Cretuceous Research (1982) 3, 187-194

Review on the Geology of the Cretaceous Sediments of the Rub’ al-Khali, Saudi Arabia

F. A. Soliman and A. Al Shamlan Ministry of Petroleum and Mineral Resources, Dhahran, Saudi Arabia

Received 7 July 1980

F. A. Soliman and A. Al Shamlan. Review on the Geology of the Cretaceous Sediments of the Rub’ al-Khali, Saudi Arabia. Creruceous Research (1982) 3, 187-194. Petroleum exploration activities in the Rub’ al-Khali were initiated in the 1950s and have continued since then to the present time. In 1968 oil was discovered in the Lower Cretaceous Shu’aiba Reservoir of the Shaybah Field. To date, this is the most important discovery in the Rub’ al-Khali area.

The Rub’ al-Khali is an_ elongated basin that plunges gently north-east from Bir Hadi towards the Arabian Gulf. The north-eastern part of the trough apparently extends across Abu Dhabi and Oman and continues in the Arabian Gulf almost as far as the coast of Iran. The width of the basin is relatively uniform throughout its length, averaging about 300 km. This basin is primarily a Tertiary feature, possibly with some Late Cretaceous history.

It appears that the Cretaceous sediments were laid conformably over the Jurassic in this area. Seven formations belong to the Cretaceous. ‘I’hese are (oldest first): the Sulaiy, Yamama, Buwaib, Biyadh, Shu’aiba, Wasia and Aruma Formations.

Several marine transgressions and regressions took place during this period, and as a result, these seven formations represent a wide variety of sedimentation environments and thus contain a wide range of rock sequences. Even one formation, due to the vastness of the area, may present several of these environments.

In this paper, the geologic features of each separate formation are discussed. Also a hint is given concerning oil occurrences in Rub’ al-Khali. A structure contour map on the top of the Cretaceous in the Rub’ al-Khali is introduced.

Ministry of Petroleum and Mineral Resources, Dhahran, Saudi Arabia.

KEY WORDS: Petroleum exploration; Transgressions; Regressions; Lower Cretaceous.

1. Introduction

The Rub’ al-Khali is one of the largest sand bodies in the world. It is situated in the southern part of Arabia. It stretches from near the north-eastern borders of the Yemen all the way to the south-western part of the Arabian Gulf, a distance of more than 1300 km.

This study deals with the Cretaceous sediments in an area bounded from the north by the Jafura and Summan, from the east by a line passing by ST-well No. 18, and from the west by the Yemen highlands and southern Tuwaiq mountains.

The majority of the Rub’ al-Khali basin is of sand dunes of many shapes except for some patches of outcrops in the west and sabkhas in the extreme eastern parts. Sometimes valleys or shuqq develop between these dunes. These shuqq are barren of sands and may have on the surface recent rocks, gravel or even sabkhas.

Climate is very severe with high temperatures. It is also windy. Temperatures during the warmest months generally exceed 49°C and thus the Rub’ al-Khali is classified as an arid region.

0195-6671/82/010187+08 $03.00/O 0 1982 Academic Press Inc. (London) Limited

188 F. A. Soliman and A. Al Shamlan

Sporadic herbs and shrubs grow after the rare rainfalls in the Rub’ al-Khali region.

The purpose of this paper is to present a summary of the stratigraphic sequences of the Cretaceous in the area and to demonstrate oil and gas shows discovered.

2. Cretaceous formations

The Cretaceous system in the Rub’ al-Khali is divided lithologically into seven formations. It appears that there was no break in sedimentation at the Jurassic- Cretaceous boundary, and the Lower Cretaceous followed the Upper Jurassic quite conformably.

Age Formplion

rertiary Umm Er Rodhuma

Maastrichtian

Campanian

Santoman

Aruma

Coniacian

Turonian

Cenomanian

Albian

Was10

Aptian

Barremian

Hauterivian

Voianginion

Berrlasion

Shu’olba

Biyadh

Buwalb

Yomomo

Sulory

Jurassic Hith I

Figure 1. Subsurface stratigraphic sequence of Cretaceous sediments, eastern Saudi Arabia.

2.1. Sulaiy and Yamama Formations

A marine transgression of limited extent affected the Rub’ al-Khali area in the Early Cretaceous. In the central and eastern Rub’ al-Khali, rather quiet conditions of sedimentation are indicated by the deposition of fine-grained limestone in the Sulaiy and Yamama Formations. The presence of neritic carbonates indicates nearness to deeper parts of the basin. The transgression did not occur in the southern and western parts to an equal degree and thus transitional conditions might have occurred.

