Seismic Sequence Stratigraphic Model and Hydrocarbon Potential of Yamama Formation in Al-Fao Area, Southeastern Iraq

Abstract


Introduction
The Cretaceous period petroleum system holds immense economic value due to the various stratigraphic sequences that can generate, emigrate, store, and preserve hydrocarbons within oil and gas reservoirs (Aqrawi et al., 2010).Iraqi southern and southeastern regions contain significant hydrocarbon reserves in the Cretaceous rocks, making them economically valuable.Yamama is considered one of the most important reservoirs of the Lower Cretaceous period because it contains large oil reserves due to the presence of shallow coastal facies (shoal), as its sediments have calcareous-granular facies, which enhance the porosity and permeability of the reservoir, and accordingly the hydrocarbons within Yamama reservoir will be trapped within the geological structures (structural traps).The facies and their wide horizontal extensions deposited on the Yamama platform with a low slope (Ramp) play a role in the possibility of stratigraphic traps (Al-Sakini, 1992).The stratigraphic model is commonly used for analyzing seismic data.This model divides the section into time-stratigraphic units based on seismic information.In seismic sequence analysis, two sets of terminology are used-one from geology concepts and the other from seismic measurements.Understanding both sets of terms is important to interpret the data accurately (Sheriff, 1980).Yamama Formation in Siba Field is composed mainly of limestone and dolomite in other locations.Overall, the formation is considered clean because there is a low volume of shale present in Siba wells (Ali et al., 2021).In the southern Fields of Iraq, Yamama Formation has several limestone types, including mudstone, mudstone-wackestone, wackestone-packstone, packstonegrainstone, and grainstone (Al-Iessa and Zhang, 2023).Yamama Formation in southern Iraq is a good source rock in the mature oil zone (Al-Shahwan, 2002).Yamama basin is a stable flank on the carbonate platform characterized as a ramp setting.The ramp setting impacts the stacking patterns of the Yamama Formation (Idan et al., 2020).The study area is located in the southeastern part of Iraq in Basrah Governorate, along the Iraqi-Iranian border, approximately 20 km from Basra city.Shatt Al-Arab penetrates the area from the northwest towards the southeast.It is surrounded from the west by Khor-Abdullah and from the south by the Arabian Gulf (Fig. 1).Previous geological studies dealt with the Yamama Formation in the study area in terms of the type of stratigraphic sequences, the vertical and horizontal sedimentation of the components of the sedimentary basin, and the factors that controlled the creation of this basin.The current research aims to interpret seismic lines from the two-dimensional surveys previously conducted in the study area to build a stratigraphic model within the Yamama Formation and identify the most promising zones for hydrocarbon exploration.This research provides valuable information about the geology of the area that can help guide future hydrocarbon exploration and production efforts.

Stratigraphic and Tectonic Setting of the Study Area
In the Early Lower Cretaceous period, the sedimentary basins in southern Iraq took a suitable and appropriate position for the formation of calcareous sediments.Yamama sedimentation platform include multiple sedimentary environments in the form of sedimentary zones parallel to each other and parallel to the sea coastline of the sedimentary basin (Mohsin et al., 2022).
Yamama Formation is related to the Late Berriasian-Valanginian Sequence (Douban and Medhadi, 1999).It represents one of the major Lower Cretaceous carbonate reservoirs in southern Iraq, also the formation contains important source rocks that may be responsible for the generation of some of the oil stored in the Cretaceous reservoirs in the area (Al-Khafaji et al., 2022) (Fig. 2).It was initially identified as outcrops in Saudi Arabia (Khorshid and Salman, 2014).Yamama-Sulaiy Formation was referred to as a 257 m interval in borehole Ratawi-1 by Bellen et al. (1959).The upper 203 m, which is referred to as the Yamama Formation, is composed of 191 m of micritic limestone and oolitic limestone, which is topped by 12 m of specular and brown detrital limestone with thin shale beds (Jassim and Goff, 2006).In the southeast of Iraq, the Yamama Formation can be up to 360 m thick (Jassim and Goff, 2006) (Fig. 3).The Garagu and Zangura formations are the two variations of the Yamama Formation.The initial one, Garagu, comprises biological detrital limestone in the middle and oolitic sandy limestone in the upper and lower regions (Bellen et al., 1959).The second Formation, Zangura, comprises Pseduoolitic limestone, argillaceous limestone, and calcareous mudstones (Sadooni, 1993).Yamama Formation consists of three deposition sequences, each lasting 2 to 3 million years.These sequences are made up of parasequences that show highly developed cyclicity over a range of a few to ten meters (Alhakeem et al., 2019).The study area is located in the Basra Block, which is affected by Pre-Cambrian transversal faults (Fig. 4) (Jassim and Goff, 2006).The shape and pattern of current structures are heavily influenced by the late stage of the Alpine Orogeny, indicating that traps began to form in the Early Cretaceous period due to compressional forces caused by Arabian plate subduction (Jafar, 2010).

