Abstract

In 1992, when this project began, the stratigraphy of the Early Miocene deltaic sediments of the Kutei Basin was poorly understood. Previous exploration had identified the major structural fabric of the area and that the Early Miocene deltaic sediments were in excess of 5000 m in thickness. Outcrops are dominated by sandstone and limestone and their position is structurally controlled, mostly occurring in the cores of major folds. Most of the sediments are poor in age-diagnostic foraminifera and nannofossils, and it was not possible to use planktonic biostratigraphy to establish a usable, sufficiently detailed stratigraphic zonation scheme.

This paper presents the results of a study to determine the stratigraphy of these sediments prior to drilling. This has been achieved by synthesizing outcrop, biostratigraphic and seismic data to subdivide the Lower Miocene into a number of depositional sequences. Several improvements over previous 'pre-drilling methods' were implemented as follows : 1) routine lithology logging and sampling of seismic shot-holes enabled the construction of true-thickness lithological columns for chosen seismic lines, 2) combining lithological and palaeontological data enabled the construction of palaeoenvironmental logs for each of these columns, 3) a local, high-resolution palynological zonation scheme was established by detailed analysis of shot-hole samples.

Using these approaches, flooding surfaces were identified and used to divide the lithological columns into depositional sequences which were tied to the seismic and correlated locally within each syncline. Palynology was used, with high precision, to correlate these local schemes across major structural discontinuities enabling the stratigraphy and evolution of the Early Miocene Delta complex to be understood in detail.

The methodology employed is outlined in this paper together with a detailed example including the construction of a true-thickness lithological column from seismic shot-hole data, a palaeoenvironmental log, and a discussion of the main criteria used to produce the detailed local palynological zonation scheme. The utility of the approach is illustrated with examples of a strike correlation over 40 km, and a dip correlation over 50 km which show clear evidence of west to east progradation of the delta and also that depocenters can be identified for particular sequences. The establishment of this detailed understanding of the local stratigraphy has provided a valuable tool with which to high-grade exploration targets prior to drilling.