Facies Distribution and High-Frequency Carbonate Cycles of Midyan Rift Basin: the Early Miocene Musayr Formation, Northwestern Saudi Arabia

A.I. Koeshidayatullah¹, K. Al Ramadan¹, G. Hughes², and R. Collier³

Abstract

In Northwestern Saudi Arabia, an extensive syn-rift carbonate system of Early Miocene age is exposed in three-dimensional outcrops displaying a variety of carbonate facies associated with well-preserved depositional architectures. The carbonates are assigned to the Musayr Formation of the Tayran Group, and were deposited on a hangingwall dip-slope ramp during the early rift phase of the Midyan rift basin of the Red Sea region. The age is based on the combined presence of the benthonic foraminiferal genera Miogypsinoides and Miogypsina. Based on outcrop and thin section analyses, the carbonate facies in this study location consists of: 1) pebbly sandstone with wackestone and mud-rich packstone facies (F1), 2) quartz and oolitic grainstone facies (F2), 3) mud-lean to mud-rich packstone and rudstone oyster bed, with peloid as a dominant constitutes (F3) and 4) wackestone and packstones, also characterized by the presence of stromatolite deposit (F4). All these facies types provide an interpretation of the carbonate ramp subenvironment division as follows: the presence of F1 indicates inner ramp environment with some localized intertidal sand, whereas the existence of F2 indicates the ramp-crest shoal with some inundation of high-energy wave activity. Facies F3 and F4 represent proximal and distal middle ramp environments respectively. Early Miocene carbonate facies of the Musayr Formation in the Maqna area are dominated by a suite of parasequence cycles that were deposited in ramp environment. The depositional cycles were defined by the vertical stacking from the succession exposed in the area, supplemented by detailed sedimentary log interpretation. Discrete packaging of the cycles is evident and represents the repetition of transgressive and highstand system tracts. Separation of tectonic from eustatic controls on deposition is under investigation as there is clear evidence for a tectonic control on regional thickness patterns, due to local extension faults in some parts of the succession,. Dolomitization has obliterated the primary feature in much of the carbonate facies within the middle ramp part. Reservoir quality based on the identified porosity from thin section is poor to fair (1-10%). This poor reservoir quality is mainly attributed to the pervasive dolomicrite filling the pore space.

AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain