Geological Evolution of the Central Marib-Shabwa Basin, Yemen

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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.