The Middle East contains some 65% of the world’s remaining oil reserves and some 35% of its gas reserves. The vast majority of these reserves are located on the Arabian Plate. In 2000, daily world oil production averaged 76 million barrels of oil per day (MM bopd) (H.E. Rilwanu Lukman, personal communication, 2000). Some 30 MM bopd (c. 40%) was produced by OPEC and some 22 MM bopd (c. 30%) supplied by the Middle East, primarily OPEC members Iran, Iraq, Kuwait, Saudi Arabia and the United Arab Emirates. In the 4th quarter of 2000, daily world oil production averaged 78.4 MM bopd (P. Shammas, personal communication, 2000), close to global supply capacity.
The current OPEC World Economic Model (OWEM) predicts that global oil demand will rise by 27 MM bopd (35%) to 103 MM bopd by 2020 (H.E. Rilwanu Lukman, personal communication, 2000). The model predicts OPEC taking more than 50% market share between 2007 (M. Naraghi, personal communication, 2000) and 2016 (H.E. Seyed Mehdi Mirmoezi, personal communication, 2000). Should this forecast transpire, Middle East oil production will need to be raised by at least 15 MM bopd, and possibly by as much as 22 MM bopd (i.e. double current levels) to meet this demand. The speedy and efficient development of this oil production capacity will be essential to power the global economy. Understanding the relationships between the reservoirs producing this oil, and their respective source and seal lithologies, is thus of paramount importance.
This GeoArabia Special Publication 2 presents the results of a study of the sequence stratigraphy of the late Precambrian and Phanerozoic sedimentary successions of the Arabian Plate, carried out by LASMO plc, a British independent oil company.
The sequence stratigraphic and chronostratigraphic interpretation is supported by a tectonostratigraphic review of the plate, and the identification, dating and correlation of 63 Maximum Flooding Surfaces (MFS). These should be regarded as candidate surfaces, as it is yet to be proved that they are isochronous and, in some cases, that they represent true maximum flooding. Although conceptually they are here each regarded as a surface, perhaps more accurately they should be considered as “maximum flooding intervals” - full integration of seismic, outcrop and well data being required to accurately identify the surface itself. The interpretation allows the existing disparate lithostratigraphic schemes across the plate to be placed for the first time within a unifying, sequence stratigraphic framework.
Maximum flooding surfaces were chosen as the primary layering tool due to their relative ease of identification, dating and correlation in the subsurface and outcrop (instead of sequence boundaries sensu Vail et al., 1977). The sequence stratigraphic interpretation here is thus presented using genetic stratigraphic sequences (GSS) sensu Galloway (1989). The study is based on a comprehensive review of the literature, together with the experience and observations of the authors. It is illustrated with numerous figures, together with a large-scale tectonic elements map and a plate-wide chronostratigraphic cross section from the Mediterranean coast of Lebanon to the Arabian Sea coast of Oman.
Until quite recently in geological terms (c. 34 million years ago) the Arabian Plate lay on the southern margin of Neo-Tethys, which stretched from northwest Africa, across Arabia and northern India to northern Australia. GeoArabia Special Publication 2 is the first publication of this nature to cover a significant segment of this important continental margin. Should the majority of the MFS presented here prove to be mainly eustatic in origin, as appears likely, the results of this study will have implications for the many other petroleum systems that are developed along the length of this ancient continental margin.
The primary benefit of the study is that for the first time it allows the plate-wide sedimentary succession to be sub-divided into isochronous packages of sediment, placing oil source rocks, reservoirs and seals within their systems tract positions. At the exploration scale, this allows greater prediction accuracy (i.e. reduces risk) in the likely geometric juxtaposition of source, reservoir and seal facies. This should lead to improved exploration well locations targeting structural and stratigraphic traps. At the field development scale, it allows the insertion of reservoir models into their appropriate systems tract position, and will lead to better definition of reservoir flow units and flow barriers, and thereby lead to reduced costs and improved recoveries. This should in turn allow more efficient field developments.
The structural review of the Arabian Plate identifies eleven tectonostratigraphic megasequences (TMS) separated by major unconformities. These megasequences reflect the evolution of the north and north-eastern plate margin from an intra-cratonic setting, through back-arc, to passive margin, and finally to the active margin setting of today. Within each of these megasequences the general pattern of accommodation space is interpreted to have remained essentially the same, allowing an understanding to be developed of the sedimentary response to fluctuating relative sea levels and sediment supply. The definition of megasequences thus allows the identified MFS to be placed within an appropriate structural framework.
The 63 MFS have been identified using biostratigraphic data, integrated with sedimentological and lithological data (e.g. the presence of shallow water limestones, representing transgression, within an otherwise proximal coarse clastic succession), and by the application of sequence stratigraphic concepts. They are located between upward-deepening (i.e. transgressive systems tracts) and upward-shallowing (i.e. highstand systems tracts) successions of sediments, and may be represented by a variety of lithologies, but most commonly by either deeper water outer shelf shales, or shallow water limestones. Each MFS has first been assigned a geological age, and then attributed a chronometric date following the time scale of Gradstein and Ogg (1996) for the Phanerozoic and Harland et al. (1990) for the late Precambrian. In general terms, the present day distributions of Mesozoic and Tertiary MFS are a function of both original extent and subsequent erosion, whereas the distribution of Palaeozoic and late Precambrian MFS are also a function of more limited well penetrations.
Of the 63 MFS, 33 are interpreted as being preserved over most of the Arabian Plate and provide the most reliable ‘lower order’ cyclicity within the succession. Of the remainder, 25 are interpreted as being preserved ‘sub-regionally’ over large areas of the plate, and 5 preserved only ‘locally’. These MFS are believed to be effectively isochronous and result primarily from global eustatic fluctuations. Subsidence probably controlled the development of MFS in the early period of each megasequence, when following a phase of plate-wide structuring, an extensional regime tended to pre-dominate. Progressive inversion and/or uplift towards the end of each megasequence is also thought to have played a role in the deposition and erosion of MFS. It is hoped that critiques and amendments of this new framework will provide further impetus to the unravelling of the subsurface architecture of this unique and economically important hydrocarbon province.