Loma Vieja Field: Structural Geology and Related Velocity Fault Shadow in the Upper Wilcox (Fandango) in S. Texas
Abstract:
A new Upper Wilcox (Fandango) field discovery was made at Loma Vieja field in 1989 in Zapata County,
Texas. Production from the field to date is 44.1 BCF from numerous Fandango sands that trend from Bob
West field northward to E. Seven Sisters field.
The principal trapping mechanism for Loma Vieja field is a high side closure against a very large down-to-the-coast fault. The fault trends across Zapata County through Escobas field, which is immediately updip to Fandango field, northward to NE Thompsonville field in northwest Jim Hogg and southeast Webb counties.
The large down-to-the-coast fault that creates the trap for the field has a definitive velocity shadow resulting in a significant time sag on conventional migrated seismic data. Expansion of Weches, Queen City, and Reklaw shales on the downthrown side of the fault causes major changes in the average and interval velocities across the trapping fault at Loma Vieja field. Seismic data and time structure maps over the field have southeast dip toward the fault (Fig. 1) where actual dip as measured from well log correlations and dipmeter data is northwest away from the fault. Figure 2 is a depth structure map that is significantly different structurally from the time structure map.
Although the field was discovered with 2D seismic data, 3D seismic data confirmed the presence of a velocity gradient across the field and reduced structural risk during continued drilling in the field. However, structural complexities in the geologic section above the Fandango sands have a major impact in the velocity regime across the field. To reduce risk in drilling wells, it is crucial for the explorationist to understand the effects of velocity in a 3-dimensional domain by considering all structural features, as well as stratigraphic bodies, that may have any effect on the ray paths and velocity of seismic energy.
Biographical Sketch:
Jim Meyerhoff is a geophysicist for Samedan Oil Corporation working the Vicksburg, Yegua, Queen City, and Wilcox trends in South Texas. Prior to his 3-year tenure at Samedan, Jim worked 12 years for Diamond Shamrock — Maxus Exploration Company and four years at Amoco Production. He has worked various Gulf Coast trends in Texas, Louisiana, Mississippi, Alabama, and Florida throughout most of his career and has done international work in Argentina, Bolivia, and Peru. He received a B.S. in mathematics in 1978 and an M.S. in geology in 1983, both from Baylor University.
"Location and Depth Determination of Buried Ferro-Magnetic bodies in Environmental Site Assessments Using Euler's Homogeneity Equation"
Abstract:
Geoscientists apply magnetic and gravity data to determine the depth to the top of the geologic features that produce observed anomalies. For hydrocarbon exploration, this is usually equivalent to determining thickness of the sedimentary section. For minerals exploration, depth estimates help locate geologic structures that produce a magnetic or gravity anomaly.
Since the application of Euler's homogeneity equation by Thomas (1982) and Red et al (1990), it became clear that the location and depth determination of buried ferro-metallic bodies could be achieved if the object's delineation could be based on Euler's relationship. The conventional technique required manual and/or computer-assisted interpretation procedures that were time consuming and, as such, expensive. The results were always dependent on the geophysicist's capabilities.
Euler's homogeneity relationship offers a quasi-automated way to derive plan location and depth estimates of buried objects from a gridded potential data set (magnetic or gravity). The equation relates the potential field and its gradient components to the location of the source, with the degree of homogeneity expressed as structural index, SI (Thompson, 1982). Structural index is a measure of the rate of change of the field versus distance from the source (fall off rate) and is directly related to the source of the observed magnetic anomalies. The technique, called Euler deconvolution, is advantageous over the conventional depth interpretation methods and can be directly applied to large grid data sets. It reduces interpretation time significantly.
The Euler deconvolution method has been applied to data collected over four sites (three magnetic and one gravity). Objects buried at the sites were drums, pipes, and underground storage tanks. Analyses of the data sets have provided characteristic Euler deconvolution signatures and structural indices associated with ferro-metallic features. The solutions obtained indicate the ability to quickly and accurately map the location and depth of buried ferro-metallic objects from gridded potential survey data.
