Journal of Geochemical Exploration 78 – 79 (2003) 297 – 300
www.elsevier.com/locate/jgeoexp
Abstract
Fluid escape from reservoirs: implications from
cold seeps, fractures and injected sands
Part II. The fluids involved
R. Jonk *, A. Mazzini, D. Duranti, J. Parnell, B. Cronin, A. Hurst
Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE, UK
Abstract
Petroleum fluids escape from hydrocarbon reservoirs through permeable networks of fractures, injected sands and by
seepage through low permeability host rocks. Carbon stable isotope analysis of carbonate cement associated with such
structures shows that the escaping petroleum fluid is intimately involved in precipitating carbonate cement. Within fractures and
injected sands, oxidation of chained hydrocarbons supplies bicarbonate to the co-existing aqueous solution from which
carbonate precipitates (y13C around 20xV-PDB). y13C values within carbonate crusts associated with seeps are lower (as
low as
50xV-PDB) suggesting a component of carbon derived from methane within these structures. This suggests a
separation within the escaping petroleum fluids, with chained hydrocarbons remaining trapped within sand injectites and
fractures, whereas more buoyant fluids (methane) continue to escape through low permeability host rocks. The abundance of
fluorescing (chained) hydrocarbon inclusions within cement associated with fractures and injected sands versus the scarce
occurrence of fluorescing inclusions within carbonate crusts associated with seeps is in agreement with results from stable
isotope analysis. Oxygen stable isotope ratios indicate that carbonate cement within fractures and sand injectites precipitates at
temperatures between 30 and 50 jC in the subsurface, whereas carbonate cement associated with seeps at the seafloor
precipitates at temperatures around 0 jC.
D 2003 Elsevier Science B.V. All rights reserved.
Keywords: Injected sandstones; Cold seeps; Carbonate cement; C/O stable isotopes
1. Introduction
Hydrocarbon reservoirs in the North Sea exhibit a
close association between fractures, sand injectites
and seeps, the combined network of which has a
significant impact on fluid escape from reservoirs
* Corresponding author. Tel.: +44-1224-273435; fax: +441224-272785.
E-mail address: r.jonk@abdn.ac.uk (R. Jonk).
and aquifers (Mazzini et al., 2003). A link between
hydrocarbon reservoirs and seeps has in several
instances been suggested. Examples of released reservoir gases include hydrocarbon seeps from the Gulf
of Mexico (Roberts and Aharon, 1994), Southeastern
Mediterranean (Coleman and Ballard, 2001), North
Sea (e.g. UK block 15/25, Judd et al., 1994) and
Southern and Eastern Skagerrak (Hovland, 1991). As
with cold seeps, injected sandstones are observed
above numerous hydrocarbon reservoirs (Jenssen et
al., 1993; Dixon et al., 1995; Duranti et al., 2002). The
0375-6742/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0375-6742(03)00048-7
298
Abstract
role of reservoir fluids in activating injections and
subsequently using injected sands as fluid flow pathways has, to our knowledge, never been investigated.
In outcrop, injected sands and cold seeps can be
spatially related and a possible common origin of
the two phenomena is inferred (Schwartz et al.,
2001; Weberling et al., 2001). In the present study,
we focus on escape of petroleum fluids through fluid
escape systems as recorded in carbonate cements
observed in the samples described in Part I (Mazzini
et al., 2003).
2. Methods of study
Carbon and oxygen isotopic analysis was undertaken at the SUERC facility in East Kilbride using an
AP2003 triple-collector mass spectrometer. The reproducibility of the system is F 0.1xfor both y13C and
y18O values. All data are reported as per mil deviation
from the V-PDB international standard. Samples for
fluid inclusion analysis of carbonate cements were
prepared as doubly polished wafers and examined
using a Linkam THM600 heating – freezing stage
attached to a Nikon Optiphot2-POL microscope. Fluorescence of the fluid inclusions under ultraviolet
light (illumination source 435 nm) was investigated
using a Nikon Eclipse E600 microscope with a UV2A filter block.
3. Results
3.1. Isotopes
Fig. 1 shows the results of the oxygen and carbon
stable isotope analysis of carbonate cements associated with fractures, injected sandstones and seeps
encountered within Tertiary to Recent sediments
above North Sea hydrocarbon reservoirs. Isotopic
results show three main groups of cold seeps. The
first group includes the samples from modern cold
seeps retrieved from the seafloor (1x< y18O < 3x
)
and very low y13C values ( 50x< y13C < 39x
);
the second group are samples from modern seeps
formed in the subsurface, with similar positive y18O
values (mean value 2x
) but less depleted y13C values
(mean value-20x
). Samples from paleo-seeps (group
3) have y18O values varying from 1.5xto 5–
5x
, and y13C values from 8xto 35x
. A clear
trend can be detected, with less depleted y13C values
corresponding with more depleted y18O values (Fig.
1). Injected sands have consistent negative y18O
values ( 11x< y18 O < 3x
) and y13C values
Fig. 1. C/O stable isotope values of carbonate cemented seeps, injected sandstones and fractures. See text for explanation.
Abstract
around
20x
. The fractures analysed show two
clusters: both groups have similar y18 O values
( 10x< y18O < 4xPDB), but reveal two distinct groups with negative and positive y13C values.