Cretaceous Geology of Rub’ al-Khali 189

Thickness. The Lower Thamama Group, perhaps attains its maximum thickness to the east of the Rub’ al-Khali. But its exact thickness there cannot be estimated because it is difficult to separate the Yamama Formation from the overlying Buwaib Formation.

In the central Rub’ al-Khali, it is 900 ft (275 m) thick, and decreases to the west. To the south it is much thinner. South of latitude 20”N, Berriasian and Valanginian were removed by the pre-Hauterivian erosion.

Lithology. To the east the Lower Thamama Group consists entirely of carbonates, of which very fine-grained limestones form the bulk. Very fine crystalline dolomite is also present with rare beds of calcarenite. Northward, fine-grained limestone is predominant but shale is also present. Southward, argillaceous elastic deposits such as clays, marls and shales form the characteristic lithologies.

2.2. Buwaib Formation

By the end of the Valanginian, the whole area was uplifted. It suffered erosion before the Hauterivian sediments were laid down. This affected mostly the areas south of latitude 20”N where the Berriasian and Valanginian were eroded.

With the deposition of the Buwaib Formation, a widespread transgression covered the depositional area. A thick development of the formation in the north, central and north-eastern Rub’ al-Khali defines the basinal areas of sedimentation. To the south and to the west, shelf areas and probable non-marine sediments are encountered.

Thickness. In many wells, the Buwaib Formation cannot be separated from the underlying Yamama or from the overlying Biyadh Formation. The Buwaib is absent in the southern Rub’ al-Khali and increases in thickness in the direction of the north and east.

Lithology. Very fine-grained elastics such as clays, shales or marls form more than 5Oo/o of the sequence in the southern and north-eastern Rub’ al-Khali and here, fine-grained sands do not form more than 16%.

In the central and eastern parts, the carbonates replace the argillaceous deposits and thus form the predominant lithology. Dense, compact limestone is common. Calcarenite and calcarenitic limestone are less frequent. Dolomite may also be present. In the south-western parts, limestone occurs throughout the formation with varying amounts of marl and shale.

2.3. Biyadh Formation

West, to longitude 49”E, the elastic facies of the Biyadh Formation, consisting mainly of sands and clays associated with lignite and devoid of marine fossils, indicates an essentially continental, deltaic or fluvial environment of deposition. The character of the sands, being medium to coarse-grained, sub-rounded to sub- angular may indicate the proximity of the source. It appears that marine conditions prevailed to the centre and to the east, evidenced by the occurrence of fossiliferous, fine-grained carbonates and shales. These represent quiet-water shelf conditions of sedimentation. Going more to the east, the effects of oscillation of sea-level are in evidence.

Thickness. The formation is irregular in thickness over the area. In the north-eastern Rub’ al-Khali, the Biyadh Formation cannot be separated from the underlying Buwaib as in the Shaybah and Ramlah areas.

190 F. A. Soliman and A. Al Shamlan

Lithology. In the south-western Rub’ al-Khali, clays and claystones are dominant. In the western parts, arenites, commonly quartz-sands, appear to be more developed. To the east, there is a decrease in elastic deposits and carbonates become an important constituent. In some places, as in well ST-14, the sequence can be divided into a lower, non-marine part consisting of coarse-grained quartz-sands and an upper part of compact vuggy calcarenitic limestone. Going eastward, the formation is wholly carbonate with aphanitic limestone, calcarenitic limestone, calcarenite and dolomite. Further east it is mainly fine-grained, dense limestone with minor dolomite and calcarenite. Still further to the east, shale may be developed. To the north, the Biyadh and Buwaib cannot be separated.

2.4. Shu’aiba Formation

This is the uppermost part of the Lower Cretaceous. It follows the Biyadh without noticeable break in sedimentation. It is mainly composed of marine, fine-grained carbonates. To the east, at the Shaybah Field, a reef complex is developed. Back reefs or lagoonal type deposits are also encountered in the area.

Thickness. The formation is absent in all the wells west of longitude 48”E. It is relatively thin to the south and gradually thickens to the north-east and attains a maximum thickness at the Kidan North and Shaybah areas.

