Biostratigraphy is a key tool for reducing risk and costs in the exploration process, but it risks getting lost in today's rush to rely solely on new 3D seismic and advanced well log tools. A threat to the long-term health of the upstream geoscience process is the reduced emphasis on specialist technologies.
These technologies, such as biostratigraphy and sedimentary petrology, often require substantial human input to acquire basic data, which has made them seem quaintly old-fashioned in an era of terabytes of high quality seismic and well log data. The reduction in costs and risks that the specialist technologies can provide in today's cost-conscious upstream geoscience is certainly not old-fashioned, however.
Thorough biostratigraphic analysis, including integration with other wells, most often costs less than 1% of the total cost of drilling an offshore well. As industry activity increasingly moves to deeper water, the costs of a single misplaced well increase dramatically, and the cost of a specialist technology such as biostratigraphy is small relative to the utility received.
Bonnie Field in the Gulf of Mexico demonstrates the cost effective use of biostratigraphy to answer geologic and drilling questions.
Improved technology allows biostratigraphy to add value in the development and production cycles of the upstream geoscience process as well as its well-recognized value in basin exploration.
Biostratigraphy has been used in the oil industry for more than 70 years to answer three main questions: correlation, age estimation, and paleoenvironment. Biostratigraphy is an important additional contributor to value in correlation, even in today's era of high-resolution 3D seismic and detailed log suites. All correlation tools are indirect indicators of the subsurface, and biostratigraphy can help resolve ambiguities or contradictions that exist in and between seismic and well logs.
Biostratigraphy remains the primary tool for numerical estimates of the age of strata as an input to basin modeling programs. Additionally, biostratigraphic analysis of paleoenviromental variables, such as paleobathymetry or paleoclimate, provide useful information for constraining sequence stratigraphic interpretations for predicting source, reservoir, and seal rock environments.
In the electronic age
Basic data acquisition in biostratigraphy is largely by visual examination with little automation. This results from the sophisticated analysis that the human visual system makes in recognizing a specific fossil and the difficulty that software engineers have had in trying to match this human skill. As a result, this observational step is the most labor-intensive in biostratigraphic analysis, and there is no substitute for an experienced paleontologist.
Low-cost biostratigraphy was quicker than a check-shot survey and showed that the objective section had been penetrated. The well was side-tracked and found gas-charged sands downdip adding more than 100 bcf of reserves.
The storage, analysis, and plotting of biostratigraphic data is generally computerized and can be readily integrated with geological and geophysical datasets to maximize its utility in the exploration process. Effective integration of biostratigraphic data requires knowledgeable input and control. This control can be supplied by a geoscientist with substantial knowledge of biostratigraphy, but is most efficiently supplied by a paleontologist who keeps up with changes in the field.
Applications
Effective and cost-efficient application of biostratigraphy is not simply a matter of ordering "standard" biostratigraphy of new wells from consultants and posting the results with any existing data on cross-sections and maps. Effective biostratigraphy requires several steps:
- Careful interaction with the consultants generating the basic data: This interaction ensures that their work answers immediate questions and will remain useful as geoscience analysis continues throughout the project cycle. This interaction involves both design of the initial analysis as well as communication of the data and interpretation and consideration of alternative interpretations.
- Integration of all biostratigraphic data available for a basin: The key element here is creation of a system so data developed at different times and by different sources is incorporated in a consistent stratigraphic framework. This method must then be followed rigorously to ensure that biostratigraphic data remain comparable over the entire geoscience cycle allowing for a clear separation of the internally consistent paleontologic data from its stratigraphic interpretation. The biostratigraphic system can be constructed within the databases used for well log interpretation.
- Interpretation of the consistent biostratigraphic database: Interpretation includes selection of useful correlation markers, analysis of their reliability, and even inclusion of data from outside the hydrocarbon industry (for example, the Ocean Drilling Program) where appropriate.
- Making these data readily available to geoscientists in their interpretation environments (workstation well log and seismic interpretation tools).
- Iterative discussions among geoscientists and biostratigraphers over inconsistencies and data reliability to achieve a robust final stratigraphic interpretation.
Example
Shell's Bonnie prospect in Eugene Island 89 was an attempt to find deeper reserves in an existing field adjacent to salt. The well location was based on seismic bright spots and correlation elsewhere in the field. The well plan intended no biostratigraphy during drilling. The original hole encountered no sand at the depths interpreted to contain the seismic amplitude anomaly. Two alternatives were then discussed:
- The objective section was below current total depth, so the well should be deepened.
- The well was in the objective section, but the sands had pinched out against the salt so the well should be side-tracked down-dip.
A check shot survey was considered to resolve this, but it would take several days to get the tool to the rig. Biostratigraphy could start quickly on hot-shot samples from the rig and demonstrated that the objective section had been penetrated. The well was sidetracked and encountered gas-charged sands downdip. Thus, less than $20,000 worth of biostratigraphy played a major role in adding more than 100 BCF of reserves.
