Journal of Geochemical Exploration 78 – 79 (2003) 293 – 296
www.elsevier.com/locate/jgeoexp
Abstract
Fluid escape from reservoirs: implications from
cold seeps, fractures and injected sands
Part I. The fluid flow system
A. Mazzini *, R. Jonk, D. Duranti, J. Parnell, B. Cronin, A. Hurst
Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, Aberdeen AB24 3UE, UK
Abstract
Fluid escape from reservoirs can take place through (hydraulic) fracturing, sand injection and seepage. Above several
Tertiary hydrocarbon reservoirs in the North Sea, substantial amounts of fractures, sand injectites and seeps occur. Petrographic
observations of these features show that all have carbonate cement associated with them that contains fluorescing hydrocarbon
inclusions. The petroleum fluids escaping were partially trapped during the cementation and analysis of the cements allows
understanding of the fluid flow system and the fluids involved. Cathodoluminescence indicates that sand injectites and fractures
have one phase of cement associated with them. Seeps, however, show zoned carbonate cement, suggesting precipitation in a
varying geochemical environment. This suggests that fractures and sand injectites were short-lived fluid-conduits, whereas
seeps can act as fluid escape pathways over prolonged periods of time.
D 2003 Elsevier Science B.V. All rights reserved.
Keywords: Cold seeps; Injected sands; Hydrocarbons; Cathodoluminescence
1. Introduction
Fluid escape from hydrocarbon reservoirs in the
shallow crust is a common phenomenon, but the fluid
flow pathways can be diverse and are presently not
fully understood. Fluids follow different pathways and
generate diverse geological structures and phenomena
(Fig. 1). The occurrence of fluid seepage on the sea
floor is manifested by the presence of hydrothermal
vents (metal-rich deposits) and cold seeps (typically
carbonate deposits). Cold seeps are associated with
* Corresponding author. Tel.: +44-1224-273435; fax: +441224-272785.
E-mail address: a.mazzini@abdn.ac.uk (A. Mazzini).
carbonate mounds, coral reefs, pockmarks, mud volcanoes and seamounts. It is at these sites that seeping
gases sustain chemosymbiotic communities inducing
the precipitation of authigenic carbonates (Hovland et
al., 1987; Ritger et al., 1987; Paull et al., 1992). Seep
deposits can assume different scales and shapes, such
as exposed and buried mineralised chimneys (Jorgensen, 1989), cemented fluid conduits and veins, micro
and macro slabs, and cemented sedimentary layers
(Hovland and Judd, 1988). In the subsurface, rapid
fluid escape can take place through networks of
fractures and injected sands. Often, fluid overpressures generate the pathways for fluid escape through
(hydraulic) fracturing and sand injection (Cosgrove,
2001), but more importantly, once created they remain
fluid escape pathways. Fluids may be trapped where
0375-6742/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0375-6742(03)00046-3
294
Abstract
Fig. 1. Schematic submarine cross section showing a range of features through which fluids can escape from a reservoir: (1) diapiric structures
eventually reaching the sea floor as mud volcanoes where chemosynthetic communities can occur; (2) dense networks of faults and fractures act
as conduits for seepage (conventional cold seeps) or (3) can be used by sand injection and pockmarks. Injected sandstones can reach the sea
floor or can terminate at various depths eventually allowing fluids to reach the sea floor through fractures and seepage. Grey boxes correspond
to carbonate cemented features analysed.
fractures or injected sandstones terminate or may
continue to seep to the surface. It is important to
know whether structures are truly trapping and sealing
or whether fluid escape continues to take place,
eventually manifesting itself as seeps on the sea floor.
Carbonate is the most common cement associated
with these features at shallow burial and the cements
yield valuable information on the origin of the fluids
and its various components. Our goal is to understand
the linkage between reservoirs, fractures, injected
sandstones and seeps via a study of carbonate cementation.
2. Methods of study
Polished slabs and thin sections were studied
using standard petrographic and cathodoluminescence techniques in order to analyse and distinguish
the cementation phases. Cathodoluminescence was
conducted with a Citl Cold Cathode Luminescence
8200 mk3 linked with a Nikon M32 microscope to
evaluate the compositional variations during the
crystal growth.
3. Study area
The North Sea is a major producing hydrocarbon
province. Some of the earliest descriptions of fluid
escape features on the sea floor (Hovland and Judd,
1988) and descriptions of injected sandstones (Jenssen
et al., 1993; Dixon et al., 1995; Lonergan and Cartwright, 1999) associated with hydrocarbon reservoirs
are from this province. In the present study, samples
were retrieved from the sea floor and from cores
associated with several Tertiary hydrocarbon reservoirs showing substantial degrees of carbonate cement
associated with seeps, fractures and sand injectites
(Duranti et al., 2002).
