Over the years geochemists have explored the application of brine benzene analysis as a proximity indicator of petroleum deposits (Collins, E.G.). Such studies have been shown to be highly effective in confirming the approximate distance from a dry hole to a reservoir in the same hydrogeologic system by mapping the plume of dissolved petroleum constituents emanating from the nearby accumulation (Zarrella et al., 1967).

Major sources of error in these initial studies were due to the difficulty in storing, handling and analyzing brine samples without loss or fractionation of volatile constituents. With the advent of standardized environmental sampling and analytical techniques, brine analysis for benzene as well as ethylbenzene, toluene and xylene (BTEX) aromatics is now routine and reliable. This relatively low cost environmental analytical method, assisted by next-day air courier service, can now be effectively applied to both domestic and international petroleum exploration drilling programs as an economical formation evaluation tool.

Background

Extensive reservoir fluid compositional studies and research have shown that soluble aromatic hydrocarbons, such as, benzene, toluene and xylenes make up a large proportion of the dissolved hydrocarbons found in brines associated with oil reservoirs (Wiesenburg, et. al 1981). Figure 1 and Table 1 show a comparison of oil and co-produced brine from the Buccaneer oil and gas field located in Galveston Area Blocks 288 and 296, offshore Gulf of Mexico. As can be seen from these illustrations, BTEX aromatic hydrocarbons are the primary volatile liquid hydrocarbons dissolved in the brines due to their high solubility in water. As summarized in Table 2, empirical studies by Zarrella, et al, 1967, indicate that the benzene content of brines from typical oil reservoirs range from about 5 to 20 ppm depending on oil composition and source.

Benzene and related aromatic constituents, which are in chemical equilibrium with adjacent oil accumulations, emanate from the vicinity of the reservoirs, forming a plume of decreasing magnitude with increased distance from the reservoir edge. Research by Zarrella et al, (1967) has confirmed that the distance to an oil reservoir is directly proportional to the log of the benzene concentration of the adjacent brines. This relationship, as plotted on Figure 2, is due to solubility and diffusion factors.

Analyses of benzene concentrations in brine samples from non-productive exploration wells can therefore be used to predict the distance to a nearby petroleum accumulation with a reasonable level of confidence. With tests from the same formation, in two or more nearby wells, results can often be used to provide a much more accurate prediction of the distance and direction, to potential undiscovered reservoirs within the stratigraphic horizon tested. As a data base of local wells is developed, the benzene magnitude to distance relationship can be further refined for a specific basin or formation of interest, improving the quality of the distance predictions.

This exploration method was used extensively by the Gulf Oil Company in the late 1960's and 1970's and has been discussed in several books on petroleum geochemistry including Hunt, 1979 and Collins, 1975.

Applications

Brine analysis for dissolved aromatic hydrocarbons can be applied in new field exploration programs, development drilling, and predicting extensions of existing fields.

New Field Exploration. Brine sample analysis can be used to confirm if, despite an apparent dry hole, the formation has an oil accumulation within a radius of up to about 12 miles (20 km). This information is extremely valuable when assessing the results of rank wildcat areas and international concessions, when decisions must be made with respect to the general prospectiveness of a lease position, in addition to its obvious use in selecting step-out locations and for evaluation of similar structures in the basin. Results provide a lease evaluation tool by predicting the presence or absence of petroleum within a known radius of an exploration well location.

Development Drilling. During development drilling and step-outs, brine analyses can be used to help confirm the location of a well with respect to the edge of the reservoir zone. If samples are collected from each well during initial production and testing it may be possible to reduce the number of dry holes by helping to vector step-outs toward areas of higher magnitude benzene content, which would be indicative of more economical reservoir zones.

Field Extensions. Extensions to existing fields may be predicted by comparing brine samples from groups of producing wells. As fluids are extracted, formation waters from adjacent areas are drawn to the producing wells. If the brine is drawn from barren areas, the benzene content will continue to decrease over the life of the well. Wells which draw brine from beneath adjacent, undrained accumulations, or untested extension areas in the field, often show consistently higher benzene content, since produced brines from these wells were in contact with fresh, unproduced petroleum deposits. This technique is also useful to confirm whether individual wells are influenced by water injection or other secondary and tertiary recovery operations which would alter the magnitude and composition of indigenous formation brines.

Sample Collection

Brine samples from drill stem tests, flow tests or repeat formation testers can be acquired using standard environmental sampling methods by collecting duplicate 1.35 ounce (40 ml VOA) sample jars from each test interval. Samples are collected with no headspace and stored on ice or refrigerated to a temperature of about 5 degrees centigrade. Care should be taken to obtain samples with minimal drilling fluid dilution or contamination from petroleum products used during the drilling operation.

In existing wells, which are currently under production, it is better to obtain a sample, if possible, from the flow line close to the pump rather than from the stand tank. Well fluid samples should be collected into a clean separation funnel and allow to phase separate for about 5 minutes. Water samples for analysis are then drawn from the bottom of the funnel and placed into sample jars. It is very important that good quality brine samples, without appreciable free oil, be collected to reduce the possibility of alteration during sample handling and transport.

Samples can be delivered directly to the laboratory in coolers or sent by air courier on ice in insulated shipping containers. With proper planning and documentation, samples can easily be shipped or hand carried from international locations without significant reduction in sample quality.

