Unlocking the Mysteries of the Mishrif Formation: Seismic Data Reinterpretation and Structural Analysis for Reservoir Performance Optimization in the Garraf Oil Field, Southern Iraq

Abstract


Introduction
A seismic survey provides geophysical data collected by a seismograph on subsurface rock formations.Seismic data may link well-log and core analyses on the one hand and tracer and well-test analyses on the other.Most geologic maps and models are based on 3D seismic data, which must describe reservoir heterogeneity, vertical zonation, lateral compartmentalization, and the factors that lead to anisotropy or directional fluid flow in the reservoir (Mondol, 2010;Baker and Al-Rikaby, 2017;Al-Rubaye and Hamd-Allah, 2019;Almahdawi et al., 2023).Seismic reflections produce accurate graphical depictions of the subsurface and geologic features.They provide seismic data, velocity, and time contour maps for identifying structural, stratigraphic, and seismic facies traps (Trezzi et al., 2022;Alher et al., 2018;Al-Jawad and Kareem, 2016).
Moreover, seismic reflections are used to interpret the interior sedimentary architecture in terms of environmental paleogeographic depositions (Ali et al., 2018;Khawaja and Thabit, 2021).Therefore, it is vital to interpret the 3D seismic reflections, post-stack times migrated data, and well data for the Garraf Oil Field.The oil reserves of the four reservoirs (Mishrif, Zubair, Ratawi, and Yamama) were confirmed by the crew of the National Oil Company in 1984 using a 2D seismic survey.As a result, three exploratory wells were drilled.In 2009, a three-dimensional seismic survey was conducted by the Oil Exploration Company (OEC), on the basis of which a final development plan was drawn up (Petronas, 2017).Since then, an additional 107 productive wells have been drilled and penetrated the Mishrif Formation, and more are planned in the future.In the study area, there was a problem with the bubble point discrepancy, which prompted a thorough re-evaluation of the Mishrif Formation.
To investigate the issue and identify its causes, including faults and reefs, the study re-evaluated the seismic survey with a subscription of log data (density, sonic, VSP) obtained from Thi-Qar Oil Company (TOC)-an essential component of the comprehensive re-evaluation of the formation or reservoir.This paper aims to generate a high-resolution structural map of the Garraf oil field and identify its structures such as faults and reefs.The initial step is to determine whether faults or reefs were responsible for the problem as reservoir comparting and determine the correct probability by historical matching of the reservoir model after construction of a static model based on two probabilities, fault, and reef.It is considered a new approach to verify this rare problem in geosciences and reservoir engineering.

Geological Settings
The Garraf oil field is located in Thi-Qar Provinces, the southern part of Iraq, about 5 km northwest of Rifai City and 85 km north of Nasiriya (Petronas, 2017).Oil gatherings in the field were discovered in 1984 by the seismic survey crew of the National Oil Company.As a result, three exploratory wells were drilled to confirm the oil reserves of the main four reservoirs, namely Mishrif, Zubair, Ratawi, and Yamama Formations.The Mishrif Formation is at a depth of 2283-2417 m, the Zubair Formation at a depth of 3036-3323 m, the Ratawi Formation at a depth of 3436-3569 m, and the Yamama Formation is located at a depth of 3362-3717 m.
The Mishrif Formation, a heterogeneous carbonates reservoir, is the most important reservoir in the Garraf oil field as well as in other fields in southern Iraq.The Khasib Formation bounds the formation at the top and the Rumila at the bottom.The Mishrif Formation is at the top of an anticlinal fold structure at the Ga-1 well (about 31 km in length, 10 km in width, and 2283 m in depth).The thickness of the formation is about 130 m.Currently, 107 wells have been drilled in the Mishrif reservoir, with a production capacity of 110,000 barrels/day based on variations in BP pressure, with water injection of 120,000 barrels/day from the Third River or officially named main outfall drain (MOD).
The research region is located in an unstable region within the Mesopotamian basin of the Arabian plate (Al-Ameri et al., 2009) (Fig. 1).It is surrounded by several oil fields that produce hydrocarbons from NW-SE-trending anticline formations, which are compatible with the direction of the Zagros folded axis (Jassim and Goff, 2006).
The deposition of the Mishrif reservoir in Iraq is governed by tectonic and isostatic processes, and it belongs to the Cenomanian-Turonian Supersequence of the uppermost component of the tectonic, stratigraphic Megasequence deposited along a passive border (Oil Exploration Company, 2013).The area is situated within the Mesopotamian structural zones, split into Tigris, Euphrates, and Zubair tectonic subzones (Jassim and Goff, 2006;Sharland, 2001).The stratigraphic profile of the Tigris subzone reveals a noticeable thickness of the Mishrif Formations, indicating significant subsidence rates.The deformation of the northeastern Tethyan border of the Arabian Plate during the Cenomanian and Early Turonian epochs resulted in the construction of distinct high and low structures in each subzone (Buday and Jassim, 1987).
The presence of negative residual gravity under some supergiant field structures indicated formations such as Rumaila and Zubair in southern Iraq from salt diapirism (Khawaja and Thabit, 2021;Karim, 1989;Karim, 1993).(Jassim and Goff, 2006) Several of these structures started to form during the Early Jurassic period (Sadooni and Aqrawi, 2000).Fig. 2 depicts the location of faults in Iraq.Jurassic source rocks, Cretaceous and Tertiary reservoirs, and Paleozoic and Tertiary structural traps are the fundamental geological components necessary for hydrocarbon accumulations in the Mesopotamian Basin (Alsharhan and Nairn, 1997).Fig. 3 gives an overview of the basin-specific hydrocarbon system dynamics (generations, migrations, and accumulations).(Ahlbrandt et al., 2000).

