SEQUENCE STRATIGRAPHY, PALAEOGEOGRAPHY AND PETROLEUM PLAYS OF THE CENOMANIAN – TURONIAN SUCCESSION OF THE ARABIAN PLATE: AN UPDATED SYNTHESIS

A. D. Bromhead ^1*, F.S.P. van Buchem ², M.D. Simmons ¹, and R.B. Davies ³

¹ Halliburton, 97 Jubilee Avenue, Milton Park, Abingdon, OX14 4RW, UK.

² King Abdullah University of Science and Technology, Thuwal 23955, Saudi Arabia.

³ Davies Geoconsulting Limited, London.

^* Corresponding author: alex.bromhead@halliburton.com

In order to facilitate the search for new play concepts and exploration opportunities, a sequence stratigraphic synthesis of the Cenomanian–Turonian interval of the Arabian Plate has been compiled. The synthesis is based on published datasets which have been analysed within a temporal framework constrained by biostratigraphy and isotope stratigraphy. The high stratigraphic resolution allows the palaeogeography of the study area to be mapped within 3rd order depositional sequences, and the relative influence of eustacy and tectonics on basin development to be evaluated. This significantly improves the prediction of stratigraphic architecture and depositional morphology at the scale of the entire tectonic plate.

Conceptual models informed by outcrop and subsurface observations have been applied to characterize the development of intrashelf basins in depositional settings that are either isolated from siliciclastics (symmetric intrashelf basin model) or influenced by siliciclastics (asymmetric intrashelf basin model). The application of a sequence stratigraphic model across regional well log transects facilitates an understanding of stratigraphic architecture and acts as an important control for the generation of a new suite of gross depositional environment (GDE) maps. These maps characterize the palaeogeography at a previously unprecedented resolution and scale during both periods of high relative sea level (maximum flooding surface and highstand systems tract) and low relative sea level (lowstand systems tract). The maps are complemented by sequence isopachs which reveal changes in accommodation through time and space.

This approach helps characterize the preserved distribution and stratigraphic configuration of petroleum systems elements. In the Shilaif (UAE), Natih (Oman) and Sarvak (Iran) intrashelf basins, condensed, organic-rich carbonate source rocks were deposited in restricted, anoxic conditions. These basins resulted from differential aggradation of the carbonate platform during the transgressive systems tract. Grainy, rudist-debris –rich carbonate reservoir rocks developed along the margins of the intrashelf basins during highstand progradation. Claystones in the overlying sequence may form intraformational seals and were deposited during retrogradation of the shoreline associated with sea-level rise.

By contrast in the Najaf intrashelf basin (Iraq), there is a significant siliciclastic component sourced from the Arabian Shield. This influx resulted in a mixed carbonate-siliciclastic ramp depositional system on the proximal, western margin of the basin, and an apparent absence of organic-rich intervals within the central basin succession. Grainy carbonate reservoir rocks are restricted to the eastern margin and are charged by older source rocks within the underlying stratigraphy.

The GDE maps record the configuration of these petroleum systems elements within the Cenomanian–Turonian interval and form the basis for play screening. The Cenomanian–Turonian interval is punctuated by the major mid-Turonian unconformity which is a tectonostratigraphic boundary of considerable geological and economic significance. In the southern and eastern part of the Arabian Plate, this unconformity has a differential erosion profile that facilitates a subcrop play with exploration potential. Identifying subtle stratigraphic traps is challenging, but by combining GDE facies with the mapped preservation limit for each sequence, it is possible to identify areas where rudist-rich reservoir facies with potential karst enhancement are overlain by a regional claystone seal, high-grading areas with subcrop trap potential.

Key words: Mid-Cretaceous, Arabian Plate, sequence stratigraphy, Gross Depositional Environment, intrashelf basins, reservoir, source rock, seal, subcrop trap.

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