ABSTRACT

During interpretation of a 3-D seismic survey in southern Oman a solitary, 2.5-km-wide circular basin with a central peak and raised rim was identified in the subsurface 35 km west of the Marmul oil field. The feature is the only one of its kind in the area. The basinal structure is probably of Late Cretaceous (Albian) age and the regional geology strongly suggests that it is neither a volcanic crater nor related to salt-dome tectonics or salt dissolution. It possibly represents a crater formed by a terrestrial impact event and has been named the Murshid crater. This report does not constitute a detailed investigation of the possible impact crater but rather records the 3-D seismic observations and the drilling that has taken place near the structure so far.

INTRODUCTION

During interpretation of a newly acquired 3-D seismic survey for oil exploration in southern Oman, a solitary 2.5-km-diameter circular basinal feature was identified as a possible impact structure and was named the Murshid crater. It lies 35 km west of the Marmul oil field in the South Oman Salt Basin (Figure 1). The center of the structure is at latitude 18°10′59″N, longitude 54°55′08″E, and it is buried at a depth of approximately 380 m below mean sea level (680 m below the ground surface).

The authors are petroleum geologists who felt that the Murshid basinal structure needed reporting to the wider scientific community. It is their hope that, by doing so, geoscientists with more knowledge of impact structures will be stimulated to study the structure in the detail it deserves.

Seismic Data

The structure is in an area covered by a 3-D seismic survey acquired in 1996. The survey provided good-quality data down to the base of the Cretaceous. Imaging problems due to multiple reflections from high-velocity Cretaceous carbonates obscured the Permian to mid-Cretaceous intervals below the major basal Nahr Umr unconformity. Various predictive multiple elimination techniques were used in order to highlight the dips in the pre-Nahr Umr section.

The map of the crater-like structure (Figures 2 and 3) corresponds to the top Natih-E member of the Natih Formation in the Murshid wells, and to the laterally equivalent reflector in the basin itself where the top Natih-E is missing. The structure cuts as deep as the Permian Gharif Formation. Exploration wells Murshid-2 drilled in 1988 and Murshid-3 drilled in 1998 (Figure 3), are located 3.6 km southeast and 6 km south of the structure, respectively, and provided stratigraphic control for the interpretation of the seismic data. A composite stratigraphic log from Murshid-3 is shown as Figure 4.

The seismic display (Figure 5) is a ‘Reverse SEGY’, which means that a red loop corresponds to a soft-to-hard acoustic impedance jump. The data are zero-phased. The red loop at 405 msec corresponds to just above the top Natih, and the blue loop at 530 msec to the base Natih. The Natih Formation was uniformly 200 m thick in the Murshid wells. A synthetic seismogram derived from logs and check shots matched the seismic well with a bandpass filter with corner points at 8, 14, 40, and 60 Hertz. The seismic interval velocity for the Natih Formation was 3,000 m/sec based on well data.

In Figure 5, the hint of a seismically chaotic zone occurs toward the top of the Gharif beneath the pre-Nahr Umr unconformity, which could be an additional 90 m deep. However, strong seismic multiples mirror the shallower basin shape. Partial infill of the basin from within the Natih Formation is evident, although differential compaction of the fill (presumably finer-grained deeper-water facies) continued well into post-Natih times.

Several other possible buried impact craters have been found on seismic data elsewhere in the World; for example, from Kansas (Carpenter and Carlson, 1992), Alaska (Kirschner et al., 1992), and the Barents Sea (Dypvik et al., 1996). Plawman and Hage (1983) and Hodge (1994) list examples.

MURSHID CRATER

Description

The basinal structure—the shape of the upper surface of a possible impact structure—is 2.5 km wide and broadly circular (Figures 2 and 3). The base of the structure could be within or beneath the mapped seismic reflection, the loop width of which gives a depth uncertainty (Figure 5). The basin has a maximum depth of about 60 m (± 15 m) with respect to the average surrounding elevation. About 40 m of the relief is expressed as a thinning or removal of the Natih-E, -F, and –G, and as much as 20 m as an indentation into the top of the underlying Permian Gharif Formation (Figure 6). The crater has a raised lip that is 10 to 15 m high at the Natih-E level, and a halo of raised acoustic impedance seen as an arc of higher amplitude that could represent reduced porosity, particularly on the southern and eastern sides (Figure 7). The central peak is 500 m wide at its base and about 40 m high. Irregular indentations on the northern and western sides of the basin (Figures 2b and 6) appear to represent terraces such as those caused by flow toward the center of a crater.

