Basintype: Tectonic; cratonic interior basin generated by sag or growth of peripheral bulge to .... and Simmons, 1991) and Iran (for example, James and. Tertiary ...
Chapter 16
Mishrif Formation (Cenomanian~Turonian),Southern Arabian Gulf: Carbonate Platform Growth Along a Cratonic Basin Margin Trevor P. Burchette BP Exploration Sunbury on Thames, Middlesex, U.K.
SUMMARY Name: Shilaif basin in the Rub al Khali (Arabian) basin Author: Trevor P. Burchette Location: From 520 to east longitude and 240 to 260 north latitude, southern Arabian Gulf Geologic time interval: Late Albian—early Turonian Tectonic—sedimentary setting: Passive margin of Arabian craton; drowned foreland cratonward of peripheral bulge Basin type: Tectonic; cratonic interior basin generated by sag or growth of peripheral bulge to Oman mountains; restricted and density stratified Paleoclimate: Subarid in the Albian-Cenomanian to humid in the Turonian Platform type: Ramp or low gradient rimmed shelf (shoal with patch reefs); leeward and windward margins recognizable Platform geometry: Linear length of platform margin >2000 km encircling an elongated basin about 300 km across. Accretion caused a sedimentary wedge that advanced 75 km into basin. Width of shoal margin is several kilometers; thickness is 100—200 m. Facies and fossils: Basin: cyclic organic-rich pelagic foraminiferal packstone. Protoglobigerinids, hedbergeffids, oligosteginids, Exogyra, and Placunopsis. Slope: bioturbated bioclastic packstone, grain size fining basinward. Diverse irifaunal bivalves. Shoal: bioturbated bioclastic packstone and grainstone. Scattered radiolitid rudists and other bivalves. Biostrome: rudstone and floatstone with caprinid and radiolitid rudists. Chaetetids and bivalves. Back shoal: interbedded lagoonal lime mudstone and bioclastic packstone. Chondrodonta, radiolitid rudists, bivalves. Platform interior: bioturbated lime mudstone with benthic foraniiniferal fauna. Systems tracts and stacking patterns: Two third-order sequences. Sequence 1: vertical accretion followed by lateral accretion. Upper sequence boundary well defined. Lower sequence boundary and transgressive stage poorly known. Parasequences poorly defined on platform, well defined in basin by flooding events. Salt withdrawal and diapirism created intra-platform sequences and local unconformities. Sequence 2: Muddy lowstand and transgressive ramp (Tuwayil Member) followed by highstand shallow water carbonates (Ruwaydha Member). Upper sequence boundary type 2(?), but tectonically enhanced unconformity continued in east. 550
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INTRODUCTION The Mishrif Formation, part of the Wasia Group, is a widespread Cenomanian—Turonian carbonate succession in the Arabian Gulf and surrounding areas (Figure 1). It is a prolific hydrocarbon reservoir and host to a number of giant oil fields. This paper briefly synthesizes information on the Mishrif platform in the southern Arabian Gulf and examines its tectonic setting and the controls on platform growth and demise.
Database In the Arabian Gulf, Oman and Saudi Arabia (Figure 1), middle Cretaceous sediments have been penetrated by numerous petroleum exploration and development wells. Large volumes of data exist in the archives of national and international oil companies with interests in the area, but access to this database is limited. Extensive seismic coverage is available over the area; three-dimensional seismic datasets exist over some fields. There is a trend to fully integrated studies of the Mishrif Formation, particularly in field development.
Previous Work The structural evolution of the Cretaceous cratonic margin in Oman has been reviewed by Glennie et al. (1973, 1974), Graham (1980), Coleman (1981), Searle et al. (1983), Searle (1985), Robertson (1987a, b), Patton and O’Connor (1988), and Warburton et al. (1990). Patton and O’Connor (1988), in particular, have discussed the tectonic events that culminated in the formation of the Oman Mountain foredeep and have placed middle Cretaceous formations in this context. The regional distribution, fades, and stratigraphy of the middle Cretaceous around the Arabian Gulf have been discussed by Murris (1980), Harris et a!. (1984), and Scott et al. (1988). Previous studies of the Wasia Group have been reviewed by Aisharhan and Nairn (1988) and Aisharhan and Kendall (1991). Sedimentologic, diagenetic, and biostratigraphic work as components of regional and reservoir studies have formed the main thrust of investigations on the Mishrif Formation, but most work remains unpublished. Limited outcrop studies of its correlatives in Oman (Glermie et al., 1974; Harris and Frost, 1984; Hughes-Clark, 1988; Kennedy and Simmons, 1991) and Iran (for example, James and Wynd, 1965) have supplemented subsurface studies. In the southern Arabian Gulf and northern Saudi Arabia, semiregional sedimentologic studies of the Mishrif Formation using information from exploration wells have been made by Newell and Hennington (1983), Burchette and Britton (1985), and Jordan et al. (1985). Published studies of more local significance,
Figure 1. Arabian Gulf and region, showing area considered in this paper (stippled).
