Oil Hydrogeology:
The Role of Water in Hydrocarbon Migration and Storage

Collaborators:

(1) Prof. Tom Torgersen, University of Connecticut.

(2) Thjis van Soest, University of Connecticut and Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory.

We have found that production fluids from oil and gas reservoirs tend to be enriched in heavy noble gases (krypton and xenon) of atmospheric origin. For instance, hydrocarbons produced from the Elk Hills National Petroleum Reserve in the southern San Joaquin Basin, California have ¹³²Xe/³⁶Ar ratios up to ~576 times the ratio in air (solid and open circles, Figure 1). These represent the highest Xe/Ar ratios observed in natural terrestrial fluids and the enrichment factors and/or gas concentrations are significantly greater than that found in other hydrocarbon reservoirs, such as the Paris and Alberta Basins.

Figure 1: Xenon enrichment factors [F(¹³²Xe) values defined as the ¹³²Xe/³⁶Ar ratio in the sample normalized to the air ratio] are plotted as a function of the ³⁶Ar concentration. Open and solid circles represent samples from the southern San Joaquin Basin where the highest Xe enrichment factors have been observed. The colored areas represent the range of values observed in other hydrocarbon fields.

If air saturated groundwater is the only noble gas source for the water-hydrocarbon system, then the heavy noble gas enrichments observed in the San Joaquin Basin cannot be explained by equilibration of oil with air saturated water (blue dashed line, Figure 1) and/or secondary enrichment by Rayleigh or batch distillation of an oil-water-gas system (red dashed lines, Figure 1).

Therefore, we believe the source of the excess heavy noble gas component was adsorbed air initially trapped in petroleum source rocks and that this component was expelled and mixed with hydrocarbons during oil-gas expulsion and primary migration. Support for this hypothesis is provided by laboratory studies of potential petroleum source rocks. If our hypothesis is true, then the heavy noble gas component is a natural tracer for fluid related processes occurring during expulsion, migration, and storage.

One such process is hydrocarbon interaction with air saturated groundwater, as depicted in Figure 2 which shows that the San Joaquin noble gas data define a mixing trend between the a kerogen-rich fraction isolated from shale and analyzed for noble gas content by Frick and Chang (Frick and Chang, Proc. Lunar Sci. Conf 8, 1977) and air saturated water (ASW). Apparently, the heavy noble gas-enriched component mixed with the hydrocarbons during oil/gas formation and expulsion is subsequently diluted by noble gases extracted from air saturated groundwater as the oil and gas phases interact with water during secondary migration and storage.

Figure 2: The Kr/Ar and Xe/Ar enrichment factors for the San Joaquin Basin data (open and filled circles) define a mixing trend between air saturated groundwater (ASW, solid triangles) and the composition of a kerogen-rich phase isolated from shale and analyzed by Frick and Chang (1977). The open triangles and the light green shaded region depict the possible range in values for the oil-water-gas distillation model portrayed by the dashed curves in Figure 1.

The ability to use noble gas geochemistry to quantify oil-gas-water interaction has several important applications.

(1) The extent to which the heavy noble gas-enriched component is diluted by noble gases extracted from air saturated water places firm limits on the effective volume of water that has been in contact with the hydrocarbons (Figure 3).

Figure 3: The oil-water dilution model. The data symbols (open and filled circles) are the same as in Figures 1 and 2. The composition of carbon-rich sediments depicted by the shaded region covers the range in measured compositions with the highest degree of Xe-enrichment and represents the assumed initial hydrocarbon compositions prior to water interaction. The solid lines are dilution trajectories depicting the change in noble gas composition of oil as it quantitatively extracts noble gases from air saturated groundwater during secondary migration and storage. The tick marks along the trajectories are water dilution factors in units of gram-water/gram-carbon. The San Joaquin hydrocarbons have been in contact with only ~6-80 gm-H₂O/gm-carbon, indicating that water has played a very limited role in the very large San Joaquin system which has produce over 2 billion barrels of oil.

(2) The limits on the volume of water that has been in contact with the hydrocarbons constrains oil and gas formation/migration models and the time scales for these processes.

(3) Field wide systematic trends in water dilution factors can be used to map oil and gas migration pathways and to delineate different hydrocarbon source regions.

Related Publications:

Torgersen, T. and Kennedy, B.M., Air-Xe enrichments in Elk Hills oil field gases: role of water in migration and storage, Earth Planet. Sci Lett. 167, 239-253, 1999.

Funding:

This project was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Engineering, and Geosciences Division of the U.S. Department of Energy (http://www.er.doe.gov/production/bes/bes.html).