ABSTRACT

In order to determine the composition of brines derived from seawater, studies were carried out on fluid inclusions in evaporites from recent coastal sabkha (halite and gypsum) and Miocene gypsum from the Gulf of Suez. The types of inclusions observed in the studied samples were mainly aqueous-brine, and hydrocarbon bearing inclusions. At room temperature, both types of inclusion occur in monophase, two-phase (L+V), and poly-phase (L+V+S) form. They exist in association and are distributed as primary inclusions throughout the growth zones of chevron and hopper structure of halite, as well as along the growth zones or short lines in gypsum. Hydrocarbon bearing inclusions are dominant in both recent sabkha and Miocene gypsum. No differences were observed between the types and composition of fluid inclusions in gypsum from the recent sabkha and Miocene. Melting behavior and eutectic temperature (Te between –28°C and –33°C) of halite indicated that the salt system was composed mainly of Na ± Ca and/or Mg. In gypsum, eutectic temperature between –47 to – 52 °C were consistent to pure Ca ± Mg chlorides. Salinity, calculated on the basis of final melting temperature of ice (Tmice), hydrohalite melting temperature, and dissolution temperature of daughter crystals, ranged from 22 to 30 wt % NaCl eq., in halite, and from 19 to 26 wt % NaCl eq. in gypsum. The total homogenization temperature (Th) of aqueous – brine inclusions was measured for halite at temperatures between 25°C and 55°C, and for gypsum from 35°C to 75°C. The homogenization temperature of halite (25-55 °C) may represent the surface water temperature, while its range may reflect daily or seasonal variations in temperature. IR investigation confirmed the presence of hydrocabon in the studied samples. Microthermometric measurements of hydrocarbon-bearing inclusions indicated that methane and/or nitrogen are the main organic compounds in the studied samples. Total homogenization temperatures were achieved between 80 and 130 °C.
The presence of hydrocarbon-bearing inclusions in the studied samples could be attributed to the existence of oil fields in the Gulf of Suez, possibly a source of organic compounds in the recent Ras Shukeir, and Gebel El-Zeit Middle Miocene evaporites.