Lithology. Carbonates form the main constituent of the Shu’aiba Formation. Very fine-grained limestone is the most common type in the wells of the central and northern Rub’ al-Khali. To the south, dolomite is dominant. Fine grained elastics such as shales and marls are present at the Kidan area. Sands are encountered to the west at well ST-21. In the Shaybah and Ramlah areas, the facies are more compli- cated and indicate a reef-complex environment for this formation.

2.5. Wasia Formation

Towards the end of the Aptian, there was a general uplift among the whole Rub’ al-Khali area. The period of erosion was relatively short and in the Albian, the sea advanced once again. This Albian transgression covered the eastern and central parts.

In fact, the Wasia, over the whole Rub’ al-Khali presents a complex series of rocks which represent many sedimentational environments ranging from deltaic to open marine. In the Kidan area, the presence of a reefal environment in the upper part of the Wasia is indicated. The centre of deposition is in the central Rub’ al- Khali where clays are dominant over the carbonates. The western parts have non- marine deposits.

Thickness. The Wasia attains its maximum thickness in the central Rub’ al-Khali. From this point, the formation thins out in all directions. It thins rapidly to the north-east. In the Shaybah area, the whole Upper Wasia and a great part of the Middle Wasia are missing while in the nearby Ramlah area all the Wasia sequence is found. To the west, the formation thins out and it becomes difficult to separate the Wasia from the Biyadh Formation.

Lithology. The Wasia is divided into seven rock units which are finely demarcated only in few areas, as in the Kidan area. These are (oldest first): the Khafji, Safaniya, Mauddud, Wara, Ahmadi, Rumaila and Mishrif members.

The lithology of the Wasia is characterized by a sand-clay facies in the west. The sands are mainly sub-rounded to angular and associated with variegated clays and

Cretaceous Geology of Rub’ al-Khali 191

marls with minor sandstones. The clays and marls tend to increase eastward at the expense of sands. Also to the east there is a marked increase in carbonates and shales.

2.6. Arum Formation

A major regional unconformity separates the Wasia Formation from the overlying Aruma Formation, indicating that the area experienced uplift and erosion. In the western Rub’ al-Khali, non-marine or deltaic conditions are prevalent. Eastward the Aruma is mainly marine. At the Ramlah area, the succession suggests an open marine environment, whereas small intervals of Lower Aruma shale may indicate tectonic instability in this area by that time. In the Late Cretaceous, the trans- gression increased to the west where the non-marine succession is capped by fossili- ferous carbonates.

Thickness. In the extreme west, the Aruma is absent or truncated and unconform- ably overlain by the Neogene. Eastward, it thickens but further eastward it thins again. This irregularity in thickness is due to the fact that this formation is under- lain and overlain by two unconformities that affected it greatly.

r Age

Coniac- ian

Turonion Mishrif

Rumaila

Cenoman- ian

Ahmadi

Wasia

Wara

Mauddud Albian

Safaniya

Khafji

-_?-

Aptian Shu’aiba

T Formation Rock units

Aruma

Figure 2. Shaybah Field, Shu’aiba Reservoir, Shaybah well No. 1, lithology.

192 F. A. Soliman and A. Al Shamlan

Lithology. The Aruma in the western Rub’ al-Khali is represented by elastic sedi- ments of which sands and sandstones form the bulk of the sequence with marls and clays in subordinate quantities. North-eastward, as in the Abu Bahr area, the Aruma can be divided into three units: a lower sandy, a middle carbonate, and an upper shale and clay unit. To the south, elastic facies are well developed where quartz- sands form the bulk of the sequence. In the central and eastern Rub’ al-Khali, the sand facies is poorly developed or absent.

3. Oil occurrences in the Rub’ al-Khali Cretaceous sediments

The Upper and Lower Ratawi Reservoirs belong to the Yamama and Sulaiy Formations, respectively. In Niban well No. 1, traces of gilsonite were reported in the Lower Ratawi. In Kidan North area, tar was reported in the same interval. In the Shaybah Field, on testing the Upper Ratawi, gas and oil-cut formation water were recovered, while the Lower Ratawi included minor oil quantities in some wells.

The well drilled at Bir Hadi in 1971 was reported to have fluorescence in the cuttings of the Biyadh reservoir. Some 25 ft (7.5 m) of oil were recognized on the logs of some Shaybah wells where the Biyadh, Buwaib and Yamama Formations cannot be differentiated.

Density log

op Shu’C

leservoir

Oil

Neutron log

m Limestone ~Shole m Marl

Figure 3. W&a Formation rock units. (1 ft= 30.48 cm.)

