Abstract The Jurassic Arabian Intrashelf Basin provides the setting for the world's greatest conventional oil reserves, including the world's largest oilfield, the supergiant Ghawar field. The stratigraphic interval corresponding to the development and infill of the Arabian Intrashelf Basin is from the uppermost Dhruma Formation to the top of the Hith Anhydrite Formation, spanning the late Bathonian–early Callovian to Tithonian. Many areas of the intrashelf basin have been well described in recent years and the stratigraphic succession has been defined in sequence concepts, but the regional development of the intrashelf basin has not been well synthesized. This Memoir builds on published data to give a regional interpretation of the geological evolution of the Arabian Intrashelf Basin. This introductory chapter reviews some of the earlier work, summarizes the key events and elements in the geological history of the Arabian Intrashelf Basin and gives a brief review of the history of petroleum exploration in this region. It is intended to serve as an extended abstract to introduce the general setting and summarize the contents of this Memoir, including some of the proposed revisions of depositional models, correlations and the sequence nomenclature, providing a context for considering and evaluating each subsequent chapter. The themes summarized in this chapter are documented and discussed in much greater detail in the subsequent chapters of this Memoir.

Abstract This chapter includes 11 cross-sections and one well log profile to show the depositional geometry and setting in specific areas around the basin: the Saudi Arabia outcrop belt; the Rimthan Arch; and the eastern and central areas of the intrashelf basin in Saudi Arabia, Bahrain, Qatar, Abu Dhabi and Oman. These cross-sections are used to demonstrate the similarity and degree of continuity of the upper Dhruma Formation, the Tuwaiq Mountain Formation, the source rock, the Hanifa, Jubaila–Arab and Arab–Hith formations and depositional sequences in these different locations in the basin. They show the manner in which the underlying platform formed, the rim developed, the source rock was deposited and the basin progressively filled. The blanket deposition of the Arab-D anhydrite was followed by the Arab-C to Arab-A and Hith carbonate and evaporite sequences. The cross-sections provide the framework used in subsequent chapters to make a series of facies maps and other interpretative diagrams and cross-sections that summarize and, for some intervals, revise the interpretation of the settings and geological events that formed the Arabian Intrashelf Basin.

Abstract This is the first of two chapters in which the data presented in the first four chapters of this Memoir are reviewed and interpreted. A summary cross-section across the basin from the Saudi outcrop to Abu Dhabi and the Tethyan margin is used to summarize the regional setting and sequences as interpreted in this Memoir. The key points illustrated by the summary cross-section are stated and discussed in the first part of this chapter. Two different interpretations for the eastern margin of the intrashelf basin are reviewed. In the first of these interpretations, the margin with the deeper waters of the Tethys Ocean shelf is adjacent to the intrashelf basin rim in Abu Dhabi, whereas in the second, preferred in this Memoir, a broad, shallow Tethyan shelf platform (200–300 km wide) extends from the intrashelf basin rim to the Tethys continental shelf edge. The implications of Late Jurassic exposure and erosion on the adjacent Tethyan shelf are discussed. The development of the Arabian Intrashelf Basin during the Callovian–late Oxfordian (the Tuwaiq Mountain Formation, the source rock and the Hanifa intervals and associated sequences) is interpreted and discussed with illustrations, including step-by-step facies maps. The interpretation integrates depositional, eustatic and tectonic factors in the evolution of the intrashelf basin. The interpretation in this Memoir is that the Arabian Intrashelf Basin began with isostatic and extensional subsidence on top of a broad Dhruma Atash Member platform, which had largely filled the accommodation space up to the wave base and to near sea-level. It developed fully into an intrashelf basin during the deposition of the Callovian Tuwaiq sequence, with rising sea-levels coincident with a productive shallow water carbonate factory resulting in a rim of shallow water carbonate. An end-Callovian to early Oxfordian lowstand terminated the Tuwaiq sequence on the basin rim. During the lowstand, restricted conditions in the basin deposited the rich Lower Hanifa source rock as a lowstand systems tract. As more normal conditions returned in a subsequent sequence, the source rock facies graded upwards into Hanifa reservoir facies, which partially filled the basin. Hanifa progradation was terminated by another lowstand during which a subaqueous gypsum/anhydrite marker bed was deposited in at least part of the remnant basin. Earlier interpretations of these sequences are also discussed.

Abstract Exploration of the Jurassic hydrocarbon system in the Arabian Intrashelf Basin area is in a mature state. Given the scale of the present day anticlinal structures and the adjacent synclines, all of the supergiant conventional fields trapped in huge anticlines have already been discovered. The theme throughout this Memoir has been to present the evolution of the self-contained Callovian–Tithonian Arabian Intrashelf Basin hydrocarbon system. Its size, c. 1200 × 450 km, is greater than that of the UK, larger than the Black Sea and almost as large as Turkey or the area of Texas and New Mexico in the USA. It is geologically much simpler than these regions, both in the exceptionally remarkable continuity of facies within the sequences that developed and filled the intrashelf basin and its relative tectonic simplicity, including up to the present day. The cross-sections, facies maps, depositional profiles and other data and interpretations presented in this Memoir have documented this remarkable continuity. The source rock interval is well-defined everywhere it occurs and is mature; enough oil has been generated and migrated so that every sealed trap with reservoir facies will have oil. Around and within the basin, shallow water ramp facies in each sequence are in the reservoir facies and the early-formed porosity has been preserved. The carbonate seals and, even more so, the evaporite seals are remarkably laterally continuous. Therefore the big issue in future exploration is finding a sealed trap with potential reserves large enough to be worth drilling when compared to known reserves and estimates of future production. This chapter discusses some possibilities for stratigraphic traps and unconventional plays. Potential plays have been and/or can be identified, but finding them in the present day structural setting is likely to be very difficult.