Al Jawf Research Articles

A tectono-stratigraphic framework for Yemen and its implications for hydrocarbon potential

The Late Jurassic Marib Al Jawf Basin is part of a rift system extending southeastwards into the less well known Wadi Hajar Basin. This rift system may be one arm of a Late Jurassic failed triple rift which also included the Masilah Basin to the east, and the Sir Basin to the north. Late Jurassic rifting is shown to be diachronous, younging eastwards. Renewed rifting during the Early to Mid Cretaceous was accompanied by deposition of the newly recognized Furt Formation. This second rift phase initiated movement on the N-S striking 47 degrees E Fault, which probably extends southwards into Somalia. Extension in the Jeza-Howarime Rift System may also have commenced at this time. Oman Mountain orogenesis led to renewed subsidence in the east, and was followed by polyphase Tertiary rifting, inversion and localized gravity-driven extension on the continental margins. As a result of these events, a number of distinct petroleum systems developed.

Jabali, a Zn-Pb-(Ag) carbonate-hosted deposit associated with Late Jurassic rifting in Yemen

The Jabali deposit (3.8 Mt at 16% Zn, 2% Pb and 132 g/t Ag) is hosted by dolomitized platform carbonates of Kimmeridgian age at the southwestern edge of the oil-producing Wadi al Jawf rift basin in northern Yemen. Paleogeographical reconstructions demonstrate that tensional synsedimentary tectonic activity from the Late Jurassic to Early Cretaceous was responsible for the thick accumulation of argillaceous and evaporitic sediments in the subsident rift basin, the unstable margin of which was the site of rapid facies changes, local disconformities and periods of emergence, as well as of dolomitization along the WNW- and NNW-striking boundary fault system. In the Jabali area, the upper part of the Jurassic sequence underwent two stages of dolomitization before emergence and deep karstic erosion. Solution cavities and depressions in the eroded surface were filled by dolomite sand and black pyritic mudstone prior to a last marine transgression of limited extent. Subsequent ore deposition and associated late dolomitization sealed the network of solution cavities, impregnating the dolomite sands and the host dolomites. Sphalerite I and wurtzite, followed by silver-bearing zoned sphalerite II associated with galena, crystallized from a cyclic influx of low-temperature (75-100°C) saline solutions. Lead isotope geochemistry indicates that the lead, zinc and silver probably originated from an Early Proterozoic basement. The dissolved metals were likely derived from the basal aquifer (detrital material of basement origin) of the evaporite-bearing sequence filling the Wadi al Jawf trough. Migrating metalliferous brines from the basin to the uplifted Jabali area, where ore deposition was favoured by a reducing environment, were probably channelled by the boundary fault system during the last stages of synsedimentary tectonic activity.