Materials and Methods
The current study was conducted using a methodology that involved several steps.Firstly, the seismic data in SEG-Y format and well data in LAS and ASCII formats, along with their respective coordinates, were uploaded.Subsequently, a base map was prepared for the study area, as illustrated in Fig. 5.To link the seismic information with the well information, a synthetic seismogram (seismic well tie) was created to help define the reflectors of the studied formations (top and base Yamama Formation) on the seismic sections.Seismic facies analysis was used based on observational criteria, such as reflection amplitude, continuity, and geometry to understand the depositional architecture of Yamama Formation.Following this, the instantaneous phase attribute was applied to identify features that serve as excellent indicators of hydrocarbon accumulation; this allowed us to interpret the seismic sections and describe the stratigraphic features.Finally, a stratigraphic model was created for the Yamama Formation in the Al-Fao area.

Basic Concepts
This study relied on the concepts of "depositional systems, systems tracts, and stratigraphic surfaces" for seismic stratigraphic analysis (Mitchum et al., 1977).Fisher and McGowen (1967) define a depositional system as a grouping of sedimentary rocks deposited within a specific environment, distinguished by a distinct set of sedimentary processes and characteristics.This term encompasses all types of sedimentary environments, ranging from terrestrial to marine, and all forms of sediments, including clastic, chemical, and biochemical.The primary unit of sequence stratigraphy is the sequence, a collection of sedimentary rocks bounded by surfaces of erosion or non-deposition that can be correlated laterally over a relatively extensive area.These sequences typically contain a series of genetically linked depositional systems, such as shorelines, deltas, or deep-water turbidities, reflecting changes in sea level and sediment supply (Catuneanu, 2006) (Fig. 6).Sequence stratigraphy serves as an effective tool for interpreting the depositional history of sedimentary basins, identifying potential reservoir rocks for oil and gas exploration, and comprehending the evolution of sedimentary environments over time.Systems tracts are sedimentary units defined by their depositional environments and sequence stratigraphy.There are several types of systems tracts, such as Lowstand systems tracts (LSTs), transgressive systems tracts (TSTs), and Highstand systems tracts (HSTs) (Mitchum et al., 1977).Fig. 6.Architecture of depositional systems, systems tracts, and stratigraphic surfaces (Catuneanu, et al., 2009).Abbreviations: e-FR-early forced regression; l-FR-late forced regression; e-T-early transgression; l-T-late transgression

Seismic Facies Analysis
Seismic stratigraphic interpretation began with the analysis of seismic facies (SF), which refers to interpreting facies type from seismic reflector information.Six distinct seismic facies have been identified based on critical observational criteria, such as reflection amplitude, continuity, and geometry (Table 1).SF-1, for instance, is a high-amplitude, relatively continuous facies found in the inner platform, whereas SF-2 is a discontinuous to semi-continuous facies with low to moderate amplitude, representing the barrier shoal.SF-3, on the other hand, is semi-continuous to continuous with moderate to high amplitude and is located on the slope (carbonate shedding).SF-4 is a low to moderate amplitude, semi-continuous facies in the slope (carbonate progradation), whereas SF-5 is moderate amplitude, semi-continuous, representing the lower stand shoal.Finally, SF-6 is a continuous, high amplitude facies in the basin.These distinct seismic facies result from variations in the composition and depositional architecture of the Yamama carbonate platform over time and in response to tectonic, environmental, and eustatic controls.

Facies
Seismic Reflections Geometry Description

SF-1 Wavy
Mostly parallel with a small degree of reflector thickening and thinning, high amplitude, relatively continuous.

SF-2 Mound shape
Discontinuous to semi-continuous, low to moderate amplitude.

SF-3 Oblique parallel
Semi-continuous to continuous, moderate to high amplitude.

SF-5 Chaotic
The reflections have no continuity with other reflectors in a unit, moderate amplitude, semi-continuous.

Seismic Attributes Analysis
The seismic attributes are becoming increasingly popular for better understanding subsurface conditions in a specific area.This study used instantaneous phase for stratigraphic analysis.The measurement of instantaneous phase is represented in degrees (-л, л) and is independent of amplitude.It provides insight into the continuity and discontinuity of events and is particularly useful in displaying bedding.In principle, the phase along the horizon should remain constant, but changes can occur due to picking errors or lateral layer shifts caused by sinkholes or other phenomena.This attribute is a reliable indicator of lateral continuity and corresponds to the phase component of wave propagation.It can also calculate phase velocity, instantaneous frequency, and acceleration (Subrahmanyam and Rao, 2008).The results of the analysis indicate the presence of carbonate buildups and progradation stacking patterns.For instance, Fig. 7a shows a carbonate buildup with a flat spot as a direct hydrocarbon indicator (DHI) on seismic line F6.The instantaneous phase attribute was utilized to detect the termination of seismic reflectors and carbonate buildup, as depicted in Fig. 7b.Carbonate buildup and DHI were identified in seismic lines F4 and F7, as shown in Figs. 8 and 9, respectively.

