BRUCE D. TRUDGILL

RESEARCH

1.                 Salt tectonics

Salt tectonics is a fascinating area of structural geology. Unlike most other rock types, salt is ductile at relatively low pressures and temperatures and will flow under differential pressure over time. Geologically rapid (up to meters per year) flow of salt produces a wide variety of complex geological structures, and strongly influences depositional systems. The interaction between salt tectonics and sedimentation is of great interest to the petroleum industry as understanding these processes will lead to a better understanding of reservoir distribution in salt controlled settings. I am currently working on a number of salt related projects in different areas around the world.

The rapid rise in availability of 3-D seismic data has revolutionized imaging of complex salt structures. Combined with more realistic analogue models and sequential restoration techniques, 3-D seismic data have also helped change the way we view how salt structures evolve over time. Pre-stack depth migrated seismic data in the northern Gulf of Mexico are now revealing the complex nature of the flanks of these salt structures that were previously poorly imaged even on high quality, conventionally time migrated data. However, the structural and stratigraphic geometries of the sequences flanking salt diapirs and walls can still be difficult to interpret, particularly where wells encounter overturned strata, rapid lateral facies changes, salt welds and complex faulting.

A.     The Paradox Basin in SE Utah

The Paradox Basin is an asymmetric foreland basin, developed along the southwestern flank of the Uncompahgre uplift in southeast Utah and southwest Colorado, USA. This large basin (265km by 190km) developed during the Middle Pennsylvanian-Permian Ancestral Rocky Mountain orogenic event. Salt structures in the northern Paradox Basin form a variety of structural styles ranging from deeply buried salt pillows to complexly faulted diapirs and salt walls exposed at the surface. Complex intra-formational unconformities and rapid lateral stratigraphic facies variations indicate that salt structures were active over at least 75 Ma.

Analysis of field exposures, sub-surface well and 2D seismic data across the northern part of the basin reveals a complex relationship between crustal shortening, loading, creation of accommodation space, differential sedimentation and salt movement. Salt flow through time across the northern part of the basin reflects the varying basin geometry and its response to sediment depositional systems. From the early stages of salt movement in the Upper Pennsylvanian, through passive growth of a series of large (up to 4 km high) salt walls, the dynamics of salt movement were a strong control on both structural development and stratigraphic facies architecture.

(Abstract from Trudgill, B.D., Banbury, N., and Underhill, J.R., 2004, Salt Evolution as a Control on Structural and Stratigraphic Systems: Northern Paradox Foreland Basin, SE Utah, USA, in GCSSEPM 24^th Annual Research Conference)

Outstanding exposures in the Paradox Basin, SE Utah

Structures in the northern Paradox Basin (after Doelling, 2002)

Salt controlled stratigraphy on the banks of the Colorado River near Moab, Utah

Exposures along the flank of the Onion creek salt diapir in the Paradox Basin

Satellite view of the Colorado Plateau and eastern Basin and Range

B.    Gulf of Mexico Salt Tectonics

Bathymetry of the Gulf of Mexico (image courtesy of Shell) showing the complex geometry of the Sigsbee salt nappe

Salt systems of the northern Gulf of Mexico (after Diegel et al., 1995)

Sample seismic profile showing salt geometry (black) and dasted horizons in the West Delta/South Pass area

Three-dimensional depth models of salt systems in the West Delta/South Pass area, offshore Louisiana, Gulf of Mexico: (A) Map view of the simplified 3-D model. Present day salt is shown in pink, salt welds in red, counter-regional “fault welds” in green, and two large roller faults from an adjacent roho system in blue. (B) Perspective view of the simplified 3-D model viewed from the north; (C) View towards the southwest with faults and fault welds removed to highlight the multi-layered and linked geometry of the salt systems. The depth model was constructed using Midland Valley’s 3Dmove software. Tick marks on frame edges in (a) and (b) are spaced every 10 km, but the scale in each frame varies because of the perspective view.

3D Movie of the West Delta/South Pass salt systems

Animation of the simplified 3-D depth model of the WDSP salt system constructed using Midland Valley’s 3Dmove. Present day salt is shown in pink, salt welds in red, and counter-regional “fault welds” in green.

