Institute for Geophysics Theses and Dissertations

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Andean deformation and basin evolution during changes in subduction zone geometry (29–33°S)
(2023-05-01) Mackaman-Lofland, Chelsea; Horton, Brian K., 1970-; Ketcham, Richard A; Stockli, Daniel F; Constenius, Kurt N; Folguera, Andres; Fosdick, Julie C
Understanding the surface, crustal, and tectonic processes controlling deformation and crustal evolution along subduction margins remains an outstanding challenge in Earth science, with implications for characterizing Earth systems, resource potential, and natural hazard assessment. The southern Central Andes of Chile and Argentina define type examples of deformation, arc magmatism, and basin evolution during long-lived subduction, and provide unique opportunities to study the overriding plate response to changing plate margin conditions and tectonic regimes. This dissertation presents new geo- and thermochronological assessments of sediment provenance, basin accumulation, and deformational timing at ~29–33°S that provide critical insight into retroarc deformational patterns and subsidence mechanisms over the past ~200 Myr. Mesozoic to mid-Cenozoic deposits recorded the unroofing of basin margins and sediment contributions from the Andean magmatic arc during Late Triassic to Early Cretaceous extension, thermal subsidence, and possible slab rollback, followed by initial shortening and sediment derivation from western sources during the Late Cretaceous. Newly dated volcanic and sedimentary deposits in the retroarc hinterland provide evidence for a late Paleogene episode of intra-arc and proximal retroarc extension preceding Negoene growth of the modern Andes. Geochronological and thermochronological data for thrust sheets and Neogene foreland basin fill indicate Andean shortening initiated with the inversion of the intra-arc basin system, followed by sequential eastward migration of the fold-thrust belt–foreland basin system along a regionally connected, hybrid thin- and thick-skinned decollement. Finally, new structural interpretations and flexural thermo-kinematic model results quantify the influence of inherited structures, precursor stratigraphic architectures, thrust belt critical taper, and non-flexural geodynamic processes during Neogene construction of the modern Andes
A mixed forward/inverse modeling framework for earthquake deformation problems
(2023-08-11) Puel, Simone; Becker, Thorsten W.; Lavier, Luc; Ghattas, Omar; Hesse, Marc A; Johnson, Kaj M
Subduction is responsible for the most powerful earthquakes and dangerous volcanic eruptions, resulting in significant human casualties and economic losses. However, the prediction of these natural events remains challenging due to an incomplete understanding of the underlying physics that govern these phenomena. Key questions persist regarding stress accumulation and dissipation, rock behavior under extreme pressure and temperature, and the influence of fluids and melt in these processes. Recent advancements in space geodesy and seismic networks have enabled the measurement of seismic responses and surface displacements, revealing the complex dynamics of subduction zones. To enhance our comprehension of these processes, computational modeling that integrates various types of observations and constraints is crucial. This can be achieved through forward modeling, where model parameters are adjusted to better fit the observations, or through inverse modeling, which extracts critical parameters and the underlying physical mechanisms directly from the data. However, a comprehensive numerical physics-based modeling framework that combines both forward and inverse capabilities, using adjoints, within a unified infrastructure is currently lacking. The objective of this dissertation is to address this gap by developing an open-source, flexible, transparent, and easily extendable framework capable of handling multi-physics coupled problems. This framework will also drive the advancement of innovative techniques for analyzing earthquake systems. It introduces a novel implementation of fault discontinuity within the finite-element model, an improved fault slip inversion method that does not require Green’s function computations, and a novel approach to infer material structure solely from surface displacement data, eliminating the need for seismic velocity analysis. Furthermore, by incorporating these approaches, it offers a novel joint inversion of surface geodetic data, facilitating the simultaneous recovery of subduction zone structure and coseismic slip distribution. This provides valuable insights into the interplay between heterogeneous material structure and fault processes. As a demonstration, the framework successfully recovers the coseismic slip distribution and subduction zone structure by inverting the coseismic surface displacements recorded during the 2011 M9 Tohoku-oki earthquake in Japan. The results reveal weaker material beneath several volcanoes in the same region where local coseismic subsidence was reported during the earthquake. Accounting for heterogeneity in fault slip inversions is crucial for accurately matching the surface displacement data, as suggested by previous studies. Overall, the proposed framework represents a significant advancement in subduction zone modeling, providing a comprehensive tool for understanding and analyzing these complex phenomena, thereby paving the way for improved hazard assessment and risk mitigation strategies.
