Evaluating kerogen type according to source quality, compositional heterogeneity and thermal lability

Abstract

Kerogen typing is today an established and routine tool for high-grading petroleum exploration acreage, especially in terms of perceived oil-prone versus gas-prone nature. Source quality and thermal lability are essential components of these assessments but are not incorporated into routine kerogen typing per se. This article shows how these criteria are readily determinable. A suite of kerogen analyses illustrate the point, using rocks from mainly fluviodeltaic-lacustrine settings. One-step pyrolysis-gas chromatography (Py-GC) and micro-scale sealed vessel (MSSV) pyrolysis provide the crucial information on source quality in the form of chain length distribution, which not only adds new degrees of resolution to this aspect of source evaluation but also provides an essential link between gross petroleum composition (e.g. gas to oil ratio) and kerogen pyrolysate composition. Two-step Py-GC and MSSV pyrolysis allow kerogens to be further differentiated according to the relative ease with which they generate petroleum during progressive maturation. One compositional inference in the latter regard is that some kerogens generate petroleums with a fixed C 2+ n-alkyl composition throughout catagenesis and possibly metagenesis. Accordingly some overmature kerogen pyrolysates reveal the once prolific oil-prone nature of earlier kerogen histories.