PART I: This is Part 1 of a two-part series on the use and effectiveness of geochemistry in different phases of an oil and gas field's life. Part II will appear in a subsequent issue.
Hydrocarbon assets have a life cycle. They begin as a location on a map and an idea, and proceed through stages including basin evaluation, prospect evaluation, discovery drilling, developmental drilling, production, enhanced recovery, abandonment and reclamation. During this life cycle there are constant changes in the types of information needed to make decisions about developing and managing the asset.
In other words, the questions continually change. For example, in the earliest stages of asset life, explorationists need to know if the basin contains economic quantities of oil and gas, locations of the largest accumulations, the distribution of oil versus gas, and of oil quality.
Later, the significant information includes locations of missed opportunities, reservoir compartmentalization, pay allocation, and proper functioning of production equipment. After that, production engineers need to know if enhanced recovery techniques are working properly and if reservoirs are fully tapped. Throughout the asset life, it is important to know that the environment is being properly preserved.
Among the tools available to asset managers to address these questions, petroleum geochemistry has historically been utilized during the exploration phase. However, geochemistry has proven to be a useful tool in all phases of petroleum asset life.
Exploration geochemistry
Application of geochemical techniques to petroleum exploration can provide an increased understanding of the hydrocarbon generation, migration, and accumulation processes within a basin prior to drilling. As more samples become available, geochemical techniques can outline complex basin filling histories, explain unusual oil and gas distributions, and identify the sources of oil and gas. When considered against random drilling, geophysics (trap size) alone provides a forecasting efficiency of 28%, while geophysics in conjunction with geochemistry provides a forecasting efficiency of 63% for locating hydrocarbons during exploration (Sluijk and Parker, 1986). Petroleum geochemistry has proven to be an effective and inexpensive tool for reducing exploration risk.
In applying petroleum geochemistry to exploration problems, a major focus is the analysis and interpretation of the compositions of the complex hydrocarbon fluids. The composition of oil and gas is dependant on many factors, including the original source material, the source maturity, the migration distance, the attributes of the carrier bed, and post accumulation processes such as biodegradation, leakage, thermal stress and water-washing. Each of these processes establishes or alters the fluid composition in predictable ways. Thus, the fluid composition can be used to detect and characterize these processes.
An example of applying geochemistry to determine oil source can be seen in a study of an offshore Tertiary rift basin in eastern Asia (Bissada, Elrod et al., 1993). The basin contains three large deltaic systems along a northeastward trend. Examination of the oil compositions from each of the three systems indicated that the northernmost oil accumulation was sourced from a hypersaline lake facies, while the southernmost was from a freshwater lake facies. The center accumulation has compositional and isotopic characteristics of both types of sources and was determined to be a mixture of the two oil types.
When the geochemical findings were placed in the geologic setting, it was clear that the two oil types were generated from two separate generative troughs and the mixed oil was accumulated on a ridge situated such that it could receive oil from both troughs. The findings had significant implications with regard to further exploration in the area.
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 decisions. This is illustrated by a study done on the Barinas Basin in Venezuela by PDVSA and HARC (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 (green area) 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.
One of the problems with applying geochemical findings to exploration has been quantifying the impact of geochemical results on exploration risk. Although the geochemical findings are generally useful for ranking prospects and establishing hydrocarbon charge and quality, it is often difficult to quantify the impact of the findings on exploration risk.
In recent years, software packages (ex: HARC's RapidVal) have enabled the geoscientist to evaluate geologic and geochemical data in a risk-based framework that can directly address the probability of reaching an economic threshold. Such packages can be used at any stage of the exploration process and can pinpoint which additional data would be most valuable for more accurately assessing the risk.
Enhancing 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 compo sitions. Production and en hanced 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 in creasingly applied to development, production, and enhanced recovery pro cesses. Petroleum geochemistry is proving to be an effective and inexpensive tool these processes.
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.
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 by Pemex and HARC (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.
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), Intevep, Texaco, and HARC analyzed hydrocarbons from different sand units 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.
References
A list of references is available upon request.
Applying geochemistry effectively:
- Collect samples: During exploration and development drilling, sediment and all encountered hydrocarbon fluids should be sampled and stored.
- Analyze hydrocarbon samples when initially taken: Simple compositional and isotopic analyses are not expensive, but can be valuable.
- Think geochemistry: Virtually everything that happens to hydrocarbons during generation, migration, accumulation and production affects their composition, making geochemistry a useful tool in every stage of asset life.