dealmg with facies and stratigraphy, are by Harris and Frost (1984) in Oman, Sharief et al. (1989) in Saudi Arabia, and Reulet (1982) and Alkersan (1975) in Iraq. More specialized work on diagenesis and aspects of seismic interpretation have been published by Buruss et al. (1985), Videtich et al. (1988), and Ayes and Tappmeyer (1985). The paleontology and biostratigraphy of the Mishrif Formation is well known from oil company reports, but only a small fraction of this information has been published. Notable exceptions include Scott (1990) and field studies in the Oman Mountains (Simmons and Hart, 1987; Smith et al. 1990; Kennedy and Simmons, 1991).
GEOLOGICAL SETTING AND STRATIGRAPHY The Mesozoic succession of the Arabian Gulf was deposited along the eastern passive margin of the Arabian craton. In common with much of the Tethyan belt, this margin evolved from rift in the Permian and Triassic to drift in the Jurassic and Early Cretaceous, followed by suture in the Late Cretaceous and early Tertiary. In response to the growth of the Oman and Zagros mountains, a linear foreland basin developed at the margin of the craton during the Late Cretaceous (Murris, 1980; Searle et al., 1983; Patton and O’Connor, 1988). Several large, restricted basins developed within the middle Cretaceous Arabian shelf (Murris, 1980). Where rimmed by carbonate platforms, these formed self-contained hydrocarbon systems sealed regionally
16. Mishrif Formation (Cenomanian—Turonian), Southern Arabian Gulf
ml
Figure 2. Chronostratigraphy of the southern Arabian Gulf and Oman showing locations, distribution, and durations of major stratigraphic breaks (vertical hatching). (After various sources; time scale for middle Cretaceous after Haq et al., 1988.) beneath the marly Laffan and Fiqa formations (Aruma Group). It is largely in these areas, with interest driven by petroleum exploration, that limestones of the Mishrif Formation and its equivalents have been studied. In the southern Arabian Gulf, the largest such basin is the Shilaif basin, about 300 km across, which forms the focus of this review. Basinal deposits largely time equivalent with the Mishrif Formation (Figure 2) are known as the Shilaif Member of the Salabikh Formation in Abu Dhabi, the Rumaila or Khatiyah Formation in the northern Emirates, the Natth Formation (Member F) in Oman, or the “lower Mishrif” in northern Saudi Arabia. The Mishrif Formation (early Cenomanian— Turonian) is developed throughout the region in the upper part of the Wasia Group (Figure 2), an extensive middle Cretaceous (Albian-early Turonian) lithostratigraphic unit comprising a supersequence, up to 600 m thick, bounded by two regionally important unconformities (Harris et al., 1984). Over much of the Arabian Gulf, the Wasia Group consists of several major depositional sequences separated by less significant and discontinuous unconformities. Distinctive lithostratigraphic units within these sequences in the southern Arabian Gulf are the Nahr Umr, Mauddud, and Mishrif formations (Figure 2). Each of these units exhibits regional differentiation into shallow and deeper water facies, the distribution of which closely reflects the
tectonic evolution of the area (Murris, 1980; Alsharhan and Naim, 1988). The Nahr Umr Formation (duration about 8 m.y.) represents a broad, east-facing, storm-dominated, largely clastic ramp (Murris, 1980). Marginal deposits in Saudi Arabia and Kuwait are sandstones of the Burgan Formation. The Mauddud Formation (duration about 4—5 m.y.) represents establishment of a middle Cretaceous carbonate ramp and the first subtle differentiation into intrasheif basins and platforms. In most places, Mauddud limestones shoal upward from Orbitolinadominated packstone and wackestone into shallow water rudistid limestones (Harris et al., 1984). The Mishrif Formation (duration about 5 m.y.) represents a broad carbonate ramp or low gradient shelf that fringed the margin of the Arabian craton and interior basins during the Cenomanian and early Turonian. The Tethys ocean margin is now deformed in the Oman and Zagros mountains, but was characterized by grainstones, rudist reefal tracts, and slope deposits (see Glermie et al., 1974; James and Wynd, 1965; Watts and Blome, 1990). Other correlatives of the Mishrif Formation occur in the upper part of the thick Sarvak Formation in Iran Uames and Wynd, 1965) and in the Natih Formation in Oman and the northern Emirates (Scott, 1990). Similar carbonate platforms developed widely in the middle Cretaceous along the northern and southern Tethyan margins.
