Although marine hydrocarbon "sniffers" have been used to detect
anomalous concentrations of dissolved gases in bottom waters all over
the world, the ability to predict the oil versus gas potential of buried
reservoirs in frontier areas provides the most significant accomplishment
of the marine seep detector (Williams et al, 1981).
All hydrocarbon reservoirs, even those which produce primarily liquids,
contain low molecular weight hydrocarbon gases. The composition of these
gases generally shifts toward higher molecular weight components in oil
reservoirs as compared to gas reservoirs. Previous publications have demonstrated
the use of several methane through butane light hydrocarbon ratios for
making compositional correlations (Bernard et al, 1976; Drozd et al, 1981;
Jones and Drozd, 1983). For the marine seep detector, a compositional
cross-plot scheme can be demonstrated to be useful for classifying marine
hydrocarbon seeps as to their oil, condensate, or gas potential and for
relating these seeps to their associated source reservoir types. Marine
hydrocarbon seeps from the Gulf of Mexico will be shown to correlate with
producing wells using well analysis data published by Rice et al, (1978)
in a USGS open file report.
Recent advances also include illustrations from a color imaging sonar
system which is capable of providing a color image of all microseeps large
enough to produce bubbles in the water column. These high quality images
provide actual profiles of the bottom sediments and aid considerably in
defining the location of the fault or fracture that issued the seepage.
A final example correlating marine seep data with hydrocarbons analyzed
from dart cores will be shown for a "sniffer" anomaly which
preceded the discovery of the Beta Field in offshore California.
Marine hydrocarbon seep detectors are designed to analyze seawater near
the bottom for the presence of dissolved hydrocarbons, which are an indication
of potential deep sedimentary oil and/or gas deposits, or the presence
of man-made leakage from oil and gas pipelines or well casings. The first
information published on offshore geochemical sniffing was by Dunlap et
al, (1960), followed by Dunlap and Hutchinson, (1961). Over the next ten
years, programs were initiated by many of the major oil and service companies,
Anonymous, (1964); Jeffrey and Zarrella, (1970); Schink et al, (1971);
Rogers and Edwards, (1975); Sigalove and Pearlman, (1975); Reitsema et
al, (1978); Mousseau and Williams, (1979). Additional research was conducted
on stripping techniques and establishing baseline values, Lamontagne,
(1973, 1974); Bernard et al, (1976); Brooks and Sackett, (1973, 1976);
Sackett and Brooks, (1974, 1975); and Sackett, (1977).
Also during this period, Gulf Research scientists designed, built, and
operated several marine seep detectors which were employed aboard various
research vessels, such as the R/V HOLLIS HEDBERG, along with its predecessor,
the R/V GULFREX, Mousseau and Glezen (1981), Mousseau, (1981a, 1981b,
1983). These ships have circumnavigated the earth and conducted extensive
detailed surveying in areas such as the Gulf of Mexico (Mousseau et al,
1979). The R/V HOLLIS HEDBERG system employed three separate water inlets
which continually supplied sample streams from the near surface, intermediate
depths to 450 feet and a deep towed sample inlet which operated at 565
ft. depth while the ship was underway at normal seismic survey speeds.
Each sample stream was analyzed for seven (7) hydrocarbon gases once every
three minutes with a sensitivity which depends upon the hydrocarbon, but
for example, is about 50 picoliters of propane at STP per liter of seawater.
The purpose for using three inlets is to differentiate between surface
contamination and microseeps. As shown in Figure-1, which is a 3-D perspective
plot of propane from the hull and deep inlets, surface contamination can
be a major interference to shallow sampling, but is not a factor in producing
the seeps observed by the deep inlet.
The most typical form in which the "sniffer" data is deployed
when used in conjunction with seismic as an exploration tool is illustrated
in Figure-2. Geochemical data from a deep tow inlet in profile form is
shown superimposed to scale on a seismic record. Such records were produced
at sea by Gulf oil Co. to aid the explorationist in making real time evaluations
of hydrocarbon potential of structurally significant areas. The anomaly
represented in Figure-2 is considered a "localized" anomaly
because of the relatively short duration of the hydrocarbon signal and
the magnitude of the hydrocarbon concentrations relative to regional background.
Several "bright spots" may be seen on the seismic section at
depth as well as shallow gas-charged sands presumably sourced by migration
along the observed fault plane.