Seismic Reflection
Seismic reflection is a geophysical exploration technique that estimates the earth's subsurface parameters through an analysis of reflected seismic waves.The general principle involves sending artificially generated acoustic waves down the column into the core of the earth, where the different structures and objects within the Earth's crust reflect this energy back according to their acoustic impedance (Al-Aaraji and Karim, 2021).Controlled seismic energy sources such as dynamites, air pistols, or seismic vibrators, generate seismic waves which are mechanical disturbances that travel through the ground at a speed determined by the acoustic impedance of the material through which they pass.Z, the acoustic impedance, is calculated using the formula Z= v x ρ, where ρ is the layer's density and v is the velocity wave.
The objective of seismic interpretations is to get geological data from the interpreter's maps of seismic reflections.Seismic interpretation involves the analysis of traces, correlations, and continuous reflectors across the 3D dataset, which are fundamental to geological interpretation (Avseth et al., 2005).The goal of seismic interpretation is to create a structural map that displays the spatial variations in the depths of certain geological strata.These maps are used to identify hydrocarbon traps and develop a model of the reservoir that allows for volume calculations.

Materials and Methods
Two appraisal wells were drilled in the study region based on the findings of prior 3D seismic reflection investigations.These wells pierced the Mishrif Formation and reached the Yamama Formation.The first well, Ga-1, was drilled to a depth of 3900 meters in 2011 and produced crude oil from the Mishrif reservoir.The appraisal well, Ga-5, was drilled 12 kilometers northwest of the Ga-4 well in 2013 at a depth of 3930 meters.
Data preparation is the basis for constructing the seismic model, in addition to using the Petrel platform to set up this model, which is considered an application for subsurface modeling and interpretation.Based on research area characteristics, the data preparation for this seismic model are 3D-seismic surveys, well tops, and well logs.These data include: • 3D-seismic survey: The data included a comprehensive three-dimensional survey of the study area with a length of 31 km and a width of 10 km by the seismic crew • Well tops: represent significant points (well picks) along the well path, normally a change in stratigraphy.
• Well logs: The data include sonic, density, and vertical seismic profile (VSP) logs along the well path.Seismic data were collected to achieve the objectives of this study and update the subsurface images of the Mishrif reservoir in Garraf oil fields.A synthetics seismogram is created as a result of 1D forward modeling of earth layers as the actual well seismic trace.For a good correlation between wells' data and seismic section or 3D block, it is important to convert well tops from depth to time domain in order to track the behavior of each one in the oil field (Figs. 4 and 5).Also, a synthetic seismogram differentiates the true primary reflector of the formation tops from the multiple reflectors, which could prevent the interpreter from tracking the false structure.This was followed by structural interpretations using times and depths maps and the building of a three-dimensional depositional reservoir model.

The Seismic Reflection Horizons of the Mishrif Formation
The Top Mishrif was selected near the bottom of the wavelet's amplitudes due to its negative reflectivity value.This value is due to the fact that the Top Mishrif has a lower interval velocity than the overlaid Khasib Formation.The base contact of the Mishrif Formations was identified at the peak of the wavelet's amplitudes due to its positive reflectivity.The interval velocity of the base portion is smaller than that of the underlying Rumaila Formations (Khawaja and Thabit, 2021;Jassim and Goff, 2006).Numerous sharp discontinuities were observed in the seismic reflectors of the Mishrif formation's reservoir units, indicating the presence of either a reef or a fault (Fig. 6).The correct interpretation can be verified by conducting a historical match with the actual behavior of the reservoir as per the reservoir model or by running a more precise 3D vertical seismic profile (VSP) log than the previously used seismic survey.

