Can Biomarker Ratio Data Be Used Quantitatively to Constrain Thermal Histories? A Case Study from East Kalimantan, Indonesia

Abstract

The determination of an appropriate thermal history in an exploration area is of fundamental importance when attempting to understand the evolution of the petroleum system. In this paper results are presented from a single-well modelling study in which bottom hole temperature data and three different biomarker ratio datasets were available to constrain the modelling. Previous quantitative investigations using biomarker ratios have been hampered by the wide variety of published kinetic parameters for biomarker evolution. Generally, these parameters have been determined either from measurements in the laboratory and extrapolation to the geological setting, or from downhole measurements where the heat flow history is assumed to be known. In the first case serious errors can arise because the heating rate is being extrapolated over many orders of magnitude, while in the second case errors can arise if the assumed heat flow history is incorrect. The utility of biomarker ratio data for constraining the heat flow has been assessed for a well in offshore East Kalimantan. In this case published kinetic parameters do a poor job of constraining the heat flow. In addition, compensation effects enable a wide range of activation energy and frequency factor pairs to produce equally good fits to the downhole data. This makes it difficult to invert downhole data to determine the heat flow history and the biomarker reaction parameters simultaneously. If independent estimates of the activation energy can be made, it is possible to use this information with an inversion procedure to constrain both the heat flow history and the frequency factors for the biomarker reactions. However, the compensation effect outlined in this paper may severely limit the practical, quantitative use of biomarker ratio data to constrain thermal histories.