CHAPTER 1 INTRODUCTION
Published:January 01, 2010
What it is and what it is not
The Lexicon is intended to be a reference and practical, operational guide for all those involved with the subsurface geology of Oman. It provides a systematic overview of the current state of knowledge of the stratigraphy of Oman’s hydrocarbon basins. The intention is to summarise and describe objectively and not to enter too deeply into specific debates.
The Lexicon draws heavily on numerous published and unpublished reports and documents, most notably Hughes Clarke (1988) and Mohammed et al. (1997). In some cases, for rock units with historically limited academic or economic interest, the ‘current state of knowledge’ relates directly to these two publications. It is therefore a testament to the coverage and quality of these two publications that many of their conclusions and descriptions still apply today.
It is intentional that the descriptions remain general. They are not based on sequence stratigraphic principles, although the megasequences of Sharland et al. (2001, 2004) are referred to where relevant and other references may be cited for further detail. The text discussions do attempt to be as comprehensive as possible and placed within a meaningful and up-to-date geological context. The ultimate aim is to educate the reader in terms of the available data and the key issues relevant to each rock unit. Where lines of evidence conflict, this will be noted but not necessarily resolved. The Lexicon can be a starting point for more detailed individual investigation.
The Lexicon will help the wellsite geologist to pick his formation tops, whilst for the production geologist or the explorationist it provides the geological context to review, correlate and understand their reservoir, seal or hydrocarbon system.
The Lexicon does not cover Oman’s surface stratigraphy as mapped and published by others, notably Glennie et al. (1974) ‘Geology of the Oman Mountains’ and the Geological Maps of Oman compiled by the Bureau de Recherches Géologiques et Minières (BRGM) in the late 1980s and early 1990s (see amongst others Le Métour et al., 1993). It does discuss correlation between subsurface and surface stratigraphy where relevant, but the main focus remains Oman’s subsurface stratigraphy.
Why this Lexicon?
The Lexicon is based on data from many exploration and production wells that have been drilled, mainly by Petroleum Development Oman (PDO), augmented with what is known from outcrop studies and occasionally from data outside Oman. Several generations of geologists, from both industry and academia, have been involved with the Exploration and Production of oil and gas in Oman since the late 1950s. Sharing the wealth of information accumulated through the years with the wider geological communities in Oman and beyond is essential to further refine our understanding of the rocks that are so valuable to all of us. A common, available, definition and description is a prerequisite for meaningful information sharing.
The Lexicon discusses Oman’s subsurface stratigraphy, starting with the youngest rocks and ending with the oldest basement, based on a practical ‘down-hole’ approach. The lithostratigraphic descriptions are hierarchical from Supergroup, Group, Formation to Member level.
The ‘approach and conventions’ section in this introductory chapter outlines the descriptors that are used systematically and the conventions that are followed, for each rock unit.
In additional to the formal lithostratigraphic definitions and associated descriptions, each chapter also provides an overview of the tectono-stratigraphic setting as it is currently understood. A general introduction to the tectono-stratigraphy of Oman is provided in the last section of this introductory chapter.
A wall chart showing Oman’s complete subsurface stratigraphy is included as Enclosure 1, together with a CD containing the full content of the Lexicon.
A general location and structure map, a legend for the reference well figures and a summary of the Oman subsurface stratigraphy are provided together as a foldout at the back of the Lexicon (Enclosures 2a, 2b and 2c).
APPROACH AND CONVENTIONS
The Lexicon provides systematic stratigraphical information and definitions covering the full stratigraphic column from the Recent to the Basement. Most of the material used for this Lexicon comes from numerous Petroleum Development Oman, unpublished, in-house as well as published studies and reports. The relevant authors have been fully acknowledged and listed in the References.
North, Central and South Oman as regional indicators are capitalised in the text and refer to the areas as indicated on each of the chapter (Group-level) location maps. Interior Oman is referred to as the flat desert region south of the Al Hajar Mountains and north of the Qara Mountains in South Oman.
Location names are according to the National Survey Authority (NSA) of Oman. These names may differ from field, well and stratigraphic names that are referred to in relation to common/historical usage in reports and publications.
Well names are unabbreviated and follow normal oil-industry convention ‘name-number’, with ‘name’ referring to the well name and ‘number’ referring to the well number. An additional hole number ‘H#’ is added only if a reference section refers to any other than the first hole of the well. For example, Fahud-43H2 refers to Hole 2 of well Fahud-43, whereas Fahud-44 refers to Hole 1 of well Fahud-44.
Authors: All authors of formal descriptions relevant to the stratigraphic unit, including historical references and changes through time are listed. Particularly useful update, clarification or summary references may also be listed.
Introduction: The introduction generally comprises a summary of the regional tectono-stratigraphic setting and other significant aspects of the unit, supported by key publications.