The Murshid structure, if it is a crater, would be classed as ‘complex’ because of the central peak, the flat floor and the terraces on the northern and western sides. No evidence, such as an elliptical shape or asymmetry in relief, occurs to indicate oblique impact and there are no signs of secondary or related smaller craters. Grieve and Cintala (1999) stated that terrestrial impact craters in sedimentary rocks typically become complex rather than simple (i.e. flat-floored with a central peak and interior terraces instead of bowl-shaped with no central peak) at crater diameters of more than 2 km. The same transition for crystalline rocks is at greater than 4 km diameter. The Murshid structure could therefore represent one of the smaller complex craters.

Grieve and Cintala gave the relationship of diameter to depth as follows:

Log (depth) = log (0.12) – 0.3 × log (diameter) where depth and diameter are expressed in kilometers.

They stated that the equation was based on a sample of about 15 terrestrial craters, so illustrating the paucity of proven craters in this size class.

The diameter of the Murshid structure is better known than its depth due to uncertainties in picking the base of the apparent crater. The diameter of 2.5 km implies an actual depth of 157 m based on the equation above and the depth of 60 m given above would be merely the depth of the ‘apparent’ crater. If the feature is truly an impact crater then an additional thickness of impactite (shock-metamorphosed rock) should be added to this estimate (Melosh, 1989; Price, 2001).

The minimum width/depth ratio based on the basin depth as mapped, is about 50:1 (maximum about 40:1). Both values are high with respect to the quoted averages for impact craters given by Grieve and Cintala (1999). However, the depth dimension of the Murshid structure is only approximate given the lack of constraints on the nature, and hence velocity, of the basin fill.

Impact Origin

Evidence in favor of an impact origin for the basinal feature is its morphology, in particular its circular form, raised lip, and central peak—and possibly the width/depth ratio, although this deviates from the published mean values. A clearly raised lip and a slope away from the crater for about 500 m (at least toward the north) can be seen, but there is no evidence from the seismic maps for an extensive ejecta blanket or ejecta rays.

Several exploration wells have been drilled in the vicinity of the Murshid structure. The Murshid-3 well was drilled for oil in 1998 as a result of the 3-D seismic interpretation. It was sited about 6 km south of the crater and thus provides no direct evidence of the crater’s origin. The well was drilled through an entirely normal, regionally correlatable section in both the Cretaceous limestones of the Natih and the unconformably underlying Permian clastics. A re-examination of Murshid-2 drilled in 1988 on 2-D seismic data 3.6 km southeast of the crater (Figure 3), also failed to reveal any anomalous stratigraphy. Cuttings are available from both wells but they have not been analyzed for any geochemical signatures that might relate to an impact.

Other circular structures in Oman are clearly related to uplift, or to collapse above salt stocks due to dissolution over their crests. One such feature on Jidat al Harasis in central Oman had been erroneously interpreted as a surface impact feature (see below). The salt-stock interpretation can be ruled out in the case of the Murshid structure, which sits between linear salt uplifts that are responsible for the arcuate grabens seen on the structural maps to the south, east, and west of the structure. In fact, underlying Murshid is a ‘turtleback’ structure that consists of anomalously thick clastic sediments accommodated by the withdrawal of salt into the surrounding highs (Figure 2a). The turtle structure has been proven by three wells drilled for oil on the flanks and crest. A sinkhole origin can also be ruled out since the structure clearly cuts into, or depresses, the Permian Gharif clastics. Precambrian carbonates 2 km beneath the Paleozoic clastics show no evidence of dissolution at the scale of the 3-D seismic.

The evidence does not support a volcanic (caldera) origin for Murshid. Latest Precambrian and mid-Ordovician volcanics are known from the area, as are Tertiary volcanics in southern Oman and Yemen, but none of Permian to Cretaceous age have been identified in southern Oman. This is despite the drilling of at least 400 petroleum exploration wells. In particular, Murshid-2 and Murshid-3 provided no evidence of anomalous stratigraphy in the form of volcanics or anomalous changes in thickness. The absence of gravity or magnetic anomalies over Murshid, in combination with an absence of volcanics in the wells, means that a caldera origin can be safely excluded. The Murshid structure is interpreted as an isolated impact crater on the basis of morphology and negative evidence.