Abstract
4. Results
Carbonate cements in seeps, fractures and sand
injectites were analysed. Fluid inclusion and isotope
295
data indicate an intimate association with hydrocarbons. (Jonk et al., 2003, this issue), Seep deposits
(Fig. 2A) are cemented with blocky and fibrous
calcite, which is often recrystallised aragonite. Cath-
Fig. 2. Plane light (left) and corresponding cathodoluminescence (right) pictures of the three carbonate cemented features analysed. Cold seeps
at the sea floor consist of blocky calcite (A) that shows oscillatory zoning and no evidence of dissolution (B). Injected sandstones (C) show
extreme minus cement porosities and one phase of carbonate cement (D). Fractures (E) contain unzoned carbonate cement (F).
296
Abstract
odoluminescence shows zones of dull and bright
luminescent calcite. It also shows a continuous
precipitation history of carbonate with no evidence
of dissolution events (Fig. 2B). Cement in injected
sandstones is poikilotopic calcite. Minus cement
porosities around 50% suggest that cementation
took place early in the burial history (Fig. 2C).
Cathodoluminescence shows one phase of cement
(Fig. 2D). Fractures are around 1 mm thick and are
cemented with calcite. The calcite exhibits either a
beefy or a blocky texture (Fig. 2E). Again, the
cathodoluminescence shows one phase of cement
(Fig. 2F).
5. Discussion and conclusion
Although all fluid escape features found above
hydrocarbon reservoirs are characterised by carbonate cement, the cathodoluminescence shows a striking difference between seeps versus fractures and
sand injectites. Seeps show zoned cement, suggesting varying geochemistry of the cementing fluid. The
likely prolonged history of fluid seepage could be
responsible for this. However, both sand injectites
and cemented fractures show one phase of cement
and thus one fluid phase responsible for cementation.
This, together with the observed high minus cement
porosities in sand injectites suggests rapid cementation following fracturing and sand injection. Crosscutting relations between cemented fractures and
sand injectites are often contradictory, suggesting
coeval fracturing and sand injection. Fluid overpressure in the reservoir causes rapid fluid escape
through hydraulic fracturing and sand injection.
The newly created fluid flow pathways are quickly
cemented. Once cemented, fluid escape can continue
to take place through seepage at the top of the
structures (Fig. 1). Seepage takes place more slowly
over a prolonged time, recording the longer-term
geochemical changes in the cementation environment. This study suggests that fluid escape from
reservoirs can occur catastrophically over short timespans through (hydraulic) fracturing and sand injection, but continued fluid escape can take place less
catastrophically over a prolonged time through seepage. This conclusion is supported by fluid inclusion
analysis. UV light observations on the described
samples (Jonk et al., 2003, this issue) show that
similar types of fluorescing hydrocarbon inclusions
occur in injectites, fractures and cold seep features
sampled from the same location. This suggests that
fluid escape from reservoirs can only be fully understood if the spatial and temporal distributions of both
components are analysed.
References
Cosgrove, J.W., 2001. Hydraulic fracturing during the formation
and deformation of a basin: a factor in the dewatering of lowpermeability sediments. AAPG Bulletin 85 (4), 737 – 748.
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, vol. 94. Geological Society of London,
London, pp. 77 – 94. Special Publication.
Duranti, D., Huuse, M., Cartwright, L., Hurst, A., Cronin, B., Mazzini, A., Flanagan, K., 2002. Unusual facies and geometries of
the Paleogene deep-water systems in the North Sea: effects of
sand remobilisation, EAGE Florence, Annual meeting, abstract
P057.
Hovland, M., Judd, A.G., 1988. Seabed Pockmarks and Seepages.
Graham & Trotman, London. 293 pp.
Hovland, M., Talbot, M.R., Qvale, H., Olaussen, S., Aasberg, L.,
1987. Methane-related carbonate cements in pockmarks of the
North Sea. Journal of Sedimentary Petrology 57 (5), 881 – 892.
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. Geological Society of
London, London, pp. 135 – 143.
Jonk, R., Mazzini, A., Duranti, D., Parnell, J., Cronin, B.T., Hurst,
A., 2003. Fluid escape from reservoirs: implications from cold
seeps, fractures and injected sands. Part II: the fluids involved.
Journal of Geochemical Exploration (this issue).
Jorgensen, N.O., 1989. Holocene methane-derived, dolomite-cemented sandstone pillars from the Kattegat, Denmark. Marine
Geology 88, 71 – 81.
Lonergan, L., Cartwright, J.A., 1999. Polygonal faults and their
influence on deep-water sandstone reservoir geometries, Alba
Field, United Kingdom Central North Sea. AAPG Bulletin 83,
410 – 432.
Paull, C.K., Chanton, J.P., Neumann, A.C., Coston, J.A., Martens,
C.S., 1992. Indicators of methane-derived carbonates and chemosynthetic organic carbon deposits: examples from the Florida
Escarpment. Palaios 7 (4), 361 – 375.
Ritger, S., Carson, B., Suess, E., 1987. Methane-derived authigenic
carbonates formed by subduction-induced pore-water expulsion
along the Oregon/Washington margin. Geological Society of
America Bulletin 98, 147 – 156.