Figure 3. Schematic cross section of the Mishrif platform in the southern Arabian Gulf, showing environments of deposition of lithofacies associations 1 through 6 and their constituent sedimentary rock types.
MISHRIF DEPOSITIONAL FACIES Mishrif platform carbonates throughout the southern Arabian Gulf can be divided into six “lithofacies assodations” (Burchette and Britton, 1985) that represent lateral and vertical transitions from (1) basinal silt-grade pelagic foraminiferal wackestone, through (2) a slope succession that coarsens upward to (3) rudist—ostreiid shoal and (4) biostromal packstone and grainstone (Figures 3 and 4). In proximal situations, shoal and biostromal facies are overlain by (5) bedded back shoal packstone and (6) nodular, micritic shallow water deposits. The biotas that characterize these sedimentary rocks are shown in Table 1. Lithologies have been illustrated by Burchette and Britton (1985) and Jordan et a!. (1985).
Lithofacies Association 1: Basin Description. This association is composed of siltgrade to fine sand-grade, organic-rich lime packstone and wackestone containing a pelagic biota (Oligostegina, hedbergeffid, and other planktomc foraminifera). These Shilaif limestones are well bedded but nodular, have abundant wispy microstylolites, and grade up into the Mishrif platform via an interbedded transition zone comprising thin (5—20 cm) bioturbated and graded beds. Over the whole basin, Shilaif sedimentary
deposits exhibit cycicity (10-40 m scale) between dark, organic-rich and light-colored, fine-grained bioclastic limestone. In the light-colored lithologies, unidentifiable silt-grade bioclastic debris and peloids are abundant. In the northern part of the Gulf in the upper Cenomanian—lower Turonian Ruwaydah and Tuwayil subfacies, these sediments are more argillaceous. Interpretation. The pelagic microbiota in these deposits indicates an open marine, basinal environment with water depths as much as 200 m, although generally less than 100 m (Figure 3). Dark brown, organic-rich intervals, interbedded on a 20—30 cm scale with the light-colored bioturbated sediments, suggest that the environment was periodically restricted or that sedimentation rates fluctuated with short-term cycicity. Beds are graded in places in the transition zone, which suggests that storms may have been prominent as mechanisms of deposition.
Lithofacies Association 2: Slope Description. These sedimentary rocks form the lower part of the Mishrif succession, grading over 40—70 m from moderately well-bedded, silt-grade to fine sand-grade bioclastic wackestone, containing sparse pelagic foraminifera in the transition zone with the
Figure 4. Block diagram showing depositional environments of the Mishrif—Shilaif succession in the southern Arabian Gulf. Circle numbers indicate lithofacies associations: (1) basinal facies of the Shilaif Formation; (2) slope; (3) shoal; (4) rudist biostrome; (5) back shoal; (6) interior lagoon. Shilaif Member, to poorly bedded, cream-colored, friable, coarse bioclastic packstone (Figure 3). Grains are highly micritized, but largely of molluscan and echinoderm origin. The middle and upper parts of the sequence contain a diverse infaunal and semi-infaunal molluscan assemblage. Small, abraided radiolitid rudists occur sparsely in the upper part of the succession. Bioturbation is pervasive throughout. Stylolitization also decreases upward as the proportion of coarser biodastic material increases. Interpretations The succession, coarsening upward from basinal limestone to shallow marine packstone, reflects shallowing of the depositional environment in response to progradation of a carbonate slope. Bioturbation has homogenized the sediments and has obliterated most depositional structures. The absence of resedimented material or slumps indicates that the slope was of low gradient.
Lithofacies Association 3: Shoal Description. The shoal association is composed of off-white, poorly sorted bioclastic packstone, grainstone, and rudstone. These beds gradationally overlie the slope deposits and are locally intercalated with them. Bioclasts are predominantly molluscan, mostly rudistid. Small rolled radiolitid rudists are abundant. Bedding is ifi-defined and few other sedimentary structures are present, although cross stratification occurs locally.
Interpretation. This is the most widespread coarsegrained facies in the Mishrif Formation, and in most places it is the culmination of the coarsening upward sequence (Figure 3). It represents the deposits of low energy shoals and banks along the platform margin. The absence of sedimentary structures is due to pervasive bioturbation. Well-sorted grainstone, present locally near the top of the succession, may represent beach deposits.
