Abstract

Carbon isotopes have been used extensively in source rock environmental interpretations and to differentiate between oils of marine and non-marine origin. This has proven less than successful because carbon isotope genetic significance is incompletely understood.

A large volume of literature exists on marine oils but few studies have been published for non-marine sourced oils or lacustrine source rocks. A better understanding of the genetic significance of carbon isotopes is important because of the increasing recognition of the worldwide significance of non-marine sourced oils.

We review δ¹³C values of 174 lacustrine sourced oils and 109 lacustrine source rocks from 23 rift basins in Southeast Asia and Southern China. Mechanisms of isotopic fractionation within lacustrine systems are reviewed and integrated with known depositional conditions where possible. Specific examples illustrate the δ¹³C variations in time and space relative to potential fractionation mechanisms.

Dominant factors affecting isotopic fractionation in lacustrine environments are availability of CO₂, source of carbon (atmospheric CO₂ vs recycled organic CO₂), and photosynthetic mechanism. Depositional environments (shallow vs deep) and variables such as salinity, temperature and pHB are complexly interrelated in the fractionation process.

Botryococcus, an important alga in Tertiary lacustrine systems, is show to impart a specific isotopic signature.

Carbon isotope data, combined with other geochemical data and integrated with geology, provides an improved basis for understanding the depositional environment.