Palaeogeographic Reconstruction of a Tide-Dominated Oolite Shoal Complex in the Lower Arab Formation, Onshore UAE

Abstract

The Late Jurassic Arab Formation is being developed in a major gas accumulation located onshore southern UAE. The lower Arab Formation consists of wackestones to mudstones of mid ramp to basinal setting overlain by foreshoal and oolitic grainstone shoal deposits. Despite the number of well penetrations, several published models have been proposed for the palaeogeographic evolution of the shoal complex. The grainstone-dominated interval comprises skeletal and ooid-rich wackestones and packstones grading upwards into oolitic grainstones interpreted as the result of progradation of foreshoal to ooid shoal environments. A key element to understanding reservoir architecture has been the integration of core descriptions with borehole image logs, permitting recognition and re-orientation of the main bounding surfaces and cross-bedding sets. Existing depositional models for grainstone shoals are highly schematic and not useful for defining reservoir architecture. Two key surfaces constrain correlation and reservoir architecture: a basal hardground typified by dolomitised burrows and an upper erosion surface which terminates grainstone deposition. Shoal initiation was in the southwest of the study area with progradation of bioturbated foreshoal deposits with isolated planar cross-sets. Tidal reversals become more frequent upwards, with deposition mainly in offshore flood-oriented ooid sand ridges. The main part of the grainstone interval is dominated by a stacked parabolic sand shoal complex comprising cross-bedded oolitic and skeletal grainstones. Initial primary macroporosity in this facies association is pervasively occluded by calcite cements. Behind the shoal complex, planar laminated oolitic grainstones accumulated in flood-tidal deltas in a broad lagoon. Progressive infilling of the lagoon by sediment supplied from the shoal resulted in basinward (northeasterly) shifting of the shoreline and development of linear beach ridges. Bioturbated to massive grainstones capping shoal complexes are clean and only patchily cemented retaining a well-connected primary macropore network. Geometries, scales of cyclicity and facies distributions indicate analogs to modern tidally-influenced ooid ridges and parabolic bars of the Bahamian system.

Comparison of the diagenetic and reservoir quality evolution between the anticline crest and flank of an Upper Jurassic carbonate gas reservoir, Abu Dhabi, United Arab Emirates

“…Moreover, an exposure surface has been recognized on top of the upper Arab D (Lawrence et al, 2015). For a more detailed depositional and sequence stratigraphic framework of the studied field the reader is referred to Lawrence et al (2015), Hollis et al (2017) and Marchionda et al (2018).…”

Controls on porosity preservation within a Jurassic oolitic reservoir complex, UAE

“…The grainstones are thickest in the north eastern wells, whilst planar laminated oolitic skeletal grainstone facies (Gol) dominate in the southwest. Low angle lamination and accretion surfaces show southwest to westerly dips over most of the field with some dips to the northwest in the south (Lawrence et al, 2015). This suggests a tide-dominated, NW-SE trending shoreline protecting a back-barrier lagoon.…”

“…A depositional model and sequence stratigraphic framework for the Upper Arab D Member was presented by Lawrence et al (2015) and is summarized in Table 1, Figure 1C and Figure 3. Overall, six facies associations are interpreted to have been deposited along a south west-north east dipping, platform to basin transect from a shallow water, low energy lagoon fringed by ooid grainstone shoals and offshore slope and basinal peloidal skeletal pack-wackestones and lime mudstones.…”

“…The layer is capped by an irregular surface that is characterised by intercalations of green clay and sub-angular to rounded pebble-grade clasts of peloidal oolitic grainstone, peloidal packstone and lime mudstone. This surface is interpreted as an emergent surface and is referred to as the Upper Sequence Boundary (USB) (Lawrence et al 2015; Figure 2). …”

Limited thermochemical sulfate reduction in hot, anhydritic, sour gas carbonate reservoirs: The Upper Jurassic Arab Formation, United Arab Emirates

“…The mudstones and wackestones were deposited in an outer ramp environment and the floatstones represent storm events (Marchionda et al, 2018). The upper Arab D is composed dominantly of cross or parallel laminated or massive grainstones and packstones (Lawrence et al, 2015;Marchionda et al, 2018). The allochems in the limestones include ooids, peloids, and bioclasts.…”