Journal of Geochemical Exploration 69–70 (2000) 545–549
www.elsevier.nl/locate/jgeoexp
Structural control of fluid flow: offshore fluid seepage in the Santa
Barbara Basin, California
P. Eichhubl a,b,*, H.G. Greene a,c, T. Naehr a, N. Maher a
b
a
Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA
Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA
c
Moss Landing Marine Laboratories, Moss Landing, CA 95039, USA
Abstract
Evidence of active and dormant fluid seepage in the Santa Barbara Basin is observed as active venting of gas and oil, bacterial
mats, precipitates of authigenic carbonate, and mud and tar volcanoes. Fluid seepage occurs preferentially in the proximity to
faults and faulted anticlines, and to slump scarps. Seepage next to faults and anticlines indicates that hydrocarbon migration and
pore fluid expulsion is controlled structurally, with faults acting as preferred conduits for fluid flow across units of low matrix
permeability. 䉷 2000 Elsevier Science B.V. All rights reserved.
Keywords: fluid seeps; authigenic carbonate; faults
1. Introduction
The Santa Barbara Basin, part of the continental
borderland of southern California, is composed of a
Cretaceous to Holocene sequence of clastic and hemipelagic units. The sedimentary sequence includes
organic-rich siliceous mudstone of the Miocene
Monterey Formation that is both source and fractured
reservoir of hydrocarbons. While the northern flanks
of the basin are being actively folded and exhumed as
part of the Santa Ynez Mountains, the central part of
the basin undergoes sedimentation and prograde
burial. Transpressive shortening of the basin flanks
is accommodated by high-angle oblique reverse and
strike-slip faults and by folds with faulted anticlines
(Figs. 1 and 2). Prograde burial and diagenesis in
synclines and in the basin center drive hydrocarbon
* Corresponding author. Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA.
E-mail address: eichhubl@pangea.stanford.edu (P. Eichhubl).
generation and pore fluid expulsion, leading to
submarine seepage of natural gas and oil (Vernon
and Slater, 1963; Allen et al., 1970; Wilkinson,
1972; Fischer and Stevenson, 1973).
Due to the low matrix permeability of the siliceous
mudstone, fluid flow within and out of the Monterey
Formation depends on the presence of conductive
fracture and fault systems. Evidence of focused fluid
flow along faults is seen in surface outcrops and core
samples across faults that are extensively cemented
with carbonate (Eichhubl and Behl, 1998). Based on
mass balance estimates of fluid involved in fault
cementation, Eichhubl and Boles (2000) inferred
that faults channel fluid migrating up along the tilted
flanks of the basin, providing cross-stratigraphic pathways for fluid expulsion to higher structural levels and
to the surface.
In an effort to assess the structural control of basinal
fluid expulsion to the surface, seep location and fluid
composition have been correlated to the subsurface
structure and to the fluid composition of formation
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved.
PII: S0375-674 2(00)00107-2
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P. Eichhubl et al. / Journal of Geochemical Exploration 69–70 (2000) 545–549
Fig. 1. Bathymetry of the Santa Barbara Channel, based on Symrad EM300 30 kHz multibeam data. ROV dive tracks in gray: A, active gas
seepage; M, tar mounds and mud volcanoes; C, authigenic carbonate; B, Beggiatoa mats; cross-hatch, areas of bedrock exposure; Circles, Water
column anomalies suggesting gas seepage based on USGS data; buoy symbol, oil seeps after Wilkinson (1972). Faults after Yerkes et al. (1981).
fluids in producing hydrocarbon reservoirs. Seeps
were visually inspected and sampled using MBARI’s
ROV Ventana. Sampling included pore fluid extracted
from up to 1 m long sediment cores and grab sampling
of authigenic carbonate. In preparation for ROV
dives, the basin was mapped using high-resolution
30 kHz swath bathymetry and sidescan sonar. Dives
were focused on known areas of gas and oil seepage
and on potential seep sites such as fault and slump
scarps, mud volcanoes, and on areas of high-sidescan
reflectivity that may represent authigenic carbonate.
2. Evidence for active fluid seepage
Active seepage of hydrocarbons occurs predominantly in areas of no or shallow Holocene sediment
cover on the northern shelf of the Santa Barbara Chan-
nel and on the Mid-Channel trend, a structural and
morphologic high in the eastern part of the Channel
(Fig. 1). Active seepage was observed as continuous
or intermittent release of gas bubbles and to a lesser
extent of oil droplets from crevices in rocky substrate
or from circular openings in muddy substrate (Fig.
3a). At Coal Oil Point, gas seepage occurs along
two linear trends following faulted anticlines that
form producing hydrocarbon reservoirs in fractured
Monterey Formation (Quigley et al., 1999). South of
Gaviota, active gas seepage was observed from a
series of mud volcanoes that are aligned along the
Molino anticline, a producing gas reservoir. Active
growth of these mud volcanoes, measuring about
10 m in diameter and 4 m in height, is indicated by
recent tar extrusions on top of these edifices (Fig. 3b).
Less vigorous gas venting was observed at several
locations along the head scarp of the Goleta slump
P. Eichhubl et al. / Journal of Geochemical Exploration 69–70 (2000) 545–549
547
Fig. 2. Structure contours of top of Monterey Formation, contour interval 1000 ft. After Heck (1998). Other symbols same as in Fig. 1.
