DETAILED MICROBIAL SURVEYS HELP IMPROVE RESERVOIR CHARACTERIZATION

James Tucker, Daniel Hitzman
Geo-Microbial Technologies Inc.
Ochelata, Okla.

Application of the Microbial Oil Survey Technique (MOST) as a reservoir characterization tool expands a surface geochemistry technique from exploration into the production industry.

The three examples presented show how tightly spaced microbial sampling grids over producing fields are effective tools to delineate reservoir drainage patterns associated with and related to reservoir heterogeneities. Using this surface microbial information with geology and geophysics is a powerful combination for identifying infill target and stepout potential within existing fields.

Distinct microbial population patterns identified in these surveys are:

Decreased microbial populations over or adjacent to producing wells indicating hydrocarbon withdrawal drainage patterns,
Linear high and low variations of the gas withdrawal patterns which illustrate the compartmentalization of the reservoir; and
Exploration type, broad microbial highs which indicate either missed horizons or stepout potential in the field.

NEW APPROACH

The MOST technique used in this survey was first developed by Phillips Petroleum Co. in the 1950s.1

The technique is based on analyzing shallow surface soil samples for a specific suite of microorganisms which indicate the presence of light hydrocarbon gases. Where elevated clusters of these specific microbial populations are identified, anomalous hydrocarbon microseepage from oil and gas reservoirs is indicated.2 3 4

In the past MOST has been used primarily for exploration reconnaissance surveys, for ranking and comparison of wildcat prospects, and for hydrocarbon microseepage measurements of new and offset prospects.5 6 7 This new application of MOST within producing fields broadens the ability of surface geochemical surveys to identify reservoir heterogeneities and bypassed pockets of reserves.

Anomalous hydrocarbon microseeps are identified by observing MOST population concentrations and distribution patterns within a survey area. There is a direct, positive relationship between the light hydrocarbon concentrations in the soils and these microbial populations. Surface contamination of produced oil and changing soil types do not affect these light hydrocarbon indicating microbial population distributions.

The specific MOST populations are measured by standard microbiological screening techniques for hydrocarbon-indicating microorganisms. A selective growth media is utilized which cultures only microorganisms capable of utilizing light hydrocarbons.

After incubation, microbial colonies are counted and reported as Microbial Values. Soil samples normally are processed within 48 hr after field collection. However, a new stabilization technique extends this period to one month for exploration areas or producing fields where express transport services are not available.

PRODUCTION PATTERNS

Hydrocarbon microseepage is dependent upon pressurized reservoirs driving the light hydrocarbon microseepage upward. The pattern of reduced microbial counts adjacent to producing wells has been a commonly observed phenomenon for older producing fields. Over some well-drained gas reservoirs, the microbial values have been found to even be anomalously low.

The phenomenon of apparent microseepage shutdown over producing fields is thought to be due to a change in the drive mechanism controlling microseepage. When a well is brought into production, the drive mechanism changes from a vertical, buoyancy driven force to horizontal gas streaming to the pressure sinks created around producing wells.

When this occurs, microseepage greatly decreases or ends and microbial populations at the surface decline rapidly. This change in drive mechanism and microbial population densities can be used to define reservoir drainage direction, radius, and heterogeneities around existing wells in producing fields.8

In undrilled areas this phenomenon will not occur and there will be a direct relationship between high microbial populations, microseepage, and potential reservoirs.

Rapid alterations in light hydrocarbon microseepage are required to explain the observed microbial population patterns identified in these surveys. Although more stingent research is required to completely explain this phenomenon, initial investigations suggest the migrations of gases and liquids within the crust are controlled by solid earth tides.9 This seems to be the most logical mechanism for controlling the opening and closing of microfracture pathways within the crust allowing rapid microseepage to the surface.

PRODUCING FIELD EXAMPLES

Microbial surveys conducted over active and abandoned fields identified drainage patterns, infill locations, stepout potential, and perhaps redrill potential of repressured abandoned fields.

The hydrocarbon microseepage trends will be affected by the actual hydrocarbon withdrawal, reservoir heterogeneities, and bypassed pockets of hydrocarbons.

SACRAMENTO BASIN

In this example from one of the larger gas fields, all of the predicted microbial patterns have been identified (Fig. 1).

Clusters of low microbial populations over or adjacent to many of the producing wells identify the reservoir drainage radius around the wells. Some of these patterns are more radial while others were elongate. The elongated patterns are probably related to reservoir heterogeneities within the reservoir.

Areas of high microbial activity which were found between the producing wells indicate pockets of unproduced hydrocarbons within the field. These microbial highs were found to be either surrounded by radial withdrawal patterns or as elongated pods (Figs. 1, 2).

The first is probably related to yet undrained portions of the producing reservoir, while the second type appears to be related to reservoir fragments isolated from the producing well by reservoir heterogeneities. The second type would be potential targets for infill drilling.

Additionally, large, continuous clusters of elevated microbial values, in the northeast (Fig. 1), are similar to those associated with exploration programs. These indicate untapped reservoir potential and are either missed horizons for recompletions or flank potential for stepout drilling.

SOUTHEAST OKLAHOMA

In this example a large, elongated microbial low was identified within a larger area of microbial highs (Fig. 3).

Since most of our domestic microbial surveys are collected by clients, the results were presented to the client prior to GMT's knowledge of the production history of the survey area.

Later discussions revealed that a producing gas well is located near the center of this elongated microbial low. This microbial population pattern indicates a well drainage radius shaped by reservoir heterogeneities.

SAN JOAQUIN BASIN

The two examples presented here are a small portion of a much larger survey conducted near Buttonwillow, Calif. The results from these two abandoned fields demonstrate the use of microbial surveys to screen old fields for bypassed potential.

The first (Fig. 4) indicates anomalously low values occurring over the crest of the anticline while the flanks of the field have anomalously high values. Here there appear to be no bypassed pockets over the crest but the flanks demonstrate stepout potential. More detailed survey grids would have to be run to more accurately define the extent of this potential.

A second example (Fig. 5) has anomalously high values clustered at the crest of the anticline and low values on the flanks. This indicates either bypassed pockets, missed horizons, or recharging with hydrocarbons at the crest of the anticline. Again, a more detailed sample grid would have to be conducted to more accurately define the extent of this potential.

CONCLUSIONS

The detailed microbial evaluations of these producing fields have identified microbial population patterns related to hydrocarbon withdrawal and reservoir heterogeneities.

Integrating the microbial data with geologic and geophysical information can identify the drainage fabric within the reservoir to help define the most appropriate well spacing and well pattern and identify bypassed reserves for particular reservoirs.

Abandoned fields can be screened in the same manner for missed/bypassed potential. Ultimately this could increase economical reservoir exploitation by more effectively draining the reservoir.

ACKNOWLEDGMENTS

The authors thank IMCO Inc., Fort Worth, Tex., for release of their data in this report.

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