MICROFACIES DISTRIBUTION OF KHASIB FORMATION IN AMARA OIL FIELD, SOUTHERN IRAQ

The Khasib Formation Late Turonian-Lower Coniacian (Late Cretaceous) is an important reservoir rock in the Amara oil field. Hence, the importance of Petrographic study in this research of Khasib Formation is to know the facies distributions that are summarized in facies models and digenesis affected in the field. Through focusing in AM11, AM9, AM5, AM3, AM12, AM13 wells by using 33 thin sections, RG-log, density and sonic logs that used to the Layering models that have wide engineering applications. The result shows that Khasib Formation consists of limestone and dolomitic limestone. The Khasib Formation has been divided into three microfacies that are deposited in pelagic deep marine environment (deep Cratonic water). The microfacies analysis of the Khasib Formation also is showing that the oil production units in Khasib Formation KA unit (upper part) is made up of Packstone and Grainstone carbonate, but KB unit (lower part) only from Grainstone. The skeletal grains of the Khasib Formation include variety of planktonic foraminifera, Calpionellids and bioclast. The bioclast consists of bivalved shells that belong to Mishrif Formation embedded in micrite and microspare groundmass. The Authogenic minerals consist of pyrite and iron oxide. The rocks of the formation are affected by diagenesis which have a significant impact on the reservoir architecture. The dolomitization is a very important factor to improve petrophysical properties and forming Intercrystalline pores in KA unit. Besides, dolomitization interprets the relatively height of the RG-log in the lower part of KA unit and BAR2. The dissolution has an important role in forming mold


INTRODUCTION
1.The Khasib Formation in Late Turonian-Coniacianis is one of the most important reservoirs in the Amara oilfield.The type locality of Khasib Formation was in type well locality as defined by (Owen and Nasr, 1958) in Zb-3 consist of dark gray-green shale in the lower part 20m and gray marly limestone in the Upper part by thickness 30 m, the total thickness in type locality up to 50m (Dunnington et al., 1959).It seems difficult to apply these divisions in all areas of Khasib Formation distributions because of the difference in the petrographic components.For example, in the case of the absence of gray marly limestone, the upper part has disappeared in the area (Buday, 1980).Hence the importance of the Petrographic study is to define the upper and the lower parts component of the Khasib Formation in the Amara oilfield and the Paleoenvironment.The thickness of the Khasib in the study well is shown in (Table 1).The study aimed to Determining the microfacies type depending on the (Dunham, 1962) classification, distributions of these facies and sedimentary environment according to (Wilson, 1975) standard through the petrographic study, Determining the digenesis processes and their effect on the basin architecture, Design the cross section and Well study correlation of AM11 well and Design the facies and Layering models depended on the petrel software by using GR-log, Density Log (FDS, DRHO, RHOB) and sonic log.

LOCATION AREA
The well study area is located southern Iraq in the Amara oil field about 10 Km southern Missan Governorate as it is shown in the Satellite image (Fig. 1) that is obtained from satellites Sentinel-2 according to wells coordinates (UTM) (Report of well.11 MOC, 2011) in (Table.1).

TECTONIC FRAMEWORK
Missan Governorate is located in the Tigris Subzone in the Mesopotamian Zone.It contains wide synclines and narrow anticlines trending often NW-SE extending by long normal faults (Jassim, 2006).The western boundary synchronizes with a palaeoridge that is effected by and by long gravity gradient and line of residual gravity anomalies between Qal at Salehin southeast and Fallujah northwest (Jassim, 2006).

RESULTS AND DISCUSSION
This section includes the result of microfacies analyses (a type of skeletal grains, Matrix, pore space and relationship with reservoir unit and sedimentary environment).The digenesis effects on microfacies.All these pieces in the formation are summarized in the cross section of well AM11.