(Fig. 1) and southeast of Point Conception adjacent to
a fault scarp.
Slow seepage of methane is indicated by mats of
the sulfide oxidizing bacterium Beggiatoa sp. (Bernhard
et al., 2000) (Fig. 3c) and by high sulfide and alkalinity
values of pore water extracted from shallow sediment
cores within these mats (Fig. 3d). In addition to the
sites of active gas venting, evidence for slow seepage
was found in one of the feeder canyons of the Conception fan, at the toe of Goleta slump, and along a linear
depression in the southern part of the basin.
3. Authigenic carbonate precipitation
Authigenic carbonates form irregular vuggy
concretions (Fig. 3d), slabs (Fig. 3e), or crusts. Except
for carbonate found within the sediment collected in
push cores, most samples were recovered from the
sediment surface, either isolated or as pavements.
Carbonate samples are typically composed of highMg calcite, aragonite, and dolomite to varying proportions, cementing the silty and locally sandy to pebbly
sediment (Fig. 3f). Aragonite occurs as micrite and as
pore-filling acicular botryoids that are rhythmically
layered (Fig. 3g). Due to the agglutinated nature of
these carbonates, formation is inferred to occur within
the sediment. Their occurrence on the sediment
surface suggests either secondary winnowing of
surrounding sediment after carbonate precipitation
or redeposition of carbonate rocks due to slumping.
The carbon isotopic composition of carbonate
ranges from ⫺58 to ⫹ 26‰PDB, reflecting the dominance of shallow organic matter diagenesis over other
sources of carbon such as thermogenic methane and
biogenic carbonate. The d 18O composition of authigenic carbonates varies between ⫺5 and ⫹ 8‰PDB
and is likely to reflect mixing of ambient sea water
with seeping formation water and meteoric water.
Potential mixing with formation water is inferred
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P. Eichhubl et al. / Journal of Geochemical Exploration 69–70 (2000) 545–549
Fig. 3. (a) Active gas seepage at Coal Oil Point; (b) Tar extrusion on top of mud volcano, Molino anticline, south of Gaviota; (c) Beggiatoa mat
on active seep; (d) Pushcore sampling at an active seep; (e) Vuggy carbonate crust composed of aragonite and high-Mg calcite from the head
scarp of Goleta slump; (f) Dolomite slab from the Mid-Channel trend; (g) Isopachous dolomite cement in conglomerate, southeast Point
Conception; (h) Micritic and acicular aragonite.
based on the possible range of fluid temperature
during precipitation, resulting in inferred fluid d 18O
values of ⬎ ⫹ 2‰SMOW for several of the collected
carbonate samples. The analyzed formation water
from oil wells that produce from the Monterey Formation in the basin ranges between ⫹ 2 and ⫹ 6‰SMOW
(Eichhubl and Boles, 1998).
4. Structural control on seepage
Active gas venting is clearly controlled by subsur-
face structures such as faults and faulted anticlines. At
Coal Oil Point, Molino-Gaviota, and on the MidChannel trend, seepage results from leakage of underlying hydrocarbon reservoirs. Fault control of seepage
is consistent with the model of focused basinal fluid
migration along faults as inferred from outcrop and
core observations. In addition to the structural control,
seepage occurs preferentially along slump scarps and
in submarine canyons. Enhanced seepage at slump
scarps and within canyons is likely due to steepened
pore fluid pressure gradients within the exposed older
sediment units that are undergoing compaction and
P. Eichhubl et al. / Journal of Geochemical Exploration 69–70 (2000) 545–549
organic matter diagenesis. Increased fluid seepage
next to slump scarps and the resulting increase in
pore fluid pressure may also increase slope instability
promoting slumping and canyon incision (Orange and
Breen, 1992). This feedback between fluid seepage
and slope instability may be affected by subsurface
structure as well, localizing fluid seepage and thus
controlling the location of slumps. The head scarp
of Goleta slump where increased seepage was
observed does indeed follow fault and anticlinal
trends suggesting a structural control on mass wasting
imposed by upward fluid migration.
5. Conclusions
Active and dormant fluid seepage is observed as
active release of gas and oil, bacterial mats, mud
and tar volcanoes, and precipitates of authigenic
carbonate. Seepage occurs predominantly along the
northern shelf of the Santa Barbara Basin and on the
Mid-Channel trend, areas that are characterized by
thin Holocene sediment cover. Evidence of fluid
seepage is found preferentially in proximity to faults
and faulted anticlines, and to slump scarps. Seepage
next to faults and anticlines indicates that hydrocarbon migration and pore fluid expulsion is
controlled structurally, with faults acting as preferred
conduits for fluid flow across units of low matrix
permeability. The association of seepage with slump
scarps is explained by steepened pore pressure gradients adjacent to scarps due to sudden erosion associated with slumping and the resulting focusing of
fluid towards these scarps. In addition, slumping
may be triggered by upward migration of fluids
along faults resulting in a reduction in slope stability,
thus providing a potential link between subsurface
structure and mass wasting.
Acknowledgements
Funding for this study was provided by the David
and Lucile Packard Foundation.
549
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