Numerical modeling of viscoplastic mantle convection with damage rheology to investigate dynamics of plate tectonics
(2023-08-11) Heilman, Erin; Becker, Thorsten W.; Faccenna, Claudio; Lavier, Luc; Hesse, Marc; Dannberg, Juliane
Mantle plumes are typically considered secondary features of mantle convection, yet their surface effects over Earth's evolution may have been significant. We use 2-D convection models to show that mantle plumes can in fact cause the termination of a subduction zone. This extreme case of plume-slab interaction is found when the slab is readily weakened, e.g. by damage-type rheology, and the subducting slab is young. We posit that this mechanism may be relevant particularly for the early Earth, and more generally, plume "talk back" to subduction zones may make plate tectonics more episodic in certain cases. When these models are carried out in a 3-D geometry, we see the same plume-slab terminations take place and can observe the effect of lateral extent on the dynamics. We examine the dynamics of these terminations through their geometry, frequency, and effect on the surface. By varying the proportion of internal heating, we show the effect of mantle temperature on the efficacy of plume-slab terminations and draw parallels to the evolution of the Earth's mantle temperature. A subdued version of these plume-slab interactions may remain relevant for past and modern subduction zones. Such core-mantle boundary – surface interactions may be behind some of the complexity of tomographically imaged mantle structure, e.g., for South America. Continuing the exploration of our damage rheology, we investigate spreading ridges, which are another feature integral to plate tectonics. We carry out 3-D internally heated mantle convection modeling to produce discrete spreading ridges and transform faults in a freely convecting model. The inclusion of damage in these models allows for transform faults to develop more easily than in previous modeling attempts. We vary the strength of the damage in its weakening and healing proportions to understand the effect on the dynamics and lifespan of the transform faults. These transform faults match well with observations from Earth, and as a result these models are a stepping stone to a new class of global mantle convection modeling.
Mechanisms of lithospheric failure during late continental rifting and early subduction
(2021-08-13) Shuck, Brandon Douglas; Van Avendonk, Harm J. A.; Gulick, Sean P. S.; Bangs, Nathan L.B.; Becker, Thorsten W.; Lavier, Luc L.; Shillington, Donna J.
Two fundamental components of plate tectonics are the separation of continents, leading to new ocean basins, and the initiation of subduction zones, which facilitate recycling of the Earth's outer shell into its interior. In order for continental rifting and subduction initiation to succeed, tectonic driving forces must overcome resisting forces and strength of the lithosphere. If achieved, the lithosphere undergoes failure and a new plate boundary is established, wherein subsequent strain is localized along a narrow weak zone, such as a subduction zone megathrust or seafloor spreading center. Though these processes are conceptually straightforward, many aspects remain elusive. In particular, intact lithospheric strength is thought to be far greater than available tectonic forces, yet observationally continental breakup and subduction initiation occur frequently throughout Earth's history. The goal of this dissertation is to further investigate this force paradox by exploring the weakening mechanisms that assist lithospheric failure during late continental rifting and early subduction. Active-source seismic data are used to image geologic processes and the tectonic evolution along two study areas - the Eastern North American Margin and the Puysegur Margin, New Zealand. Along the Eastern North American Margin, I show that new mafic crust was emplaced above a thinning subcontinental mantle lithosphere that resisted breakup despite abundant magmatism. I propose a new model in which continental crust separated before the lithosphere and complete breakup was not achieved for ~25 Myrs after the arrival of melts. I then image mantle dynamics near the lithosphere-asthenosphere boundary during final stages of rifting and show that rupture was enabled by highly organized crystallographic textures that focused melt and deformation into a narrow weak zone. At the Puysegur Margin, I argue that subduction initiation was aided by previous phases of continental rifting and strike-slip. Rifting stretched continental crust of Zealandia and later dextral strike-slip translated thin and dense oceanic crust from farther south and juxtaposed it with thick continental crust at a collisional restraining bend. Ideal conditions ensued, where buoyancy contrasts and pre-existing fault zones weakened the lithosphere and facilitated subduction nucleation. Since initial underthrusting, subduction initiation became more efficient as the trench propagated southward over time. I conclude with a novel 4D model where subduction initiation is resisted at the site of nucleation but followed by mechanically easier and faster initiation and lateral propagation as the plate boundary develops along-strike. Inherited lithospheric heterogeneities and weak zones are the dominant mechanism allowing the plate tectonic cycle to persist on Earth.