Sequence Stratigraphy
Seismic stratigraphic interpretation is considered one of the most important tools for interpreting seismic data, as it can give a more inclusive view of the subsurface geological condition of the target formations, especially in the presence of a small number of drilled wells in the study area, and thus provide more clarity for the sedimentary basin of the targeted formations.Fig. 10 shows the logs P-sonic, gamma ray (GR), and density (RHOB) for Sb-well in the study area.Yamama Formation was deposited in a ramp setting, which is characterized by the accumulation of carbonate sediments in shallow marine conditions (Aqrawi et al., 2010) (Fig. 11).After analyzing the seismic data, it was found that there are two types of sedimentary facies: transgressive and regressive.Two depositional cycles were identified: the first was represented by the Sulaiy Formation, which began with the appearance of sediments of the transgressive system tract (TST) and showed onlapping patterns due to facies belts migrating landward.This cycle ended with the maximum flooding surface (MFS), diagnosed by the progradation stacking patterns.The facies above this surface represent the Highstand system tract (HST) in the Yamama Formation (Fig. 12).These two tracts were created in alternating oolitic shoal and deep inner shelf environments.The second cycle began above the Yamama Formation in the Ratawi Formation, representing a transgressive system tract.One of the most significant characteristics of the HST is the gradual accumulation patterns that gradually prograde towards the basin in the form of grain-supported bodies.These bodies are formed due to the gradual slope towards the basin caused by the coastline retreat.On the other hand, the TST is characterized by a deepening upward, caused by the coastline's progress toward the land.The MFS ultimately terminates this package (Idan et al., 2020).
In the Al-Fao area, three main seismic stratigraphic representing potential stratigraphic traps have been identified.These indexes reflect the facies of shoal carbonate sediments from the last depositional cycle of the Yamama Formation (Highstand), followed by the depositional cycle of the Ratawi Formation (Transgressive).The thickness map of Yamama Formation was created with a 50 m contour interval to identify the varying thickness of the studied formation in the depth domain.In the study area, the thickness ranges from (200 to 550) m.The increase in thickness extends from southeast to northwest and represents the shoal carbonate buildup.The location of potential stratigraphic traps on the thickness map of Yamama Formation in the study area is illustrated in Fig. 13.

Stratigraphic Model
A stratigraphic model for the Yamama Formation has been created to understand its depositional environment better and predict the location and characteristics of potential hydrocarbon reservoirs.This model draws upon previous findings, including seismic facies analysis, attributes analysis, and seismic stratigraphic interpretation, as shown in Fig. 14.Utilizing this information can improve the overall understanding of Yamama Formation in the Al-Fao area.

Conclusions
Iraqi southern and southeastern regions contain significant hydrocarbon reserves in the Cretaceous rocks, especially in Yamama Reservoir.This reservoir contains large oil reserves because of its shallow coastal facies (shoal), increasing its porosity and permeability.Yamama Formation in Al-Fao area is divided into two main units representing transgressive and regressive facies deposited in HST.Six seismic facies were identified (wavy, mound shape, oblique parallel, sigmoid, chaotic, and parallel), which reflected the deposition of Yamama Formation in a ramp setting, which allows for the widespread accumulation of carbonate sediments over a broad area extends from the coastline to the deep basin.The seismic stratigraphic interpretation was supported by seismic attributes that focused on the lateral seismic facies changes and provided detailed information about the architecture of Yamama Formation depositional basin.The results of the seismic attributes analysis included identifying carbonate buildups and progradation stacking patterns with the presence of DHI.Based on this, three main seismic stratigraphic indexes representing potential stratigraphic traps have been identified.These three stratigraphic features reflect the facies of shoal carbonate sediments in the last depositional cycle of Yamama Formation (Highstand).The stratigraphic model showed the best image of the depositional environment of Yamama Formation that corresponds with the seismic data interpretation and identifies the promising hydrocarbon traps.

Fig. 1 .
Fig.1.Location map for the study area

Fig. 5 .
Fig.5.Base map of the study area

Fig. 7 .
Fig.7.Seismic profile for the seismic line F6 showing carbonate buildup and flat spot (a) original amplitude, and (b) instantaneous phase.For seismic line location see Fig.5.

Fig. 8 .
Fig.8.Seismic profile for the seismic line F4 showing carbonate buildup and progradation stacking patterns (a) original amplitude, and (b) instantaneous phase.For seismic line location see Fig.5.

Fig. 9 .
Fig.9.Seismic profile for the seismic line F7 showing carbonate buildup and progradation stacking patterns (a) original amplitude, and (b) instantaneous phase.For seismic line location see Fig.5.

Fig. 10 .
Fig.10.Well log curves for Sb-well are displayed from left to right, including the P-sonic log, gamma ray log (GR), and density log (RHOB)

Table 1 .
Description of seismic facies in the study area