SNIFFER SURVEY: HIGH ISLAND AREA, GULF OF MEXICO
LIST OF FIGURES
A marine hydrocarbon seep detection survey was completed over High Island Blocks A-152, and A-198, including a 54 mile long regional north-south line which extends from Block A-198 down to Block A-321 in the High Island South extension. This regional line provides a background data set over non-producing areas and a calibration data set over an area of known production. This study, consisting of 239 miles of sniffer data, was conducted aboard the RV/GYRE by Texas A&M University, in conjunction with Exploration Technologies, Inc., using a marine hydrocarbon analytical system originally designed by Gulf Oil Corporation for use on the RV/HOLLIS HEDBERG. Light hydrocarbon data was collected continuously along seismic lines of interest from a water sampling system towed approximately 30 feet above the sea floor. A total of 52 miles of gridded data (259 analyses) was completed over the Block A-152 study area and a total of 31 miles of gridded data (129 analyses) over the Block A-198 study area. Three minute intervals gave an approximate sample spacing of 1500 feet along the surveyed track lines.
The largest magnitude anomalies observed occur where the regional profile crosses the southern trend of Pliocene gas fields. Iso and normal butane reached a total of approximately 1 nl/l in the anomalies associated with these fields and clearly distinguish this more petrogenic production from the northern blocks, which appear to contain only biogenic gases. Marine compositional cross plots are presented which demonstrate the compositional differences observed.
It is significant to note that gas production was established in both of these gridded areas after the sniffer survey was completed. Gas discoveries were made on Blocks A-129 and A-154 in 1989 and on A-200 in 1990. This High Island Area Survey in the Gulf of Mexico provides an example of a marine hydrocarbon seep detection survey which successfully identified biogenic and petrogenic gas sources.
Marine hydrocarbon seep detectors are designed to analyze seawater near the bottom for the presence of dissolved hydrocarbons. These hydrocarbons 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), Albright, (1973), Geyer and Sweet, (1973), Tinkle et al., (1973), Guinasso and Schink, (1975), 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 by: Lamontagne et al., (1973, 1974), Bernard et al., (1976), Brooks and Sackett, (1973, 1976), Sackett and Brooks, (1974), and Sackett, (1977), Jones, et al., (1988).
Also during this period, Gulf Research scientists designed, built and operated several marine seep detectors employed aboard various research vessels, such as the RV/HOLLIS HEDBERG, and its predecessor, the RV/GULFREX (Mousseau and Glezen, 1980), (Mousseau, 1980, 1981a, 1981b, 1983). These ships have circumnavigated the earth and conducted extensive, detailed surveying in areas such as the Gulf of Mexico. The RV/HOLLIS HEDBERG system used three separate water inlets. The inlets continually supplied sample streams from the near surface, intermediate depths to 450 feet, and a deep towed sample inlet which operated at a 600 foot depth while the ship was underway at normal seismic survey speeds. Each stream was analyzed for seven (7) hydrocarbon gases once every three (3) minutes with a sensitivity depending upon the hydrocarbon. For example, there are approximately 50 picoliters of propane at STP per liter of seawater. The purpose for deploying the sampling inlets at different depths is to differentiate between surface contamination and natural microseeps. Figure 1 is a 3-D perspective plot of propane collected from the hull and deep inlets. Surface contamination can be a major interference to shallow sampling, however, is not a factor in producing the seeps observed by the deep inlet.
The 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 Corporation to aid the explorationist in generating 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 ranges from 300 feet to 1200 feet. 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, 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. The umbilical consists of a central nylon hose surrounded by power and signal conductors encased in a polyurethane sheath with a woven stainless braid. Low-drag hydrodynamic farings insure the towfish follows close to the stern of the vessel and achieves 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. It also 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 stripper designs available to the industry follow either a vacuum stripping or gas partitioning scheme. In the Gulf Oil Corporation marine geochemical sniffer system, which is shown diagrammatically in Figure 3, a helium or nitrogen 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 into the extracted gas stream. This dissolved gas analysis system has proven to be very reliable for conducting sniffer surveys because the stripper has no moving parts or pumps to 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 analyses available are: total hydrocarbons, gasoline range C5+, benzene, toluene, helium (using nitrogen as a carrier gas), 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 three (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. It is approximately three minutes using a 600 feet deep towfish.
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 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 has been demonstrated to be useful for classifying marine hydrocarbon seeps as to their oil, condensate, or gas potential. This scheme has proven beneficial for relating these seeps to their associated source reservoir types using well analysis data published by Rice and Threlkeld, (1978).
MARINE CROSS PLOTS AND PREDICTION OF RESERVOIR TYPE
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 Drozd, 1983).
To establish this compositional marine cross plot scheme in the Gulf of Mexico, Williams et al., (1981) compared the Gulf Oil Co. sniffer data base shown in Figure 4 to the well data base shown in Figure 5 (Rice and Threlkeld, 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 compositional crossplot 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. This is 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 Gulf Oil's, Gulf of Mexico data base are plotted in Figure 7 and show an overall distribution similar to the producing wells from this area. The character 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 functionality in worldwide productive areas regarding major changes in composition, oil vs. gas. To use these cross plots to tie surface geochemical data to well data, one must also assume that migration and mixing does 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 does alter the ratios of seepage gases, one must gather sufficient data over known fields to create new classification boundaries.
Bray and Jones, (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 basins. 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 that 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 areas of shallow biologic activity, could explain excess methane.
