High Resolution Sequence Stratigraphy and Sedimentology of the Prolific Devonian Jauf Formation Gas Reservoir: Transgressive Tidal Estuarine and Regressive Wave-Dominated Shoreface Deposits, Kingdom of Saudi Arabia

Abstract

Abstract The Devonian System in the Kingdom of Saudi Arabia (KSA) forms a significant part of the Paleozoic succession on the Arabian plate and was deposited on an extensive coastal plain to shoreline to shallow continental shelf, dipping slightly to the north and east. The Devonian succession attains a thickness of nearly 2.5 km (8200 ft), though a significant part of this thickness is from the reconstructed Jubah Formation in deep basins to the east and north of KSA. The Devonian Jauf Formation is an important siliciclastic gas-bearing and producing reservoir in eastern Saudi Arabia, and it formed within a second-order progradational megasequence, lasting from the latest Ordovician to the culmination of the Hercynian orogeny in the very early Carboniferous. It also includes proven source-rock intervals related to plate-wide flooding events. We recommend that the top of the Jauf Formation, in the subsurface of the Eastern Province, be raised approximately 120–130 m (390–430 ft) higher than the currently adopted top by Saudi Aramco staff, to include all strata that are considered as part of Jubah by Saudi Aramco current usage. The present Jauf top (Saudi Aramco’s operational Jauf top) is picked at the top of a biostratigraphic zone (D3A), a practice not recommended by the code of stratigraphic nomenclature for formational tops. The contact this chapter proposes coincides with a prominent lithological change that marks a third-order sequence boundary (SB70). We believe that this distinct disconformable contact is of regional significance. We reconstruct the Jauf depositional systems using subsurface and outcrop data, emphasizing key regressive deltaic and transgressive estuarine shorelines as well as and associated fluvial, paralic, and shelf deposits. We demonstrate the importance of the fundamental fourth-order shelf-transiting sequences and larger third-order host sequences in the very extensive Devonian Arabian shelf building. We also draw contrasts between the Jauf Formation development in the outcrops of northwest KSA (including carbonate embayments) and those in the subsurface of the east and southeast parts of the KSA (wave-dominated strandplains and deltas and tide-influenced estuaries), where there was stronger fluvial supply. The Devonian Jauf shorelines show truly spectacular regressive and transgressive transits for hundreds of kilometers across the wide shallow shelf that sloped gently off the Arabian shield. In eastern KSA’s producing fields, the revised Jauf Formation thickens toward the east-northeast from 170 m (557 ft) to 343 m (1125 ft) and comprises three third-order sequences, referred to as SQ55, SQ60, and SQ65. Five new paleogeographic maps are presented for these three sequences. Each of the third-order sequences consists of several fourth-order sequences. Sequence SQ55 is dominated by a spectacular falling stage systems tract (forced regressive shoreface), which prograded from west to east across a distance of over 200 km (125 mi). The overlying sequences SQ60 and SQ65 each have a thick transgressive systems tract (TST) and a thinner highstand systems tract (HST). Depositional environments were mostly coastal plain and nearshore and ranged from wave-dominated shorefaces to tidally influenced estuarine embayment fills with tidal channels and bars to tidal and fluvial-dominated coastal-plain channels. Reservoir quality rocks are preferentially those deposited during the TST of SQ60 in tidal estuarine environments as channel-fills, bars, and bay-fill deltas. In these reservoirs, porosity was protected by grain-rimming clays from subsequent quartz cementation. The well-sorted shoreface facies, which dominates SQ55, tends to be of lower reservoir quality because of finer grain size and pervasive silica cementation.