Journal of Petroleum Geology, vol. 19(2), April 1996, pp. 183-198 183

THE TECTONIC AND DEPOSITIONAL HISTORY OF KUWAIT FROM SEISMIC RELECTION DATA

F. Bou-Rabee*

A series of structural and isopach maps have been constructed to model the tectonic history of Kuwait. These indicate that the geological history of the country has been one of general subsidence, interrupted on a number of occasions by periods of uplift. A major unconformity in the mid-Cretaceous is observed above the Burgan High, which is a structure of great importance fo r hydrocarbon accumulation. The thinning of sediments, as seen on the isopach maps, suggests that the development of this structure began as early as the Jurassic.

INTRODUCTION

The State of Kuwait is located in the NE part of the Arabian Peninsula at the NW end of the Arabian Gulf. Kuwait covers an area of approximately 17,180 sq. km, and borders Iraq to the north and west and Saudi Arabia to the SW. Since the Triassic, the Kuwait region appears to have occupied an intermediate position between the Arabian Gulf geosyncline to the NE and the Arabian massif to the SW, and a thick sedimentary sequence is present in the subsurface (Fig. 1). Drilling has shown that the region is underlain by about 6,000 m of sedimentary rocks, whose age ranges from Triassic to Pleistocene. These sediments include a Jurassic interval up to 1,500-m thick, a Cretaceous interval up to 3,300-m thick, and an Eocene interval up to 1,500-m thick.

The regional dip of strata is about 2 m per km towards the NE (Bou-Rabee, 1986). This regular dip is interrupted by the Kuwait and Dibdibba Arches and other smaller structures, which are present at the Raudhatain, Umm Gudair andMinagish oilfields. These structures are very gentle, and no dips over 3" have been identified. The gentle dips and the similarity of the lithologies have made the correlation of rock units difficult. However, subsurface information indicates that these structures developed steadily since the Middle Cretaceous, and that they may be as old as Late Jurassic (Bou-Rabee, 1986). These data indicate that they were uplifted due to horizontal compression, especially during pre-Miocene times.

* Kuwait University, Geology Department, PO Box 5969, Safat 13060, Kuwait.

184 The Tectonic and Depositional History of Kuwait from Seismic Reflection Data

In particular, the Kuwait Arch in the vicinity of Ahmadi Ridge appears to be the result of horizontal compression during post-Eocene times, and is probably related to the Zagros Orogeny (Bou-Rabee, 1986).

Regional stratigraphy (Fig. 1) The Miocene-Pleistocene interval comprises arenaceous sediments, including local

evaporites of middle Miocene age which thin to the SW (towards the Iraq desert) and pinch-out in north Kuwait. The Eocene section is represented by a limestone sequence interrupted by a thin but very widespread anhydrite interval.

Upper Cretaceous rocks are predominately composed of limestones with less important marls and thin shales; these lithologies extend down into the upper part of the Middle Cretaceous section. The lower part of the Middle Cretaceous and most of the Lower Cretaceous section is composed mainly of sandstones with intervening shale layers, and there are at least two limestones beds. The sand content of this sequence increases substantially southwards toward Burgan (location on Fig. 2), where it constitutes about three-quarters of the lithologies present. In northern Kuwait, sandstones comprise only 30% of the Middle Cretaceous rocks present. The lowermost part of the Lower Cretaceous is represented by a limestone and shale sequence.

Jurassic rocks in SE Kuwait mainly consist of limestones with a thick (400 m) anhydrite-salt-evaporite unit. Sedimentary rocks of probable Triassic age are composed of shales and sandstones.

A major break in sedimentation occurred during late Eocene and Oligocene times, with a possible minor break in the Early Cretaceous (pre-Maastrichtian), and another clearly-recognized break at the end of the Middle Cretaceous (post-Cenomanian-Turonian). All the sedimentary intervals above the Middle Cretaceous show progressive thinning across the crest of the drilled structures (near the Burgan oilfield) in Kuwait, indicating that folding and uplift began around that time.

Because only one well has penetrated into the Paleozoic in Kuwait, the pre-Mesozoic stratigraphy is poorly understood. Below the Sudair Formation (Permian-Triassic), the Permian Khuff Formation consists of 460-610 m of limestones with some clastic and carbonate shelf deposits (Murris, 1980).