Type and reference sections: Under this header are listed the original type section of the unit and any relevant additional reference well sections. For each stratigraphical unit, where possible, a number of reference wells have been selected from North, Central and South Oman to illustrate the variations and/or similarities in lithological development throughout Oman. Where available, such well sections include (and illustrate) relevant parameters, which aid stratigraphic identification including basic wireline log patterns (Gamma, Sonic, Density and Neutron logs, and Rate of Penetration), specific lithology, and criteria for general unit recognition. In particular, boundary definitions are explained. The legend for reference well figures is included on the foldout at the back of the Lexicon (Enclosure 2b). If available for a well, fossil zones are also illustrated. Most well panels are taken, and adapted, from Mohammed et al. (1997).
Lithology: Comprises a summary lithological description of each unit. This will be more extensive where more data are available, which is generally the case for established hydrocarbon reservoirs.
Subsurface recognition: These sections will provide a useful reference for (wellsite) geologists covering recognition criteria, both whilst drilling and post drilling. Mud logs, wireline logs, and ditch cuttings, sidewall cores and core descriptions as well as biostratigraphic and non-biostratigraphic analyses are all employed. Similar descriptions and additional notes are also included in the Type and Reference Well figures.
All changes in lithology and log character refer to downward changes unless otherwise stated.
The separation of Neutron and Density logs overlay is often a more reliable indication of shale content than the Gamma log. In this Lexicon the Density-Neutron separation is referred to as positive with reference to shaly lithologies and negative with reference to sandy lithologies after Rider (1996). A change in Rate of Penetration is called positive when it shifts to the left (faster or less minutes per metre).
Boundaries: Details the definition and description of upper and lower boundaries of the unit.
Distribution: The regional distribution of the unit is discussed, linking, where relevant and possible, with outcrop or Arabian scale correlation. Location references, as used in the text, are shown on maps per chapter, with additional location references labelled in the foldout section at the back of the Lexicon (Enclosure 2a).
Depositional environment: Discusses the depositional environments for unit and sub-units.
Subdivisions: A discussion of formal and informal subdivision of the stratigraphic units and their description.
Sequence stratigraphy: This is generally only included at Group and Supergroup level as a brief statement of assignment to the Arabian Plate (AP) megasequences of Sharland et al. (2001, 2004). If relevant and available, sequence-related observations are made elsewhere in the text.
Age: Age estimates are both expressed as Epoch (Series) and Age (Stage) ranges as well as numerical age range estimates with reference to the Ogg et al. (2008) International Stratigraphic Chart. The chart is also available and regularly updated on the website of the ICS (International Commission on Stratigraphy, www.stratigraphy.org). This results in age ranges quoted to 1 decimal point, here used without any error range qualification, suggesting a better accuracy than can often be expected based on the available constraints. Boundary ages not tied to an actual stage boundary, with qualifiers like Late or ‘mid-’ are simply rounded up or down. There may be a brief discussion of the key evidence for any given age, which may also be discussed further under Biostratigraphy. This and the biostratigraphy section may also present non-biostratigraphical age calibration data such as radiometric dates and chemostratigraphical interpretations, e.g. carbon and sulphur isotope trends.
Biostratigraphy: Many formations are defined, or in part constrained, with the aid of biostratigraphy. Biostratigraphic zones, and where appropriate their recognition criteria, are included as part of the discussion of each unit. These include PDO defined microfossil zones, with ‘F’ designating ‘foraminiferal’ zones (Enclosure 1, note that other fossil groups such as ostracods and calpionellids can also define F zones), and numeric palynological zones. The foraminiferal ‘F’ zones are important for the biostratigraphy of the Cenozoic and Mesozoic. Palynology is an important tool for the Palaeozoic rocks, but is also still used in specific sections in the Mesozoic.
The foraminiferal F zonal numbering system is relatively consistent, whereas the palynological zonal numbering appears anything but consistent. This is because a palynological zone originally related to a database code of the zonal index fossil. In either scheme where an index taxon is defined this fossil name is also quoted in the text, otherwise key and additional marker species may be listed.
The zonal schemes presented are of various vintages. The most relevant and current ones relate to units of most economic interest, e.g. Haushi and Haima palynology. Other schemes, notably the majority of foraminiferal zones date back to the 1990s and may be in much need of revision. Similarly the fossil names listed may not comply with current academic usage, they are simply reproduced here in terms of their initial and, in most cases, ongoing PDO usage.
It has not been possible to critically assess all zonal and biostratigraphical interpretations, especially where different fossil groups are involved. This is particularly the case for micropalaeontological and nannofossil related data. Where possible all alternative interpretations and any general uncertainties are presented and discussed as objectively as possible.
For the ‘unfossiliferous’ Cambrian and older stratigraphic units there is also a slowly expanding framework of radiometric dates correlated tentatively within Oman and globally by comparison with carbon and sulphur isotope trends (Brasier, 1999; Brasier et al., 2000; Le Guerroué, 2006; Allan, 2007; Bowring et al., 2007).
