Abstract

Compared to good source rocks, marginal source rocks have questionable potential to provide an adequate charge of liquid hydrocarbons. Certain basins of Indonesia, Southeast Asia, Australia, and other regions contain large volumes of continental, deltaic, paralic, nearshore marine and shallow lacustrine deposits. These sediments usually contain Type II/III and III kerogen, which consists of variable mixtures and types of oil- and gas-prone, and inert organic matter. Although traditional source rock analyses suggest most of these sections are gas-prone, some have sufficient petroleum potential. Two factors control the liquid hydrocarbon potential of these marginal source rocks: (1) their ability to generate adequate amounts of liquid hydrocarbons, and (2) expulsion efficiency. Accurate identification of oil- and gas-prone facies in marginal source intervals is important to define hydrocarbon charge and oil/source relationships. An integrated geological and geochemical evaluation is essential to effectively define the oil- versus gas-prone potential of marginal source rocks.

Quantitative pyrolysis-gas chromatography and pyrolysis gas chromatography-mass spectrometry helps unravel the wide variation in origin, organic precursors, generated products, and hydrocarbon generative potential of kerogens associated with marginal source rocks. Pyrolysis - GC parameters further define the gas versus oil generative capacity, and the quantity and distribution of different hydrocarbon components generated from Type III and II/III kerogens. The large variation in the gas/oil proneness and other quantitative pyrolysis-GC parameters of Type II/III and III kerogens indicate variable source potential for these marginal source rocks.

Detailed maceral analysis integrated with bulk geochemical and pyrolysis-GC results provide a comprehensive evaluation of source rock kerogen. This multi-level integrated approach provides a refined evaluation of the petroleum potential for kerogen traditionally considered predominantly gas- prone.

Step-wise pyrolysis of isolated kerogens in sealed tubes is a useful technique to define oil/source relationships. Comparison of the bulk and biomarker composition of artificially generated and naturally occurring oils help identify specific source facies in a marginal source rock sequence.

Expulsion efficiency is a critical factor in quantifying the potential of marginal source rocks to provide an adequate hydrocarbon charge. The composition and distribution of oil-prone organic matter, composition of generated products, rock matrix properties, and kinetics and timing of generation affect expulsion characteristics. Expulsion efficiency in marginal source rocks can be highly variable due to the amount of hydrocarbon generation and adsorptive capacity of certain organic matter assemblages.