Bull
247
Aiad Ali Hussien Al-Zaidy
Bull. Iraq nat. Hist. Mus.
June, (2019) 15 (3): 247-262
MICROFACIES ANALYSIS AND BASIN DEVELOPMENT OF THE
CENOMANIAN - EARLY TURONIAN SEQUENCE IN THE RAFAI,
NOOR AND HALFAYA OIL FIELDS, SOUTHEASTERN IRAQ
Aiad Ali Hussien Al-Zaidy
Department of Geology, College of Science, University of Baghdad,
Baghdad, Iraq
Email: aiad.alzaidy@gmail.com
Received Date: 19 August 2018 , Accepted Date: 15 October 2018, Published Date: 27 June 2019
ABSTRACT
The stratigraphic sequence of Cenomanian-Early Turonian is composed of Ahmadi,
Rumaila, and Mishrif formations in the Rifai, Noor and Halfaya Oil Fields within the
Mesopotamian Zone of Iraq, which is bounded at top and bottom by unconformity surfaces.
The microfacies analysis of the study wells assisted the recognition of five main
environments (open marine, basinal, shallow open marine, Rudist biostrome, and lagoon);
these microfacies were indicative of a normal lateral change facies from shallow water
facies to deeper water and open marine sediments.
Ahmadi Formation (Early Cenomanian) is characterized by open marine sediments
during the transgressive conditions, and would become deep basinal environment upward to
deposition the Rumaila Formation. Rumaila Formation (Middle Cenomanian) was deposited
in the deeper part of the intrashelf basin; it comprises basinal sediments mainly, and
includes an abundant of open marine fauna supportive of Middle Cenomanian age. Rumaila
Formation is represented as time equivalent basin to the Mishrif Formation, where they
were deposited during highest and system tract (HST). The Cenomanian- Early Turonian
sequence can be subdivided into three cycles displaying coarsening upward cycles :Mishrif
A, Mishrif B, and Mishrif C; which comprises a highest and system tract dominated by
rudistid packstone to grainstone or rudistid biostrome facies separated by transgressive units
(CR I and CR II).
Keywords: Cenomanian, Microfacies, Mishrif, Southeastern, Turonian.
INTRODUCTION
Ahmadi, Rumaila, and Mishrif succession was deposited during Cenomanian-Early
Turonian cycle. The intra-shelf basin developed during the Cenomanian age by dominated
shallow water of carbonate ramps (Robertson, 1987; Patton and O'Connor, 1988; Sharland
et al., 2001). This event was due to growth of Oman-Zagros peripheral bulge associated
with obduction of the ophiolites, but the possibility resulted from compressional tectonic
system along Arabian Plate margin (Al-Badry, 2005).
The Ahmadi Formation was deposited during the Cenomanian age in a shallow marine
basin with fine clastic sediment supply from the south. The Ahmadi Formation is usually
https://doi.org/10.26842/binhm.7.2019.15.3.0247
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Microfacies analysis and basin development
overlain conformably and gradationally by the Rumaila Formation and unconformably
underling by the Mauddud Formation (Jassim and Goff, 2006).
The Rumaila Formation is the most widespread Cenomanian formation in South and
Southwest of Iraq and extends as far north as the Makhul area in the North. This formation
was deposited in a relatively deeper basin which was locally restricted in the north. In the
south of Iraq the formation is conformably overlain by the Mishrif Formation (Jassim and
Goff, 2006).
The Mishrif Formation represents a heterogeneous formation originally described as
organic detrital limestones, with beds of algal, rudist, and coral-reef limestones, capped by
limonitic fresh water limestones (Bellen et al., 1959). The Mishrif Formation is deposited
above the high barrier or on the detached platform, which extend from south Kuwait to the
southeastern Iraq (Burchette, 1993). In parts of Southern Iraq, where the Kifl Formation is
present (i.e. in the basin roughly to the west of the Musaiyib-Nahr Umr palaeoridge) the
upper contact is conformable. Where the Kifl is absent the top of the Mishrif Formation is
marked by an unconformity (Jassim and Goff, 2006).
The study area is located in the southeastern of Iraq (Missan Province) within the
Mesopotamian Zone, which includes the study of the wells Noor -1 Rifai-1 and Halfaya-1
(Map1).
The purpose of the present study is a microfacies analysis and stratigraphic development
with knowledge of the tectonic events for this succession during Cenomanian-Early
Turonian period.
MATERIALS AND METHODS
Field work
Five oil wells were selected for the study that contains the largest amount of thin sections,
and then the description is made so as to identify texture, grains size, and type of pores
between the grains, and to determine depositional environments.
The Sampling was made by taking rock samples from the cutting and core available to
the Cenomanian-early Turonian succession, and then making a thin section, the sampling
was done one sliced per meter.