Animation of the 2-D restoration of a cross section across the Dome A system (line orientation shown in Fig. 8), constructed using 2DMove and the methodology of Rowan (1993). Salt is in black, and pairs of dots indicate salt welds. See also Fig. 10 and text for detailed discussion.

Animation of 3-D restorations for the WDSP study area constructed using 3DMove from 9.10 Ma to the present day. Views are from the northeast. Volumes of salt at each stage are shown in bright pink. Welds that develop during evolution of the salt system are shown in red and late stage counter-regional “fault welds” in green.
 

C.   Other examples of Salt Tectonics

Salt structures revealed by seismic reflection data from offshore Brazil

Anticlines pierced by salt diapirs in the Zagros foldbelt, Iran

Detailed satellite image of a salt glacier flowing from a salt diapir in Iran

In southern Iran, the collision between the Asian landmass and the Arabian platform has folded rocks and pushed up the rugged Zagros Mountains. In places, underlying deposits of salt have ascended in fluid-like plumes. Some of these plumes have pushed through the rock above, like toothpaste from a tube, and they are now visible as darkish irregular patches. This image shows a few of over 200 similar features—called diapirs, or salt plugs—that are scattered about this part of the Zagros Mountains.

 Gravity has caused the salt to flow like glaciers into adjacent valleys. The resulting tongue-shaped bodies are more than 5 kilometers long, with repeating bow-shaped ridges separated by crevasse-like gullies and with steep sides and fronts. The darker tones are due to clays brought up with the salt, as well as the probable accumulation of airborne dust. This ASTER perspective view was created by draping a band 3-2-1 (RGB) image over an ASTER-derived Digital Elevation Model (2x vertical exaggeration), and was acquired on August 10, 2001.

Image courtesy NASA/GSFC/MITI/ERSDAC/JAROS, and U.S./Japan

2.                 Growth of normal fault systems

How faults grow and link in time and space is an important mechanism in structural geology

A 3D seismic image of an array of normal faults from offshore Lebanon

A.   The Canyonlands Grabens of SE Utah: A natural laboratory for

examining fault growth

The Canyonlands Grabens in southeast Utah form an active extensional fault array covering 200 km² southeast of the Colorado River in Canyonlands National Park. The fault array formed as a result of gravity gliding above a thick layer of salt. Growth of this fault array within the last 0.5 my (possibly last 0.1 my) has produced a spectacular array of linked normal fault geometries. A wide variety of relay ramp structures are developed in regions of fault overlap, and their evolution can be clearly defined in the field.

The rapid growth of the fault array resulted in major changes in the stream drainages across the area through processes of stream capture and diversion. During growth of the fault array, relay ramps between overlapping fault segments form topographic lows along the graben margins. These commonly act as access points for captured streams to enter a graben system. As fault segments continue to propagate laterally, linkage leads to breaching of the relay ramp structures. This causes changes in the course and gradients of the streams, often shifting the locus of alluvial sediment deposition away from grabens that were previously infilling. This complex evolution of drainage networks in a growing fault array may provide a valuable analogue to the early structural and stratigraphic development of larger continental rifts systems.

Current research is now focused on obtaining rates of fault growth from an integrated analysis of drainage evolution with dating techniques and GPS measurements. This will ultimately lead to constraints on the evolution of the fault array through time and help enhance our understanding of fault growth processes.

The geometry of the fault array at depth is less well constrained in the field. However, the presence of a salt detachment level 500m below the land surface means that models of graben formation and vertical fault propagation can also be tested in this unique natural laboratory. By happy coincidence the Canyonlands Grabens lie within one of the most spectacular National Parks in the United States, and are reasonably accessible to geologists keen to examine these features for themselves.

Detailed map of the Canyonlands Grabens fault array illustrating the present day geometry of the fault array (red), current stream drainages (blue), and interpreted paleo-drainage patterns prior to graben development (dashed black lines)

DEM of the  Canyonlands Grabens fault array

A view across the growing fault array in the Canyonlands Grabens, SE Utah

Oblique, low altitude aerial photographs of the southeast margin of the Canyonlands Grabens fault array

Breached relay ramps connecting growing faults segments in the Canyonlands Grabens

Complex bifurcating grabens bounded by linked fault segments in the Canyonlands Grabens

A)  USGS aerial photograph of the region around Cow Canyon. Shifting Wash, a major tributary to Butler Wash traverses the eastern side of the image from south to north. (B) Interpretation of fault geometries, modern streams and paleo-drainages (dashed lines) across the Cow Canyon area. Prior to the development of the fault array, stream drainages ran southeast to northwest across the area. Numbered paleo-drainages illustrate the interpreted history of channel switching of Shifting Wash.

