By Louis W. Elrod, Ph.D. Intertek Westport Technology Center, Page One
Petroleum Geochemistry plays an important role in every phase of asset life:
Virtually everything that happens to hydrocarbons during generation, migration, accumulation, and production affects their composition. Geochemistry measures these compositional changes and can detect important variations. These variations and changes can reveal information that is critical to proper and efficient management of the hydrocarbon asset. Geochemistry is a tool for decision-making during every phase of hydrocarbon asset life.
Geochemistry and Hydrocarbon Assets:
- Hydrocarbon assets begin as a location on a map and an idea. During their life cycle, they proceed through stages that include basin evaluation, prospect evaluation, discovery drilling, developmental drilling, production, enhanced recovery, abandonment and reclamation. During the progress through this life cycle there are constant changes in the types of information needed to make decisions about developing and managing the asset.
Thus, for the asset manager, 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. In later stages, required 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 assure that the environment is being properly preserved.
Many tools are available to asset managers to address these questions. Among them, petroleum geochemistry has historically been utilized during the earliest stages, the exploration phase. However, in recent years geochemistry has proven to be a useful tool in all phases of petroleum asset life.
Geochemistry in Exploration:
- Application of geochemical techniques to oil and gas exploration provide increased understanding of 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.
Identifying Mixed-Sourced Petroleum:
- 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.
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