Biographical Sketch
Mustafa Saribudak is a principal of Environmental Geophysics, 9406 Palm Shores Drive, Spring, TX 77379. He received a master's degree in geology and a doctorate in geophysics from Istanbul Technical University, Turkey. He came to the University of Houston in 1989 to work on a project funded by the National Science Foundation. Between 1990 and 1993, he worked for Tierra Environmental and pioneered the application of geophysical methods to environmental problems. Mustafa founded Environmental Geophysics in 1994 to provide near-surface geophysical services for the engineering, environmental, oil and gas industries, and real estate developers. During the last five years he has conducted geophysical surveys at more than 100 sites in the U.S. and Central America. He has published broadly in geophysical and environmental journals.
Post-Rift Hydrocarbon Systems, Greater Amazon Mouth, Brazil: Transition from Shelf to Basin and Source Distribution Controls
Abstract:
Three post-rift marine petroleum systems in the Amazon mouth are characterized, with maturation, migration, and Neogene reservoir distribution controlled by rapid deposition related to Andean tectonics. Two of the potential hydrocarbon source intervals (Upper Cretaceous and Paleogene) are present on the shelf and upper rise, but their basinward extension is only inferred (Fig. 1). Extrapolation of potential sources from the shelf into the basin requires an understanding of the controls on distribution and preservation of organic matter. Amazon mouth sources have three main controls: (1) paleogeography, (2) oceanic conditions, and (3) terrestrial river input (Fig. 2).
The Upper Cretaceous (Limoeiro) clastic source rock is well documented on the shelf, where it is of fair to good quality (TOC 1%–5 %, HI 150–500). The source rock quality should improve basinward where terrestrial input and siliciclastic dilution decrease. Source rock thickness may be a risk, but oil-prone, clastic-starved marine shales are predicted in the deep basin. Primary controls on the distribution of the Turonian source rock are productivity and oceanic circulation.
The Paleogene (Amapa) source rock is also occurs on the shelf, but is less oil-prone (TOC 1%–5%, HI 200–350) than the Upper Cretaceous source rock in the area. Based on map distribution and biomarker data, we interpret this source to be limited to a back-reef lagoonal environment and absent seaward of the carbonate shelf edge. The key to better organic enrichment is interpreted to be paleogeography: carbonate highs cause restricted circulation and increase the potential for organic preservation.
Source rock potential is also postulated for the Miocene–Pleistocene (Pirarucu) interval. Tropical Tertiary age depocenters around the world have produced hydrocarbons with distinctly terrestrial signatures, and some of these systems produce large amounts of oil. The Amazon mouth region possesses many attributes which could allow terrestrial sources to be capable of oil generation, but more data are required to test this concept.
Hydrocarbon maturation is driven by rapid Miocene-to-present burial that causes the Limoeiro to be locally overmature. Postulated Pirarucu sources may be mature for oil in some areas. Rapid sedimentation resulted in deep listric faults, shale diapirs and toe-thrust structures that provide abundant vertical migration pathways. The presence of interbedded Tertiary sources and sands would allow for simple migration scenarios.
Biographical Sketches
David M. Advocate (M.S., 1983, California State University, Northridge) is a geological associate at EEC with 16
years of E&P experience. His main area of expertise is hydrocarbon systems analysis.
Steven W. Young (Ph.D., 1975, Indiana University) is a geophysical associate at EEC with 22 years of
experience in minerals, oil, and gas exploration. His main interests are sequence stratigraphy, and reservoir geometry
and quality.
Art H. Ross, Jr. (M.S., 1965, Virginia Tech) has more than 33 years of domestic and international oil and gas
exploration experience. He has conducted geological surveys in Colombia as a captain in the Corps of Engineers.
His main area of expertise is seismic interpretation and regional geology.
Thomas P. Buerkert (completed doctoral studies at Louisiana State University, 1997) has been with EEC for nearly
two years as a senior geophysicist. His main area of expertise is trace element geochemistry and application in
paleoceanography.
Jack E. Neal (Ph.D., 1994, Rice University) is a senior research geologist and serves as group
leader for source evaluation and petroleum geochemistry studies at EPR. He has nearly five years of oil and gas
industry experience, and his principal interests are tectonics and sequence stratigraphy.
Keith I. Mahon (Ph.D., 1996, University of California, Los Angeles) is a senior research geochemist at EPR and
is conducting studies in basin modeling, thermochronometry techniques, and uncertainty analysis. He also has nine
years of experience developing aerodynamic vehicle simulations and real-time software for Northrop and Hughes.