Host Rock

The top Natih-E is the youngest unit to clearly show the crater form (Figure 5). Any impact therefore would have been into the Lower Natih shallow-marine limestones. The Natih-E to–G members, deposited before the impact took place, form a 50- to 60-m-thick limestone section interbedded at the base with marls. Porosities in the correlative rocks average 12 percent and they consist mostly of packstone and wackestone lithofacies. The porosity and fabric of the crater materials are unknown, but there is no clear seismic amplitude expression (Figure 7) of the crater itself to suggest higher or lower acoustic impedance and therefore lower or higher porosities, respectively. The crater structure clearly penetrates through the Nahr Umr Formation into the top of the Gharif Formation. Beneath the Nahr Umr is a several-kilometer-thick section of Paleozoic clastics assigned to the Haushi and Haima supergroups.

In an impact crater, a lens-shaped body of impactite would be expected between the base of the apparent crater (in this case, the mapped reflector) and the base of the true crater. At Murshid, the base of the impact melt and impact breccia, if present, cannot be interpreted. This could be due to seismic multiples that were generated by the high-velocity carbonate rocks above. Drilling in the crater itself is needed to obtain the evidence of shock metamorphism that would establish an impact origin. Regional impact fracturing of the 14-m-thick Nahr Umr Formation, an Albian mudstone, which acts as a widespread top seal for underlying hydrocarbon-bearing formations, was recognized as a clear risk for prospects down-dip of the cratered area. No oil was found in the Murshid-1, -2 or -3 wells despite the evidence of a valid structural closure. However, this failure was probably due to the lack of westward-charge migration from the salt windows through which oil from the intra-salt Precambrian source rocks had to migrate. This is likely to limit future exploration interest in this structure, at least for shallow objectives.

Age

The crater form is most complete at the top Natih-E level. From regional correlations, the top of the Natih-E member is of late Albian age. The age has not been substantiated from rocks in the vicinity of the crater, but regional correlation is robust as the Natih-E is present throughout much of Oman. It is possible that the top Natih-E reflector drapes the basin and is therefore older. If this was the case, the age of the basin would be very poorly constrained since the underlying Nahr Umr, also Albian in age, is only 14 m thick and is not seismically mappable. The Nahr Umr in turn overlies a regional unconformity above dipping clastics of the Permian Gharif Formation. If the crater were cut into this unconformity it could be of any age between Permian and Albian. However, this interpretation is very unlikely, as it would require a passive drape of the basin form by Lower Natih limestones. Since these are of shallow-water origin, it is improbable that this would occur without any infill of the paleorelief. For these reasons, an intra-Natih (post Natih-E) Albian age is deduced.

Crater Infill

The seismic data (Figure 5) suggest that the crater was filled by deposition of the younger Natih limestones (Natih-D through -A) that, away from the crater, are up to 150 m thick. There are hints of progradation into the basin by these cyclical limestones and marls, and essentially all relief was reduced to zero by the time of deposition of the Cenomanian Fiqa Formation. Only minor amounts of differential compaction occurred over the basin after that time.

OTHER METEORITE SITES IN OMAN AND NEARBY AREAS

No surface impact craters are known from Oman. The nearest is the famous Saudi Arabian impact crater at Wabar (lat. 21°30.2′N; long. 50°28.4′E) in the Rub’ al Khali (Figure 1) that may date from 1863 (Wynn and Shoemaker, 1997), but is probably about 600 to 100 years old.

One supposed surface impact crater in Oman is on the gravel plain of Jidat al Harasis) near Habhab (Figure 1), in the area of the White Oryx Reserve (Hughes-Clarke 1990, p. 78). It is visible as a ring 6 km in diameter, known to the local people as ‘Lehob’. The ring structure, caused by subtle vegetation changes, is visible on satellite imagery. Seismic surveys showed that the structure was formed by a salt dome rising from deep beneath the surface and lifting up the overlying rock (J. Terken, personal communication, 1998).

