Origin of Beach-Stranded Tars from Source Rocks Indigenous to Seychelles

Abstract

Tarballs have been found stranded as traction deposits on a beach on northwest Mahe Island, Seychelles, with every ebbing tide. Weekly collections from mid-1993 to mid-1994 gathered nearly 4750 samples of two types of tar: a dominant black malleable tar and a less common brownish-black hard tar. Both tars represent moderately degraded extracts of crude oils seemingly derived from mature source rocks dominated by a combination of oil-prone phytoplanktonic and algal organic matter over gas-prone land-plant organic matter. Source rock deposition occurred principally in paralic to open-marine environments. The presence of minor oleanane and bicadinane, and a general preponderance of diasteranes and tricyclic terpanes in the hard tar suggest derivation from a Late Cretaceous c astic source rock with possible carbonate interbeds. By contrast, no samples of the malleable tar contain oleanane or bicadinane, but all contain significant norhopane and tetracyclic terpanes, which, along with significant sulfur and low diasterane contents, indicate derivation from a carbonate source rock of perhaps pre-Late Cretaceous age. Both these tars differ markedly from a tar that is stranded only on Coetivy Island that contains minor amounts of steranes, oleanane, bicadinane, and botryococcane, but has high pristane and tricyclic terpane contents, suggesting derivation from a Late Cretaceous clastic deltaic source rock. All three tar types can be related to depositional environments that developed during the multiphase rift-drift origin of the Seychelles microcontinent from eas Africa, Madagascar, and India. Additionally, maturity and limited geochemical data from proven and potential source rocks present in the offshore wells enable the malleable and hard tars to be equated with specific units in the Seychelles stratigraphy; the malleable tar is seemingly derived from a Middle Jurassic carbonate of Tethyan affinity and the hard tar possibly originated from either Campanian or Maastrichtian-Paleocene shales. These tars are thus concluded to be natural seepage products derived from source rocks indigenous to Seychelles, and their persistent beach stranding significantly reduces the risk of hydrocarbon exploration in this frontier province.