GeoArabia, Vol. 7, No. 4, 2002 Gulf PetroLink, Bahrain A possible Albian impact crater at Murshid, southern Oman Bruce Levell1, Pascal Richard2 and Folco Hoogendijk2, Petroleum Development Oman 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 18º10’59"N, longitude 54º55’08”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. 1 Present address: Shell International Exploration and Production, The Hague, The Netherlands 2 Present address: Shell Technology, Rijswijk, The Netherlands 721 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/4/721/4564329/levell.pdf by guest on 27 September 2021 Levell et al. Location Map IRAN N 0 500 km Arabian Gulf Gulf of Oman UNITED ARAB EMIRATES Muscat OMAN Fahud Salt Basin Wabar Crater Ghaba Salt Basin SAUDI ARABIA Masirah Island Sayh al Uhaymir Jidat al Harasis Ghubarah South Oman Salt Basin Rima Arabian Sea Caspian Sea Murshid crater Marmul Med N Sea IRAN D h o f a r Salalah Arabian UAE Shield SAUDI ARABIA OMAN YEMEN R ed S Crater site/meteorite debris ea Oil field Gas field YEMEN Arabian Sea 0 300 Gulf of Aden km Figure 1: Location of the Murshid crater in southern Oman. 722 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/4/721/4564329/levell.pdf by guest on 27 September 2021 Albian impact crater at Murshid, Oman 3-D View of Murshid Structure a Murshid structure b Figure 2: (a) A vertically exaggerated 3-D view of the Murshid structure looking north-northeast at the mapped top Natih-E level. Colors show structure contours in meters below sea level: red, less than 540 m; green more than 580 m––total basin depth is about 60 m. Deep, U-shaped ‘valleys’ on either side of the Murshid structure are grabens related to withdrawal/dissolution of salt along the elongate salt walls that surround the Murshid ‘turtleback’ structure. Isometric block is about 20 km wide and about 25 km long. (b) Close-up of the Murshid structure. Terraced features on the northern and western sides, together with the central peak, allow the basin structure to be categorized as a possible ‘complex’ crater. 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 723 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/4/721/4564329/levell.pdf by guest on 27 September 2021 Levell et al. Seismic Line and Well Locations A' A Murshid-2 N Murshid-3 0km5 Murshid-1 1 km Figure 3: A shaded vertical relief view (artificial light from N) of the Murshid structure at Natih-E level; diameter 2.5 km. A–A’ is seismic line of Figure 5. 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. 724 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/4/721/4564329/levell.pdf by guest on 27 September 2021 Albian impact crater at Murshid, Oman Well Murshid-3 The diameter of the Murshid structure is better Gamma- known than its depth due to uncertainties in picking Ray Density the base of the apparent crater. The diameter of 0 API 150 1.95 g/cm 3 2.95 2.5 km implies an actual depth of 157 m based on the Porosity µ MEMBER (Mud log) Depth (m) 500 sec/m 100 45 NPHI -15 equation above and the depth of 60 m given above would LITHOLOGY FORMATION be merely the depth of the ‘apparent’ crater. If the feature is truly an impact crater then an additional thickness of Fiqa impactite (shock-metamorphosed rock) should be added to this estimate (Melosh, 1989; Price, 2001).