Planktonic Foraminifera
The majority occurs in the upper part of the water column within the upper 50 to 100 m near the surface (photic zone).Below 200 m, they are virtually absent but after the death of these groups are transported by waves to a deep environment to gather and build deep limestone.
Planktonic fauna avoids terrigenous input.Therefore, they accumulate in the bathyal depths of open-marine basins.
They represented the rock-building limestone in a Cretaceous deep marine and they have an importance in biostratigraphic markers in the Early Cretaceous (Flugel, 2010).These are the most important species of Planktonic foraminifera in Khasib Formation: 1. Globigerina Spherical test.Wall calcareous is common in Middle and Late Miocene deep shelf and basinal limestone (Flugel, 2010)

Calpionellids
Calpionellids are extinct bell-shaped and very small calcite microfossils.Pelagic limestones of Late Jurassic and Early Cretaceous Tethyan ocean.The planktonic fossils are considered as a group of the subphylum Ciliophora (Flugel, 2010).Calpionellid stratigraphy is generally combined with biozonations based on calcareous nannoplankton (nannoconids, coccolithophorids) and radiolarians (Flugel, 2010).Calpionellids are found in common presence in Khasib Formation PL.2E.

Bioclast
Organoclastsare organic particles of various origin with or without structure (Tucker, 2001).In Khasib Formation the, univalved shells are thin bivalved shells is the main type of bioclast at a rare presence up to 2-3 % with subangular -subroundedin sphericity PL.3D.

Matrix
As it is noted in the Khasib Formation, the major constituents that are seen in thin sections of carbonate rocks are 'groundmass'.The spray calcite is latter developed as fine-grained carbonate 'matrix' or as representing pore-filling cement or recrystallized from micrite.
The micrite is commonly regarded as an indicator of low-energy depositional environments in protected shallow areas or Pelagic sea floor (Flugel, 2010) as noted in the Khasib Formation lamination micrite with abundant planktonic foraminifera PL.2B.

Pore Space
This study uses (Choquette and Pray, 1970) classification that has shown the importance of relating pore space to depositional and digenetic fabrics and of distinguishing between pore space types in reservoir study which are shown in below: 1. Moldic pores are fabric selective secondary pores from the selective dissolution of grains by solvent liquid.Preferentially, Molds form in rocks of mixed mineralogist in burial settings.Generally, pore sizes are 0.1 to 10 mm which are very commune in KB unit (lower reservoir unit) in Khasib Formation are the basic pores PL.2F.

Interparticle pores:
Pores between individual particles generally agree with depositional primary porosity, pore sizes are 0.05 to 1 mm.Interparticle pores are high in modern carbonate deposited (up to about 50%) and in the study reservoir is not more than (2-3 %) in Khasib Formation PL.3A.both of KA (upper reservoir unit) and KB unit.
3. Vug pores: non fabric selective formed by synsedimentary to early digenetic biogenic methane freedom organic matter is common in KB unit Khasib Formation in PL.3C.
5. Fractures: non fabric selective uncommon in KA unit in the Khasib Formation PL.3B.
6. Intercrystalline pores: a medium to coarsely crystalline replacement dolomite are the main pores in KA unit PL.3C.

DIGENETIC AND POROSITY MODIFICATION
The Khasib Formation is characterized by homogenous components.This led to regularity in the distribution of pore system in the description of the Microfacies.The basic digenetic processes are discussed below:

Dolomitization
The diagenetic process that converts limestones to dolostones through a microchemical process of calcium carbonate dissolution and dolomite precipitation.In the Khasib Formation, the partly dolomitization doesn't form distinctive rhomb crystals of dolomite PL.1F.The dolomitization is indicated to replace from grain-dominated fabrics and has a positive role in petrophysics which is characterized in AM11, AM9, AM5 wells in (KA) unite.Therefore, dolomitization is a very important factor to improve petrophysical properties because of (1) increasing particle size (2) developing intercrystalline pores and (3) raising resistance to compaction (Lucia et al., 1995).
When the dolomitization forms dolomitic limestone, which is destroying the pore space system and building the (BAR2) between the two reservoir units in Khasib Formation PL.2A.
Because of the grain dominated are usually consisting of grains much larger than the dolomite crystal size, that dolomitization does not have an important influence on the pore-size distribution (Lucia, 2007).
The main dolomitic texture in Khasib Formation is Aphanotopic crystals of dolomite in (0.002) mm size.It is formed by the homogeneous dolomitization of micrite (Randazzo and Zachos, 1984).When crystals size is more than 0.002 mm, it is known as Microcrystalline texture.According to (Randazzo & Zachos, 1984) it was noted that this type of dolomite is formed by early dolomitization but it is commonly presented in Khasib Formation.