Seismic Reflection Data Time Unit Interpretation
Accurate seismic analysis is crucial in sedimentary basins as it considerably increases the probability of effectively identifying hydrocarbon traps (Sheriff and Geldart, 1995).Hydrocarbon reserves, porous carbonate reefs, and faults can be identified on amplitude seismic sections using zones of abnormal reflection events such as velocity drops, frequency decreases, and amplitudes-phase reversals with substantial signal attenuation (Burgess et al., 2013).Fig. 7 shows a seismic section with many irregular amplitudes of reflected events and several abrupt discontinuities, particularly in the region of the Ga-3 and Ga-5 wells.These seismic features reflect either a reef or a fault.In contrast, the segment going through the Ga-4 well reveals a different seismic image, which demonstrates the near-absence of severe discontinuities, confirming that the wells have distinct lithofacies (Alameedy et al., 2023).

Two-Way Time Map of Mishrif Formations
The structural evaluation of 3-D seismic reflection data relies on the time maps of reflection horizons produced by detecting and selecting seismic reflectors in the two-way time units correlated with the wells' data.The contour time maps for the Top Mishrif reservoir were created using a contour interval of 3 ms and a closure time value of 80 ms to indicate the geometry of the recorded reflection horizon ( Fig. 8).

Depth Map of Mishrif Formation
By building a velocity model for the Mishrif Formations, two-way time (TWT) maps were translated into depth maps.The depth map provides a more accurate representation of the area's structure than the times map of the Top Mishrif Formations depth map, where the closure is 130 meters.The Top Mishrif reservoir is situated at approximately 2170 meters (Fig. 9).

Building of a Dimensions Depositional Environment Model
The depositional environment modeling enables the visualization of the internal architecture of the Mishrif Formation in the Garraf oil field ( Figs. 10 and 11).It displays the structural geometry of the reservoir's depositional layers.It provides a better understanding of the sedimentary environment, the lateral and vertical distribution of different lithofacies, and the geometry of the depositional layers (Brown, 2011;Khawaja and Thabit, 2021).Utilizing Petrel software a 3-D depositional environment modeling of the Mishrif Formation is constructed in the Garraf field (Fig. 10).It indicates that the Mishrif Formation has a monocline structure with a typical east-southeast dip.
Two perpendicular portions of 3-D depositional models were developed to illustrate the geometry of the Mishrif Formation layer sequences (Fig. 11).It demonstrates that the Mishrif Formation levels between Ga-3 and Ga-5 have several abrupt discontinuities that have not been conclusively identified as reefs or faults and may be validated later via the historical matching of the reservoir pressure and the production behavior.

Discussion
The Cenomanian-Early Turonian cycle is represented by the Khasib, Mishrif, and Rumaila formations.The Mishrif reservoir is characterized by an upward shallowing cycle related to the tectonic system's ongoing compression.The Khasib Formation is overlaid at the top of the Mishrif Formation unconformably.Seismic waves move through the earth at a material's acoustic impedance, Z = v x ρ, to generate a structural map that depicts the spatial variations for the depth of certain geological strata and identifies the kinds of structures, such as faults and reefs.Numerous sharp discontinuities were detected in the seismic reflectors of the reservoir units of the Mishrif formation, indicating either a reef or a fault.
The higher probability can be proved by making a historical matching in the reservoir model with the actual behavior of the reservoir or by running a 3D vertical seismic profile log (VSP), which is more precise than the seismic survey previously used in the exploration of faults.Therefore, it is essential to confirm the existence of minor faults by applying the 3D VSP logging.The measured bubble point pressure of the considered reservoir's oil varies among the samples from different wells.
The reservoir compartment could be the reason behind this discrepancy.Accordingly, this work reevaluates the seismic survey along with the density, sonic, and VSP logs.This re-evaluation shows anomalies that may be attributed to either faults or reefs by testing the probability against the measured reservoir production data.The duplicates of the measurement technique will be adopted.The new methodology represents a useful tool to solve some reservoirs' abnormal behaviors which could caused by unpredictable reservoir compartments.

Conclusions
The Garraf oil field is flanked by several hydrocarbon fields that produce hydrocarbon from massive NW-SE-trending anticline formations, which are typically compatible with the direction of the Zagros folded axis.It contains one dome that comprises a structural trap with a hydrocarbon pay zone.Jurassic source rocks, Cretaceous and Tertiary reservoirs, and Paleozoic and Tertiary structural traps are the fundamental geological components necessary for hydrocarbon accumulations in the Mesopotamian basins, including the study region.It is crucial to emphasize the significance of seismic interpretation in effectively identifying and detecting faults, reefs, and other geological structures.Time and depth maps are crucial in the structural evaluation and more accurate representations of the area's structure of the Mishrif formation.
The study, based on a 3-D reflections survey that permits the building of 3-D modeling for the description of the depositional environment, indicates that the Mishrif formation has a monocline structure with a typical east-southeast dip.

Fig. 4 .
Fig. 4. Synthetic seismograms for the location's times of Mishrif reservoir in Garraf field by well (Ga-1)

Fig. 11 .
Fig. 11.Two portions perpendicular to 3-D depositional layer modeling of the Mishrif Formations in the Garraf oil field.