Advances in Arabian stratigraphy: Comparative studies of glaciogenic Juwayl and lower Unayzah strata (Carboniferous – Permian) of Saudi Arabia

The Juwayl Member of the Wajid Formation, which crops out in the Wajid region of southwest Saudi Arabia, and the Unayzah C and B members of the lower Unayzah Formation in the subsurface of eastern and central parts of the country are all demonstrably late Carboniferous to early Permian in age, based on palynological analysis. Comparative studies of these two stratal units confirm strong genetic similarities between them, suggesting stratigraphic equivalence between the lower part of the Juwayl Member and the Unayzah C member, and the upper part of the Juwayl Member and the Unayzah B member, respectively. The boundary between the lower and upper parts of the Juwayl was not specifically recognized in outcrop, being covered with thick deposits of modern desert sand. The lower Juwayl and the Unayzah C are both interpreted to have been deposited in large glacial valley systems. In the Unayzah C, subsurface isopach mapping suggests down-valley transport from the north, implying that at least some of the ice was situated in localized, upland (alpine) ice caps. Multiple phases of glacial advance and retreat occurred. Glacial outwash sands and gravels (representing retreat phases) were subsequently cannibalized and severely deformed by re-advancing ice and they are identified now only as relict deposits. Glaciotectonic deformation is manifest in the lower Juwayl at outcrop as high to low-angle thrust sheets, and throughout the subsurface numerous shear zones are recognized in the Unayzah C member, both in core as well as in downhole wireline log responses (spectral gamma-ray and image logs). The multiple phases of glacial advance and retreat produced push moraine nappe complexes within which wholesale explosive disruption and redistribution of the sediment is observed. This was a consequence of fluidization related to glacially-induced overpressuring. Potentially significant reservoir heterogeneity arises in the subsurface Unayzah C member in relation to the shear zones, by the creation of reservoir compartments of widely varying extent and with unknown but potentially very poor interconnectedness. The upper Juwayl Member is represented at the outcrop by pebbly sandstones and conglomerates that were laid down upon a braided fluvial, glacial outwash plain. These pass upwards into boulder-bearing siltstones (stratified diamictites) that were deposited in a glaciolacustrine setting. Similar depositional sequences are seen in the subsurface Unayzah B member in the western (basin-marginal) part of the study area. Locally, subsurface data from the Unayzah B in these western areas suggests sustained ice-contact conditions, interpreted as evidence for local sustained “alpine” ice caps throughout Unayzah B time. Farther east, the more basin-central deposits of the Unayzah B member comprise a wide variety of depositional facies, all of which are nonetheless attributed generally to a glaciolacustrine setting. These include: (1) minor ice-contact push moraine deposits indicative of minor glacial re-advance during overall glacial retreat; (2) thick sequences of ice-proximal sublacustrine gravity flow sandstones and massive diamictites; and (3) significantly more ice-distal sublacustrine sandstones and mudrocks, including stratified diamictites and dropstones. In both basin-marginal and basin-central settings, the Unayzah B facies associations everywhere display strong evidence for sustained melting during terminal retreat of the Gondwanan ice sheets. This resulted in sustained infill and deepening and eventual overspilling of the basin-central lakes leading to widespread flooding throughout the basin. This flooding was ultimately manifest also at the basin margins in the west of the study area. A high degree of reservoir heterogeneity occurs within the subsurface Unayzah B member as a result of the wide variety of depositional facies. Reservoir quality correspondingly ranges from excellent to non-reservoir.