Carbonate Cementation
Cementation can be formed by sin sedimentation or may occur later during burial.Later cementation may occur by a complex geochemical-hydrological meteoric flow system.
Therefore, the pore space has been cleared totally from carbonate from some around and reprecipitation in other areas (Budd, 2002).
A characteristic of all cement is that they are growing from pore wall into pore space and therefore reducing pore size.The methodology of reduction in pore size accounts for the observed systematic changes in permeability PL.3B when filling cavern and fracture.In the Khasib Formation and forming impermeable rock (BAR1) in the upper part of the Globigerina Grainstone microfacies (GG3).
Cementation in the Khasib Formation form the granular cement which is calcite cement that is consisting of relatively equals pore-filling small crystals texture PL.3F.

Compaction
The compaction is resulting from the increased overburden pressure due to the burial.Textural effects include the loss of porosity, reduction of pore-size, grain penetration, grain deformation, grain breaking and fracturing (Budd, 2002).The least contact phase is observed between the grains in Khasib Formation which is described as concavo-convex, tangential, sutured contacts and two sets of cleavage forming in the high physical compaction PL.3G (Tucker, 2005).
Khasib Formation in thin section for the same sample display irregular stylolite PL.3E.Also, in other state, thin section shows irregular parallel sutured seams Bedding-parallel (micro stylolite set) PL.1E.The effect of compaction varies depending on the type of the microfacies if grain-supported limestones grainstones, packstones.The Stylolites are very clear as it is Shawn in Khasib Formation (Tucker, 2005).

MICROFACIES DISTRIBUTION IN KHASIB FORMATION
The lithology of Khasib Formation consists of grain dominated carbonate (planktonic microfacies) which is divided into three Microfacies (Fig. 3)

Globigerina Grainstone microfacies (GG3)
This microfacies has Globigerina genus high ratio about (90%) embedded in microspar ground mass with lamination Bituminous limestone.Depth is from 2831 to 2872 m and thickness is 41m.
This microfacies mostly is affected by dolomitization ranging between 40-45% which causes deformations to all skeletal grains.Cementation generated granular cement.The visible pore space intercrystalline and intrapartical ranging between 4-5%.Oil show is ranging about highmedium.
This microfacies (GG3) is repeated in the other depth which is ranging between 2809 -2817 m and thickness is 9 m.This microfacies consists of Dolomitic limestone which involves planktonic foraminifera PL.1A, B. Skeletal Grains are the basic components and one of these components is the planktonic foraminifera which is about 60% and the other components is Calpionellids which is about 30%.The groundmass is modified completely to isopachous and Blocky cement.
Digenesis are summarized by compaction which is shown in sutured contact and in the last five-meter dolomitization which causes deformations to all skeletal grains.Pore space is mostly formed by selective dissolution moldic and vug PL.2F.and is ranging between 6 to 8%.
Oil show is sometimes medium and sometimes high.This microfacies is forming KB unit in Khasib reservoir PL.3H.This microfacies has a high ratio of pyrite at a depth 2817m.
Another appearance for this microfacies at depth 2798 Khasib top -to 2807 m and thickness about 9m.This microfacies contains skeletal grains more than 90% from planktonic foraminifera only (Globigerina, Globigerinid, Praeglobotruncana) PL.1C and embedded in microspare groundmass.
Cementation consists of granular cement.The chemical compaction creates sutured PL.1D and stylolite PL.1E.Physical compaction led to forming large crystals of calcite manufacture having two sets of cleavage and this is the phenomena which is the result of of high compaction.
Vug and mold are the main pore spaces type with low-medium oil show.This part is forming BAR1unreservoir unit in Khasib reservoir.According to (Wilson, 1975) this microfacies is similar to the standard Microfacies (SMF3) which is originally precipitated in (FZ1) Gratonic deep water basin.

