D.L. Hall1, S.M. Sterner1, W. Shentwu1, and M.A. Bigge1
Search and Discovery Article #40042, (2002)
*Adapted from D.L. Hall’s presentation to the Tulsa Geological Society, March, 2002.
Mechanical crushing of core or cuttings samples, followed by bulk analysis of evolved organic and inorganic fluid inclusion volatiles by quadrupole mass spectrometry using a rapid, automated sampling system, allows for nearly continuous stratigraphic mapping of paleofluids in the subsurface. This technique is called Fluid Inclusion Stratigraphy (FIS). Application to over 6000 wells worldwide has established the utility of this technique in addressing elements of risk associated with petroleum exploration and production.
Key indicator compounds and compound ratios are used to document petroleum migration through specific stratigraphic intervals, evaluate product type and quality, and establish a time-integrated effectiveness for potential seals. Additionally, FIS has proven proficient in identifying zones that are near reservoired petroleum, even if that petroleum did not migrate directly through the analyzed section. In these cases, water-soluble hydrocarbons, particularly benzene, toluene and/or organic acids, occur in anomalous concentration and can often be shown to have migrated by diffusion through an aqueous phase away from known petroleum accumulations.
More recently, production applications have been developed, particularly in the general area of reservoir and resource characterization. Known applications include delimitation of pay zones, petroleum-water transition zones and fluid contacts for reserve estimation and EOR applications, as well as assessment of reservoir compartmentalization and connectivity. Application of this technology in real time on drilling wells has been used to influence drilling and testing decisions.
Integration of FIS with classical fluid inclusion methodologies provides additional and more specific data on petroleum fluid properties, salinity of associated aqueous pore fluids and timing constraints for petroleum migration and reservoir charging. These data can be used as input into basin models and to assess timing of petroleum charge as it relates to trap configuration and reservoir quality. Analysis by solvent-extraction or thermal-extraction GCMS of samples pre-screened and targeted by FIS as being petroleum-inclusion-rich effectively establishes links among petroleum-fluid-inclusion-bearing intervals and source rocks, thereby providing better control on migration pathways. Specific examples below illustrate how to use the above techniques to:
• Evaluate whether or not petroleum has moved through a dry hole with no shows that is adjacent to a prospect.
• Determine whether oil or gas should be anticipated in a given area.
• Assess the probability of encountering a deep, hydrocarbon-charged reservoir below a shallow boring.
• Pick perforation points in a problematic pay zone.
• Delineate a pay interval and fluid contact.
• Identify a failed seal.
• Infer a nearby oil field from a wet well with no shows.
• Explicitly evaluate bound-water salinity in the absence of a water analysis.
Fluid inclusions are micron-scale, fluid-filled, isolated cavities in or between crystals in rock material (Figures 1, 2, and 3). They form during subsurface diagenetic processes in which mineral cement is added to intergranular pore space or microfractures. Fluid inclusions are representative of past or near-present-day pore fluids, and they track movement of aqueous and petroleum fluids.
Fluid inclusions may be the freshest samples of reservoir fluids we have. They remain even after pore fluids change (correspondingly with applications for fossil migration paths, flushed reservoirs and tilted oil-water contacts). Also, they record multiple charges, temperatures and pressures.
Figure 7. Updip prospectivity from wet well. Fluid Inclusion Stratigraphy (FIS) of downdip dry hole documents migration of liquids through reservoir section. FIS of subsequent oil discovery verifies off-structure signal.
Figure 8. Local prospectivity / deeper potential. Fluid Inclusion Stratigraphy (FIS) of rich gas condensate discovery indicates gas and liquids prospectivity of area. FIS of dry hole from same basin shows little encouragement for liquids prospectivity.
Figure 10. 3-D stochastic modeling of FIS data; Haltenbanken area, offshore Norway. From FIS, anomalous concentrations of methane, other light hydrocarbons, and sulfur compounds in the shallow subsurface probably reflect anaerobic alteration of vertically seeping gas-range compounds from depth by sulfate-reducing bacteria.
Figure 14. Seal effectiveness shown by analyses of part of the Devonian section in a western Canada well, which, based on FIS, appears to indicate seal breach in the Lobstick and Ireton and evidence of a paleocolumn of gas in wet Leduc reservoir (Compartment 3).
Inferring Updip Prospectivity from a Wet Well (Figure 7)
Predicted Drilling Success Rate (Figure 11)
Regional Evaluation (Figure 12)
Identifying Seals (Figure 13)
Seal Effectiveness (Figure 14)
Shown in Figure 20 is an excellent top seal to gas reservoir. The gas column is delineated; chemistries track porosity. The present-day gas-water contact is defined; TSR products are identified; and moderately sour gas is indicated. The interpretations were verified with production tests.
Figure 21 shows the log suite, along with FIS results, for a well that was an oil discovery, where there was a shallow hole that was not logged. The shallow FIS anomaly is in regionally productive interval with porosity and staining, and the chemistry is analogous to deeper known pay zone.
EOR Application in a Mature Field (Figure 24)
Figure 27. Timing of biodegradation. (Left) Relatively early carbonate vein contains interpreted original and biodegraded oil. Degradation may have occurred during vein formation. (Right) Later vein is barren, hence may postdate both petroleum migration and biodegradation. Collection of fluid data on inclusions from these respective veins may help constrain timing of these events.
Figure 28. Example of CSIRO’s GOI (grains containing oil inclusions) technique for paleo-saturation determinations shows empirical threshold separating oil and water zones (see Liu and Eadington, 2000).
Bernard, B. B., 1978, Light hydrocarbons in marine sediments: PhD thesis, Texas A&M University, College Station, Texas, 144 p.
Burtell and Jones, 1996, Benzene content of subsurface brines can indicate proximity of oil, gas: Oil & Gas Journal, June 3, 1996, p. 59-63.
Faber, E., and W.Stahl, 1984 Geochemical Surface Exploration for Hydrocarbons in North Sea: AAPG Bulletin , v. 68, p. 363 – 386.
Larter, S.R., and A.C. Aplin, 1996, Geochemical Application to Reservoir Assessment: AAPG Short Course Notes.
Liu, K., and P.J. Eadington, 2000, A new method for identifying oil migration pathways by combining analysis of well logs and direct oil indicators (Abstract): AAPG Bulletin, v. 84, no. 13 (annual meeting abstracts).