Reservoir Prediction Using the Forest and the Trees: Reducing Reservoir Risk and Uncertainty in Deepwater GOM Exploration
Abstract:
Reservoir prediction in exploration may be enhanced by following six axioms:
Our analysis proceeds from regional to prospect-scale evaluation of reservoir potential, and we use an example exploration well to illustrate the methods used, ranges of uncertainty, and insights gained at each scale.
Regional analyses provide the depositional and petroleum systems framework within which exploration is focused. Reservoir evaluation is based predominantly on a 2D seismic grid, calibrated using key well information, structural controls, and biostratigraphy. Key products are a chronostratigraphic and sequence stratigraphic framework, a regional understanding of the architecture and distribution of major depositional systems, and an associated regional reservoir risk pattern.
In the deepwater Gulf of Mexico, a range of risks on the amount and type of reservoir facies present may be applied at a regional scale. The location of the major sediment input sites migrates with time, such that the ages of prospective reservoir intervals and their provenance are different in different regions. Well-developed sands are commonly found in a middle or lower slope setting directly down dip from the major coeval shelf depocenter, which leads to a low "regional" risk for the reservoir. Higher risk is associated with the lateral edges of the deposystem and the upper slope and shelf margin (often bypassed or characterized by complex reservoirs). Reconstruction of the subregional structural and stratigraphic evolution of an area provides insight into the range of depositional processes and controls on reservoir geometry and distribution. Overall slope gradient, subsidence rate, and local structures (faults, salt withdrawal) may generate accommodation space where sediment can aggrade or pond, even in a generally sand-poor setting such as the upper slope. Local bathymetric highs may lack reservoirs, but may restrict or impede flows and concentrate sand accumulation in adjacent areas.
Subregional analysis is typically built on a framework of 3D seismic surveys and any available well data. Data include detailed biostratigraphic analyses, seismic facies maps (geometries, textures, and seismic attributes), log facies and lithology interpretations, and structural analysis of subsidence patterns, fault movement, and salt migration. Key products are a detailed chronostratigraphic framework and a series of paleogeographic maps showing the nature and distribution of potential reservoir facies and their controls through time. The details provided by a robust subregional analysis allow us to better understand the details of potential reservoir systems and to corroborate or modify the risk associated with the regional framework.
On a prospect scale, prediction is focused on reservoir thickness, extent, quality, and continuity. These parameters provide input to reserves ranges, well positioning, definition of stratigraphic trap edges, and t he distribution of potential reserves within a trap. Detailed seismic and well log facies analyses are utilized to high-grade potential reservoir-prone intervals. Seismic attribute analysis tied to a rock properties database may be used to predict the range of possible lithologies for a target horizon. Delta-t/interval velocity, AI, and AVO techniques may be used to predict thickness and net-to-gross variations across the prospect. Facies mapping and fault analysis are used to predict reservoir continuity. At the prospect scale, multiple reservoir models are described, risked, and carried for each target interval, with risk and a range of reserves calculated for the most likely reservoir prediction.
In summary, the integration of a variety of methods, data types, and geologic disciplines across a range of scales yields more robust results for reservoir prediction than any one particular method of analysis. Different information and aspects of risk are derived from investigations at different scales, but sometimes the appropriate level of analysis is controlled by the availability and quality of data. For example, the regional picture may be the only tool available for reservoir prediction in some wildcat areas. Well tests give us confidence that, using the approach described above, we can often predict the types of reservoir and general facies within a deposystem.
Biographical Sketch
Cindy A. Yeilding is team leader—frontiers, Gulf of Mexico exploration, BP Exploration. Her specialities and technical focus include stratigraphy and sedimentology of shelf margin clastic systems and analysis of stratal architecture/facies relationships from regional to production scale. Her experience includes 14 years in industry (with BP) as an operations, development, appraisal, and exploration geologist and R&D manager. She is also experienced in regional to 3D seismic scale salt-sediment interactions. In addition, she leads deepwater field workshops for BP. Her work has focused on the U.S. Gulf Coast, but she has worked exploration in Venezuela and subsurface imaging in Colombia.