Lithofacies Association 4: Rudist Biostrome and Patch Reef Description. This association is made up of very coarse-grained, shelly bioclastic and lithofacies floatstone containing a more diverse intactrudstone fauna than association 3, dominated by radiolitid and caprinid rudists with accessory monopleurids. The ostreiid Chondrodonta and small branching corals are locally common. Interskeletal matrices are composed of finer molluscan wackestone and packstone. These sediments occur in scattered patches up to 15 m thick and several square kilometers in area at the top of the Ivlishrif coarsening upward successions (Figure 4). In places this facies is localized by diapiric highs. Interpretation. The diverse fauna, distribution, and context of this fades at the top of a coarsening upward succession all indicate an origin as biostromes and small bioherms within the prograding margin.
Table 1. Mishrif Uthofacies Associations, Sedimentary Rock Types, and Characteristic Blotasa Association Basin
Lithology Well-bedded silt-grade bioclastic packstones and wackestones; Stylolitized
Biota Pelagic foraminifera: Favuseila, Hedbergeila, Ratalipora, Globigerineloides casey~Heterohefix globulosa, and oligosteginids, including Pithone//a avails and Ca/cispheru/ina innomata
bioturbated silt-grade bioclastic packstones to poorly bedded
local bioturbation washitensis, Praeg/obotruncana; rare benthonic foraminifera Diverse bivalves, including Lima semiornata, Lithophaga, Plagiostoma,
shoal
grained bioclastic packstones, grainstones, and rudstones
Praealveo/ina, Dukhania, Chrysalidina, dicyclinids Rare planktoriic foraminifera: Praeglobotruncana Heteroheilx,
bioherm
bioclastic rudsiones
monopleurids, Chonocjroejonta
grained bioclastic packstones,
Nezzazata conica, Dukhanfa
~
benthonic—foraminferal and peloidal mudsiones and wackestones
aAfter Burchette and Brftton (1955).
Dukhania, Va/vu/am/na picardi, Nezzazata coriica, Taberina, Biconcava, Pseudedomia, miliolids Common ostracodes, rare molluscs, Lithocodium, Permocalcujis, dascyclads; abundant bioturbation
16. MishrifFormation (Cenomanian—Turonian), Southern Arabian Gulf
Lithofacies Association 5: Back Shoal Description. Thin- to medium-bedded, fine to very coarse grained bioclastic packstone, wackestone, and grainstone characterize this association. The sediments are more varied, indurated, and stylolitized than other coarse fades. Coarser deposits in discrete beds contain a fauna dominated by Chondrodonta, Tylostoma, and small in situ clusters of radiolitid rudists. Bioturbation is extensive and includes Callianassa-type burrows up to 3 cm in diameter. Ripple cross lamination occurs locally. Interpretation. These deposits represent a zone of sediment mixing between shoal and interior lagoon (Figures 3 and 4). They overlie the coarsening upward succession and are interbedded in several-meter-thick intervals with lagoonal sediments. Chondrodonta and radiolitid rudist beds probably represent thin (about 1 m) biostromes, while sparser packstone and grainstone beds are mostly shoal-derived carbonate sand sheets and washovers.
Lithofacies Association 6: Lagoon Description. The lagoon association is characterized by indistinctly bedded benthonic foraminiferal and peloidal lime mudstone and wackestone. These sedimentary rocks are extensively burrow mottled and in places have a characteristic nodular fabric. Molluscan (including rudist) debris, echinoderm, and ostracodes are locally important. Uncommon algal-laminated clasts are present.
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widely correlatable on logs and can be used to map progradation of the Mishrif platform margin into the basin for distances up to 75 km (Burchette and Britton, 1985) (Figure 6). On both sides of the Shilaif basin, the MishriI platform shows initial vertical, and then strong lateral, accretion concomitant with thickening of the slope facies (Burchette and Britton, 1985), pointing to an increase in slope height and thus an increase in basinal water depth during the period of Mishrif deposition. During the middle Cretaceous, the area of the Arabian Gulf was equatorial, with a semiarid or humid tropical climate (Murris, 1980; Parrish and Barron, 1986); no evaporites have been described from Mishrif platform interiors. Dominant winds were probably from the southeast (see Frakes, 1979; Habbicht, 1979). Thus, the eastern Mishrif margin was probably a leeward one, while the western side of the Shilaif basin exhibits slightly more windward characteristics (steeper margin and more grainstone sediments). Major salt diapirs and basement structures formed highs in the southern Gulf during the middle Cretaceous, which modified Mishrif facies distribution by localizing high energy grainstones and rudist reefs and generating local intraformational unconformities (Ayes and Tappmeyer, 1985; Burchette and Britton, 1985; Videtich et al., 1988). Windward and leeward fades distribution within the Mishrif Formation over these structures have not been described.