Globigerina packstone Microfacies (GP1)
This microfacies consists of bituminous limestone and dolomitic limestone including low percentage of skeletal grain 50-55% and maintain a good variety Globigerina PL.2D embedded in micrite ground mass and it includes Pyrite.
The depth of this microfacies is about 2826 to 2830 m and its thickness is about 4m.The effective digenesis process is represented by dolomitization that destroys facies features.
Compaction is existed by sutured and stylolite.The pore space is ranging about 13% with mold spores, channel pores and intercrystalline.This microfacies includes high oil shows and forming the lower part of KA unit in Khasib reservoir.
According to Wilson (1975), this microfacies is similar to the standard Microfacies SMF4 in originally precipitated in FZ1 Gratonic deep water basin.

Calpionellids Packstone Microfacies (CP2)
This microfacies showed an increasing of the Calpionellids fauna about 75% bioclasts embedded in micrite.In addition to, high percentage of Pyrite PL.2C.
Thickness of this microfacies is about 8m and depth is about 2818 to 2826 m.The effect of digenesis is cementation (isopachous and drusy types) and Compaction that is summarized in laminated limestone PL.2B sutured, stylolite contact and twinning calcite.This microfacies is also featured by dolomitization that deforms facies style in some time PL.1F and PL.2A.The Pore space is intercrystalline, mold, intrapartical and vug which is ranging between 8-10%.Oil show is low.This microfacies is forming the middle part of KA unit in Khasib reservoir.
According to (Wilson, 1975) this microfacies is similar to the standard Microfacies SMF3-CAIP and it is originally precipitated in FZ1 Gratonic deep water basin.

CROSS AND CORRELATION SECTIONS
After completing the above-mentioned comparison (Microfacies and Reservoir unit to determination the type of facies for these units and with Oil show observer in thin section) the following points are observed: 1. Cementation has a negative role in Khasib Formation by forming BAR1(impermeable rope) which is included in Globigerina Grainstone microfacies (GG3).The cementation proses converted Grainstone facies to Cap rock reservoir.
3. The oil production units in Khasib Formation KA unit is made up of Packstone and Grainstone carbonate.The KB unit is only made from Grainstone which is deposited in deep Cratonic water environment.
4. The GR-log reading is relatively high in BAR2 and lower part of KA unit and the initial interpretation through the microfacies analysis due to the dolomitic limestone according to (Hassan, 1973) and partly dolomite in the lower part of KA unit where the KB unit is a clean reservoir.
5. Oil shows which might be indicated to hydrocarbons concentration, is high in the upper part of KB unit and medium in the lower part for the same unit.In the KA unit, the concentration of the hydrocarbons is high in the medium and the lower part.

DEPOSITIONAL ENVIRONMENT OF KHASIB FORMATION
Microfacies distribution for Khasib in Amara oil field is an indicator to one environment setting in deep water basin.The first microfacies Globigerina Grainstone indicates SMF 3-FOR pelagic foraminifera according to (Wilson, 1975) classification and featured with good diversity for Globigerina genus in microspar ground mass.Most researchers link between this genus (Globigerina) and the deep marin environment (under wave of the effect).This follows Globigerina packstone microfacies and also have the same criteria to deep water environment in (FZ 1) with a higher percentage of micrite.Another physical indication of description for deep water environment dark thin limestone beds, dark shale beds rock color dark brown, black (due to organic matter) and reddish due to slow sedimentation as notable in the core of study wells (Luterbacheret et al., 1984).
Towards the highest of the formation, Calpionellids packstone microfacies was noted.
The calpionellids fauna is 75% percentage in miritic groundmass specified in SMF 3-CALP (calpionellids) according to (Wilson, 1975) classified and give another guide of (FZ1) in deep sea water.According to (Cook, 1983)

THE KHASIB FORMATION FACIES MODELS
To build 3D facies model, the model requires two key steps.The first step is facies characteristics and direction trend that should be explained by sedimentological description.
The second step is application for this data and correlation in the simulation model.This model can be used for quality control and development plans (Cannon, 2018).
In the current study, according to (Schlumberger, 2015) it uses algorithm truncated aussian with trends for 2D and 3D Facies model of the Khasib Formation (Figs. 5 & 6).These were built depending on the results of the microfacies description (analyses) and interpretation of sedimentary environments from thin section and also core description and log interpretation of five wells in the Amara oil field area in the petrel software (Schlumberger, 2015).
The 2D and 3D facies models noted the distance between AM12 to AM3 reached to 10.5

