Microfacies Architecture and Stratigraphic Development of the Yamama Formation, Southern Iraq

The Yamama Formation belongs to the late Berriasian-Aptian succession, which was deposited during the Lower Cretaceous period within the main shallow marine depositional environment. Petrographic study and microfacies analysis enabled the recognition of six main microfacies for three association facies. These are the Semi-restricted, Shallow open marine and Shoal environments. The study succession represents deposition of three third order cycles, these cycles where deposited during successive episodes of relative sea level rises and still stand. The presence of shoal association facies (oolitic packstone microfaces) between the Sulaiy and Yamama formations refer to continue the deposition during the same stage, and may suggest the end of Sulaiy Formation was maximum flooding surface (mfs). The first stage started with occurrence of the shallow open marine association facies underlain by semi-restricted association and then shoal association facies. There are three cycles of this sequence consistently in the south of the study area, so that it continues to the lower part of the Ratawi Formation to be the upper contact of the Yamama Formation of a conformable and continuous in sedimentation To the north of the study area (near of Rf-1 and Hf-5 wells) the shoal association was only shown once at the bottom of the Yamama Formation and these cycles to became unclear. This suggest that the paleo-high was developed to the south of studied area, while the open sea was characterized the northern part. The presence of shoal association facies (oolotic packstone microfaces) between the Sulaiy and Yamama formations refers to continuous deposition during the same stage, and may suggest the end of Sulaiy Formation was represented by maximum flooding surface (mfs). This first stage stated with the shallow open marine association facies underlain by semi-restricted association and then shoal facies association.


Introduction
The Yamama Formation is a part of the late Berriasian-Aptian succession, which was deposited during the Lower Cretaceous period within the main retrogressive depositional cycle (Berriasian -Aptian) south of Iraq. This sequence is represented from shore to deep basin by the Shuiaba, Zubair, Ratawi, Yamama, and Sulaiy formations [1].
The basin of the Yamama Formation was located over two tectonic regimes, the north eastern part of the stable Arabian platform and the Mesopotamian Zone. This situation and the probable syntectonic deposition of the formation over growing structures created complex carbonate lithologies. From this tectonic setting and the facies distribution, it seems that the Yamama Formation was deposited within a range environment from inner to an outer ramp [2].
Five Oil fields represented by Rachi-2, Rifaei-1, Ratawi-3, Halfaia-5, and Luhais-12 are included in this study. They are located in the southern part of Iraq. This basin is covered by loess and fluvial plain sediments of Quaternary deposits within the Mesopotamian Zone (Buday ,1980). The study area is located in the southern part of Iraq within the Mesopotamian Zone between (30º 58′-32º 08′ latitude) and (46º 52′ -47º 56′ longitude). At the present day, the basin forms a flatlying area located between northern central Iraq and Kuwait. Anticline and horst lie beneath undeformable or gently deformed Neogene cover and frequently related to long-lived paleo-structure in the Basrah area. The aim of this study are microfacies and well log facies analysis of the studied succession to determinate the depositional environment and establishing the sequence stratigraphic framework.

Methodology
The present study was completed by two major stages: 1. Field observation and sampling stage: This stage is represented by going to the Luhais and Rachi oil fields where the core and cutting samples of the studied sections were collected and sampled of the core and cutting samples of the core and cutting samples of Table 1.

2.
Laboratory and office works  Make thin sections in the petrology lab of Geology Department/College of Science/ University of Baghdad; for microfacies study.  Digitizing well logs using Digger software  Study of available well logs and relate the log response to facies changes for the studied succession intervals.    [7]. From its stratigraphic position an age range of Upper Berriasian to Lower Valanginian age is likely (140 to 136 Ma.). In Kuwait the Yamama Formation is assigned a Berriasian age [5]. The Yamama Formation was deposited in alternating of oolitic shoal and deep inner shelf environments, probably controlled by paleo structural highs within a carbonate ramp [8]. The Minagish Formation in Kuwait is the equivalent of the Yammama Formation [5]; was deposited as a transgressive unit within the inner shelf environment.

Microfacies Analyasis
Seven major microfacies are recognized in the studied succession which represented by Yammama Formation depending on Dunham classification [8] of Carbonate rock which deals with depositional texture that used to determine their characteristic grain types and depositional texture enabled the recognition of paleoenvironment.

Oolitic grainstone
Oolite grainstone is defined as a grain-supported carbonate rock that contains less than 1% mudgrade material. This definition has recently been clarified as a carbonate-dominated rock that does not contain any carbonate mud and where less than 10% [8].

Semirestricted environment
The semirestricted environment is represented by Orbitolina wackestone to packstone microfacies and Milliolids wackestone microfacies. The microfacies with Orbitolina as main components with few small benthic foraminifera may refer to the semirestricted environment (deep part for this environment), but the microfacies with the Milliolids as the major component may refer to more shallow environment [11].

Shallow open marine environment
The Orbitolina -Milliolids wackestone to packstone is the main microfacies which reflect open marine conditions [9,12]. It is characterized by high diversity of components where orbitolina and milliolids as the major components in addition to Echinoderms, gastropod fragments, small benthic foraminifera and Lithothamnium algae.

Shoal environments
Two major microfacies characterized the shoal environment [12], they include; the peloidal grainstone characterized by fine, well sorted peloids and ooidal to peloidal grainstone which consist mainly of ooids and peloids with some bioclasts and benthonic foraminifera.

E F Stratigraphic development
Sequence stratigraphy is the study of the sedimentary rocks relationships within a chronostratigraphic framework of repetitive genetically related strata bounded by the unconformity surface, or their correlative conformities [13,14]. The stratigraphic signatures and strata patterns in the sedimentary rock record are the result of the interaction of tectonics, eustasy, and climate parameters. [15].
In order to study the development of this cycle in the southeastern Iraq in more details, the Mesopotamian foredeep has experienced active syntectonic sedimentation leading to the formation of giant structures that were apparently simultaneously growing during deposition of the Yamama Formation. These structures probably were induced by diapiric warping caused by the Infracambrian Hormuz Salt Series which is believed to underline parts of southern Iraq [1]. The formation extends (Figure-8) and facies association ( Figure-9) show the basin is characterized by one main depocenter to the east of the studied area. This cycle was divided into four stages of highstand system tracts, the first and the last were interplay with the older formations (Sulaiy) and younger (Ratawi). This sequence is repeated three times consistently in the south of the study area, so that it continues to the lower part of the Ratawi Formation to be the upper contact of the Yamama Formation of a conformable and continuous in sedimentation To the north of the study area (near of Rf-1 and Hf-5 wells) the shoal association was only shown once at the bottom of the Yamama Formation and these cycles to became unclear. This suggest that the paleo-high was developed to the south of studied area, while the open sea was characterized the northern part.
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 [16]. The Yamama Formation is conformably underlain by the Sulaiy Formation except in some parts of the Salman Zone where it is unconformably overlain by the Zubair Formation [17].

Al Mafraji and Al-Zaidy
Iraqi Journal of Science, 2019, Vol. 60, No.5, pp: 1115-1128 1127 The presence of shoal association facies (oolotic packstone microfaces) between the Sulaiy and Yamama formations refers to continuous deposition during the same stage, and may suggest the end of Sulaiy Formation was represented by maximum flooding surface (mfs). This first stage stated with the shallow open marine association facies underlain by semi-restricted association and then shoal facies association.