GEOHISTORY ANALYSIS AND BASIN DEVELOPMENT OF THE LATE BERRIASIAN-APTIAN SUCCESSION, SOUTHERN IRAQ

The studied succession is deposited during late Berriasian-Aptian interval, which is represented by the Zubair, Ratawi, Yamama formations. The present study includes stratigraphic development and basin analysis for 21 boreholes (Rachi-1, 2; Rifaei-1, Diwan-1; Ratawi-1, 2; Halfaia-5; West Qurna 12, 15; Nahr Umr-7,8; Zubair-47,49; North Rumaila-72, 131, 158; Suba-7; Majnoon-2, 3 and Luhais-2, 12) distributed within 13 oil fields in the southern Iraq. The back-stripping process determined the original direction of basin depocenter for the studied succession. The Yamama basin in the study area stretches from southeast to southwest with single depocenters, it was located in the southeast of the study area near wells Mj-2, Mj-3.NR-8 and WQ-15 and stretched to the new position southwest near wells NR-7, RU-131, RU-158, ZB-47 and ZB-49. The Ratawi Formation is represented by one depocenter located in the southwest of the study area near wells Rc-1, Rc-2, Lu-12 and Ru-72. The Zubair basin in the study area has two depocenters, the first depocenter is located to the north of the study area near well Dn-1, while the second one is located at the northwest area near wellsSu-7, Lu-12, Rc-1, RC-2 and Rt-1.


INTRODUCTION
The studied succession (late Berriasian-Barremian interval) represented by the Zubair, Ratawi, Yamama formations are located over two tectonic regimes.The northeastern part of the stable Arabian Platform and the Mesopotamian Zone.This situation and the probable syntectonic deposition of the formations over growing structures created complex carbonate lithologies.From this tectonic setting and the facies distribution, it seems that the Berriasian-Aptian succession was deposited within a range environment from deltaic to the outer ramp (Sadooni, 1993).The present study includes stratigraphic development and basin analysis for 2; Rifaei-1, Diwan-1; Ratawi-1, 2; West Qurna 12,15;8;49;131,158;12.
The petrography and microfacies study included only five boreholes, while the basin analysis included all of them.The study area is located in the southern part of Iraq within the Mesopotamian Zone between 30º 58′-32º 08′ latitude and 46º 52′-47 º 56′ longitudes.At the present day, the basin forms a flat-lying 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 (Fig. 1).The late Tithonian/ early Turonian Megasequence was represented by deposits in intra shelf of a wide area in a basin that coincides with a new phase of ocean floor spreading in the Southern Neo-Tethys that cause a differential in subsidence due to change in thickness along the transverse fault.The axis of the intra shelf basin shifted toward the eastern Mesopotamian position on Salman zone and western Mesopotamian zone (Buday and Jassim, 1987).A new passive margin formed along the NE margin of the Arabian Plate as a result of opening the southern Neo-Tethys that caused drifting away of a narrow microcontinent.The Rutba uplift formed the western margin of Mesopotamian basin the NE margin was formed by a carbonate ridge along the facing of North passive margin of the Southern Neo-Tethys (Sharland et al., 2001).
This study aims to interpret the basin development during the Early Cretaceous through geohistory analysis where using back stripping and subsidence analysis to find the changes in accommodation and consequently its effect on sequence development.

METHODOLOGY AND REQUIREMENTS
The geohistorical interpretation depends on many stratigraphic data such as the following: 1.The stratigraphic column showing the thickness of the present-day of the sequence units.

Ages of horizons which used to estimated paleo-water depth.
Input geohistory data in the Rock Work V.16: 1. Backstripping.

Basin depth.
The data required were represented the present lithologic and thickness, age of lithologic units and the paleo bathymetry and relative sea level.The data has been taken from 15 boreholes distributed within the study area, as thickness and lithologic description for subsurface geological sections (Table .1).

d. Relative sea level
One of the basic tenets of standard sequence stratigraphy that the sequences and their systems tract mainly controlled by sea-level change (Van Wagoner et al., 1988;Emery et al., 1996).
In addition, there are several assumptions and uncertainties that are built into this analysis.
Most of these problems can be overcome if thick stratigraphic sections of relatively shallowwater deposits are used and only long-term, large-scale changes are studied (Angevine et al., 1991).The flow diagram in figure 2 shows steps of geohistory analysis (Al-Zaidy, 2013).