Tectonic Control on Mishrif Formation Deposition
Mishrif shallow water carbonates prograded centripitally into the Shilaif basin as an almost continuous encirclingplatform with a linear length of over 2000 km
During the late Cenomanian and Turonian, uplift and erosion of the Mishrif platform occurred in a north—south tract paralleling the Oman Mountain front (Figure 5c). Erosion increased westward, in places down to the Nahr TJmr Formation, and erosional relief on the unconformity amounts to more than 240 m (Patton and O’Connor, 1988). The Mishrif succession, as much as 150 m thick, terminates in a karstic unconformity characterized by breccia-fified dolines, paleosols, and mature flint claystones (Burchette and Britton, 1985). Regional geologic studies in the northern Emirates and Oman (Robertson, 1987a, b; Patton and O’Connor, 1988) suggest that this event was due to growth of a peripheral bulge associated with obduction of the Semail Ophiolite (Figures 5 and 7), but the possibility that some early uplift may have resulted from compression along the southeastern side of the Dibba transform zone cannot be excluded. Even prior to major
(Figure 5). The margin varies in style locally from ramp to low gradient rimmed shelf, probably reflecting both the dominant wind direction and the subsidence regime, but isolated rudist buildups with steeper slopes developed locally over diapirs or basement highs. Parasequences in the Shilaif basinal succession are
uplift, extensional faulting in the area of the arch appears to have controlled Mishrif facies distribution and diagenesis in parts of eastern Oman (Harris and Frost, 1984). Subsequent collapse of the bulge and deepening of the foreland basin (Aruma basin) due to continued
Interpretation. These deposits form the uppermost unit of the Mishrif succession in the most proximal areas (Figure 3). They represent deposits of broad, sheltered, shallow platform interiors. Intercalations of coarser bioclastic sediments probably represent sporadic extension of back shoal sand flats.
PLATFORM MORPHOLOGY, DISTRIBUTION, AND CONTROLS ON GROWTH Platform Morphology
Figure 5. Middle Cretaceous paleogeography of the Arabian Gulf area. (A) Middle Cenomanian, early Mishrif; development of cratonic interior basin at the margin of the Arabian craton. A—A’ shows line of section in Figure 7. (B) Middle to late Cenomanian; progradation of shallow water Mishrif platform into basin (sequence 1). (C) Late Cenomanian—early Turonian; initial development of peripheral bulge exposes Mishrif platform in eastern Gulf and Oman and establishment of Tuwayil—Ruwaydha cyclic muddy ramp and basin in subsequent depositional sequence (sequence 2). nappe emplacement in the Coniacian—Maastrichtian resulted in deposition of the Laffan, Muti, and Fiqa formations (Figures 3 and 7), which overlie the Mishrif Formation and the Shilaif Member throughout the area (Patton and O’Connor, 1988). Early development of an arch appears to have influenced the trend of, and contributed to the isolation of, the Shilaif basin (Patton and O’Connor, 1988) and thus to the growth and differentiation of the Mauddud and Mishrif carbonate platforms around its margins (Figures 5 and 7). Tectonism was a major stimulus for the reactivation of basement features, salt-withdrawal basins, and diapirs during the late Cenomanian and Turonian in the eastern Arabian Gulf (Ricateau and Riche, 1980). On the western side of the Shilaif basin, the Mishrif consists of a shallowing upward succession, typically 200 m thick, of similar character and age to that in east. This is onlapped by argillaceous, silty pelagic and outer ramp sedimentary rocks of the Tuwayil and Ruwaydah members (late Cenomanian—early Turonian) (Figure 2). Shallow water facies appear to have backstepped toward the Arabian shield (for example, see Newell and Hennington, 1983) following the lowstand phase. The Tuwayil—Ruwaydha section thins from more than 300 m thick to the west of the area to zero some 200 km to the east, where it onlaps the karstified eastern Mishrif margin (Figures 5 and 7). This whole unit is correlative with the unconformity at the top of the Mishrif in the east. In the west, there appears to be no significant erosional unconformity between the Tuwayil— Ruwaydha section and the overlying Turonian Laffan Formation. This thick basinal section was thus deposited over the flooded foreland during uplift of the
platform in the Oman—Zagros peripheral bulge and
Figure 6. lsochron map for the base of shallow water Mishrif carbonates (sequence 1) in the southern Arabian Gulf, showing progradation of the platform into the Shilaif basin. Contours indicate number (circled) of Shilaif basinal gamma log picks prior to arrival of Mishrif slope facies. Stipple area represents the area of the Shilaif basin that was not covered by shallow water Mishrif facies in either depositional sequence. Black dots show well control points. Paleowind direction was probably from the southeast. indicates major drowning, probably corresponding with an early Turonian sea level highstand (Haq et al., 1988). Correlative sections in Iran, Kuwait, and Saudi Arabia show that this Turonian deepening was regional. This event may correspond with the upper of two shoaling cycles recognized in the Mishrif of Iraq (see Reulet, 1982). Syndepositional halokinesis significantly influenced Mishrif platform development in eastern offshore Abu Dhabi and Dubai (Burchette and Britton, 1985), where the Mishrif Formation shows two phases of aggradation and progradation separated by uplift and erosion centered on the Fateh salt diapir (for example, Videtich et al., 1988). The Fateh diapir is one of several rimming an area of salt withdrawal approximately 20—30 km across, the “Sir Abu Nu’air” basin, located to the northeast of Sir Abu Nu’air island. Following exposure of the Mishrif in this area, the Sir Abu Nu’air basin subsided and infilled with Tuwayil and Ruwaydha
facies. In the Sir Abu Nu’air basin, these overlie the karstified Mishrif platform and probably formed a marine reentrant into which thin rudist buildups of the Fatah field upper Mishrif cycle prograded (Videtich et al., 1988). On the eastern side of the Shilaif basin, deposition of Turonian deeper water sediments was thus restricted to reentrants and depressions along the margin of the exposed platform.
