Geological Evolution of the Central Marib-Shabwa Basin, Yemen
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GeoArabia, Vol. 4, No. 1, 1999 Marib-Shabwa Basin, Yemen Gulf PetroLink, Bahrain Geological Evolution of the Central Marib-Shabwa Basin, Yemen Joe Brannan, Nimir Petroleum Ltd., Gurdip Sahota, Oryx Energy Company, Keith D. Gerdes, Triton Resources (UK) Ltd. and Jonathan A.L. Berry Shell UK Exploration and Production ABSTRACT The Marib-Shabwa Basin is part of an extensive west-northwest oriented, petroliferous rift system straddling Southern Arabia and the Horn of Africa. The history of the basin has been unravelled using well and seismic data gathered by Nimir Petroleum Company between 1992 and 1995. Four megasequences have been defined using seismic data and these have been further subdivided using integrated well and seismic information. A fifth megasequence is identified from regional information but has been eroded within Nimir's Block 4. Pre-Rift Megasequence sedimentation began in the Middle Jurassic when transgression from the southeast resulted in the deposition of paralic clastic rocks and shallow-marine carbonates of the Kohlan and Shuqra formations. Rapid deepening in the Oxfordian resulted in the deposition of anoxic shales in the basin immediately prior to rifting. The Syn-Rift Megasequence is of Kimmeridgian-Tithonian age. Adjacent to basin margins and elevated intra-basinal highs, thick turbidites of the Lam Formation accumulated. However, over much of Block 4, rift geometries produced sediment-starved areas where Madbi Formation carbonates accumulated. As rift topography was infilled, fine-grained clastics of the upper Lam Formation spread throughout the basin. Following minor fault reactivation, rifting stopped in the mid-Tithonian. Carbonate deposition (Ayad Formation) in early post-rift times was rapidly followed by isolation of the basin from the open ocean to the southeast. As a result an extensive salt basin (Sabatayn Formation) developed throughout the Marib-Shabwa system. However the salt basin was short- lived and marine carbonate deposition was re-established by late Tithonian times. Initially these carbonates were very clean (Lower Naifa Formation) but the clastic component gradually increased (Upper Naifa Formation) before sedimentation abruptly stopped in the Berriasian. The Ayad, Sabatayn, Lower Naifa and Upper Naifa formations comprise Post-Rift Megasequence 1. Sedimentation resumed in the Barremian (Post-Rift Megasequence 2) with the deposition of mixed marine clastics and carbonates of the Qishn Formation. Sediment loading at this time mobilised the salt which flowed updip and formed a series of elongate ridges overlying the footwall crests of major fault blocks. As a result, Qishn clastics were deposited in a series of discrete, elongate salt withdrawal basins. Paralic clastics prograded eastwards into the accommodation space left by retreating salt. The most distal basins were initially starved and contain condensed limestones. These basins were infilled by Tawilah Group fluvial sandstones which constitute the upper part of Post-Rift Megasequence 2. Following resubmergence in the Early Tertiary the widespread Hadramaut Group carbonates and evaporites (Post-Rift Megasequence 3) were deposited but uplift and erosion in the Late Tertiary has removed them from large parts of the Shabwa Basin, including Block 4. INTRODUCTION The Marib-Shabwa Basin (Redfern and Jones, 1995) is a west northwest-east southeast trending Late Jurassic rift system which lies in southwestern Yemen (Figure 1). It is part of an extensive system of basins which trend across southern Arabia and the Horn of Africa, from Sana’a in the northwest to the east coast of Somalia in the south. To the east, the system extends almost to the island of Socotra, which, prior to Tertiary sea-floor spreading lay just south of Salalah in southwestern Oman (Figure 1; Beydoun, 1970; Richardson et al., 1995). 9 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/4/1/9/4552973/brannan.pdf by guest on 25 September 2021 on 25 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/4/1/9/4552973/brannan.pdf Brannan etal. 45°E 46°E 47°E 48°E 16°N Sayun AL-JAWF Raydan Basin GRABEN Jabal MARIB Meem Samadan GRABEN Azal Wadi Al-Tahreer Bana Alif Dostour SHABWA Lam Al-Wahdah GRABEN Asakir High Ma'een Shaharah Al-Shura Jabal Al-Wihdah Nuqum MARIB-SHABWAMawza BASIN Central Jabal Ayad High Habah Halewah/ Sabatayn Shabwa 10 Arch West Ayad 15°N Amal NIMIR Ayad Block 4 East Ayad ° ° ° ° ° 40 45 50 55 60 ATAQ MARIB-SHABWA BASIN ARABIAN GULF GRABEN Mahfid Uplift 25° GULF OF OMAN Rift Margin Rollover Margin 20° Highs R E D Marib-Shabwa Basin S E A Cretaceous Basin Study 15° Balhaf Area Volcanics ARABIAN SEA Basin Basement Socotra Oil Field 14°N 10° 0 50 Balhaf 0 500 Gas Field Km Salt Dome Km GULF OF ADEN Figure 1: Tectonic framework Marib-Shabwa Basin. Marib-Shabwa Basin, Yemen The Marib-Shabwa Basin can be subdivided into several linked