New Insights on the Depositional Environments of the Amin and Miqrat Formations in Huqf Region Outcrops, Central Oman

Abstract

The early-middle Cambrian Amin and Miqrat formations of the Mahatta Humaid Group (Haima Supergroup) are extensively distributed in the subsurface of the Oman Interior Basins and act as deep, complex tight gas-producing reservoirs. The formations are well-exposed in the south to north of the Huqf region (central Oman) and thus serve as analogue for the subsurface equivalents. Depositional environment and sequence stratigraphic interpretations, which are essential for a better understanding of reservoir architecture and internal characteristics, are a key component in any exploration, appraisal, development, production, and enhanced recovery strategies. This study reports new insights on the depositional environment interpretation of Amin and Miqrat formations based on field observations in the Huqf outcrops region. In its basal part, the Amin Formation contains 4 m thick, poorly exposed red claystone and siltstone interbeds containing lenses of greenish-grey, very fine-grained sandstone overlain by brown, fine- to medium-grained, trough cross-bedded sandstone. This claystone, siltstone, and sandstone sequence lack any marine influence and was deposited in a relatively low topographic depression on top of the Huqf Supergroup surface suggesting its deposition in isolated fresh-water lacustrine environments. About 50 m thick unit of conglomerate grading upward into pebbly- to granule sandstone, coarse-grained sandstone and sometimes, siltstone and claystone interbeds rests either on top of the fresh-water lacustrine sediments or the Huqf Supergroup surface. The conglomerate sequence is dominantly clast- to matrix-supported containing up to a meter-thick large-scale trough- to tabular cross-bedding with low relief erosional base. The conglomerate sequence amalgamates laterally and vertically forming continuous bodies that tend to pinch out towards the northeastward. The interbedded pebbly- to granule-sandstones contain cosets of high- to low-angle trough and tabular cross-bedding exhibit pebble lags at the base at places. The claystone that sometimes interbeds with the sandstones is yellow in color and parallel laminated containing lenticular-shaped bodies of siltstones. The erosional scours with lag deposits, finning upwards trend and lack of any marine influence suggest deposition by channelized flows possibly associated with braided channels and the claystone with siltstones indicate overbank; floodplain and crevasse splay environment The uppermost fine- to very fine-grained sandstone unit of the Amin Formation exhibits trough to tabular cross-bedding and ladder to wave-ripples suggesting deposition in the beach to shoreface environment. The overlying Miqrat Formation, in its lowermost part, contains 80 m thick unit of dark brown to reddish-brown mudstones to sandy siltstones that grads into very fine-grained sandstones representing coarsening upwards cycles. This unit is traceable along most parts of the Huqf outcrops. The sandstones show parallel to low-angle cross lamination and wave ripples, whereas, mudstone and siltstone interbeds display large scale desiccation cracks. This may suggest deposition in marginal marine tidal; supratidal to subtidal environments. Miqrat Formation also contains 40 m thick light-brown to buff, fine- to coarse-grained sandstone containing low- to high-angle tabular to trough cross-bedding and wave ripples. Near the uppermost part of this unit, the sandstone contains Thalassinoides trace fossils, reported here for the first time, indicating the marine influence in subtidal environments. Miqrat Formation ends with a poorly exposed unit having a similar lithology and characteristics of the marginal marine tidal; supratidal to subtidal environments. Keywords: Mahatta Humaid Group, Amin Formation, Miqrat Formation, Huqf, Oman, Fluvial, Tidal, Marginal Marine

AAPG Datapages/Search and Discovery Article #90360 © 2019 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition of the Siliciclastic Reservoirs of the Middle East, Muscat, Oman, October 28-30, 2019