The Eocene Rusayl Formation, Oman, carbonaceous rocks in calcareous shelf sediments: Environment of deposition, alteration and hydrocarbon potential

During a subsequent stage II, an increase in the temperature of alteration was partly induced by burial and a high heat flow from the underlying Semail Ophiolite. Type-III kerogen originating from higher plants and, in addition, some marine biota gave rise to the generation of small amounts of soluble organic matter during this stage of diagenesis. The average reflectance of humic particles marks the beginning of the oil window and the production index reveals the existence of free hydrocarbons. Further uplift of the Eocene strata and oxidation during stage IIII caused veins of satin spar to form from organic sulfur and pyrite in the carbonaceous material. Lowering of the pH value of the pore fluid led to the precipitation of jarosite and a set of hydrated aluminum sulfates dependant upon the cations present in the wall rocks. AMD minerals (= acid mine drainage) are not very widespread in this carbonaceous series intercalated among calcareous rocks owing to the buffering effect of carbonate minerals. These carbonate-hosted carbonaceous rocks are below an economic level as far as the mining of coal is concerned, but deserves particular attention as source rocks for hydrocarbons in the Middle East, provided a higher stage of maturity is reached.