HIGH ISLAND SNIFFER SURVEY
A marine hydrocarbon seep detection survey was completed over High Island Blocks A-152, and A-198 and surrounding areas on April 22-23, 1988, as shown on the site location tract map in Figure 10. This study, consisting of 239 miles of sniffer data, was conducted aboard the RV/GYRE by Texas A&M University, in conjunction with Exploration Technologies, Inc., using a marine hydrocarbon analytical system originally designed by Gulf Oil Corporation for use on the RV/HOLLIS HEDBERG. Light hydrocarbon data was collected continuously along seismic lines of interest from a water sampling system towed approximately 30 feet above the sea floor. A total of 52 miles of gridded data (259 analyses) was completed over the Block A-152 study area and a total of 31 miles of gridded data (129 analyses) over the Block A-198 study area. Three minute intervals gave an approximate sample spacing of 1500 feet. Three regional profiles are included as Figure 11A, Figure 11B, and Figure 11C.
Survey tracks, as shown on Figure 10, include a 54 mile regional north-south line, from Block A-198 to Block A-321 in the High Island South extension. This regional line plotted on Figure 11A, provides a calibration data set within the area of the known gas fields. Background values are observed on the north end of this profile near Blocks A-211 to A-223, where methane drops to about 100 nl/l; ethane is below 0.70 nl/l; and propane is below 0.50 nl/l. These thresholds are typical of Gulf of Mexico backgrounds from previous study data (Mousseau and Williams 1979).
The largest magnitude anomalies observed on this survey are also noted on this regional line (Figure 11A), where it crosses the center of Block A-268 and traverses the major trend of the known gas producing fields. Within this producing trend, methane is over 500 nl/l, ethane ranges from 1-2 nl/l up to 5 nl/l; and propane rises from 0.50 to 1 nl/l. In addition, iso and normal butane reached a combined total of about 1 nl/l in anomalies associated with these known gas fields.
The presence of butanes in the sniffer data clearly separate the southerly gas producing trend from that data gathered to the north of Block A-252. The grids over Blocks A-152, A-198 and the regional line north of Block A-252, are shown in Figure 10. These areas exhibit a clear lack of propane and butane anomalies. In contrast, however, there were several methane and ethane anomalies in these northern areas suggesting the potential for future gas production. The observed presence of primarily methane in the northern areas suggests these anomalies are sourced by biogenic gas sources. It is significant to note that gas production was established in both of these gridded areas after the sniffer survey was completed. As shown on Figure 10, gas discoveries were made on Blocks A-129 and A-154 in 1989 and on A-200 in 1990.
Marine compositional cross plots from the anomalies observed in Block A-152 and A-198, and from the regional profile, are shown in Figure 12 for comparison. All three areas fall as expected, based on the known oil and gas producing reservoirs, and are indicative of gas potential. Block A-198 appears to contain even drier gas reservoirs than Block A-152. In contrast, both of these blocks plot below the cluster associated with the major Pliocene gas producing trend that lies to the south of Blocks A-152 and A-198. The increase in ethane, propane and butanes in this southern gas producing area implies the gas fields in the southern part of the area surveyed contain Pliocene gas from a petrogenic rather than a biogenic source. The areas to the north appear to be dominated by biogenic gas sources which do not contain appreciable C2 plus components.
This marine compositional cross plot model was originally defined from fairly regional reservoir analysis data published in an open file report by Rice and Threlkeld, (1978). This original open file data set did not contain any actual analyses from High Island wells. More recently, Rice et al. (1983, 1990) has published some additional reservoir data that does include several wells from the High Island area. A marine compositional cross plot of this new data is included as Figure 13. In general it shows good agreement with the High Island survey data plotted in Figure 12. Obviously wells that fall outside the shaded areas should be used to further refine the model (i.e., these should be included within the shaded fields).
Of additional interest is the observation that several of these High Island wells cluster within an oil-condensate area on Figure 13. To understand this observation we can examine a study published by Walters (1990) on the origin of the gases and condensates produced from High Island Block A-511 in the South Addition. Gas analysis data from this study is plotted in Figure 14 for comparison. This data falls as predicted by the model, and clusters tightly within the oil condensate and condensate-gas windows. Walters' presents both chemical and isotopic data from eight wells suggesting the A-511 reservoir produces a mixture of biogenic and thermogenic gases along with small amounts of condensate from shallow Pleistocene sands. The presence of very low trace levels of butanes, relative to ethane and propane in the Block A-511 data, is in good agreement with the sniffer data gathered within the southern Pliocene gas and condensate trend.
As shown by the first two sniffer comparisons taken from West Cameron (gas area, Figure 8) and Vermilion (oil area, Figure 9), and this much more detailed gridded survey conducted in the High Island area (Figure 12), this scheme appears to work at any scale, and suggests that subtle changes from block to block can be mapped and ultimately related to subsurface sources at depth. Such changes should be related mainly to source/migration phenomena which can be confirmed from appropriate well analysis.
Dissolved gas analysis systems have been used to detect anomalous hydrocarbon concentrations in bottom waters all over the world. The final products of a marine system are contour maps and line profiles delineating 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.
The High Island Area Survey in the Gulf of Mexico provides an example of a marine hydrocarbon seep detection survey which successfully identified biogenic and petrogenic gas sources. The analysis was based on light hydrocarbon data collected continuously from a water sampling system towed above the sea floor. The presence of butanes in the sniffer data collected in the south separated itself from data collected to the north. There was a clear lack of propane and butanes in the north. In contrast, there were several methane and ethane anomalies in the northern areas that have been substantiated by known gas discoveries made after the sniffer survey was completed. All the areas within the survey plot as expected, based on the known oil and gas producing reservoirs, located within the survey area.
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 is that it is relatively inexpensive and provides large numbers of statistically significant analysis data 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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