Wayne D. Pennington

Role of shallow phase changes in the subduction of oceanic crust.

Detailed studies of the seismicity of several subduction zones demonstrate that shallow-dipping thrust zones turn to steeper angles at depths of about 40 kilometers. An increased downward body force resulting from shallow phase changes in subducted oceanic crust may be the cause of this increased dip angle. In addition, the volume reduction associated with phase changes may produce sufficiently large stresses in neighboring rocks to cause the seismicity of the upper Benioff zone.

The effect of oceanic crustal structure on phase changes and subduction

Abstract Recent numerical models for the thermal regime of subduction zones are in disagreement with observations of depth to formation of eclogite, which require higher temperatures than those found in the models. This discrepancy may be reconciled if more accurate geometric depictions of the subducted slab are incorporated in the modelling studies. The transition from 15° dip along the thrust plane to 30° along the Benioff zone starts at depths of about 40 km. Dehydration reactions and the gabbro-eclogite phase change are presumed to increase the average density of the lithosphere and to cause the steepening of the subducted lithosphere. The Benioff-zone seismicity, which starts at about 40 km depth, may itself be a result of stresses caused by the phase changes. Since the gabbro-eclogite transformation occurs in gabbroic sections of the crust, thickened crustal sections beneath oceanic plateaus will affect subduction. First, the width of the thrust zone is reduced due to the increased dip angle because of early transformation of lower and warmer crustal sections; second, incomplete conversion of the entire crustal thickness results in an average density intermediate between unsubducted oceanic lithosphere and fully transformed “normal” lithosphere. The first effect leads to decreased thrust-mechanism seismicity because the coupling between the plates is reduced. The second effect leads to flat-lying subcontinental subduction or arcward advance of the trench in suboceanic subduction.

An introduction to this special section Development & Production

It is perhaps the careful integration of geophysics with petrophysics and reservoir performance which distinguishes development and production geophysics from exploration geophysics. This is not to imply exploration geophysics is inferior, or substandard. Rather, it is a statement of fitness for purpose. In exploration, the primary objective is usually structural definition. Anomalous values of attributes such as amplitude may warrant close attention, especially when related to structural closure, but the physical cause of an anomaly may be difficult to explain and physical models ambiguous. Well data is in short supply and analysis tends toward the qualitative.

Seismic time-lapse surprise at Teal South: That little neighbor reservoir is leaking!

Why perform time-lapse seismic monitoring? Is it to verify the reservoir model? No! We should conduct time-lapse seismic surveys in order to find out what is incorrect in the reservoir model, in a way similar to the production history matching familiar to reservoir engineers as they look for improvements to the model. This being the case, it is difficult to determine in advance of monitoring just what it is we should be monitoring. Thus, surveys designed specifically to test one feature of a reservoir model may be missing other important features. In this paper, we present a set of very surprising results from the Teal South time-lapse multicomponent (4-D/4-C) study, in Eugene Island Block 354 in the Gulf of Mexico. We will show that time-lapse seismic observations have revealed that an undrilled reservoir near a producing reservoir is exhibiting time-lapse changes consistent with expansion of a free gas phase, and that this implies that oil is being lost through the spill point, never to be recovered, ev...

2D Seismic Data Processing With Seismic Un*x

Our objective is to introduce you to the fundamentals of seismic data processing with a learn-by-doing approach. We do this with Seismic Un*x (SU), a free software package maintained and distributed by the Center for Wave Phenomena (CWP) at the Colorado School of Mines (CSM). At the outset, we want to express our gratitude to John Stockwell of the CWP for his expert counsel. SU runs on several operating systems, including Unix, Microsoft Windows, and Apple Macintosh. However, we discuss SU only on Unix. Detailed discussion of wave propagation, convolution, cross- and auto-correlation, Fourier transforms, semblance, and migration are too advanced for this Primer. Instead, we suggest you refer to other publications of the Society of Exploration Geophysicists, such as “Digital Processing of Geophysical Data – A Review” by Roy O. Lindseth and one of the two books by Ozdogan Yilmaz: “Seismic Data Processing,” 1987 and “Seismic Data Analysis,” 2001. Our goal is to give you the experience and tools to continue exploring the concepts of seismic data processing on your own. This Primer covers all processing steps necessary to produce a time migrated section from a 2-D seismic line. We use three sources of input data: Synthetic data generated by SU; Real shot gathers from the Oz Yilmaz collection at the Colorado School of Mines (ftp://ftp.cwp.mines.edu/pub/data); and Real 2-D marine lines provided courtesy of Prof. Greg Moore of the University of Hawaii: the “Nankai” data set and the “Taiwan” data set. The University of Texas, the University of Tulsa, and the University of Tokyo collected the Nankai data. The U.S. National Science Foundation and the government of Japan funded acquisition of the Nankai data. The University of Hawaii, San Jose State University, and National Taiwan University collected the Taiwan data. The U.S. National Science Foundation and the National Science Council of Taiwan funded acquisition of the Taiwan data. Chapters 1–3 introduce the Unix system and Seismic Un*x. Chapters 4–5 build three simple models (complexity slowly increases) and acquire a 2-D line over each model. (These chapters may be skipped if you are only interested in processing.) Chapters 6–9 build a model based on the previous three, acquire a 2-D line over that model, and process the line through migration. Chapters 10–11 start with a real 2-D seismic line of shot gathers (Nankai) and process it through migration. Chapters 12–13 and 15–16 start with a real 2-D line of shot gathers (Taiwan) and process it through migration.