SEQUENCE STRATIGRAPHIC CONTEXT OF THE MISHRIF FORMATION The sequence stratigraphy of the Wasia Group has not been discussed in detail in the literature and thus the following observations are, in part, speculative. Regional investigation is hindered by lack of an adequate published biostratigraphic framework and inconsistencies in the location and interpretation of
Figure 7. Middle to Late Cretaceous tectonic evolution of the Arabian craton margin in the area of the Oman Mountains. (A) Albian. Passive margin sedimentation of Nahr Umr and Mauddud formations followed by initial differentiation into intracratonic basins and platforms. (B) Cenomanian. Continued development and accentuation of interior basins and strong progradation of Mishrif—Natih shallow water carbonate platforms (sequence 1); possible start of swell development. (C) Early Turonian. Development of initial peripheral bulge and exposure and karstification of carbonates in east, flooding following minor lowstand of foreland in west. Deposition of Tuwayil and Rhuwaydha Members (sequence 2). (D) Coniacian—Campanian. Emplacement of Semail Ophiolite and associated nappes and depression of foreland and foreland basin sedimentation. (E) Campanian—Maastrichtian. Continued basin development and filling. Sections not drawn to scale, but total length is about 1000 km. Line of section shown by A—A’ in Figure 5A. Symbols: fine stipple, thrust stack; coarse stipple, Mishrif Formation shallow water carbonates; black, basinal deposits of Shilaif Formation. (Adapted from Patton and O’Connor, 1988.)
16. MishrifFormation (Cenomanian—Turonian), Southern Arabian Gulf formation boundaries and in lithostratigraphic nomenclature. Well studies have yet to be adequately tied to seismic stratigraphic interpretations. Several attempts have been made to relate middle Cretaceous successions of this area to global sea level curves (Newell and Hennington, 1983; Harris et al., 1984; Scott et al., 1988; Alsharhan and Kendall, 1991). However, this has been in areas where any global eustatic signal has been significantly modified by local halokinesis and foreland restructuring, thus emphasizing the fact that sections used for such correlation must be selected with great caution. Bias in sequence interpretation is also commonly introduced by exploration and development drilling since this has concentrated on structural highs that have demonstrably influenced facies and the expression of unconformities from the Permian to the present day. Published information (for example, Harris et al., 1984; Scott et al., 1988; Burchette and Britton, 1985) suggests that the Wasia Group represents a secondorder sequence comprising a number of third-order depositional sequences of varying style; these are most clearly expressed in the passive basin margin areas of Saudi Arabia and Kuwait. The basal megasequence boundary, between the Nahr Umr and Shuaiba formations (Thamama Group) (Figure 2) is a complex one (Simmons et al., 1992), but it has been shown to represent a karstified regional hiatus corresponding to a major sea level lowstand (Scott et al., 1988). Glauconitic and phosphatic sandstone and mudstone at the base of the Basal Nahr Umr Formation (Newell and Hennington, 1983) suggest stratigraphic condensation above this sequence boundary. The Nahr Umr— Mauddud boundary (Figure 2) represents an abrupt regional facies change from siiciclastic to carbonatedominated sedimentation with little subaerial exposure or erosion. The Mauddud Formation commenced with flooding of the Nahr Umr clastic ramp and the establishment of a regional carbonate platform. Although locally shoaling, Mauddud shallow water limestones are onlapped in places by “Rumaila” basinal fades, so that the Mauddud may represent a largely retrograde sequence or sequence set, at least over the area of the developing Shilaif basin. The Mishrif Formation and Shilaif Member together probably encompass two third-order depositional sequences. Sequence 1 involved initial vertical accretion followed by strong lateral accretion. The upper sequence boundary is well defined, but varies regionally in character from a major karsted unconformity (type 1) in the east to a less pronounced boundary with little evidence for exposure (type 2) in the west. The “maximum flooding surface” for this sequence probably corresponds with an interval of high gamma ray response within the lower Shilaif succession. The late Cenomanian—Turonian “upper Mishrif” of Saudi