Laboratory work
1- Petrographic and microfacies investigation for the current study was based mainly on the
Dunham (1962) classification by using transmitted light microscope in the petrographic
laboratory of the Department of Geology, University of Baghdad. The petrographic study
was based on the more than (300) thin sections from cores and/ or cutting of the study
wells.
2-The well-logging tried to compare the micro-facies which were extracted from the
laboratory work (Electrofacies), diagnostic of the horizontal and vertical facies change,
and use well-log data to get the petrophysical characteristics for the study area wells.
3- Location map and columnar sections for the studied wells were draw by using the Corel
Draw X7 and Rock work 16 programs.
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Aiad Ali Hussien Al-Zaidy
Map (1): Location map of the study area with tectonic subdivisions according to Fouad
(2012).
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Microfacies analysis and basin development
RESULTS
Microfacies analysis: Carbonate depositional textures and microfacies were described
following Dunham classification (1962), and rudist- bearing facies were classified
according to Embry and Klovan’s (1971). The microfacies were compared with the models
of standard microfacies and depositional environment belts of carbonates proposed by
Wilson (1975) and Flugel (2010).
Facies association: A depositional environment can be defined in terms of physical,
biological, chemical, or geomorphic variables (Reineck and Singh, 1973); thus, sedimentary
environment is a geomorphic unit in which deposition takes place. The diagnosis of the
microfacies Cenomanian-Early Turonian succession and comparing with the standard
microfacies (Wilson, 1975), which contributed to the identification five facies associated
(open marine, basin, shoal, Rudist biostrome, and lagoon).
Basinal facies association: These facies make the beginning of the first sedimentary cycle,
which started with deep basin deposits supported by pelagic lime mudstone that contained
calcispheres, sponge spicules and rare of planktonic foraminifera (Pl.1-a); these facies zones
represent the Ahmadi and the Basal Rumaila Formations (Diags. 1, 2, 3).
Open marine facies association: Open marine facies association consists of fine grained
skeletal lime mudstones to wackstones; the skeletal grains consist mainly of planktonic
foraminifera such as, Hedbergella. The bioclasts are mostly fine and unidentifiable, spicules
and lesser amounts of small Echinoderms were also present (Pl.1-c); wells (Rf-1 and No-1)
(Pl.1- b, Diags. 2, 3).
Shallow open marine facies association: This facies association represents one of the most
common facies in the Mishrif carbonates in the study area; it consists mainly of bioclastic or
foraminiferal bioclastic wackstones and packstones, the bioclasts are silt to sand in size and
in some cases coarser. Other important fossils included in this facies association are
benthonic foraminifera, calcareous algae, coral, echinoderms, sponge spicules, and molluscs
(Pl. 1- a), (Pl. 1- d, e) (Diags. 1, 2, 3).
Shoal facies association: These sediments are sited on the marginal shelf, which is
composed of packstone- grainstone benthic foraminifera such as Plate (1-c, d), or
concentrations of their skeletal grains with rudistid debris, and culmination of the
coarsening upward sequence (Diags. 1, 3).
Rudist biostorm facies association: This facies is made up of very coarse-grained
bioclastic rudstone and floatstone containing a more diverse intact fauna than lithofacies
association shoal, dominated by rudistid debris (Pl.1- e). These are spread in the most of
studied wells, and uppermost of the Mishrif Formation in the Rf-1and No-1 wells (Diags. 2,
3).
Restricted facies association: Area of relatively shallow, quiet water separated from the
open marine conditions by a barrier (may be a coral reef). The lagoonal environment is
characterized by the presence of abundant of miliolids as Plate (1-f); associated with
mollusks, rudist debris, echinoderm, and peloidal in lime mudstone to wackestone in the
shallow marine restricted water. These deposits spread in the most of Hf-1 and Rf-1 wells
succession (Diags. 1, 2).
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Aiad Ali Hussien Al-Zaidy
Plate (1): The major microfacies of Mishrif Formation in the studied sections; (A) Basinal
facies, with abundant of Calcispheres (XPL), Well (Noor-1), at depth (3720 m),
(B) Open marine facies (Oligosteginids, Heterohelix sp., and Hedbergella sp.
(PPL). Well (Halfaya-1), at depth (3280 m), (C) Shoal facies with diversity of
skeletal grains such as (benthic foraminifera, rudist fragments, echinoderms and
mollusks (PPL). Well (Noor-1), at depth (3520 m), (D) Shallow open marine with
benthic foraminifera (Praealveolina tenuis) (PPL). Well (Noor-1), at depth (3360
m), (E) Rudist (PPL). well (Rifai-1), at depth (2880 m), (F) Restricted facies, with
abundant of miliold foraminifera (Quinqueloculina sp.). Well (Noor-1), at depth
(3320 m).