The supply of new biostratigaphers and its vital training structure are nearly gone. It is an open question whether the petroleum industry has the will to overcome the 15-year hiring gap and rebuild the profession.
Although biostratigraphy has been conducted in the Gulf of Mexico for decades, the quality and precision of biostratigraphic analysis has continued to evolve. As an example, the number of stratigraphic markers in the Gulf of Mexico Plio-Pleistocene framework increased from fewer than 10 in the 1960s to more than 50 at present. This improvement included the incorporation of events from open-ocean fossils such as calcareous nannofossils that improve resolution in deep water.
Using these basinwide markers, which appear primarily in condensed sections, and statistical analysis of the fossil assemblage through the entire section, biostratigraphers can recognize local events between the basinwide markers. In some examples, this results in recognition of several times as many local events as basinwide events and significantly increases resolution, such as separate turbidite reservoir sands within fields.
Synthesis of biostratigraphic markers within a field, region, or basin to analyze their variability and correlate them more precisely with well log or seismic markers is possible with software to implement techniques such as graphic correlation or probabilistic stratigraphy.
The Technical Alliance for Computational Stratigraphy at the Energy & Geoscience Institute, University of Utah, has developed software to automate implementation of statistical analysis and plotting of well biostratigraphic data. This software allows for both quicklook and detailed examination to maximize the utility of this specialist tool. The analysis includes plotting of stratigraphic markers themselves and recognition of their patterns as well as algorithms for maximizing return of information on biofacies patterns that constrain sequence stratigraphic and systems tract interpretations for predicting and correlating source, reservoir, and seal rock for petroleum system analysis.
Continuing application of biostratigraphy using state-of-the-art statistical analysis and digital displays adds value to the geoscience process by enhancing exploration success and making production projects more cost effective.
References
Farley, M., Armentrout, J., 2000, Fossils in the oil patch: Geotimes (October), v. 45.
Fleisher, R., Lane, H., Coordinators, 1999, Applied Paleontology: in Beaumont, E.A. and Foster, N.H. (eds.), Exploring for Oil and Gas Traps: AAPG Treatise of Petroleum Geology, chapter 17.
Jones, R., Simmons, M., editors, 1999, Biostratigraphy in Production and Development Geology: Geological Society, London, Special Publication No. 152.
Applied Biostratigraphy short course (in conjunction with GCAGS, AAPG and other meetings) ([email protected]).
In search of biostratigraphers
Biostratigraphy has much to contribute to the success of hydrocarbon exploration and production, but the industry faces the problem that without biostratigraphers, the effective application of biostratigraphy is not possible. As with the industry geological and geophysical population, biostratigraphers are an aging population with a median near 50, and there are very few industry paleontologists below the age of 40. The supply pipeline of biostratigraphers has been nearly shut off, and the industry training structure that was vital to the success of today's biostratigraphers is also nearly gone.
To be effective, a biostratigrapher must have considerable training in graduate education and in best practices in the petroleum industry. Graduate school provides fundamentals in a paleontological specialty and geological education. The initial years in industry provide experience in the basins and stratigraphic intervals important to industry and skills in integrating biostratigraphic data with seismic and well logs. Development of integration skills is key to long-term biostratigraphic effectiveness and broadens the biostratigrapher's utility to be a valued member of the exploration or production team.
Graduate study allows adequate time to acquire skills in a relevant fossil group with the guidance of a skilled faculty advisor. Graduate programs in North American universities that train micropaleontologists have gradually turned away from training relevant to the oil industry, in part due to the lack of jobs for graduating students. As faculty retire, their replacements focus on topics such as paleoceanography and climate change favored by research funding organizations. This has reduced the supply of paleontologists to industry.
As graduate education has an advisor, skills acquisition in the petroleum industry is most effective through mentoring. Historically, the large oil companies provided the continuity of acquired knowledge and the mentors required to educate each new generation of biostratigraphers. Large oil companies also provided support if it was necessary to train a biostratigrapher in a new fossil specialty because paleontologists cannot instantly switch from one group to another.
As biostratigraphic staffs at the majors have shrunk to the vanishing point, this mentoring has become nearly extinct. Most basic biostratigrahic data today is provided by consultants, who have neither the time nor incentive to train a new generation. Effective integration of these data with well logs and seismic depends on a small cadre of biostratigraphers within oil companies.
The petroleum industry has embraced high-tech tools, but their effective application requires support from geoscientists versed in specialties, including age and environmental calibration from biostratigraphers. It is an open question whether the petroleum industry has the will to overcome the barriers created by the gap in hiring over the past 15 years and the breakdown of the traditional means by which biostratigraphers reached maximum effectiveness.
