Higher Education

Field course gives petroleum geoscientists a top-to-bottom view of the structural geology of oil exploration

By David A. Ferrill, PhD     image of PDF button


Dr. David A. Ferrill is a principal scientist in the Center for Nuclear Waste Regulatory Analyses at SwRI. He specializes in investigations of faulting and fracturing of rock, concentrating on improved understanding of hydrocarbon and groundwater movement, safe disposal of radioactive waste, and structural geological training and contract consulting for the oil and gas industry. A former oil company exploration geologist, Ferrill leads SwRI's courses in structural geology for oil industry geophysicists and geologists.


Early exploration geologists identified oil reserves under the surface of the earth by mapping geologic structures that were exposed. Over time, oil exploration companies identified these exposed structures and depleted them of hydrocarbons. In the past 40 years, oil and gas exploration has been driven deeper underground, requiring the use of seismic reflection imaging to find hydrocarbon reserves.

Modern seismic reflection techniques and a new generation of interpretation software have revolutionized hydrocarbon exploration and production. However, interpreting geologic structures several kilometers below the surface remains challenging. Interpreters must have skills that enable them to identify areas of potentially complex deformation that can either enhance or ruin a petroleum exploration prospect.

The best interpreters are not only technically adept, but also have an intuitive feel for different deformation styles. They must be aware of the range of geologic structures that can occur and of the potential for structural features to influence the economic risk in drilling for hydrocarbons. Recognizing this, oil companies take training in this skill seriously, encouraging staff to hone skills to be even more effective at interpretation.

Southwest Research Institute has created a training course that provides petroleum industry geoscientists with insight into the nature of geologic structure development and interaction to help oil companies in their search for petroleum.


The map at the left shows driving routes and the areas investigated during the geology course.


Background

The course, developed originally as an outgrowth of the Institute's research in support of analyses at Yucca Mountain, Nev., for the U.S. Nuclear Regulatory Commission, has been training petroleum industry geoscientists in modern concepts of structural geology since 1997. In the field, participants analyze complex geologic structures that are analogous to those commonly found in oil and gas exploration provinces around the world. The goal is to provide geologists and geophysicists with field experience to recognize and interpret subtleties of structural style and recognize weak points in interpretations.

SwRI developed its curriculum based on the results of ongoing research in the region and new investigations conducted in preparation for the course. SwRI geologists selected field localities to provide 2- and 3-dimensional exposures that allow course participants the unique opportunity to literally walk within fault systems and associated fault blocks and fold structures that are direct analogs for hydrocarbon-bearing basins and traps around the world, such as those formed in the Gulf of Mexico, North Sea, Niger Delta, Gulf of Thailand and Bohai Bay, China. Field examples include hydrocarbon reservoir-, field- and basin-scale structures, with an emphasis on the geometry of natural fault systems and related deformation processes.

Curriculum

Key to interpreting complex geologic regions is understanding how fault systems develop over time. Participants in SwRI's course study the development of large normal faults and associated small-scale faulting. Large faults are often the bounding structures of hydrocarbon traps, whereas small faults control fluid flow characteristics within reservoirs. Studying actual fault systems helps participants refine their interpretation skills when they return to their computers.

The course focuses on four localities in southern Nevada and eastern California, between Owens Valley in the west, Death Valley to the east, Mojave Desert to the south and Long Valley Caldera to the north. This region is experiencing rapid deformation caused by the northwestward motion of the Pacific oceanic plate with respect to the North American plate. The Basin and Range region, which extends from central Utah to eastern California, is extending (widening) at a rate of about 11 mm (0.43 in) per year. The bulk of this deformation is concentrated in the relatively narrow region from Death Valley to Owens Valley. This geologically rapid deformation, plus relatively low rates of erosion and deposition, have produced 4.5 km (2.8 miles) of vertical topographic relief between Mount Whitney and Death Valley, the highest and lowest points in the contiguous United States.


The Nevada and California desert offers students an effective structural geology laboratory. Dante's View, with an elevation of 1,650 m (5,413 feet), provides an excellent perspective on central Death Valley. Shown here is part of the Black Mountains capped by the Central Death Valley normal fault. Normal faults such as this are common in areas of crustal extension.


From Las Vegas, the class heads west to Death Valley, which offers the classic example of a pull-apart basin for analysis of basin scale deformation. Instructors also review sedimentation patterns in Death Valley and discuss how this applies to oil and gas reservoirs elsewhere.

Following field stops in and around Death Valley, students investigate the structure and sedimentation patterns of the Crater Flat basin. Crater Flat basin is an asymmetrical structural depression (half graben) that has developed between Bare Mountain on the west and Yucca Mountain on the east. Although high in elevation with respect to the sedimentary basin, Yucca Mountain, the proposed site of the high-level nuclear waste repository, has been deformed by faulting and tilting in response to movement on the Bare Mountain fault.