Tectonic History of Kuwait The Paleozoic history of Kuwait is known only from sparse borehole data (e.g. the

Burgan deep test) (Kuwait Country Paper, 1989), and from data from adjacent countries. Throughout the Paleozoic, Kuwait was part of a very wide shelf on the northern passive margin of Gondwana, and was located in middle to high latitudes in the Southern Hemisphere. Clastic sedimentation was dominant. In the Late Permian, crustal extension, followed by rifting and the drifting away of the Iranian block, led to the opening of the Neo-Tethys and the formation of the NEi Arabian passive margin. Epeirogenic movements characterized the tectonism which affected this region in the Paleozoic (Beydoun, 1993).

During the Mesozoic, the Arabian Shelf consisted of a very wide platform on which carbonate deposition was dominant with subordinate evaporites (Murris, 1980), as a consequence of its location in tropical latitudes. Towards the end of the Cretaceous, ophiolite obduction on the outer edge of the platform gave rise to further epeirogenesis in the Shelf Interior, including Kuwait. The mid-Tertiary collision between Arabia and Eurasia along the Zagros line resulted in late Neogene suturing, and the overprinting of the Zagros foreland basin onto the outer edge of the NE Arabian margin, giving rise to a belt of compressional structures in SW Iran and NE Iraq, and further epeirogenesis in the Kuwait region with reactivation of earlier block movements (Fig. 3) (Beydoun, 1993). The purpose of this paper is to present a series of structural and isopach maps in order to model the tectonic history of Kuwait. A grid of 24 seismic lines covering the country,

F. Bou-Rabee 185

Fig. 1. Stratigraphic column of Kuwait (after Bou-Rabee, 1994).

together with well control, was made available to the Author by the Kuwait Oil Company. Eight wells provided control; their locations are shown on Fig. 2. Five synthetic seismograms with time-depth conversion were used for each of the wells, and seismic analysis was conducted on each seismic line.

Four principal reflectors were identified, based on the synthetic seismograms available for each well, and were used to tie the seismic lines in the area. These reflectors were identified at the top of the Rus Formation (Lower Tertiary); the top of the Ahmadi Formation (Middle Cretaceous); the top of the Hith Formation (Upper Jurassic); and the top of the Minjur Formation (Upper Triassic) (Figs. 4A to 4D). These reflectors , which are mainly due to lithological variations, are described briefly in the next section.

SEISMIC REFLECTORS

Top- Rus Formation (Lower Tertiary) This reflector is strong, continuous and marks the top of the Rus Formation anhydrite;

it represents the uppermost continuous reflection horizon over much of the study area.

186 The Tectonic und Depositional History of Kuwait from Seismic Reflection Data

Fig. 2. Base map of Kuwait with well and shot-point locations.

well locations: M1 =Mutriba LA = Reflection profile I Rl =Raudhatain LB = Reflection profile 2 s1 =Subriyu LC = Reflection profile 3 U1 =UmmGudair LD = Reflection profile 4 B1 =Burgan

Note locations of cross-sections A-A’ and E-E’ (Figs 12 and 13).

However, there are localized areas in the SW of Kuwait where the anhydrite is missing, and subsurface solution has created collapse features.

Top-Ahmadi Formation (Middle Cretaceous) The top-Ahmadi reflector is a generally strong, continuous, and reliable reflector over

most of the study area. Minor faulting is seen to affect the top-Ahmadi interval; however, this deformation is small, and does not appear to be related to the apparently deep-seated faults that affect the underlying seismic intervals.

Top-Hith Formation (Upper Jurassic) The top-Hith reflector is strong and continuous, and is one of the most prominent

reflectors in the study area. The Hith Formation consists mainly of anhydrite interbedded with some limestones and shales, and its thickness ranges from 70 m in southern Kuwait to about 100 m in the north.

Top- Minjur Formation (Triassic) Because it is overlain by an unconformity (Fig. 4 C ) , the top-Minjur Formation is not

as well defined as the overlying formations, and is also obscured somewhat by multiples. Thus, it was not possible to identify this horizon precisely at all points.

