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Geochemistry and Petroleum Asset Life

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By Louis W. Elrod, Ph.D. Intertek Westport Technology Center, Page Two

Understanding the Distribution of Oil Quality in a Basin:

• Geochemistry can also be important in assessing the distribution of hydrocarbons in a basin with regard to oil versus gas, high-sulfur versus lower-sulfur oil, heavy versus light oil, and other parameters. This is illustrated by a study done on the Barinas Basin in Venezuela (Anka, Callejon et al., 1998). An integrated geochemical-geologic approach, which included computer simulations, was able to determine that the source of the oil was the more distant La Luna formation, not the nearby La Morita formation, and that the basin had a complex filling history.
  
  The Barinas Basin study revealed that there was an initial pulse of oil that filled the reservoirs, however, the reservoirs were shallow and the oil was subsequently degraded. Next, thrusting rapidly buried reservoirs and source sequences in the northernmost part of the basin. This resulted in cracking of the reservoired oils in the deeply buried sections to condensates and additional gas condensate generation from the local source rocks. Finally, the source in the center to southern parts of the basin is currently mature and expelling oil to the reservoirs above. This filling history explains the distribution of oil in the basin, with gas condensates in the north, lighter oils (initially degraded, then lighter oil added) in the center to southern parts of the basin, and heavy degraded oils in the south.

Enhancing Hydrocarbon Asset Life:

• As we have seen, the processes that result in generation and accumulation of hydrocarbons determine the composition of the fluids and can be characterized based on the fluid compositions. Production and enhanced recovery processes also alter the composition of the hydrocarbon fluids, therefore composition of the fluids can also be used to monitor and characterize production and enhanced recovery processes.
  
  In recent years, geochemistry has been increasingly applied to development, production, and enhanced recovery processes. Petroleum geochemistry is proving to be an effective and inexpensive tool these processes.

Identifying Missed Payzones:

• Geochemistry can be utilized to identify new or missed opportunities during exploration and development. For example, in some situations, a reservoir can be uplifted or breached resulting in a sudden change in the pressure-temperature relationship in the reservoir. When the conditions are right, this can cause the oil to separate into two phases, with the lighter phase migrating to a shallower reservoir.
  
  The resulting condensate and residue have distinctive geochemical compositions that indicate the phase separation history of the hydrocarbons. When the residue portion of the hydrocarbons is encountered during drilling, it indicates that a lighter phase has migrated up dip and may have accumulated there. Conversely, if the condensate portion were encountered first, that would indicate that a residue hydrocarbon accumulation is likely somewhere down dip.

Delineating Reservoir Compartmentalization:

• During production of oil or gas, it is important to understand the geometry of the reservoir, that is, the locations of any faults or permeability barriers that could affect production. Since the reservoirs are filled with fluids and the composition and characteristics of the fluids are sensitive to reservoir processes, we can expect fluids within separated compartments of the reservoir system to have different compositions. In recent years, high-precision analyses of the compositions of oil and gas have been successfully employed to delineate reservoir compartments and communication.
  
  In a recent study (Ramos, Callejon et al., 1999), geochemistry was used to determine reservoir compartmentalization within a gas field in Mexico. Seventeen gas samples from the field were analyzed for component and isotopic compositions. As expected, the gases produced from different fault blocks had different compositions. However, the results revealed significant differences among some gases produced from the same fault blocks. Since the rate of diffusion of gas is relatively rapid, any significant difference was interpreted to be due to a barrier to fluid communication. Thus, the data were used to identify previously unknown barriers to migration within the fault blocks and provided a more in-depth understanding of fluid movement within the field. Subsequent comparison indicated that the geochemical conclusions were consistent with recently acquired 3D seismic data.
  
  In another study, geochemistry provided key information that distinguished between two geologic interpretations of a field. In the study (Kaufman, Ahmed et al. 1989), the geologic information was ambiguous leading to two interpretations of the location of significant faults in the field. To distinguish between the two models, oils from the producing wells were analyzed and the result presented in a “star diagram,” a plot of the data on a radial axis that enables easy recognition of differences. The results revealed three groups of similar oils that were consistent with one of the geologic interpretations. This clearly indicated that one geologic interpretation was more correct and improved understanding of the field for further development.

Maximizing Waterflood Efficiency:

• Geochemistry can also be useful for planning and monitoring enhanced recovery processes. In a study of the Centro Lago Field, Venezuela (Elrod, Vierma et al. 1997), hydrocarbons from different sand units were analyzed for the purpose of identifying fluid flow units within the reservoirs and optimizing a waterflood project. The data identified barriers to fluid flow that were previously undetected and reservoir connectivity among some of the units that was previously unknown. The results were critical in planning and optimizing a waterflood program in the field.

Monitoring Produced Fluids:

• The fact that hydrocarbon fluid compositions are affected by geologic and reservoir processes is what makes fluid composition useful for studying hydrocarbon generation, migration, and entrapment history. Production and enhanced recovery processes also affect hydrocarbon fluid compositions in predictable ways. Slow or sudden changes in the composition of produced fluids can indicate changes in reservoir characteristics, subsurface fluid conditions, or problems with the production equipment. Periodic monitoring of produced hydrocarbon fluids can identify such changes at low cost. Periodic geochemical analysis of produced fluids has been successfully applied as a useful tool to a number of production issues.

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