25th International Meeting on Organic Geochemistry IMOG 2011

P-292
A geochemical assessment of subsalt tar zones in the Gulf of
Mexico
Erica Morais
Petrobras, Rio de Janeiro, Brazil (corresponding author:ericat@petrobras.com.br)
Gulf of Mexico (GOM) deep water showed to be one
of the most prolific oil production provinces in the
world. Information collected during drilling operations
revealed noteworthy unexpected challenges into
subsalt plays. Among the various challenges found
into subsalt plays is the occurrence of bitumen zones
(here bitumen will be called ―tar‖) that can be
investigated from the geochemical perspective.
Tar zones have been noticed by operating companies
in deep waters of GOM (1). The presence of tar zones
is not apparent on seismic, representing an
unpredictable event this way. Thus it is important
collect information from pre drilled wells and use it in
understanding their theoretical origin. This study is
addressed in helping our understanding regarding the
origin and occurrences of tar zones.
To help understand the controls regarding to tar
origin, detailed geochemical analyses were conducted
in two tar samples. Subsequently, the results were
compared to two oil samples previously collected at
wells from the same block and to immature extracts
adjacent to reservoir intervals.
The two ―tar" samples recovered during the drilling
were submitted to extraction using dichloromethane
as solvent. The extracts then underwent the following
sequence of geochemical analysis: Medium Pressure
Liquid Chromatography, Gas Chromatography, Total
Carbon Isotope Composition and Gas
Chromatography coupled to Mass Spectrometry
(GCMS).
The extraction using dichloromethane recovered
approximately 80% of NSO compounds, and 20% of
hydrocarbons, without solid residual (mineral matrix).
This confirms the idea proposed during the drilling by
which the tar would exist as separate levels either as
layers directly below the salt or filling fault planes. The
gas chromatogram from tar samples showed
contamination by drilling mud apart from light and
heavy n-paraffins. Biomarker GCMS analyses of the
saturated fractions showed a high degree of similarity
among the tar and oil samples (Fig. 1). The samples
display low Ts/Tm ratios and significant quantities of
homomoretanes (C31-C35), which are consistent with
lower maturity from the oils recovered at this portion
of GOM. The source rock of the tar and oil is
apparently the same, possible marine carbonates
from Oxfordian Smackover Formation. The relative
lack of biodegradation indicators may denote that the
tar formation was not associated to biological
processes. The biomarker profiles recovered from the
extracts of immature rocks adjacent to the reservoir
rocks do not show similarity to the tar, suggesting that
immature material was not remobilized to the
reservoir from nearby formations. Isotopic analyses
could not be applied due to high drilling fluid
contamination.
Ts Ts
Tm Tm
H29 H29
H30 H30
Tar sample 1
Tar sample 2
Fig. 1: Fragmentograms m/z 191 from the samples analyze.
Considering the above discussed, the origin of the tar
appears to be a combination of processes over
geologic time. Probably oil trapped into reservoir
layers suffered effects of pressure release during the
fault reactivation that occurred during the salt growth.
This pressure variation could have promoted the
precipitation of NSO compounds formed either at
reservoir forming layers or along the migration
pathways due to differences in permeability.
Although the main limitation of geochemical studies is
that samples are not always available, the application
of geochemical analysis together with a geological
and engineering approach help constrain the fluid
origin, reducing uncertainties associated to each
individual exploration or developing phase and
providing a most reliable assessment.
[1] Weatherl, M.H., 2007. SPE/IADC105619.
H29 H29 H29
Ts Ts Ts
Tm Tm Tm
H28 H28 H28
H30 H30 H30
Immature extract
Well #C
C31-35
Extended
Hopanes
Oil sample
Well #A
Oil sample
Well #B
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