Application of tetracyclic polyprenoids as indicators of input from fresh-brackish water environments

摘要

C-30 tetracyclic polyprenoids (TPP) are most prominently observed in samples derived from low salinity, i.e. fresh to brackish lacustrine environments, and are generally present in low levels in samples derived from saline, i.e., marine and saline lacustrine, environments. A near-shore facies of the Chonta Formation, Peru, that has no marine palynomorphs but abundant Chlorococcalean (Green) algal nonmarine palynomorphs, has high levels of TPP, suggesting Green algae (or Chlorophyta) are a possible source for the TPP compounds. The ratio between a C-30 TPP compound and 27-norcholestanes is useful for assessing this nonmarine algal input. Moderate elevations of TPP, above what is common in marine derived samples, were found in ostensibly marine source rocks and oils from certain basins of western and northern South America (Middle Magdalena, Colombia; Maracaibo, Venezuela; and Trinidad basins). This is likely due to transport from the nonmarine to the marine environment because of an influx of fresh water into the near-shore marine environment. Alternatively, oils from these basins may have inputs from near-shore shallow marine algae with chemistry similar to that found in lacustrine settings. The TPP ratio, in conjunction with other environmental indicators such as 4-methyl steranes or hopane/sterane ratio, is useful for differentiating marine and nonmarine influences in pre-salt oils and source rocks of West Africa. The TPP ratio, used with other environmental indicators (gammacerane, C-29 steranes, C-30 steranes) and age diagnostic biomarkers (dinosteranes, 4-methylsteranes), can be useful in differentiating among notimarine source facies. For example, in the Turpan-Hami basin, China, Permian saline lacustrine and Jurassic lacustrine deltaic facies can be discriminated. (C) 2002 Elsevier Science Ltd. All rights reserved. [References: 81]