Although no surface impact craters are known, meteorites have been found lying on the surface in Oman. The largest amount of material recovered was from the Ghubarah area northwest of Rima (Figure 1; lat. 19°13′40″N; long. 56°8′34″E) where, in 1954, several hundreds of kilograms of a stony meteorite were collected. The material—an olivine hypersthene chondrite—is though to be a constituent of the early Solar nebulae. As such, it is about 4.5 billion years old.

More recently, Russian meteorite hunters found specimens of eucrite in the Dhofar area (see Figure 1). Eucrites are blobs of basalt lava that have been flung into space as a result of the impact of celestial bodies. Detailed chemical analyses of eucrites have enabled scientists to trace them to their parent body in the Solar System. The geochemistry of one of the Dhofari eucrites suggests it was ‘splashed’ onto the Earth when a celestial body hit Mars (Dhofar 019: Sky and Telescope, 2000a). Two finds have been reported from the Sayh al Uhaymir area (Figure 1), also with Martian affinities (Sayh al Uhaymir 005 and 008: Sky and Telescope, 2000a). In March and December 2000, more finds of meteorites in Dhofar (025, 026 and 081) were announced whose geochemistry indicated that they were blasted to Oman from the Moon (Sky and Telescope, 2000b).

FURTHER WORK

In order to establish a basinal structure as an impact crater, unambiguous shock metamorphic material such as shatter cones, shocked quartz, impact breccia, or impact melt must be found (Melosh, 1989; Grieve, 1999). Such material might be preserved in cuttings from nearby wells. Failing this, and the sample suites are poor, Murshid and similar structures would have to be drilled to move from ‘possible’ to ‘proven’ categories, a concept familiar to petroleum geologists.

ACKNOWLEDGMENTS

The authors thank the Ministry of Oil and Gas, Oman for permission to publish this article. They gratefully acknowledge the help of numerous colleagues at Petroleum Development Oman. Dr David Malin of the Anglo-Australian Observatory is thanked for providing the stimulus to publish this discovery. Two anonymous referees are acknowledged for their help in improving the paper. Editing and graphic design was by Gulf PetroLink.

ABOUT THE AUTHORS

Bruce Levell is Head of New Ventures and Basins in the Global Exploration Unit of Shell International Exploration and Production in The Hague. He has a D.Phil from Oxford University for work on Precambrian rocks. By background a sedimentologist/basin analyst, Bruce never expected that years later he would be exploring for hydrocarbons in Precambrian rocks of Arabia. He has been an exploration geologist in Shell International for 24 years. He has worked in the Shell Research Laboratory in Holland, and in Shell-affiliated companies in Malaysia, the USA, the UK, and Oman. Previous to his present appointment, Bruce was Asset Manager for Frontier Exploration in Petroleum Development Oman. His interest in the Murshid structure was triggered by the combination of his hobby of astronomy with his profession as a geologist.

bruce.levell@shell.com

Pascal Richard is a Structural Geologist within the Carbonate Development Team at Shell International. He has a PhD in Structural Geology on strike-slip tectonics from the University of Rennes, France. Pascal joined Shell in 1991 and spent five years in the Structural Geology Department on sandbox modeling of structural styles, fault growth, and hydrocarbon systems. In 1996, he transferred to Petroleum Development Oman (PDO) as the Structural Geology Focal Point and Seismic Interpreter with PDO’s Exploration Department. In 1999, Pascal moved to Shell’s Carbonate Development Team where he is focusing on the analysis and modeling of fractured reservoirs. He also acts as a global expert for Shell in structural geology.

p.d.richard@siep.shell.com

Folco Hoogendijk is a Senior Geologist at the Shell Technology Centre in The Hague. He has an MSc in Geology/Geophysics from the State University of Utrecht, The Netherlands. From 1990 to 1996, he worked for Shell Exploration and Production in London as a Central North Sea Exploration Team Geologist and Operations Geologist, and as a Seismic Interpreter for the Northern North Sea Exploration Team. In 1996, he transferred to Petroleum Development Oman (PDO) where he was involved in prospect evaluation and maturation of Athel silicilyte and carbonate stringers. Latterly, Folco was a Senior Seismic Interpreter with the South Oman Frontier Exploration Team at PDO. He returned to Shell in 2001.

f.hoogendijk@siep.shell.com