F. Bou-Rabee 187

I

40

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Fig. 3. General tectonic map of the Arabian Plate, with the location of Kuwait relative to the major plate boundaries (after Barazangi et d., 1993).

Below the Minjur Formation, the reflection quality varies considerably across the area of study, with poor reflection continuity and the occurrence of multiples.

Economic Basement (top- Lower Paleozoic) The lowermost reflector which has been identified is believed to correspond to the top

of the Lower Paleozoic interval. In southern Kuwait, this “basement” is seen at about 3.5-4.0 sec. TWT, while in northern Kuwait, it is at about 4.2-4.3 sec. TWT. Because of the difficulty in identifying a “basement” reflector, gravity modelling was used to estimate the depth and structure of the basement.

As shown in these seismic reflection profiles, subsurface intervals in the study area are characterized by a gentle dip towards the NE, and the absence of significant deformation. In Fig. 4D, however, the continuity of the strata below the top-Minjur reflector is disrupted either by a diapir (Infracambnan) or by a basement uplift (Bou-Rabee, 1986).

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190 The Tectonic and Depositional History of Kuwait from Seismic Reflection Data

Fig. 5. Structure contour map at the top of the Rus Formation (Early Tertiary).

Fig. 6. Structure contour map at the top of the Ahmadi Formation (Middle Cretaceous).

STRUCTURE-CONTOUR MAPS

Four structure-contour maps have been compiled by the Author; these are at the levels of the top-Rus, top-Ahmadi, top-Hith, and top-Minjur Formations (Figs. 5 to 8). They are described in turn.

Rus Formation (Lower Tertiary) Fig. 5 shows that the regional dip of the Rus Formation is towards the NE, with around

365 m of relief. The formation crops out in Saudi Arabia, not far from the SW corner of Kuwait. The Rus Formation thins over the Burgan high and in the Umm Gudair area.

F. Bou-Rabee 191

Fig. 7. Structure contour map at the top of the Hith Formation (Upper Jurassic).

Fig. 8. Structure contour map at the top of the Minjur Formation (Upper Triassic).

Ahmadi Formation (Middle Cretaceous) (Fig. 6) This formation is fairly similar to the Rus Formation in terms of its regional strike and

dip, although its direction of dip is a little nearer to the north and its total relief increases to 763 m in NE Kuwait. The development of both anticlinal and synclinal structures can be observed. For example, the Burgan Arch in eastern Kuwait trends north-south; this feature is prominent on all the intervals mapped except that of the Rus Formation. A second anticlinal trend is present to the west. Similarly, the major synclinal trend to the west of the anticline appears to be located approximately 10 km to the NE compared to its development in the overlying Rus Formation.

192

Hith Formation (Upper Jurassic) (Fig. 7) The structural configuration at the top of the Hith Formation is fairly similar to that

of the overlying intervals. The dip of the structure contours has changed from north-south to approximately NW-SE in western Kuwait, and the total SW-to- NE relief has increased on average to about 122m.

Anticlinal and synclinal features are even more clearly delineated on Fig. 7 than on the maps of the Ahmadi and Rus Formations. Two distinct anticlinal trends can be seen: the Burgan Arch in the east of Kuwait; and a smaller trend located in the middle of northern Kuwait. Both of these structures trend approximately north-south.

Top-Minjur Formation (Upper Triassic) (Fig. 8) The configuration of the Minjur Formation is similar to that of the overlying Hith

Formation, although the top of the Minjur is marked by an unconformity. No significant change in the strike of the structure contours, or the location of the major highs and lows are observed between the structure contour maps of the Hith and Minjur Formations. The total SW-to-NE relief increases to about 1.83km.

The Tectonic and Depositional History of Kuwait from Seismic Reflection Data

ISOPACH MAPS AND CROSS-SECTIONS

Isopach maps were computer-generated, and data was made available to the Author by the Kuwait Oil Company. Where data points are scattered, contours may be inaccurate (for example, in Fig. 11).

Isopach of the Ahmadi-Hith Formation interval (Fig. 9) This interval spans the Lower Wasia and Thamama Groups (Fig. 1). The interval

appears to thin above the anticlinal trends and also above individual closures, corresponding with the structure-contour maps. The interval thickens toward the NE by about 457.5m and towards the east along the southern border by 305m; the relief along the eastern border is 152.5m.

