Reactions of Organic Matter in the Sedimentary System

Introduction

We are currently developing new and deeper insights into the factors controlling the composition of sedimentary organic matter. Ultimately, these developments should lead to new tools that will be utilised in petroleum exploration.

Modelling the sedimentary reaction system: Maturity

Maturity, in a general sense, reflects the extent to which chemical reactions have occurred in the sediments. We have studied aromatic compounds and their reactions in sediments in great detail and have developed a model for a sedimentary reaction system, which satisfactorily explains the distributions of many methylated aromatic hydrocarbons, in terms of the extent of the chemical reactions that they have undergone. In other words, the model describes maturity.

In this study, we examined in great detail the composition of extractable organic matter from a sedimentary sequence from the Paqualin-1 well in the Vulcan Sub-Basin, including both saturated and aromatic hydrocarbons. The variations with depth in the relative abundances of these compounds are by definition determined by the same reaction conditions. It is expected that a model can be constructed, based on our understanding of the distribution of methylated naphthalenes, to correlate the relative abundances of all components of sedimentary organic matter.

Aromatisation and hydrogenation reactions of cyclic compounds

Most geological samples contain tetralin and decalin, which are bicyclic monoaromatic and saturated hydrocarbons. In addition, most samples also contain a suite of alkylated derivatives, such as monomethyl, dimethyl and trimethyl tetralins and decalins, as well as several isomers within each class. The obvious structural similarities with methylated naphthalenes suggest that naphthalenes, tetralins and decalins are related through hydrogenation-dehydrogenation processes. In this project, we study the occurrence and formation of tetralins and decalins and their possible genetic relationship with naphthalenes through hydrogenation / aromatisation reactions.

Trans-methylation reactions in the formation of aromatic hydrocarbons

Sedimentary reactions, including trans-methylation, are currently being investigated. Sedimentary methylation processes have been proposed to account for the occurrence of numerous methyl substituted aromatic hydrocarbons which have carbon skeletons that are not obviously derived from common natural product precursors. Compounds such as alkylbenzenes, alkylnaphthalenes and alkylphenanthrenes are likely to result from methylation reactions involving all these compounds. In this study, relations between the relative abundances of methylated aromatic hydrocarbons in both crude oils and sediments are being explored. Once relationships are established, in-reservoir factors that affect these, such as biodegradation, mixing, water-washing, evaporitic fractionation (PVTX) and temperature, can be more accurately assessed.