Abstract

In deltaic petroleum systems there is often no shortage of potential source rocks but it can be difficult to work out which facies contain the effective source rocks. An oil, being by definition evidence of the latter, contains information about the key characteristics of that source. This paper shows how the distribution of oleananes and bicadinanes - land plant biomarkers abundant in many Southeast Asian and Australasian oils - can be used to better define the maturity and depositional environment of the effective source rocks.

Although oleananes are ubiquitous in oils and sediments of the region they are actually minor products of the diagenesis of terrestrial plant matter. As such, their abundance is highly sensitive to conditions during early diagenesis, especially the degree of marine influence, and their abundance can increase rather than decrease with distance from the paleoshoreline, e.g., in the South Sumatra Basin. Conversely, A-ring contracted oleananes result from diagenesis of plant matter under peat swamp conditions and their abundance decreases (with respect to oleanane) with increasing marine influence. Taken together, these observations suggest a means of distinguishing between source rocks laid down in flood-plain, paralic and marine deltaic settings. This concept is supported by data for a large set of oils from across Asia and Australasia.

Maturity estimation based on saturated or aromatic biomarkers can be misleading in deltaic depositional settings due to diagenetic interferences. Some new maturity parameters based on bicadinane isomers are less subject to diagenetic effects, continue to change into the oil window and are extremely resistant to biodegradation.