Isopach of the Rus - Ahmadi Formation interval (Fig. 10) This map illustrates the depth-interval between the two shallowest seismic horizons,

and corresponds with the seismic sections. Thus, thinning is most obvious across the structural trends and along the major anticlinal trends.

During mid- to Late Cretaceous times, crustal movements resulted in regional uplift followed by erosion, and there is therefore an unconformity between the Wasia Formation and the Aruma Group. The isopach map shows thickening from the west towards the east and the NE. From SW to NE, the interval thickens by approximately 915m.

Isopach of the Hith - Minjur Formation interval (Fig. 11) This interval extends from the top of the Upper Jurassic to the top of the Triassic, and

is relatively thin compared to the total Cretaceous interval. Fig. 1 1 indicates the presence of a linear trough, with a thickness along the axis of about 1.5 km decreasing to east and west. Its relief is about 305m . Nevertheless, the overall regional dip is still towards the NE.

Thinning of this horizon across the anticlinal trends is well developed, and the thinner areas coincide reasonably well with the structural highs previously identified.

Cross-Section A-A’ (Fig. 12) This cross-section runs north-south parallel to the regional trends and its trace is

located on Fig. 2. The intervals identified on this section show a uniform increase in thickness from south tqnorth, with relief on each horizon of a few hundred meters. Thus, the section is dominated by.a regional dip to the north.

193

Fig. 9. Isopach map of the Ahmadi Formation - Hith Formation interval.

Fig. 10. Isopach map of the Rus Formation - Ahmadi Formation interval.

Fig. 11. Isopach map of the Hith Formation - Minjur Formation interval.

194 The Tectonic and Depositional History of Kuwait from Seismic Reflection Data

Fig. 12. Cross-section A-A’. (See Fig. 2 for location).

Cross-Section E-E’ (Fig. 13) This cross-section runs east-west (see location in Fig. 2). The lower Tertiary interval

is of uniform thickness, suggesting an absence of tectonic activity during this time interval. The Upper Cretaceous interval thins across the Burgan high; this is due to tectonic activity during the Cretaceous. In addition, an erosional unconformity between the mid- and Upper Cretaceous suggests that uplift occurred. The underlying interval, of Early and mid- Cretaceous age, has a uniform thickness across the section. The thickness of the underlying Jurassic interval changes across the Burgan high, and this again appears to be due to uplift of this structure. These thickness changes in the Burgan high area suggest episodic tectonic activity, with periodic uplift in the Jurassic and Upper Cretaceous, and tectonic quiescence in the Early Cretaceous and Early Tertiary.

Additional studies and more well control are required to determine if the thinning of the Jurassic and Upper Cretaceous intervals is due to original stratigraphic variations or erosion.

Two distinct anticlines can be observed on Section E-E’ . One is located about 46 km from the left-hand (western) margin of the section; the other, the much larger anticline at the Burgan high, is located about 98 km from the left-hand margin. This cross-section is more or less perpendicular to their axial trends.

ILLUSTRATION OF REGIONAL STRUCTURE AND STRATIGRAPHY

A fence-diagram based on the available well data (Fig. 14) indicates that thinning is most obvious across the structures and along the anticlinal trends. Mid- to Late Cretaceous crustal movements caused uplift followed by erosion, resulting in an unconformity between the Wasia and Aruma Groups. The fence-diagram clearly shows thickening from the west to east and NE.

The interval between the top-Ahmadi and top-Hith horizons includes the lower Wasia and Thamama Groups, and shows thinning over the anticlinal trend. This interval thickens toward the NE by 503.25m and towards the east by 442,25m, thinning on the Burgan Arch and thickening on the margin.

F. Bou-Rabee 195

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Tectonically, the Mesozoic sequence as indicated by the fence-diagram (Fig. 14) is consistent with the cross-section, based on the interpretation of seismic data (Fig. 13). The pre-Mesozoic sequence was only penetrated by a few wells (BI, B3, U l and Sl). In the Burgan area, two faults can be traced between the Bl and UI wells. The displacement of the Triassic sequence on both faults is about 122m. In Late Triassic times, tectonic uplift is indicated by faulting in the Triassic sequence. In addition, an erosional unconfonnity between the mid- and Upper Cretaceous (Wasia-Aruma) suggests that further tectonic activity occurred at that time (Fig. 14).