STRATIGRAPHY AND TECTONIC SETTINGS
The Mesopotamian zone stretched from Baiji in the NW to the Arabian Gulf in the SE, it is represented flat terrain with a gradient of >10 cm per kilometer (Buday and Jassim,1980).In the Early Cretaceous, high subsidence was driven under the effect of sedimentary weight.After that, slow subsidence happened in the Cretaceous that was driven by mechanical extension and rifting.The differences between tectonic subsidences at wells of the study area are reducing with the age decreasing.It indicates that the new strata have smaller compaction than the old strata.The subsidence rates are different in various periods.There is a fast subsidence period of 13 Ma during Early Cretaceous.In the burial history, sedimentation, and erosion rates, it was seen that the burial depth was greater in the east and north in comparison with the south and west of the study area (Handhal et al., 2014).The geological settings of the Mesopotamia  (Buday, 1980).Buday and Jassim (1980) have referred to this area as the Mesopotamian Zone, and considered it as a separate structural unit within the Unstable Shelf.According to their considerations, it is bordered from the northeast by the first superficial and topographic prominent anticlinal ranges represented by Makhoul, Himreen, Badra and Buzurgan.
The Mesopotamian Foredeep is a continental basin which lies between the Zagros deformational fronts from the northeast and the stable interior of the Arabian Platform (Fouad, 2012).The Mesopotamian Plain occupies the central and the southern parts of the Mesopotamian Foredeep within the Iraqi territory.It is a potential region of subsidence in the Neogene, and a significant basin of alluvial sediment accumulation in the Quaternary.It is a very mobile basin and contains several evidences pointing to its recent tectonic activity.

Geohistory Analysis
The purpose of geohistory analysis (Backstripping) is to calculate and remove the effects of compaction, sediment loading, changing paleobathymetry, and sea level variations.When backstripping is combined with the search for information on the development of the depositional system, the term geohistory analysis (or burial history) is commonly used (Einsels, 2000).For further discussion see, Van Hinte (1978), Watt and Steckler (1978), Sclater and Christie (1980), Hardenbol et al. (1981), Flavey et al. (1982), Gradstein et al. (1985), Guidish et al. (1985), Haq et al. (1987), Hegarty et al. (1988), andSteckler et al. (1988).At the first stage of geohistory procedure, the thicknesses and porosities of the studied unit rocks restored to the initial thicknesses and porosities (Table 1) by removing the compaction effect.Table (1) shows the steps of decompaction for one of the studied wells (Rachi -2) to calculate the initial thickness, which will contribute to the rebuilding the isopach maps and calculation of sedimentation rates (accumulation rates).At the second stage of this procedure the tectonic subsidence and potential available space (accommodation) were calculated and compared with the accumulation rates to understand the tectonic and depositional behaviors of the late Berriasian-Aptian basin.(Fig. 3) shows the relationship between the accumulation rate and the potential available space compared with the tectonic subsidence for well Rachi-2 as an example of how the comparison and interpretation adopted in this study.The geohistorical curve (the sediment accumulation curve) represents the relationship between the age of the formations and the decompacted thickness for them and compared with the present thickness (compacted thickness).To completing the decompaction process, the geohistory curve for 21 subsurface sections was constructed.These curves removed the effects of compaction and represent the succession of the present-day thickness (Fig. 3).

Rc-2 Basin Development
After the application of geohistory procedure the isopach map that represented decompacted thickness (Figs. 4 and 5).The backstripping process determined the original direction of basin depocenter for the studied succession, the Yammama basin in the study area was stretched from Southeast to Southwest with single depocenters, it was located in the southeast of the study area near wells (Mj-2, Mj-3.NR-8 and WQ-15) and stretched to the new position Southwest near wells NR-7, RU-131, RU-158, ZB-47 and ZB-49 (Fig. 4A).While Ratawi Formation represented by one depocenter located in the southwest of the study area near wells Rc-1, Rc-2, Lu-12 and Ru-72 (Fig. 4B).
The Zubair basin in the study area has two depocenter, the first depocenter is located at north of the study area near well Dn-1, and the second one is located at Northwest area near wells (Su-7, Lu-12, Rc-1, RC-2 and Rt-1) (Fig. 5A).The Shuaiba Formation depocenter is located in the northeast direction near well (Hf-5 and Ri-1) with general direction northeast to southwest (Fig. 5B).therefore, the curve of paleobathymetry can be compared with the tectonic subsidence maps show in Fig. 6.