Large scale grabens in the Afar area of the East African rift system

3.                 Evolution of Mountain Belts

Frontal Ranges of the Canadian Rockies

Utah/Wyoming thrust belt

Cordillera mountain ranges of Colombia

The western French Alps, my PhD field area

Fold and thrust belts of western Pakistan

Central Nepal Himalaya, Everest right of centre

4.                 Structural controls on depositional facies architecture

A structurally controlled alluvial fan in the Taklimakan Desert, China

Reducing Uncertainty in Interpretation of Subsurface Geology Through Application of 3D Outcrop Models

Colorado School of Mines – Chevron Texaco Center of Excellence

Rock outcrops are the foundation upon which the geological sciences are built, and the source of much of our knowledge of sedimentary and structural geology that together yields the architecture of hydrocarbon reservoirs. This is underscored in deep marine environments, where modern sedimentary processes and responses to structural movement cannot be readily observed. The research proposed combines traditional field observation with state-of-the-art 3D modeling technology to effectively use outcrop models to improve reservoir prediction and simulation through investigation and verification of geologic controls on reservoir architecture and interrogation of how we manage this information in reservoir modeling. This will be achieved through the construction, analysis and experimentation of 3D geologic, petrophysical and geophysical models, initially in Project I, building on existing models from Permian deep-water strata of the Brushy Canyon Formation. The Brushy dataset permits us to refine the outcrop to subsurface data conversion workflow for experimentation of fine-scale and high-resolution outcrop models, create an integrated CVX-CSM modeling team and define the modeling experiment workflow, and conduct numerous experiments on an existing model where we can efficiently test geologic rules and subsurface data analysis methods that will guide selection of the outcrop(s) to be studied in Project II. Project II integrates structural methods to investigate the affect of structural movements on deep-water sedimentary patterns. Criteria for demonstrating a structural-sedimentary linkage, methods required to document these effects, screening of candidate outcrops, and review of the literature will guide selection of one or more outcrops for Project II study, where the role of synsedimentary structural development on deep-water sedimentation patterns will be investigated.

We propose two concurrently running projects in the first two years of this research program, with the expectation that methods and learning will be fully integrated in year three as the program moves forward with study of new outcrop(s) that focus on the role of synsedimentary structural development on deep-water sedimentation patterns.

Project I: Testing and Developing Rules for Reservoir Prediction

Leveraging off the Slope & Basin Consortium the project will refine an existing deterministic The primary objective of the proposed research is to develop “Rules” that can be used to predict reservoir properties (stratigraphy, geometry, facies, petrophysics) in sparse well settings. These “Rules” will be generated through observation, parameter correlation and verification using a family of deterministic outcrop models. This includes the flow response to varying reservoir architectures for better reservoir performance prediction. Flow relevance will be interrogated and assessed through iterative streamline analysis and flow simulation of full field and sector models.  Project II will investigate how these stratigraphic rules change as a function of structural setting.

Project II: Structural-Stratigraphic Linkage in Deep-Water Sedimentation

The main aim of this new project is to assess the influence of structural processes on the stratigraphic architecture of deepwater reservoirs. We propose to begin the screening and selection of syn-sedimentary structures in outcrop where we can expand the reservoir modeling tools that incorporate detailed outcrop data, as outlined in Project I.  The initial goals of this project are to develop screening criteria and efficient methodologies to select the outcrops that will provide data for a new phase of study that will build on the predictive modeling tools developed from the Brushy Canyon data.  An additional goal will be to define a workflow for the evaluation of outcrop sites and data sets. 

Growth folds in the Laingsburg basin in South Africa

Growth folds in the Jaca Basin, southern Pyrenees

5.                 Tectonic geomorphology

World topograghy and bathymetry

The Basin and Range province in the western USA

Composite satellite image of the British Isles

West Wales around the Dyfi and Mawddach estuaries and the Cadair Idris and Pumlumon mountain ranges

GEOLOGY DEPARTMENT