Thursday 16th July, a talk by Yannick Santerre: Evolution of a carbonate-evaporite system: Example of the Eocene Facha reservoir & Hon seal from the Dahab field in the Zallah Trough of the Sirte Basin (Libya)

Thursday 16th July, conveniently placed between football matches, after Wimbledon and hopefully after the latest heatwave, we’ll have a cool evening in the RCA, Conwy, with weather-suitable refreshments and an excellent talk delivered by Yannick Santerre:
Evolution of a carbonate-evaporite system: Example of the Eocene Facha reservoir & Hon seal from the Dahab field in the Zallah Trough of the Sirte Basin (Libya)
Abstract: The Eocene (Ypresian) Facha dolomite reservoir and Hon evaporite seal from the Dahab field (Zallah Trough, western Sirte Basin, Libya) recorded the evolution of a marginal-marine carbonate-evaporite system within a subsiding rift graben. The Facha reservoir is made of seven stacked dolomite sub-units (Facha A-F) delimited by continuous to subcontinuous anhydrite layers, forming a compartmentalised “layer-cake” architecture. Performed petrographic analysis reveals lagoonal microfacies assemblages dominated by dasycladacean algae, miliolids and red algae. Established paragenesis indicates important early replacements by fine dolomicrite and anhydrite, as well as intense early leaching, opening many small mouldic and vuggy pores. A major upward evolution of the sedimentary system could have been recognised: the lower part of the Upper Facha exhibits sabkha-like cycles (nodular anhydrites, thin caps, root bioturbations, common subaerial exposures) acting as baffles, whereas the upper part of the Upper Facha displays more salina-like cycles (massive subaqueous anhydrite seals). This contrast and the observed vertical sedimentary evolution reflect a progressive deepening of the Zallah Trough mini-basin throughout the Ypresian. The Facha-Hon succession is interpreted as one long-term (2nd-order) transgressive-regressive sequence, pseudo-analogous to the Arab-Hith formations of the Middle East but deposited under hothouse climatic conditions and with a marine transgressive long-term trajectory. Stacked high-frequency parasequences (4th/5th-order) recorded the marine overflow periods, when sea water level crossed the natural hydrographic barriers (horsts), leading to the filling of the Zallah graben area. This study establishes a tectono-eustatic origin for the studied Ypresian sediments of the Zallah Trough, integrating its main reservoir: the Facha Dolomite Member