Several sea water hydrocarbon analysis systems, which can be deployed
from either standard workboats or seismic vessels, are currently available
to the industry. Depth capability for the towed sampler/sensors range
from 300 ft. to 1200 ft. All of these systems consist of a towed pump/sensor
system, connected by a fared umbilical to an onboard laboratory module.
The hydrodynamic towfish usually contains a submersible pump, a conductivity,
salinity, temperature and depth sensor (CSTD), and echo sounder transducer.
Under normal operational conditions, the fish is maintained within the
range of 4 to 8 meters above the seabed. The towfish is connected to the
winch and handling gear by an umbilical which consists of a central nylon
hose surrounded by power and signal conductors encased in a polyurethane
sheath with a woven stainless braid. The umbilical usually is fully fared
with low-drag hydrodynamic farings, which results in the towfish following
close to the stern of the vessel, achieving the maximum depth for a minimum
amount of deployed umbilical. Water is pumped through the umbilical to
the laboratory module at approximately 6 to 9 liters per minute. The water
sample is usually split into two independent streams to supply a dual
gas extractor system.
Duplication of the gas extractor system allows additional independent
analytical equipment to be used, and provides redundancy when required
due to failure, or for routine maintenance. Each extractor consists of
a glass stripper chamber into which the seawater is sprayed through a
fine jet nozzle. The water level in the stripper is maintained at a constant
height by a pressure regulated flow control system.
The design of the strippers available to the industry follows either a
vacuum stripping or gas partitioning scheme. In the Gulf Oil Co. marine
geochemical sniffer system, which is shown diagrammatically in Figure-3, a helium carrier gas is equilibrated with a water phase in such a way
as to allow the stripper to be operated under pressure preventing any
contamination from the onboard laboratory getting into the extracted gas
stream. This dissolved gas analysis system has been demonstrated to be
very reliable for conducting sniffer surveys because the stripper has
no moving parts or pumps which can fail.
The dissolved gases from the stripper are then sent to a gas chromatograph
by the helium stream. Analysis of these gases by Gulf included methane,
ethane, ethylene, propane, propylene, iso-butane, and normal butane. Additional
special gas analysis which could be included are total hydrocarbons, gasoline
range C5+, benzene, toluene, helium, hydrogen, radon, and carbon dioxide.
A computer system is used to continually monitor the conductivity, salinity
and depth of the fish sensor signals, with navigation data in UTM coordinates
acquired every 3 minutes at the start of the GC analysis. The time lapse
between collection of the water sample and the navigation time must be
accounted for by the computer system.
In addition to measuring the light hydrocarbons and their ratios for recognition
of different gas sources, there is usually an onboard capability to collect
a methane gas sample which is specially purified and burned to convert
the methane to carbon dioxide. This carbon dioxide is trapped in a special
container and returned to an appropriate onshore laboratory where it is
analyzed for its delta carbon 13 ratio (13C/12C). This ratio of the stable
carbon isotopes allows a distinction between shallow biogenic methane
and the more significant methane from a deep petrogenic source.
The ability to predict oil versus gas potential of subsurface reservoirs
provides the most significant demonstration of the value of sniffer geochemical
data (Williams et al, 1981). Compositional cross plots have been established
for classifying marine hydrocarbon seeps and predicting their source reservoir
type as an alternative to simply plotting ratios of the individual hydrocarbon
components. All hydrocarbon reservoirs, even those which produce primarily
liquids, contain low molecular weight hydrocarbon gases. The composition
of these gases generally shifts toward higher molecular weight components
(more propane and butane relative to methane) in oil reservoirs (Nikonov,
1971; Pixler, 1969; Bernard et al, 1976; Drozd et al, 1981; Jones and
In order to establish this compositional marine cross plot scheme in the
Gulf of Mexico, Williams et al, (1981) compared the sniffer data base
shown in Figure-4 to the well data base shown in Figure-5 (Rice et al,
1978). Rice has published the composition of the production gases for
each of the 32 fields shown in this figure, including gas, oil, and combined
oil and gas to condensate fields.