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Arabia and the Tuwayil and Ruwaydha facies of the southern Gulf encompass an additional third-order sequence that is commonly overlooked because it is restricted to the buried foreland. This corresponds in time to the tectonically enhanced “Wasia—Aruma break” in eastern Oman and the northern Emirates. Since it onlaps the western Mishrif platform slope, it seems likely that the argillaceous and silty Tuwayil Member encompasses initial lowstand and transgressive deposits within the Shilaif basin, while the shallower water, carbonate-dominated Ruwaydha Member represents more widespread subsequent highstand sedimentation following intitial backstepping. The Shilaif succession of sequence 1 shows several prominent upward cleaning cycles (Burchette and Brifton, 1985), 10—40 m thick, which probably represent a sedimentary response to fourth- or fifth-order variations in relative sea level (influencing the rate of clastic influx or platform advance and retreat). Superimposed on these are alternating 20—30 cm scale beds of dark brown and tan bioturbated pelagic foraminiferal packstone that show local evidence for storm deposition. These basinal cydes are poorly discernible within the platform fades. The basinal section is rarely thoroughiy cored, however, and the character of the larger cycles is incompletely known. Analyses of the Shilaif facies suggest that organic-rich sediments were deposited in an anoxic or suboxic, episodically densitystratified marine basin. By analogy with the Jurassic Hanifa Formation of Qatar (Droste, 1990), such intervals are likely to have developed during the flooding stage of each relative sea level cycle.
HYDROCARBON HABITAT OF THE MISHRIF FORMATION Trap Styles The Mishrif Formation and its equivalents in the southern Arabian Gulf forms reservoir zones in a number of world class giant fields (Table 2). Traps have a number of configurations, but all are sealed by the regionally continuous basal marine shales of the overlying Aruma Group (Laffan and Fiqa formations). The Fahud and Natth fields in western Oman are structural traps that produce oil from the Natih Formation in the footwall of major extensional faults (Tschopp, 1967; Harris and Frost, 1984). In the offshore southern Arabian Gulf, in contrast, fields in the Mishrif Formation developed largely as flexures over salt diapirs. In some of the larger fields much of the Mesozoic stratigraphy produces oil or gas from such traps. These include the North Field in Qatar (Ayes and Tappmeyer, 1985); the Falah, Fateh, South West Fateh,
Table 2. Data on Selected Mishrif Fields in the Southern Arabian Gulf
Field Fateh SW Fateh Falah Rashid Umm Addalkh Fahud Natih Sirri C Sirri 0 Sirri E
Country UAE/Dubai UAE/Dubai UAE/Dubai UAE/Dubai UAE/Abu Dhabi Oman Oman Iran Iran Iran
Trap Type Structural Structural Struct/strat Structural Stratistruct Structural Structural Structural Structural Structural
Structure
Main Reservoir Faciesa
Reservoir Thickness (m)
Diapir Diapir Diapir Diapir Anticline Faulted a/dine Faulted a/dine Diapir Diapir Diapir
Reef/shoal (4/3) Reef/shoal (4/3) Shoal/reef(?) (3/4) Shoal/reef(?) (3/4) Reef/shoal (4/3) Shoal/reef(?) (3/4) Shoal/reef(?) (3/4) Slope/shoal (2/3) Slope/shoal (2/3) Slope/shoal (2/3)
81 90 45 80 150+ 460 125 10 30 35
a Numbers in parentheses refer to lithofacies associations described in the text.
bBbbl = billion barrels.