“Basement” ridges may represent Cambrian salt swells. Apart from any inherited Paleozoic structures, block faulting could have occurred during the Triassic. The structural movements which produced these faults are shown to have died out by the close of the Triassic (Fig. 15), as the thickness of the Minjur Formation is almost uniform. The block-faulted structure of the Middle Triassic interval is apparent on the regional cross-section (Fig. 15).

SUBSIDENCE CURVES

Subsidence curves presented here are qualitative, in the sense that they do not take into account differential compaction, changing water depths, or isostatic loading by sediment and water.

The subsidence curves for the six wells investigated were plotted on a single figure (Fig. 16), so that they may be more easily compared for interpretation. The subsidence history of the six wells is generally consistent, and indicates that rapid subsidence occurred from about 300 MM yrs ago to 52 MM yrs ago (mid-Eocene), interrupted by a decrease in rate during the mid-Cretaceous (97.5 MM yrs ago). This change in subsidence rate corresponds to development of the unconformity between the Middle and Upper Cretaceous intervals.

Subsidence continued during Late Cretaceous and Eocene times, although the main structures contipued to be uplifted relative to their surroundings. Down-warping is still taking place at the head of the Arabian Gulf, and has probably been occurring continuously since the Paleozoic (Mums, 1980).

196 The Tectonic and Depositional History of Kuwait from Seismic Reflection Data

I I LOCATION OF CROSS-S€CTI0)1

Fig. 14. Fence diagram showing lithologic variations across Kuwait.

F. Bou-Rabee 197

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CONCLUSIONS

The structural and isopach maps presented here indicate that the tectonic history of the Kuwait area is relatively simple. Downwarping was required to accommodate the volume of sediment and must have occurred from Pre-Khuff times (interpreted as Late Devonian - Carboniferous). Regional subsidence was interrupted by various episodes of uplift. The structures show a history of relative upwards movements during sedimentation, as can

as the Jurassic, and earlier growth is possible. In fact, the fence diagram shows slight thinning of the Lower Cretaceous units in the oilfield areas, suggesting that tectonism possibly began even earlier than the Jurassic.

ACKNOWLEDGEMENTS

Kuwait University provided financial support for this study through research project

Journal review was by Professor Z . R. Beydoun (American University of Beirut and number SG028.

Editorial Board), whose comments improved the manuscript considerably.

REFERENCES

BARAZANGI, M., D. SEBER, T. CHAIMOV, J. BEST,R. LITAK, D. AL-SAAD andT. SAWAF., 1993. Tectonic evolution of the Northern Arabian Plate in Western Syria. In: E. Boschi et al. (Eds.), Recent Evolution and Seismicity of the Mediterranean Region, pp. 117- 140. Kluwer Academic Publishers, Netherlands.

BEYDOLJN, Z. R.,. 1993. Evolution of the NE Arabian Plate marginand shelf; hydrocarbon habitat and conceptual future potential. Rev. Znst. Franc. du P&trole, 48 (4), 31 1-345.

198 The Tectonic and Depositional History of Kuwait from Seismic Reflection Data

Fig. 16. Subsidence curves for the studied wells.

BOU-RABEE, F., 1986. The Geology and Geophysics of Kuwait. Ph.D. dissertation, University of South Carolina. 150p.

and KLOINKOPF, D., 1994. Crustal Structure of Kuwait: Constraints from Gravity Anomalies. US Geological Survey Open-File Report 94-2 10.

KUWAIT COUNTRY PAPER, 1989 (in Arabic). In: OPEC Seminar on the source and habitat of petroleum in the Arab Countries: hydrocarbon potential and exploration techniques. Proc.. Country Reports and Case Study Volume, 135-149. Abu Dhabi.

MILTON, D.I., 1967. Geology of the Arabian Peninsula, Kuwait. US Geol. Survey Pro$ Paper 560F,

MURRIS, R. J. 1980. Middle East: Stratigraphic evolution and oil habitat. AAPG Bull. 64, 597-620. 7P.