The southwestern flank of Bare Mountain reveals a normal fault system that has been tilted on edge, revealing a cross section in the mountain side similar to the view typically seen in a seismic reflection profile. Because the steep southwest flank of Bare Mountain is approximately perpendicular to the fault system, course participants are able to explore the fault system in profile, studying fault zone and fault block deformation.

Structural features in this portion of the Basin and Range are exceptionally well preserved and exposed. In some areas, relatively young volcanic layers that were once nearly horizontal have been deformed in response to Basin and Range extension. Structurally, these faulted volcanic layers are remarkably similar to deformed marker horizons mapped in 3-D seismic surveys. The course takes advantage of several of these examples (Saline Range, Yucca Mountain, and Volcanic Tableland) as natural laboratories.

Students study the Saline Range, a high structural block within the larger Eureka Valley-Saline Valley pull-apart basin system. Displacement gradients along individual faults and displacement transfer between faults at the scale of an oil field or specific trap are the subject of the participants' investigation of the Saline Range during the course.

The Volcanic Tableland in northern Owens Valley is the final field area studied and the most accessible and spectacular example of a young deformed marker layer. The Volcanic Tableland is capped by the 738,000-year-old Bishop tuff. The natural erosion of softer nonwelded volcanic ash from above the stronger welded Bishop Tuff has produced 3-dimensional exposure of the faulted hanging-wall of a classic "rollover" structure.

Consequently, topography represents a 3-dimensional structural horizon map, meaning that topographic maps can be treated as structure contour maps and used to interpret structures that can later be walked over and observed in the field. These maps reveal a network of normal fault scarps deforming the rollover above the White Mountain fault and include a crestal graben or trough bounded on both sides by normal fault systems


Bare Mountain is a block of Precambrian and Paleozoic strata exposed in southwestern Nevada. The block is bounded on the east by the Bare Mountain fault. Students study the exposure at different scales, first from an oblique aerial photograph to interpret the overall structural geometry and style (a). Next students explore faults and fault blocks on foot (b; location circled in a) to get an understanding of the mechanisms by which rocks form within the fault system (c).


The scale and style of faulting in the Volcanic Tableland is a superb analog for extensional deformation at the field to reservoir scale in exploration areas such as the Gulf of Mexico and the Niger Delta and numerous other oil producing regions around the world. Using the topographic imagery, interpretation exercises progress from low-resolution to high-resolution by increasing the grid density of the topographic data. Exercises focus on the three- dimensionality of deformation and emphasize interpretation skills that are applicable to 3-D seismic data.

SwRI's suite of field examples gives students an opportunity to study a single area with data ranging in structural resolution from kilometers to meters, an invaluable experience for studying the influence of data resolution (such as the spacing of seismic lines in a 3-D seismic reflection survey) on data quality and interpretability. Determining how the resolution of data influences an interpreter's eye and how the interpreter can overcome the pitfalls inherent in low-resolution data is an important lesson of the course.

Using the Volcanic Tableland as a natural laboratory, students start in a classroom at the White Mountain Research Station interpreting low-resolution topographic data. They then progress to higher and higher resolution until they are interpreting data based on a one-meter (3.3-foot) grid. SwRI scientists developed the high-resolution grid of data for the course by field mapping using a differential global positioning system to provide accurate horizontal and vertical positioning.


Students interpret topographic maps at varying resolutions as part of their classroom instruction. The coarsest data set (a) is based on 3-D data with a 160 meter (525 feet) pixel resolution. Students also interpret the 40 meter (131 feet) pixel resolution data set (b). The side-looking airborne radar (SLAR) data (c) provides a data set with 12 meter (39 feet) pixel resolution. The SLAR data set is at higher resolution than most 3-D seismic surveys and shows the details of fault segments and interaction in the southeastern part of the Volcanic Tableland.


After working down in scale and to highest resolution data, students go to the field and explore on foot the same structures that were previously analyzed in the classroom. They also develop and test scaling relationships for faulting in the Volcanic Tableland. Finally, students study small faults in outcrop and consider the importance of such faults for producing anisotropic permeability in hydrocarbon reservoirs. Course participants are often awed by the experience of being surrounded by direct analogs for geologic structures they see only electronically in their day-to-day data interpretation work.

Work in the Yucca Mountain and Volcanic Tableland areas provided the impetus for the origin and development of the award-winning 3DStressTM computer program. Originally developed at SwRI to analyze the behavior of faults and the resulting potential for earthquakes in the Yucca Mountain region, 3DStress has been used widely in the oil industry to assist in prospect evaluation, field development and well design.

3DStress allows geologists to analyze faults and identify those that could be either barriers or conduits for oil and gas migration and production. Research in the Volcanic Tableland and other areas has led SwRI scientists to expand the functionality of the program, which now assists users in estimating strains around faults, using rock failure data in interpreting stress fields and predicting fault orientations.