Cretaceous Geology of Rub’ al-Khali 193

To date, the Shu’aiba reservoir is the most important discovery in the Rub’ al-Khali. Hydrocarbons were discovered in the Lower Cretaceous Shu’aiba Forma- tion in the Shaybah Field in 1968. Lithologically, this reservoir consists of porous limestone and calcarenitic limestone with an abundance of rudistid shell debris and algal-growths. The facies are typically reef and back-reef deposits. The Cretaceous section in Shaybah well No. 1 consists of 3796 ft (1157 m), of which the Sh’aiba Formation occupies 461 ft (140.5 m). The top of the reservoir is marked by rudistid and algal reefs or reef detritus, predominant chalky limestone and calcarenitic limestone. The Shu’aiba reservoir, in this field, contains the greatest hydrocarbon accumulation yet known in the Rub’ al-Khali.

Some of the Wasia members are good reservoirs in some places. The Mishrif member was reported to have minor hydrocarbon accumulations in Mihrad well No. 1. Oil shows were noticed on the logs of both Kidan North and Kidan South wells. These accumulations are restricted because of poor porosity and perme- ability.

The Mauddud member was found to have oil shows on the logs of both Kidan areas. The same interval was tested in Sanam well with no result. In the Mihrad well, it produced small quantities of 36” API oil, probably due to poor porosity and permeability.

The Safaniya member has small oil shows on the logs run in Kidan North well. In the Zaynan area, the Lower Wasia and Shu’aiba Formations were found to have good reservoir characteristics but no oil or gas were encountered.

Moreover some wells of the Shaybah Field were reported to show oil-staining and fluorescence in the cuttings of the lower part of the Aruma Formation.

4. Concluding remarks

The Rub’ al-Khali is a vast area. Oil exploration work started in the 1950s. Much work has still to be done in order to arrive at a good knowledge of the geology of this region. Of course exploration should consider stratigraphic as well as structural traps.

The Arabian American Oil Company has done, and is still doing, much work in the area. Its concessions include a small part of the area. AGIP Saudi Arabia shared exploration activities in the late-1960s and early-1970s.

The Ministry of Petroleum and Mineral Resources is currently carrying out an Exploration Programme in some parts of the area.

This paper has dealt with the Cretaceous exclusively, but this does not necessarily mean that this is the only important period in Rub’ al-Khali. Other ages may prove to be as important as the Cretaceous.

Acknowledgments

The authors acknowledge the Saudi Arabian Ministry of Petroleum and Mineral Resources for its kind assistance.

They sincerely thank Mr Bilal Hibshi for his enthusiastic discussions, Mr S. Bowers for reading the manuscript and Mr Abdul Aziz Al Hawas who drafted the attached map.

Thanks are also extended to fellow geologists of the Ministry who helped to collect the data necessary for this work.

CR G

194 F. A. Soliman and A. Al Shamlan

Selected references

The Ministry files, comprising well-reports, studies made by companies that worked in the area, and studies made by the Ministry personnel were consulted. Of these we select the following references.

ARAMCO. undated. Correlation Handbook. Part I : Stratigruphy. (Report.) ARAMCO. undated. Report on structure drilling. Part II: Structure drilling in the far south-western

Rub’ al-Khali. Part III : Structure drilling in Abu Bahr, southern Summan and southwestern Rub’ al-Khali areas. (Unpubl.)

ARAMCO. undated. Report on structure drilling. Part II: Structure drilling in the southwest and south-central Rub’ al-Khali areas. (Unpubl.)

ARAMCO. undated. Report on structure drilling 1950-1960. Part I: Structure drilling in Abu Bahr and southern Summan areas. Part II: Structure drilling in the Tumaisha area. Part III: Strati- graphic drilling in the Rub’ al-Khali. (Unpubl.)

ARAMCO. undated. Structure drilling in the northern Rub’ al-Khali. (Unpubl. Report.) Hassan, T. H. 1969. Geology of the Rub’ al-Khali. (Unpubl. Report.) Powers, R. W. 1968. Lexique Stratigraphique International, Vol. III: Saudi Arabia. Powers, R. W., Ramirez, L. F., Redmond, C. D. & Elberg, E. L., Jr 1966. Geology of the Arabian

Peninsula. Washington, D.C. : U.S. Government Printing Office. Sirtajuddin Ahmad, S. 1964. Physiography of Saudi Arabia. (Unpubl. Ministry Report.)