Analytical Approach

Samples are analyzed by US-EPA method 602 by purge and trap, Photoionization Detector (PID) gas chromatography for benzene, toluene, ethylbenzene and xylene content. This method, which is routinely applied for ground water contamination studies has an analytical detection limit of about 1 microliter per liter (ppb). Samples can generally be analyzed within 24 hours of receipt in the laboratory and results sent by fax or email immediately after analysis. Confirmation samples from each interval are recommended to help insure reliable results and interpretation.

Total dissolved solids (TDS) are also measured on brine samples to help determine solubility factors of organics in the formation fluids. Additional analyses for iodine and other commonly used proximity indicators can also be completed as available sample volume allows.

Data Interpretation

As documented by Zarrella et al (1967) dissolved benzene magnitudes, when plotted versus distance from oil accumulations plot as a straight line on a semi-log scale (Figure 2). Test results are posted onto this line to provide an estimate of the distance to an adjacent reservoir. As a data base is developed for a specific geographical area more precise predictions of distance may be possible. Once a prediction is made it is up to the exploration team to evaluate the results in light of their knowledge of the areas local geologic framework, nearby production and other exploration test results. Depending on the availability of detailed geologic control from surrounding areas it may be possible to further reduce the area to be searched by eliminating areas around the well which have been previously tested.

Analytical results for benzene can also be compared and confirmed by toluene, ethylbenzene and xylene data. These compounds, which also have appreciable solubilities in oil field brines (Figure 1 and Table 1), occur in varying proportions in different reservoirs and can therefore serve as independent parameters for evaluation. As noted by Collins (1975) typically a benzene to toluene ratio of greater than 1 can be expected for brines adjacent to petroleum accumulations.

Case studies

Summaries of several case studies performed by Gulf Oil Corporation were presented by one of the authors, V. T. Jones III, during a two day short course on Geochemical Exploration Technology sponsored by the Rocky Mountain Association of Geologists, Continuing Education Committee on January 30 - 31, 1984 in Denver Colorado and in a Gas Research Institute workshop held in Dallas, Texas, May 1 - 2, 1984. Over a period of years various Gulf Oil divisions kept records of the predicted distances to petroleum reservoirs and actual distance to subsequent discoveries. Table 3 includes a list of petroleum discoveries near to 24 benzene anomalies from West Texas, Oklahoma, New Mexico, Mississippi, Offshore Gulf of Mexico and Alberta Canada. As can be seen from these results, the distance to a potential new field discovery can be reliably predicted from benzene analysis in a wide variety of environments, reservoir types and basins.

In the Alberta Basin Canada, where this tool was applied extensively for many years, records of the benzene content of Nisku Formation brines in exploration wells versus the distance to production proved very useful. As summarized in Figure 3, this observed relationship correlates extremely well with predicted benzene magnitudes as calculated using a diffusion model for benzene migration from typical Nisku reservoirs.

This model for the Nisku Formation was tested in the Stettlers Region of Alberta where an anomalous benzene magnitude was identified in a non-productive exploration well located about 6 miles from the nearest known Nisku production. As illustrated in Figure 4, a brine benzene content of 5 ppm plots well off the calculated magnitude to distance correlation line. This result was used to predict the presence of an undiscovered accumulation at a radius of 1 to 2 miles from the dry hole. Later drilling confirmed this prediction with the discovery of a new Nisku field a distance of about 2 miles from the test well. This field has an area of about 12 square miles with a 20 foot thick oil pay. Figure 4 shows the new field boundaries with respect to the original test well and confirms the brine benzene content of 5 ppm which now plots near to the predicted correlation line.

Conclusions

Analysis of dissolved benzene and BTEX hydrocarbons in formation water samples from exploration and development wells provides a reliable means of predicting the presence of, and distance to petroleum deposits in adjacent, hydrologically equivalent areas. When evaluated in light of prevailing geologic conditions, this information is invaluable in both frontier and development areas, since the results can not only reduce the area to be searched, but can significantly increase the information gained from an exploration test.

Bibliography

Collins, A.G., Geochemistry of Oilfield Waters, 1975, Elsevier Scientific Publishing Company, New York.

Hunt, J.M., Petroleum Geochemistry and Geology, 1979, W.H. Freeman and Company, San Francisco, pp. 448-450.

Jones, V.T., Overview of Geochemical Exploration Technology, 1984, Short Course on Geochemical Exploration Technology, Rocky Mountain Association of Geologists, Denver Colorado January 30-31, 1984.

Pirkle, R.J., Hager, R.N., and Jones, V.T., Second Derivative Absorption Spectroscopic determination of Benzene and Toluene at the Wellsite, Presented at the 187th American Chemical Society National Meeting St. Louis, Missouri, April 8-13, 1984.

Wiesenburg, D.A., Bodennec, G., Brooks, J.M., Volatile liquid hydrocarbons around a production platform in the northwest Gulf of Mexico, Bulletin Environmental Contamination Toxicology, 1981, Vol. 27, pp. 167-174.

Zarrella, W.M., Mousseau, R.J., Coggeshall, N.D., Norris, M.S., and Schrayer, G.J., Analysis and significance of hydrocarbons in subsurface brines, Geochimica et Cosmochimica Acta, 1967, vol. 31, pp. 1155-1166