ORIGIN OF BEACH-STRANDED TARS FROM SOURCE ROCKS INDIGENOUS TO SEYCHELLES

Tarballs have been found stranded as traction deposits on a beach on n orthwest Mahe Island, Seychelles, with every ebbing tide, Weekly colle ctions from mid-1993 to mid-1994 gathered nearly 4750 samples of two t ypes of tar: a dominant black malleable tar and a less common brownish -black hard tar, Both tars represent moderately degraded extracts of c rude oils seemingly derived from mature source rocks dominated by a co mbination of oil-prone phytoplanktonic and algal organic matter over g as-prone land-plant organic matter, Source rock deposition occurred pr incipally in paralic to open-marine environments. The presence of mino r oleanane and bicadinane, and a general preponderance of diasteranes and tricyclic terpanes in the hard tar suggest derivation from a Late Cretaceous elastic source rock with possible carbonate interbeds, By c ontrast, no samples of the malleable tar contain oleanane or bicadinan e, but all contain significant norhopane and tetracyclic terpanes, whi ch, along with significant sulfur and low diasterane contents, indicat e derivation from a carbonate source rock of perhaps pre-late Cretaceo us age, Both these tars differ markedly from a tar that is stranded on ly on Coetivy Island that contains minor amounts of steranes, oleanane , bicadinane, and botryococcane, but has high pristane and tricyclic t erpane contents, suggesting derivation from a Late Cretaceous elastic deltaic source rock, All three tar types can be related to depositiona l environments that developed during the multiphase rift-drift origin of the Seychelles microcontinent from east Africa, Madagascar, and Ind ia. Additionally, maturity and limited geochemical data from proven an d potential source rocks present in the offshore wells enable the mall eable and hard tars to be equated with specific units in the Seychelle s stratigraphy; the malleable tar is seemingly derived from a Middle J urassic carbonate of Tethyan affinity and the hard tar possibly origin ated from either Campanian or Maastrichtian-Paleocene shales. These ta rs are thus concluded to be natural seepage products derived from sour ce rocks indigenous to Seychelles, and their persistent beach strandin g significantly reduces the risk of hydrocarbon exploration in this fr ontier province.