Approximate Porosity (%) and Permeability (md) 5—25%, 5—50 md 5—25%, 5—50 md 19% (aye), 5—12 md
“Moderate” 11—25%, 19—500 md 30% (aye), 7 md-20 d? “Good” ‘Good” ‘Good” 10%
Top Mishrif Reservoir Hydrocarbon (m) Type 2410 2389 2423 2832 2312 230 900 2365 2400 2820
O/G O/G 0 G/O/C O/G O/G/C O/G O/G O/G O/G
Approximate
Original OIP (Whole Field) 3.5 Bbblb 3 Bbbl >100 MMbbI >100 MMbbl >800 MMbbl 1 Bbbl 500 MMbbI >200 MMbbl >100 MMbbI >400 MMbbI
16. MishrifFormation (Cenomanian—Turonian), Southern Arabian Gulf and Rashid fields in Dubai (Alsharhan, 1989); and the Sirri fields in offshore Iran. In the Fateh field, the Mishrif Formation was eroded from the crest of the structure at the Wasia—Aruma break, but it remains as a “halo” around the periphery of the structure (Jordan et al., 1985; Videtich et al., 1988). Because the most favorable reservoir horizons subcrop a major unconformity, truncation of this sort is a common concern in Mishrif reservoirs around the Gulf since the underlying coarsening upward succession comprises only poor quality reservoir fades in most cases. The Umm Addalkh field in southeastern offshore Abu Dhabi is a combined structural—stratigraphic trap, a strongly karstifled, small, isolated buildup (Burchette and Britton, 1985) in which much of the trap closure appears to be due to differential compaction of the surrounding Shilaif basinal facies. In the same area, attempts at demonstrating a regional stratigraphic trap at the updip pinchout of the Mishrif platform in Shilaif basinal fades have not proven encouraging, principally due to poor lateral seal in the Shilaif basinal facies (Burchefte and Britton, 1985).
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successions (commonly localized by highs) contained much aragonitic bioclastic material, largely rudistid, which allowed the formation of large volumes of moldic porosity (maximum 35%, mostly 10—25%) during meteoric leaching. In many cases, incomplete cementation by meteoric cements means that this fades commonly also has good to excellent permeabiities (maximum several darcys, mostly 10—1000 md) (Harris and Frost, 1984; Burchefte and Britton, 1985). Reservoir heterogeneity within this facies is generally fairly low, although good rudist reefal facies have a fairly local distribution. Where the Mishrif Formation consists of shoaling cydes, reservoir quality decreases downward through the section so that the finer grained slope fades (although significantly porous) contain much microporosity and generally exhibit low permeabilities (commonly just a few millidarcys). Hydrocarbon recovery may thus be complicated where this transition zone falls within the reservoir section. Several studies (Tschopp, 1967; Wffloughby and Davies, 1979; Jam and Ayoub, 1983; Crick and Singh, 1985; Trocchio, 1989) have documented development strategies and production problems encountered in Mishrif reservoirs.
Source The source for oil reservoired in the Mishrif Formation is the time-equivalent Shilaif (or Katiyah) Formation. Timing of maturation of this stratigraphic interval is variable, depending on its position with respect to the Oman foredeep. Maturity generally decreases toward the west, with only minor exceptions. Thus, in the western Emirates and northeastern Saudi Arabia, the Shilaif, although stifi a potentially excellent source rock, is mostly immature (Newell and Hennington, 1983; Alsharhan, 1989). In parts of Oman, initial maturation and charging of Mishrif reservoirs may have occurred as early as the Late Cretaceous (Buruss et al., 1985). In the southern Arabian Gulf, however, source maturation, migration, and trap charging as determined by fluid inclusion studies (Videtich et al., 1988) was initiated in the Oligocene to Miocene and may continue today. Migration in this area was aided by a regional dip to the east of about in the Oman foredeep that developed during the Late Cretaceous and early Tertiary. 30
Reservoir
Quality
Mishrif reservoir quality is facies dependent to a large degree, but it is particularly good where highstand carbonates were exposed to meteoric leaching at the Wasia—Aruma unconformity (Burchette and Britton, 1985). The coarse molluscan packstone— grainstone fades that cap the shoaling upward Mishrif
SUMMARY The Mishrif Formation in the southern Arabian Gulf is a regional shallow water limestone succession that shoals upward, due to progradation, from basinal deposits. The platform rimmed a cratonic interior basin, the Shilaif basin, at the margin of the Arabian craton for a linear distance of over 2000 km, prograding into the basin for up to 75 km in some areas. Six lithofades associations, ranging from basinal, through slope and shoal, to shallow lagoonal deposits, can be recognized. Exposure and erosion terminated platform growth in the late Cenomanian in the eastern Gulf and Oman, while in other areas of the southern Gulf and Saudi Arabia, the lowstand was less pronounced and the platform drowned during the early Turonian highstand. The Mishrif Formation possesses widespread good reservoir characteristics, particularly where it lies directly beneath the middle Cretaceous unconformity, and is a principle reservoir zone in a number of giant fields around the southern Arabian Gulf.
Many thanks to Mike Simmons (BP Exploration) for helpful comments on an early draft of this paper and for advice on biostratigraphic dating of the middle Cretaceous formations. Permission from BP Exploration to publish this paper is gratefully acknowledged. Acknowledgments
198
Burchette
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