A B
C D
E F
300µm
300µm
500µm 500µm
500µm
500µm
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Microfacies analysis and basin development
Diagram (1): Microfacies description of the Cenomanian–Early Turonian succession
response to well logs at well Halfaya-1.
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Aiad Ali Hussien Al-Zaidy
Diagram (2): Microfacies description of the Cenomanian–Early Turonian succession
response to well logs at well Rifai-1.
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Microfacies analysis and basin development
Diagram (3): Microfacies description of the Cenomanian–Early Turonian succession
response to well logs at well Noor-1.
Sequence stratigraphy: Standard carbonate microfacies models are widely used to
interpret paleoenvironment, but they do not show how carbonate platforms are affected by
relative sea level change, a realization of how the carbonate factory responds to relative sea
level changes and the role played by other environmental factors towards influencing the
formation of carbonate platforms, which allows differentiating platform type and helps
establish depositional sequence and system tract models. The sequence defined:
depositional sequences bounded by subaerial unconformities and their marine correlative
conformities (Wilson, 1975; Vail, 1987; Posamentier and Vail, 1988).
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Aiad Ali Hussien Al-Zaidy
Stratigraphic Cenomanian- early Turonian sequence: The Cenomanian-early Turonian
Megasequence started by transgressive system tract (Ahmadi Formation), and terminated in
the high stand system tract (Mishrif Formation); the studied sequence is subdivided into
three main cycles as coarsening upward cycles. These cycles consist of Mishrif A, Mishrif
B, and Mishrif C which separated by compacted rock CR I and CR II units (Diags. 4, 5, 6).
Mishrif A: This unit represents upper regressive cycle, which ends by the regional
unconformity surface; Mishrif A is characterized by the abundance of benthic foraminifera
that indicate the lagoon environment in all studied wells.
Compact rock (CR I): This unit is located below the Mishrif A unit and it is distinguished
by a high GR and low DT logs. This stratigraphic unit consists of lime mudstone and free of
fossils with pyrite.
Mishrif B: The Mishrif B unit was deposited in the differentiated basin, because it
represents lateral biofacies change from deep basin sediment as Rf-1 and No-1 wells (Diags.
5, 6), to the rudist biostrome with open shelf lagoon facies at Hf-1 wells, (Diag.4).
Compact rock (CR II): This unit is located below the unit (Mishrif B) and can be
distinguished by a high (DT and GR) logs. The stratigraphic unit consists of lime mudstone
(micrite) and free of fossils.
Mishrif C: The stratigraphic unit represents lower regressive cycle, which was deposited
during early highest and system tract; and it comprises the transitional sediments from deep
marine facies at wells Rf-1 and No-1(Diags. 5, 6), to the rudistid packstone- grainstone,
with abundance of benthic foraminiferal grainstone in the lagoonal facies at well (Hf-1)
(Diag.5).
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Microfacies analysis and basin development
Diagram (4): Stratigraphic columnar section of the Cenomanian – Early Turonian
sequence at well Halfaya-1.
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Aiad Ali Hussien Al-Zaidy
Diagram (5): Stratigraphic columnar section of the Cenomanian – Early Turonian
sequence at well Rifai-1.
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Microfacies analysis and basin development
Diagram (6): Stratigraphic columnar section of the Cenomanian – Early Turonian
sequence at well Noor-1.
Basin development: In order to study the development of this cycle in the southeastern Iraq
in more details, structural proposed model shows the vertical and horizontal facies change,
and the determining of the main factors which control the intrashelf sedimentary basin
(tectonic and sea level change); this cycle was divided into three stages (Diag. 4, 5 and 6):
Stage (A): The tectonic setting contributed to the emergence of the Passive margin in the
east and northeast Arabian plate, and made it facing the Neo-Tethys (Sharland et al., 2001).
The abrupt discontinuity of the Mauddud sediments is underlaying the studied sequence;
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Aiad Ali Hussien Al-Zaidy
then it is followed by the open marine sediments of Ahmadi Formation which deposited
during transgressive conditions on the gentle slope of the carbonate platform model; this
succession is characterized by no facies change with wide extension and the quiescent
tectonic.
Stage (B): As a result of the up growth of compressional tectonic system (initial collision),
which produced the peripheral Bulge in the Middle Cenomanian along the southeastern
Arabian plate edge; this event contributed to deposition the Mishrif Formation as coral
barrier and rudistid biostrome association facies (Chatton and Hart, 1961; Burchette, 1993).
This stage is distinguished by beginning of the emergence an intrashelf basin, which shows
a moderate slope resulting of wide extension of the Mishrif Formation.
