Abstract

In many areas of the world and in particular parts of China and S.E. Asia, non–marine sediments have proved to be significant sources of liquid hydrocarbons. In order to understand what geochemical characteristics these sources may impart to oils (so that, conversely, source type may be recognised from oil composition) we have undertaken geochemical analysis of a suite of samples from an exploration well from offshore China. This followed comprehensive palynological examination of the same well to determine the biological inputs to, and depositional environments of, the accumulated kerogen. By integrating geochemistry into other earth seciences in this way, our ability to decipher geochemical information in isolation (i.e. oils) has been gretly enhanced.

Sediments examined in this case study are Eocene and Miocene in age, overlying a pre-Tertiary basement; they were deposited in predominantly lacustrine settings with increasing fluviatile influence towards the top the sequence. Examination of the palynomorth assemblage enabled the sequence to be divided into five biozones, with differences between them related to vegetational and climatic changes affecting source rock character.

Geochemical results (from pyrolysis–GC of kerogens and GC and GCMS analysis of sediment extracts) also reflect these differences, though the information obtained is somewhat less specific. Even so, there is an encouraging match between the interpretation of source rock type derived in particular from the distribution of biological marker compounds and that coming from microscopic analysis; this is especially true in the middle biozone (which results from freshwater algal debris accumulating in a shallow, periodically drying out lake surrounded by an arid hinterland) and in the two lowest biozones (in which the kerogen is of mixed algal and higher plant origin).