fault (SE margin of the Musandam platform), but of opposing kinematics (the latter is normal), one can infer that ~ E-W sinistral wrenching had acted upon the peninsula before the shortening was transferred to the Zagros system. Coeval with them or younger, a system of normal faults trending variously around N-S, with throws up to 100 m, was mapped. The main fieldwork results reported in the paper bring significant improvements to the understanding of the Musandam belt by highlighting previously overlooked tectonic events (particularly the early rifting), refining the structural geometry of the successive fault systems and adding new constraints on the regional kinematics and evolution. Furthermore, the onset of the Musandam nappe pile erosion can be placed confidently at around the beginning of the Oligocene, older than previously considered. Through the depiction of the aforementioned reservoir beds and a direct indication of a new perspective in terms of viable source rocks in the subsurface, the accompanying suggestions may trigger a rejuvenation of the prospectivity-oriented research.