Stage (C): The continuation of the compressional tectonic system, contributed to the
development of the sedimentary basin, and the appearance of the facies change
(Differentiated basin); the marly limestone facies of the Rumaila Formation passes to the
bioclastic shoal, reef, and back- reef facies (Mishrif Formation). The high stand system tract
was caused of growth the carbonate factory, and accompanies the progradation facies
towards the basin center. In the last regressive cycle, continuance of the progradation shelf
margins (rudistid biostrome) to become overlies basinal sediments in the Rifai-1 and Noor-1
wells; while the lagoonal facies progradation, and overlie the reefal buildups is appeared in
the Halfaya-1 well. The Cenomanian- Early Turonian sequence ended with appearance of
the erosional surface in the Middle Turonian, which resulted of compressional tectonic
system that causes ophiolite obduction along the northern and northeastern of Arabian plate
(Sharland et al., 2001).
DISCUSSION
The Cenomanian- Early Turonian succession is composed of Ahmadi, Rumaila, and
Mishrif Formations; the intra-shelf basin development during the Cenomanian age was
made by dominated shallow water of carbonate ramps that was due to growth of Oman-
Zagros peripheral bulge (Jassim and Goff, 2006). Petrographic study and microfacies
analysis help recognize five main environments: open marine, basin, shoal, Rudist
biostrome, and lagoon); these are open marine facies which consist mainly of pelagic lime
mudstone which contains calcispheres, sponge spicules and rare of planktonic foraminifera,
the basinal facies consist of the calcareous sediments which consist of pelagic organisms
plus fine detritus moved off from adjacent shallow shelves. Shoal facies are represented by
packstone- grainstone benthic foraminifera and concentrations of skeletal grains with
rudistid debris, the Rudist biostrome consists of masses of organic rudstone facies, and this
facies is made up of very coarse-grained bioclastic rudstone and floatstone, lagoonal facies
which consist of benthonic foraminiferal wackestone and mudstone with miliolids and
peloidal.
The Ahmadi Formation overlies the Mauddud Formation unconformably so as to deposit
during the transgressive stage; in the studied area the lower boundary of the Rumaila
Formation with the Ahmadi Formation is conformable and gradational during the same
stage. The Mishrif Formation is deposited above the high barrier or on the detached
platform within the Rumaila basin. The studied succession is consisting of shallowing
upward cycle, and is associated with continuation of the compressional tectonic system
which led to the unconformity surface at the top the Mishrif Formation, being overlain by
the Khasib Formation.
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Bull. Iraq nat. Hist. Mus.
June, (2019) 15 (3): 247-262
التوروني املبكر في حقول نفط –تحليل السحنات الدقيقة و تطور حوض تتابع السينوماني
العراق شرق و نور و الحلفاية، جنوب الرفاعي
أياد علي حسين الزيدي
العراق, بغداد ,جامعة بغداد ,كلية العلوم ,قسم علم الارض
82/80/8891: تأريخ النشر، 91/98/8890: تاريخ القبول ، 91/80/8890: تاريخ الاستالم
الخالصة
تكوينات ألاحمدي , والرميلة , واملشرف في التروني املبكر من-للسينوماني يتكون التتابع الطباقي
حقول نفط الرفاعي والنور والحلفاية ضمن نطاق بين النهرين في العراق, والتي تحد من ألاعلى
.بسطوح غير متوافقة سفلوألا
: لقد ساعد التحليل السحني الدقيق لالبار املدروسة على التعرف على خمسة بيئات رئيسية وهي
حيث تشير . مفتوحة, و حوضية, و ضحلة بحرية مفتوحة , بيوديستروم , والغونية سحنة بحرية
ن ثم هذه السحنات الدقيقة إلى تغّير جانبي طبيعي من بيئة املياه الضحلة إلى املياه العميقة وم
يتميز برواسب بيئة بحرية مفتوحة ( السينوماني املبكر)ان تكوين ألاحمدي . االبيئة البحرية املفتوحة
.(تكوين الرميلة)خالل ظروف تقدم بحري , ولتصبح رواسب حوض عميق إلى أعلى
في الجزء ألاعمق من الرف القاري , والذي يتكون ( السينوماني الاوسط)تم ترسيب تكوين الرميلة
قة , والذي يتضمن وفرة من الاحياء البحرية لبيئة البحر املفتوح بشكل اساس ي من ترسبات عمي
يمثل حوض تكوين الرميلة حوًضا مكافًئا زمنيا لحوض تكوين . الداعمة للعمر السينوماني الاوسط
التوروني املبكر -يمكن تقسيم تتابع السينوماني . املشرف, وقد تم ترسيبه أثناء نظام الترسيب الاعلى
التي ,دورة املشرف ب و دورة املشرف ج, دورة املشرف أ -:نحو الاعلى وهي تنعم إلى ثالث دورات
بشيوع سحنة الروديستيدية املتراصة و تميز ت حيث الترسيب الاعلىترسبت خالل مرحلة النظام
CRI ) سحنة البايوستورم الروديستي التي تفصلها وحدات ترسبت اثناء التقدم البحري الحبيبية أو
.(CRIIو