A new tectono sedimentary framework of the Jurassic succession in the Merjan oil field, Central Iraq

Abstract

In the Merjan oil field (Central Iraq), the previous 2D seismic interpretation of subsurface geometry had been revealed mound facies which was interpreted as large carbonate buildup within carbonate Najmah formation of Jurassic age. The exploration well (Me-1) had been drilled depending on these results, but carbonate buildup did not exist. The current research provides new sight to interpretation of this feature in the Jurassic succession. High accurate 3D seismic reflection data have been used. Two scenarios have been presented to interpretation. Scenario (A) includes tectono stratigraphic study which finds out that mound shape is due to fault system and tectonic activity during the sedimentation which led to the formation of a graben structure and after sedimentation led to tectonic inversion. These events cause the camouflage stratigraphic feature. Scenario (B) relies on conversion of seismic section from time into depth domain using appropriate velocity model. The resultant seismic depth section helps to present another seismic analysis of feature. The proposed structural model according to the depth section interpretation is anticlinal fault-bend fold. Quantitative geometric relationships between fault and fold shapes were established. Both scenarios explain the causes of the formation of the mound seismic facies which was previously interpreted as carbonate buildup. In conclusion, the stratigraphy interpretation based on 2D data might be very risky and 3D seismic is a must for exploration of stratigraphy traps.

Crustal Structure of the Mesopotamian Plain, East of Iraq

“…According to the available subsurface geological data (e.g., Al‐Ameri, 2011; Aqrawi et al, 2010; Darweesh et al, 2017; Fadhel & Al‐Rahim, 2019; Jassim & Goff, 2006; Mohammed, 2006), eastward thickening of the sedimentary cover in the Mesopotamian basin mainly corresponds to the eastward stepping of the Triassic and Middle to Late Jurassic normal faults, which have formed a depocenter at the eastern margin of the basin. The Abu Jir‐Euphrates fault defines the southwest boundary against the Arabian stable platform (near the KAR2 where the thickness of the sediments changes sharply from 4 to 11 km; Figure 1), while the Makhul‐Amara fault zone limits the northeast margin of the basin.…”

“…Since Late Jurassic, subsidence across the graben system faults has continued in the area, and an almost continuous sedimentary succession formed in this tectonic depression (Jassim & Goff, 2006). In Middle Cretaceous, the Arabian plate transitioned from extensional to compressional tectonics leading to reverse reactivation of preexisting normal faults and marginal folding (see Aqrawi et al, 2010; Fadhel & Al‐Rahim, 2019; Numan, 1997, 2000). This tectonic inversion, however, has locally affected the basin and produced short‐wavelength fault‐related anticlines close to the inverted graben bounding faults (e.g., Darweesh et al, 2017; Fadhel & Al‐Rahim, 2019).…”

“…In Middle Cretaceous, the Arabian plate transitioned from extensional to compressional tectonics leading to reverse reactivation of preexisting normal faults and marginal folding (see Aqrawi et al, 2010; Fadhel & Al‐Rahim, 2019; Numan, 1997, 2000). This tectonic inversion, however, has locally affected the basin and produced short‐wavelength fault‐related anticlines close to the inverted graben bounding faults (e.g., Darweesh et al, 2017; Fadhel & Al‐Rahim, 2019). According to the burial thermal histories of the Middle Jurassic–Early Cretaceous source rocks in the Mesopotamian Plain, the uniform subsidence of the basin was followed by rapid subsidence at 15–10 Ma during which a ~3‐km‐thick sedimentary pile of Upper Miocene‐Pliocene deposits accumulated in the fault‐bounded depocenters (Aqrawi & Badics, 2015).…”

“…At its southwestern boundary, the Mesopotamian Plain is bordered by the Abu Jir‐Euphrates fault against the Inner Platform such that the fault separates the relatively thick (7–14 km) sedimentary cover of the Mesopotamian Plain from a thin (~2–4 km) sedimentary cover in western Iraq (Jassim & Goff, 2006; Mohammed, 2006). The Abu Jir‐Euphrates fault is clearly observed as basement fault steps in seismic lines (Mohammed, 2006) and bounds the southwest margin of the Triassic and Middle to Late Jurassic rift systems (e.g., Fadhel & Al‐Rahim, 2019). Deep‐seated normal faults such as the Abu Jir‐Euphrates fault were inherited from the extension of northeast passive margin of the Arabian plate during the Triassic time (Numan, 1997, 2000).…”

“…Deep‐seated normal faults such as the Abu Jir‐Euphrates fault were inherited from the extension of northeast passive margin of the Arabian plate during the Triassic time (Numan, 1997, 2000). The Middle to Late Jurassic rift system superimposed this initial rift system and its sedimentary sequences, reactivating the preexisting graben and horst structures in the Mesopotamian Plain (see Fadhel & Al‐Rahim, 2019 for more details). Since Late Jurassic, subsidence across the graben system faults has continued in the area, and an almost continuous sedimentary succession formed in this tectonic depression (Jassim & Goff, 2006).…”