Fig. 6: Total subsidence map of Yammama, Ratawi, Zubair and Shuaiba formations
We can classify the studied sections according to the gradient in total subsidence into: 1. Rapid gradient among Yammama, Ratawi and Zubair formations, as shown in borehole location Rc-1 with high sediment supply and high subsidence rate reflect large variation with the paleobathymetry and the amount of sediments supply between these formations.
3. Slow gradient in locations of wells Rc-2, Dn-1, Mj-1with very low sediment supply and very low subsidence rates during late Berriasian-Aptian.
From the previous definition for a subsidence rate, accumulation and accommodation this relationship could be used to understand the deposition basin behavior during that period, and whichever is the most effective and influential in these processes.Fig. 7 shows this relationship with symbols color accumulation rate (blue bar), subsidence rate (red bar) and accommodation (gray bar).We can classify this relationship for the formations Yammama, Ratawi and Zubair depending on: v Accumulation < Accommodation <Subsidence (Fig. 7) Represented by boreholes Rc-2, Rc-1, , ZB-47, ZB-49, Ri-1 and Rt-1 that show the value of accumulation rate is larger than Accommodation and subsidence rate v Accumulation >Accommodation <Subsidence (Fig. 7) Represented by borehole (Dn-1 and Hf-5), which has a large accumulation rate compare with Accommodation and subsidence rate.

Paleogeography and Tectonic Basin Evolution
After this procedure we can interpret the paleogeography, and development of depositional basin for the Yammama, Ratawi and Zubair as follows: v Yammama basin Paleogeography The high elevated (Positive) area is located in Western border at wells Zb-47, Zb-49& NR-7 while the remining area have gradually shallow elevation except two low elevation areas, the first one is located in the North represented by well Dn-1 and other one is located in the East represented by well Ri-1(Fig.8A).
v Ratawi basin Paleogeography The western area represented by many elevations, the first one is located South western basin with highest elevation in the wells Rt-3, Rt-1, Rc-2, Rc-1 & Ru-72 and the others are located Northwest with less elevation represented by Dn-1 and in North east represented by Hf-5 and Ri-1(Fig.8B).
v Zubair basin Paleography The Zubair basin is divided in to two area: The North area reflect the high elevation represented by Dn-1 and the South area represented shallow elevation to the deep are (Negative) represented by other wells (Fig. 8C).
v Shuaiba basin Paleography The positive area location in Southeastern border represented by well Hf-5 and the rest area reflect shallow depth toward the West represented by wells Nr-8 and Su-7 (Fig. 8D).

Deposition and Subsidence History
The succession is represented by Yammama, Ratawi & Zubair formations reflected approximately about 21.5 million years of restricted shallow to inner and outer shelf deposition environment in Mesopotamian basin during Beriasian-Barremian age (Jassim and Goff, 2006).
The backstripping procedure removes the effect of sediment loading from the basement subsidence, thus allowing quantification of tectonic basin subsidence.Backstripping was performed by sequential removal of the upper most layers and decompaction of the underlying strata.The mount of decompaction calculated using empirical porosity / depth relations for the specific lithologies of each layer.The last formation in the study area is Zubair Formation started with sequence cycle of transgressive system tract bounded above by maximum flooding surface reflected Delta plain environment consider with sea level fluctuation, followed by thin cycle of Highstand system tract reflected delta plain bounded upward by regressive surface (RS), followed by symmetrical transgressive system tract reflected Delta plain environment, this formation deposited during Hauterivian to Early Aptian before 10 Ma ago (Fig. 11).

Fig. 11: The stage of deposition and subsidence of Zubair Formation
This succession reflects conformable formations with changes in many variables including sea level fluctuation, accommodation and subsidence (Fig. 12).

Fig. 1 :
Fig.1: Location map of the study area with tectonic subdivisions by Fouad (2014)

Foredeep
within the tectonics framework of Iraq has been redefined and reviewed according to the modern concepts of foreland basins.The Mesopotamia Foredeep basin, which is the presented day expression of the terrestrial part of the Zagros Foreland Basin, is an integrals part of the Zagross Fold-Thrusts Belt that lies between the deformationall front of the Zagros orogenic belt and the stable interior of the Arabian Platform.The Mesopotamia basin has an elongated epicontinental basin formed above an earlier plate formal and marginal basin.Accordingly, the Phanerozoic sequence stratigraphy of the basin can be broadly categorized into three major tectono-stratigraphic assemblages; Cambrian-Early Perminan intraplate assemblage, Late Permian-Middle Cretaceous Neo-Tethys passive margin assemblage, and Late Cretaceous -present foreland basin assemblage.The Mesopotamian Foredeep is a mobile tectonics zone and contains several buried structures including folds, faults and diapiric structures.Recent activity of some of these structures was recorded through their effects on the Quaternary stratigraphy and present geomorphological landforms.The Mesopotamia Plain forms the central and the southern parts of Iraq.Early workers have considered it as a part of the Unstable Shelf of the Arabian Platform

Fig. 7 :
Fig. 7: Relationship between accumulation, subsidence and accommodation of the well Rc-2