Characterisation of organic matter formed in hypersaline carbonate/evaporite environments: Hydrocarbon potential and biomarkers obtained through artificial maturation studies

Sediments deposited under hypersaline conditions, particularly those laid d own in the transition zone between marine carbonates and evaporites, are in creasingly recognised as a potential source for oil. Recent environments th at lie in waters with elevated salinities are sites of very high biological productivity that can be used as models for evaporite related sedimentatio n in the geological record. Of particular importance is the range of elevat ed salinities well above the range for normal marine biota, in which organi c-rich cyanobacterial carbonates form and accumulate in large quantities. S uch organic matter collected from a number of modern evaporative settings h as been examined in terms of oil potential and for biomarkers characteristi c of hypersaline environments. The regions studied include marine-fed salin as (Santa Pola, Spain); marine-fed sabkhas (Abu Dhabi, UAE), and continenta l ponds and lakes (La Mancha region, Spain). High values of H/C ratio and H I demonstrate the oil source potential of this organic matter. The hydrocar bons generated during artificial maturation of these immature sediments res emble those naturally, occurring in ancient petroleum-generating evaporitic systems. Variations in the total extracts, saturare, aromatic, resin, and asphaltene fractions evolve to an oil-like composition. Similarly, the dist ributions of n-alkanes, hopanoids, and steroids evolve progressively to tho se typically found in crude oils from evaporitic environments. The most rel evant biomarkers, such as gammacerane, 2,6,10-7-(3-methylbutyl)-dodecane, C -20-isoprenoid thiophenes or chromans characteristic of hypersalinity as pr eviously described in the literature, are not always originally present in the environments studied, and their distribution can be affected by both ma turation and the mineral matrix. Therefore caution should be exercised when using these biomarkers to assess ancient environments of deposition in ter ms of salinity.