299
3.2. Fluid inclusions
Hydrocarbon inclusions can be detected using UV
fluorescence. All samples analysed reveal the pres-
Fig. 2. Plane light (left) and corresponding fluorescence (right) pictures of the three carbonate cemented features analysed. Width of all pictures
1 mm. (A and B) Yellow fluorescing hydrocarbon inclusions in a cold seep sample. (C and D) Abundant yellow fluorescing hydrocarbon
inclusions within carbonate cement in an injected sandstone. (E and F) Alternating zones of blue and yellow fluorescing inclusions trapped
along growth zones within a fibrous calcite vein.
300
Abstract
ence of fluorescing primary hydrocarbon inclusions
within carbonate cement (Fig. 2). Cold seep samples
reveal the presence of rare fluorescing inclusions at
inter and intra-crystal sites. Raman spectroscopy confirms the presence of hydrocarbon chains, including
propane, methane and ethane within fluid inclusions
(Mazzini et al., 2002). Within cemented injected
sandstones and fractures, hydrocarbon inclusions are
often trapped abundantly with co-existing aqueous
inclusions as two immiscible fluid phases. In samples
retrieved from the same locations, the fluorescing
colour of hydrocarbons within reservoir units, sand
injectites and cold seeps is similar, suggesting that all
structures together are responsible for fluid escape
from the reservoir. In most cases, the fluid inclusions
are monophase, all-liquid inclusions, suggesting trapping below 50 jC. Elevated temperatures within
certain deeper buried sections are most likely due to
continuous recrystallisation of carbonate upon burial.
4. Discussion and conclusion
The extreme 13C depletion observed within the
cold seep samples indicates a strong contribution of
carbon from hydrocarbon gases (methane). The depletion within injected sands and cemented fractures is
less and can be explained by bicarbonate derived from
oxidation of long-chained hydrocarbons. This is in
agreement with the observation of numerous fluorescing inclusions (chained hydrocarbons) within injected
sands, whereas they are relatively scarce within cold
seeps. Fractures show both an extreme positive outlier
(fermentation?) and a negative outlier (oxidation?).
The lower y18O values within fractures and injectites
is most likely related to the slightly elevated temperatures of the subsurface fluids (30 – 50 jC) with
respect to the temperatures recorded in cold seeps
(close to 0 jC). The isotopic trend within the cold
seep samples could be due to replacement of methanederived aragonite at the surface by calcite derived
from seawater (causing a shift to lower y18O values
and less negative y13C values) upon burial.
References
Coleman, D.F., Ballard, R.D., 2001. A highly concentrated region
of cold hydrocarbon seeps in the Southeastern Mediterranean
Sea. Geo-Marine Letters 21 (3), 162 – 167.
Dixon, R.J., Schofield, K., Anderton, R., Reynolds, A.D., 1995.
Sandstone diapirism and clastic intrusion in the Tertiary submarine fans of the Bruce – Beryl Embayment, Quadrant 9, UKCS.
In: Hartley, A.J., Prosser, D.J. (Eds.), Characterization of Deep
Marine Clastic Systems. Special Publication-Geological Society
of London, vol. 94, pp. 77 – 94.
Duranti, D., Hurst, A., Bell, C., Groves, S., Hanson, R., 2002.
Injected and remobilized Eocene sandstones form the Alba
Field, UKCS: care and wireline log characteristics. Petroleum
Geoscience 8, 99 – 107.
Hovland, M., 1991. Large pockmarks, gas-charged sediments and
possible clay diapirs in the Skagerrak. Marine and Petroleum
Geology 8 (3), 311 – 316.
Jenssen, A.I., Bergslien, D., Rye-Larsen, M., Lindholm, R.M.,
1993. Origin of complex mound geometry of Paleocene submarine-fan sandstone reservoirs, Balder Field, Norway. In:
Parker, J.R. (Ed.), Petroleum Geology of Northwest Europe:
Proceedings of the 4th Conference, pp. 135 – 143.
Judd, A., Long, D., Sanley, M., 1994. Pockmark formation and
activity, U.K. block 15/25, North Sea. Bulletin of the Geological
Society of Denmark 41, 34 – 49.
Mazzini, A., Li, R., Parnell, J., 2002. Spectroscopic methods for
analysing organic compounds in fluid inclusions during planetary exploration. Lunar and Planetary Science XXXIII, 1645.
Mazzini, A., Jonk, R., Duranti, D., Parnell, J., Cronin, B.T., Hurst,
A., 2003. Fluid escape from reservoirs: implications from cold
seeps, fractures and injected sands: Part I. The fluid flow system. Journal of Geochemical Exploration 78/79, 293 – 296.
Roberts, H.H., Aharon, P., 1994. Hydrocarbon-derived carbonate
buildups of the northern Gulf of Mexico continental slope: a
review of submersible investigations. In: Aharon, P. (Ed.), Geology and Biology of Modern and Ancient Submarine Hydrocarbon Seeps and Vents. Geomarine Letters. Springer Verlag,
pp. 135 – 148.
Schwartz, H., Moore, C., Weberling, K.D., Sample, J., Vrolijk, P.,
2001. Clastic intrusions and chemosynthetic paleocommunities
in the Cretaceous – Paleocene Great Valley forearc, Panoche
Hills, CA: fossil evidence for prolonged subduction-driven fluid
expulsion. EOS 82, F1304.
Weberling, K.D., Moore, C., Sample, J., Schwartz, H., 2001. Clastic
intrusions and chemosynthetic communities in the Cretaceous –
Paleocene forearc, Panoche Hills, CA: structural context of a
linked fluid system. EOS 82, F1274.