25th International Meeting on Organic Geochemistry IMOG 2011

P-311
High water pressure induced combination reactions: a new
mechanistic route for post-oil bitumen formation in deep
petroleum basins
Clement Uguna 1 , Will Meredith 1 , Colin Snape 1 , Andrew Carr 2 , Gareth Harriman 3
1 University of Nottingham, Nottingham, United Kingdom, 2 Advanced Geochemical Systems Ltd,
Loughborough, United Kingdom, 3 GHGeochem Ltd, Liverpool, United Kingdom (corresponding
author:colin.snape@nottingham.ac.uk)
Asphaltenic tars and bitumens are common in both
carbonate and clastic reservoirs, occurring either as
droplets, grain coats lining the reservoir pores, or
entire layers. The presence of tars and bitumens can
reduce field productivity, cause significant problems
during drilling operations, and in basins such as
deepwater Gulf of Mexico can also add significantly to
the cost. Currently the formation mechanisms for
bitumens and tars include gas deasphalting,
biodegradation, oil cracking, thermochemical sulphate
reduction and oil mixing. However, in the past
decade numerous wells in the Gulf of Mexico have
encountered significant amounts of anomalous high
asphaltene (~70%) tars, apparently generated from
low maturity carbonate sourced bitumens and oils
with much lower asphaltene (~5%) contents.
However, none of the current formation mechanisms
can account for these tars. This paper provides
evidence from bound biomarker profiles for a Gulf of
Mexico bitumen and high pressure water hydrous
pyrolysis experiments at pressures up to 1000 bar
that bitumen formation arises from high pressure
mediated exothermic combination reactions.
Our extensive investigations using hydropyrolysis
(hypy) to characterise oil and bitumen asphaltenes, in
relation to fingerprinting heavily biodegraded oils,
have established that the bound biomarkers are
considerably less mature than their free counterparts
(Sonibare et al., 2009). Figure 1 compares the
sterane profies for the free and bound phases in a
Gulf of Mexico tar with the same maturity being
displayed and diasteranes present in both fractions.
The only explanation for this trend is the free steranes
in the precursor oil being incorporared into the
bitumen by pressure-induced combination reactions.
It has been established that water retards oil cracking
and this trend can potentially be magnified by high
pressure. Indeed, our preliminary work indicated that
when oil cracking is conducted in pressurised liquid
water, significant retardation occurs at 500 bar
compared to normal hydrous conditions at much
lower steam pressures. The investigation has been
extended to 1000 bar and it has been found that the
disappearance of light hydrocarbons (