grabens and half-grabens (Figure 1). The gross tectono-stratigraphic development of this basin system has recently been summarised by Redfern and Jones (1995) and Ellis et al. (1996). This paper focuses on a small area in the central part of the basin centred on Nimir Petroleum Company Ayad Ltd.’s (NPC) Block 4 (Figure 1). By concentrating on this small area we provide detail that has not been possible to review in regional studies. We also provide key subsurface seismic and well data which previous authors have been unable to publish. These data will allow the reader to critically evaluate the various models proposed for the development of the Marib-Shabwa Basin. The data also allow new geological insights which may be applicable to the rift system as a whole. The interpretation is based on over 5,000 kilometres (km) of seismic data, 12 exploration wells and 73 development or appraisal wells (Figure 2). DATABASE AND PREVIOUS WORK Modern geological investigation of the area began in the late 1940s when Wetzel and Morton (1948-50) undertook surface mapping of the Shabwa salt domes (Figure 1). These were revisited by teams led by Ziad Beydoun in the 1950s (Beydoun, 1964; Beydoun and Greenwood, 1968). However, detailed understanding of the basin awaited subsurface investigations which did not begin until the late 1970s. Gravity surveys undertaken at that time, suggested the possibility of a thick sedimentary section underlying the Marib-Jawf area. On the basis of the gravity data, oil companies became interested in the area, the first licences were issued and seismic surveying began. At this time Yemen was divided into two separate states, the Yemen Arab Republic (YAR) in the northwest and the People’s Democratic Republic of Yemen (PDRY) in the southeast. Exploration in the YAR was carried out by western companies, led by Hunt Oil whilst Soviet companies explored the Shabwa region of the PDRY. Seismic data confirmed the presence of thick sedimentary basins and their prospectivity was spectacularly confirmed when Hunt discovered the giant Alif field with their first well in 1984. Progress in the PDRY was not so dramatic but by 1988 three small accumulations had been found in the area which is now Block 4 (Schlumberger, 1992). This early phase of drilling confirmed the presence of a deep, extensive Late Jurassic basin system (Paul, 1990). Following unification in 1990, the Shabwa area of the former PDRY was opened to western oil companies and an accelerated exploration drive began. Although this exploration phase, which lasted from 1991 to 1995, did not identify hydrocarbon accumulations on the scale of the Alif field, it did provide a wealth of information which helped to unravel the history of the basin and the region as a whole. The primary data set for this study consists of 1,500 km of 80-fold vibroseis seismic acquired by Nimir in 1991 and 1992, plus three wells drilled in 1992-93 (Sawaqi-1, Sulahfah-1 and North Amal-1; Figure 2). Also available were 3,500 km of Soviet seismic data and 85 exploration and development wells. The Soviet seismic data is significantly inferior to 1990s vintage western data and cannot be used for detailed sequence stratigraphic mapping. Reformatted Soviet logs (equivalent to 1950s vintage western logs; Harrison, 1995) are of considerable value, although they are inferior to recent western data. In addition to the block specific data, traded data from adjacent blocks were used to establish the regional setting. Detailed biostratigraphic and palaeoenvironmental data were obtained from the three Nimir wells and interpretative data were available from Soviet reports for many of the old exploration wells. 1,890 metres (m) of core inherited from the Soviet period were logged and described. 1,149 core plugs from 47 wells were analysed for porosity, permeability, stratigraphy and rock texture information. 630 petrographic thin sections were prepared and described for texture, mineralogy and poroperm characteristics. These data provided the basis for the stratigraphic and sedimentological description of the basin. These also provided the constraints on the well log and seismic facies interpretations. METHODOLOGY The gross tectonic and sequence-stratigraphic framework was based on the interpretation of 1,500 km of post-1990 seismic data using GeoQuest software. Additional Soviet paper data were used to supplement the workstation-based interpretation in key areas. Regional composite seismic lines were used to set the block in its basin-wide context. 11 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/4/1/9/4552973/brannan.pdf by guest on 25 September 2021 on 25 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/4/1/9/4552973/brannan.pdf Brannan etal. Figure 4 (Part) Sawaqi-1 Samad-1 Figure 14 Sulafah-1 Figure 3 West Ayad 72 Wells Ayadim-1 Nimir Data Central Ayad-2 Soviet Data 12 North Amal-1 Central Ayad-2 0 10 Km Amal 5 Wells East Ayad 6 Wells Figure 2: Well and seismic Magraf-1 database map.