Seismic Data Processing with Seismic Un*x

Our objective is to introduce you to the fundamentals of seismic data processing with a learn-by-doing approach. We do this with Seismic Un*x (SU), a free software package maintained and distributed by the Center for Wave Phenomena (CWP) at the Colorado School of Mines (CSM). At the outset, we want to express our gratitude to John Stockwell of the CWP for his expert counsel. SU runs on several operating systems, including Unix, Microsoft Windows, and Apple Macintosh. However, we discuss SU only on Unix. Detailed discussion of wave propagation, convolution, cross- and auto-correlation, Fourier transforms, semblance, and migration are too advanced for this Primer. Instead, we suggest you refer to other publications of the Society of Exploration Geophysicists, such as “Digital Processing of Geophysical Data – A Review” by Roy O. Lindseth and one of the two books by Ozdogan Yilmaz: “Seismic Data Processing,” 1987 and “Seismic Data Analysis,” 2001. Our goal is to give you the experience and tools to continue exploring the concepts of seismic data processing on your own. This Primer covers all processing steps necessary to produce a time migrated section from a 2-D seismic line. We use three sources of input data: Synthetic data generated by SU; Real shot gathers from the Oz Yilmaz collection at the Colorado School of Mines (ftp://ftp.cwp.mines.edu/pub/data); and Real 2-D marine lines provided courtesy of Prof. Greg Moore of the University of Hawaii: the “Nankai” data set and the “Taiwan” data set. The University of Texas, the University of Tulsa, and the University of Tokyo collected the Nankai data. The U.S. National Science Foundation and the government of Japan funded acquisition of the Nankai data. The University of Hawaii, San Jose State University, and National Taiwan University collected the Taiwan data. The U.S. National Science Foundation and the National Science Council of Taiwan funded acquisition of the Taiwan data. Chapters 1–3 introduce the Unix system and Seismic Un*x. Chapters 4–5 build three simple models (complexity slowly increases) and acquire a 2-D line over each model. (These chapters may be skipped if you are only interested in processing.) Chapters 6–9 build a model based on the previous three, acquire a 2-D line over that model, and process the line through migration. Chapters 10–11 start with a real 2-D seismic line of shot gathers (Nankai) and process it through migration. Chapters 12–13 and 15–16 start with a real 2-D line of shot gathers (Taiwan) and process it through migration.

Seismic petrophysics: An applied science for reservoir geophysics

Modern computational power and processing schemes have liberated reflection seismology from its primary purpose, structural mapping. It is now fairly routine to produce a number of seismic attributes, using either prestack or poststack data, or even both in combination. With these attributes, the geophysical interpreter can now make maps and look for geologically‐meaningful trends in the data…or correlate them with well observations and use them in geostatistical models…or perhaps try to use them directly to solve for the rock types and fluids in a deterministic manner.

Porosity and lithology prediction at Caballos Formation in the Puerto Colón Oil Field in Putumayo (Colombia)

At a given pressure and temperature, differences in the velocities of propagation of P- and S-waves in rocks can indicate differences in various properties of the rocks, including porosity, lithological composition, and the fluids occupying the pore space. In the absence of direct S-wave observations, we study the response of interfaces between rock units to wavefronts with different angles of incidence to infer changes in S-wave velocities. We have used modern techniques of seismic inversion that involve P- and S-impedance estimation from various angle stacks to estimate the rock properties.

Crosswell seismic imaging of acoustic and shear impedance in a Michigan reef

Acoustic and shear impedance images, obtained from deterministic simultaneous inversion of a high-resolution crosswell seismic survey, were used to obtain the internal structure of Niagaran reef in Michigan. The crosswell seismic survey was conducted using two monitor wells external to the reef. These wells had depths that extended beyond the depth of the reef, and imaging used reflections from above and beneath the reef, resulting in the best seismic images of any Niagaran pinnacle reef obtained to date. The top of the reservoir can be clearly distinguished, as well as its lateral extent or dipping edges. Reflection events internal to the reef are evident; some are fairly continuous across the reef and others are discontinuous.

Tuning of flat spots with overlying bright spots, dim spots, and polarity reversals

AbstractReflection seismic data from block F3 in the Dutch North Sea exhibit many large-amplitude reflections at shallow horizons typically categorized as bright spots. In most cases, these bright reflections show a significant “flatness” that contrasts with local structural trends. Although flat spots in thick reservoirs are often easily identified, others within thin beds or near reservoir edges can be difficult to identify and are poorly understood. Many of the shallow large-amplitude reflections in this block are dominated by flat spots. We investigated the tuning effects that such flat spots cause as they interacted with reflections from the top of the reservoir. We first studied the zero-offset “wedge-model” tuning effects of the flat spot with overlying bright spots, dim spots, or polarity reversals. We then expanded that model to examine prestack tuning effects, as well as the results from inclusion of postcritical flat spot reflections in the final stack. We observed that under certain conditions...