The Fish Slough fault system in the southeast corner of the Volcanic Tableland, shown in three views at left, provides an excellent field example for analysis of fault growth by segment linkage. After nearly 150 meters (492 feet) of vertical displacement of the capping welded Bishop Tuff, (a) fault segments have linked by curved lateral propagation. A footwall cusp at the intersection of two fault segments has been faulted from the footwall, straightening the corrugated fault trace. SwRI scientists produced high-resolution digital elevation models and slope maps by mapping the top of the welded Bishop Tuff using differential GPS. Maps shown here are based on hundreds of thousands of data points with horizontal accuracy of 20 cm (7.9 inches) (b and c). Higher precision was used to map small faults within the breached relay ramp in the center of the map with 2 cm (0.8 inches) horizontal and 10 cm (3.9 inches) vertical precision.


Conclusion

SwRI's structural geology field school incorporates world class field examples and data interpretation exercises to provide petroleum industry geoscientists with insight into the style and development of geologic structures that are common in oil and gas fields and their environments. With examples at the reservoir, field, and basin scale, this course helps hone the skills of exploration and production geoscientists alike.

Further development of the field course has produced ideas that are fed back into SwRI's research on fault system development, fault block deformation and influence of faults on hydrocarbon movement and groundwater flow.

SwRI scientists are planning to organize a Geological Society of America field forum to bring together geoscientists working in the Volcanic Tableland to share research being conducted in the Volcanic Tableland and the surrounding region.

Comments about this article? Contact Ferrill at (210) 522-6082 or david.ferrill@swri.org.

Acknowledgments

Current and former SwRI staff members and consultants Laura Connor, Mike Conway, Rebecca Emmot, Peter La Femina, Ron Martin, Larry McKague, Alan Morris, Wesley Patrick, Darrell Sims, John Stamatakos and Deborah Waiting have made major contributions to data collection, materials/figures and course instruction, and preparation and review of this article. The course benefited from early guidance from Dr. James Handschy, formerly of Shell Oil company and now chief geologist for ConocoPhillips, and later suggestions from Dr. Charles Kluth and Dr. Wayne Narr, both with ChevronTexaco. SwRI appreciates company participation from Shell, ChevronTexaco, ConocoPhillips, Petrocanada, and their precursor organizations.

Geology Course References

Ferrill, D.A., Morris, A.P., 2003. Dilational Normal Faults. Journal of Structural Geology 25, 183-196

Ferrill, D.A., Morris, A.P., 2001. Displacement gradient and deformation in normal fault systems. Journal of Structural Geology 23, 619-638

Ferrill, D.A., Morris, A.P., 1997. Geometric considerations of deformation above curved normal faults and salt evacuation surfaces. The Leading Edge 16, 1129-1133.

Ferrill, D.A., Morris, A.P., Stamatakos, J.A., Sims, D., 2000. Crossing conjugate normal faults. American Association of Petroleum Geologists Bulletin 84 (10), 1543-1559.

Ferrill, D.A., Stamatakos, J.A., Sims, D., 1999. Normal fault corrugation: Implications for growth and seismicity of active normal faults. Journal of Structural Geology 21, 1027-1038.

Ferrill, D.A., Winterle, J., Wittmeyer, G., Sims, D., Colton, S., Armstrong, A., Morris, A.P., 1999. Stressed rock strains groundwater at Yucca Mountain, Nevada. GSA Today 9 (5), 1-8.

Ferrill, D.A., Morris, A.P., Jones, S.M., Stamatakos, J.A., 1998. Extensional layer-parallel shear and normal faulting. Journal of Structural Geology 20, 355-362.

Ferrill, D.A., Stamatakos, J.A., Jones, S.M., Rahe, B., McKague, H.L., Martin, R.H., Morris, A.P., 1996. Quaternary slip history of the Bare Mountain Fault (Nevada) from the morphology and distribution of alluvial fan deposits. Geology 24, 559-562.

Morris, A.P., Ferrill, D.A., 1999. Constant thickness deformation above curved normal faults. Journal of Structural Geology 21, 67-83.

Morris, A.P., Ferrill, D.A., Henderson, D.B., 1996. Slip tendency analysis and fault reactivation. Geology 24, 275-278.

Ofoegbu, G.I., Ferrill, D.A., 1998. Mechanical analyses of listric normal faulting with emphasis on seismicity assessment. Tectonophysics 284, 65-77.

Rahe, B., Ferrill, D.A., Morris, A.P., 1998. Physical analog modeling of pull-apart basin evolution. Tectonophysics 285, 21-40.

Sims, D., Ferrill, D.A., Stamatakos, J.A., 1999. Role of a ductile décollement in the development of pull-apart basins. Journal of Structural Geology 21, 533-554.

Stamatakos, J.A., Ferrill, D.A., Spivey, K.A., 1998. Paleomagnetic constraints on the tectonic history of Bare Mountain, Nevada. Geological Society of America Bulletin 110, 1530-1546.

Published in the Fall 2002 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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