LAYERING MODEL OF KHASIB FORMATION
It is defined as the internal layering reflecting the geological deposition of a specific zone.
Layers only sub-divide the grid between the zone-related horizons (Schlumberger, 2015) For another sense of knowing stratigraphic levels represented by seismically interpretation, horizons/events and geologically significant surfaces identified in well data.The levels are identified in both data sets then the mapped seismic horizons are constrained by the well picks (Cannon, 2018).According to (Cannon, 2018) there are many applications of the layering models that might be summarized by: (1-off-lap or on-lap/down-lap sequences,2-highstand system tract tends to become eroded if it is followed by a lowstands, that is, strata are truncated by the top surface.3-usedfor laterally extensive sheet-like bodies or to replicate differential compaction in a shale sequence; in this case, there will be a constant number of cells in the vertical stack.)

Fig. 1 :
Fig.1: Satellite image of well-studied location from (Sentinel-2 satellites) in Amara oil field sedimentation in the Mesopotamian Zone in the Late Jurassic time.Separating inner shelf facies in the southwest from outer shelf facies to the north east.The Tigris subzone includes two trending groups of lines anticlines (northwest and southeast) which are relatively low spread associated with longitudinal faults and at EW transversal trend.These anticlines are affected by three faults which are Ramadi Musaiyib Fault, Tikrit-Amara Fault and Kut-Dezful Fault.The Tectonic was active from the Late Jurassic onwards.Its border by the Foothill Zone (in NE)

Fig. 3 :
Fig.3: Cross section of well AM11 in the Amara oil field of the Khasib Formation

Fig. 4 :
Fig. 4: Correlation section of study well in the Amara oil field of the Formation he suggests a Calpionellids shield which is located quiet and warming water, lived in three environments in deep water and central environment between the deep to shallow (Rudist).The study determines the environment of Calpionellids depends on the fossils associated with it such as Globigerina in studied well.calcite crystals in Khasib Formation at (2808 m) S.NO.(50) also have two set cleavage by effect of deformation in Burial environment in well AM11.H: high oil shows in KB unit in globigerina Grainstone microfacies (GG3) in S.NO (71) in depths 2831 m.
km.The dipocenter is located between AM11, AM13 wells by the deep basin facies.The facies distributions in the sedimentary basin show the Globigerina Grainstone microfacies as a main component of the Khasib Formation in the lower part.The Globigerina packstone microfacies and Calpionellids packstone microfacies are concentrated in the upper part (dipocenter) of the Khasib Formation and decreasing in facies thickness toward the east in AM3 and toward west AM12.The facies pattern in the sedimentary basin has a balance between the absorption basin and the size of the sediments and the stability of the depositional environment throughout the deposition.

Fig
Fig.5: 2D modeling facies in Khasib Formation, GG3: Globigerina Grainstone microfacies, CP1: Globigerina packstone microfacies, CP2: Calpionellids packstone microfacies The dipocenter is located between AM11, AM13 wells by the deep basin facies.The facies distributions in the sedimentary basin show the Globigerina Grainstone microfacies as a main component of the Khasib in the lower part.The Globigerina packstone microfacies and Calpionellids packstone microfacies are concentrated in the upper part (dipocenter) of the Khasib and decreasing in facies thickness toward the east in AM3 and toward west AM12.The facies pattern in the sedimentary basin has a balance between the absorption basin and the size of the sediments and the stability of the depositional environment throughout the deposition.Khasib reservoir has been divided into 45 layers depending on log behavior, petrophysical properties and facies to capture any change (Fig. 7) illustrate the layering of Khasib Formation.It was interpreted accordingly to Sequential number (Sec.NO) as it is shown in the (Figure 3) which represents the change in microfacies from the bottom to top.The number of layers has been distributed to each successive Microfacies as Sec.NO21= 14 layers, 22=4 layers ,23 = 7 layers, 24=6 layers, 25=4 layers, 26=10 layers.

Fig. 7 :
Fig.7: The Layering in the Khasib Formation in Amara Oil Field

Table 1 : The top, bottom and thickness of the Khasib Formation in the studied wells
Common in interparticle pores in burial environments are formed by recrystallization of pre-existing cement are copies present.