A cross plot of the compositions of the production gases from all of these
fields in Figure-5 is shown in Figure-6 (Williams et al, 1981). The log
of the ratio of ethane to propane plus butane is plotted against the log
of the ratio of methane to ethane plus propane. A distinctive compositional
clustering of gas anomalies signifies different kinds of production: oil
anomalies occur near the origin and become gassier as the points move
up and to the right in Figure-6. This cross plot scheme has been used
to successfully classify producing wells and their associated seepage
anomalies as to their type; oil condensate, dry gas, or biogenic gas based
upon the composition of the log of C1/(C2 + C3) and the log of C2/(C2
+ IC4 + NC4 ratios. Identification of biogenic gas from producing wells
in the northern Gulf of Mexico on this plot was based on both their molecular
and isotopic ratio data. An arbitrary boundary between oil-condensate
and gas-condensate (based upon the Rice well data base) has tentatively
been drawn midway between the other boundaries, as shown by the interpretive
line within the condensate window.
A comparison of 146 recorded geochemical sniffer anomalies taken from
the data shown in Figure-4 from the Gulf Oils, Gulf of Mexico data base
are plotted in Figure-7 and show an overall distribution similar to the
producing wells from this area. The type of typical contrast in composition
of dissolved hydrocarbon anomalies from an oil area in Vermilion and a
gas area surveyed in the West Cameron area of the Gulf of Mexico is shown
in Figure-8 and Figure-9.
The boundaries previously suggested by Williams for each reservoir type
have been demonstrated to work on worldwide productive areas as regards
major changes in composition, oil vs. gas. In order to use these cross
plots to tie surface geochemical data to well data, one must also assume
that migration and mixing do not significantly alter the ratios of light
hydrocarbons during migration to the surface. As shown above, this approach
does yield good correlations in the Gulf of Mexico where mixing of reservoir
types is expected to have considerable impact (Williams et al, 1981).
Alternatively in localized areas where mixing or migration do alter the
ratios of seepage gases, one must gather sufficient data over known fields
to create new classification boundaries.
Jones and Bray (1985) have further tested this cross plot scheme by applying
it to several onshore basins, including the Sacramento, San Joaquin, Uinta,
Paradox, San Juan, and Arkoma. Reasonable accord with known production
have been reported in all of these basins (Bray, 1986). The apparent similarity
in composition of observed interstitial gases from both onshore and offshore
implies that upward migration does not significantly segregate the four
lightest hydrocarbons. More importantly, it suggests the dominant regional
composition of near-surface gas is that which occurs in the reservoir,
or the source rock which charged the reservoir. There may, however, be
samples that contain significantly different compositions, predominantly
due to mixing of deep gases, or microbiological oxidation. In these cases,
contributions of deep dry gases along basement -related faults, or alternatively,
areas of shallow biologic activity, could explain excess methane.
It is well known that gas bubbles can become resonant scatters of acoustic
energy (Tinkle et al, 1973; Albright, 1973; Geyer and Sweet, 1973; Guinasso
and Schink, 1975). The Gulf Oil Co. marine gas sampling system also contained
a color imaging sonar system which was used to provide a color picture
of all marine seeps large enough to produce bubbles in the water column.
Actual illustrations of seeps detected by the gas sampler and imaged by
this color sonar system are included in this paper. These high quality
images provide actual profiles of the bottom sediments and aid considerably
in defining the location of the fault or fracture that issued the seepage.
Dissolved gas analysis systems have been used to detect anomalous hydrocarbon
concentrations in bottom waters all over the world. The final product
of a marine system is contour maps and line profiles which delineate areas
in which there are natural petroleum and gas seeps. This information may
be correlated with geological and geophysical data for exploration decision-making
or may be used as the basis for recommending additional survey work.
Offshore seep detection allows areas of the continental shelf to be surveyed
for seeping hydrocarbons as part of an integrated exploration program.
Seepage data can be interpreted to differentiate areas with a mature source
rock from those without, and to provide evidence for differentiating between
mature gas prone source rocks. Integrated with seismic/structural data,
survey results can be used to identify or confirm likely migration routes,
(e.g. gas chimneys), and in areas of sea floor pock marks, differentiate
a biogenic from a thermogenic source for the gas. In exceptionally simple
geological cases, such surveys have been used to identify hydrocarbon-filled
structures at depth, although in most regions the relationship between
surface anomalies and deep structure is complex, requiring an integrated
interpretation of all available geological and geophysical data.
The advantages of ship towed seawater monitoring are that it is relatively
inexpensive and provides large numbers of statistically significant analyses
on a precisely located grid. Real-time analysis also allows for informed
modification of the sampling program.
Additional applications of seawater hydrocarbon detecting systems include
the use for under sea pipeline leak detection, and for marine pollution
monitoring and prevention (Aldridge and Jones, 1987).
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