The geological development of the Arabian Plate, including the Al Hajar Mountains (the northern Oman Mountains) and Oman’s interior basins, has been described in varying degrees of detail by, amongst others, Murris (1980, 1981); Hughes Clarke (1988); Beydoun (1991, 1995); Loosveld et al. (1996); Alsharhan and Nairn (1997); Oterdoom et al. (1999); Immerz et al. (2000); Brasier et al. (2000); Konert et al. (2001); Al-Lazki et al. (2002); Grotzinger et al. (2002); Al-Husseini (2004b); Bowring et al. (2007); Allen (2007); Koopman et al. (2007); Romine et al. (2008); and van den Berg et al. (2008).
Sharland et al. (2001, revised in 2004) divided the sedimentary succession of the Arabian Plate into eleven tectono-stratigraphic megasequences, AP1 to AP11, which range in age from the Neoproterozoic to the Recent. With some minor modifications these megasequences are referred to in this Lexicon.
The oldest stratigraphic units of Oman are included in the Basement Group. The rocks of the Basement Group were amalgamated during continental accretion, most of which was completed in Oman by around 800 Ma (Mercolli et al., 2006; Allen, 2007; Bowring et al., 2007; Romine et al., 2008). Tectonic trends in the basement appear to be inherited and re-activated repeatedly through time in the overlying rock successions.
The next major unit comprises the Late Neoproterozic to Infra-Cambrian (ca. 720–520 Ma) Huqf Supergroup corresponding largely to the AP1 Megasequence. This succession encompasses the final amalgamation of Gondwana with the collision of East and West Gondwana (Immerz et al., 2000; van den Berg et al., 2008). Tectonics had a strong transpressional character in the Oman area, associated with the development of extensive salt basins and ending with the development of the regional ‘Early’ Cambrian, Angudan unconformity.
The Haima Supergroup is a major siliciclastic-dominated unit of ‘Early’ Cambrian to Early Silurian age (520–438 Ma). It represents the AP2 and base of the AP3 Megasequences. Oman was effectively a passive margin (Loosveld et al., 1996) throughout the lower Palaeozoic. This stable phase ended with the collision of Gondwana and Laurasia during the Carboniferous ‘Hercynian’ event that created the base Haushi unconformity (Konert et al., 2001; Al Husseini, 2004b; Faqira et al., 2009).
Of the entire time-span from late Early Silurian to ‘Late’ Carboniferous (AP3–AP4) only a few sections of Devonian and earliest Carboniferous sediments of the Misfar Group are locally preserved (more may be present in undrilled basin centres).
Following the widespread ‘Hercynian’ event, the northern Gondwanan terranes rifted with the opening of the Neo-Tethys Ocean in the ‘Late’ Carboniferous/earliest Permian (Konert et al., 2001). The sediments above the ‘Hercynian’ unconformity in Oman are represented by the clastics of the Haushi Group (AP5, and basal AP6, ca. 310–267 Ma).
From Mid-Permian to ‘mid’-Cretaceous times (ca. 267–91 Ma) a passive margin developed with extensive carbonate deposition, comprising the Akhdar, Sahtan and Kahmah and Wasia Groups (AP6–AP8). The two clastic formations, the Lower–Middle Jurassic Mafraq and the Lower Cretaceous Mesozoic Clastics, attest to periods of probable rift shoulder uplift and erosion. The more extensively developed clastics of the Upper Triassic Minjur and ‘mid’-Cretaceous Nahr Umr formations, represent more widespread pan-Arabian sediment pulses associated with regional uplift and subsequent flooding.
Late Cretaceous closure of the Neo-Tethys Ocean to the east led to intra-plate deformation and continent-ocean obduction in North Oman with the emplacement of the deep-water sediments of the Hawasina Group and the Semail Ophiolites (Enclosure 2a), see Glennie et al. (1974) and Glennie (2005). Transpressional movements of the Indian Plate in the east resulted in the part-obduction of the East Oman Ophiolite Complex (Enclosure 2a). This period (AP9, ca. 91–65.5 Ma) is contemporaneous with the deposition of the carbonate-dominated Aruma Group.
After the regional hiatus with much of the Danian (and latest Maastrichtian) missing, the carbonatedominated Hadhramaut Group and coeval sediments blanketed the Arabian Plate during the AP10 Megasequence (Palaeocene–Eocene, ca. 61–37 Ma).
Following Late Eocene uplift, the Gulf of Aden rift developed in the south in the Early Oligocene, with sea-floor spreading from the Late Miocene onwards. Uplift of the Al Hajar Mountains is correlated in time with the separation of Arabia from Africa along the Red Sea and continent-continent collision of the Arabian Plate with Eurasia. This period corresponds to the AP11 (ca. 33–1.8 Ma) Megasequence and deposition of the Fars Group in Oman. Not surprisingly this is a rather incomplete, variable and complex succession with many sedimentary hiatus.
Continent-continent collision is still active and responsible for the drowning of Musandam’s northern coastline, the dominant northwest-southeast stress field in Oman (Filbrandt et al., 2006) and strong earthquake activity along the Zagros collision belt and the adjacent Makran subduction zone in Iran and offshore Pakistan (see also Musson, 2009).
The biostratigraphy, lithostratigraphy and chronostratigraphy of Oman is summarised in the stratigraphic chart (Enclosure 1).