Seismic traveltime inversion in three-dimensional heterogeneous media
(1990) Finn, Christopher Jude, 1960-; Backus, Milo M., 1932-
The measured traveltimes of specular reflection events are inverted to obtain a low spatial frequency, three-dimensional model of the reflector geometry and the compressional wave propagation speed. B-spline functions are used to describe the shapes of the interfaces and the lateral variations in velocity. The inversion is performed by optimizing a maximum likelihood criterion using a Newton based iteration. Model updates are obtained by iterative forward modeling and solution of the linearized equation set derived from the maximum likelihood criterion. In the forward problem, the ray tracing equations are solved as a two point boundary value problem with appropriate internal boundary conditions at velocity discontinuities. Analytic expressions for the Frechet derivatives necessary to obtain the model updates are given. Conventional methods are compared to the traveltime inversion technique using synthetic examples. For a relatively simple earth model containing only moderate lateral velocity variations hyperbolic moveout analysis followed by a Dix inversion produces a biased estimate of the velocity and depth. This is a consequence of the simplifying assumptions of the method. In this case, the more general traveltime analysis provides a better result. This is also true for a more complex earth model containing lateral velocity variations and interfaces with large dips and curvatures where the conventional methods fail badly. Picked traveltimes are used as the data in the inversion although the use of the data semblance or the stack power along the predicted traveltime trajectory is also explored. These criterion are shown to be more nonlinear than the least-squares data residual measure. Thus, it is difficult to converge to a global minimum using these criterion and more accurate initial guesses are necessary. An application of the traveltime inversion technique to a 3D marine data set is presented. In this application the effects of the seismic source and the recording system on the measured traveltimes are estimated. The time delay between the first break and the main pulse of the minimum phase source wavelet and the effect of the ghost reflections from the free surface are compensated for in the prediction of the measured traveltimes
Hydrate-bearing sands in the Terrebonne Basin record the transition from ponded deposition to bypass in the deep-water Gulf of Mexico
(2022-12-02) Varona, Gabrielle Marie; Flemings, Peter Barry, 1960-; Covault, Jacob A; Cook, Anne E
Herein, I show how seismic stratigraphy can be used to describe the transition from ponded deposition to bypass within a gas hydrate system in deep-water Gulf of Mexico. In Walker Ridge Block 313 (WR 313), the Green and Orange sands are gas hydrate bearing sheet sands incised by a paleo channel system within the southwestern lobe of the Terrebonne Basin. I discuss two intervals that characterize the deposition of the sheet sands in conjunction with the channel. The Green interval captures the first appearances of coarse-grained material which I classify as the Green sand. The Orange interval encompasses channelized deposits and corresponding muddy levee deposits and is capped by the Orange sand. Within WR 313, the channel is oriented NW-SE and flowed towards the SE where salt related uplift took place. During the Green interval, salt movement influenced the pattern of deposition upstream and incision downstream. During the Orange interval, the channel aggraded, encountered the sheet deposition of the Orange sand, and then shut off. Well log data from two different wells support my interpretation of the Green and Orange sands as sheet sands due to their thickness patterns ~2000 m away from the channel axis. The WR 225 001 (WR 225-1) well penetrates the Green and Orange deposits upstream and the WR 313 H001 (WR 313-H) well encounters these deposits downstream. The gamma ray and resistivity logs from the WR 225-1 well record two coarsening upward signatures several meters apart which are interpreted as the Green sand (74 meters thick) and the Orange sand (22 meters thick). I correlate these gamma ray signatures with thinner packages of the Green sand (35 meters) and the Orange sand (12 meters) in the WR 313-H well on the downstream end. Due to the thickness of the sands away from the channel and the corresponding seismic character, I interpret that the Green and Orange sands record the last episodes of high energy deposition that interact with a submarine channel system.
Terrestrial constraints on the distribution of brine in Europa’s ice shell
(2022-09-26) Wolfenbarger, Natalie Soheila; Blankenship, Donald D.; Soderlund, Krista Marie, 1982-; Hesse, Marc; Goudge, Timothy A; Heimbach, Patrick; Moore, William B
Jupiter’s ice-covered moon Europa is considered a prime target in the search for habitable worlds within our Solar System. Freezing at the ice-ocean interface (accretion) entrains material from the ocean, including salts and potential biosignatures. In this work, we first examine how accreted ice on Earth can be used to estimate the bulk composition of Europa’s ice shell through a review of published ice core data, sampling low temperature gradient environments. We find that ice forming beneath ice shelves can serve as a valid analog for ice accreting beneath the ice shells of Europa and Enceladus and that the mechanism of ice accretion (frazil vs. congelation) will serve as the primary factor governing the amount of salt entrained in the ice shell from the ocean. We then introduce a framework to model the brine, salt, and ice volume fraction for a given composition as a function of bulk salinity and temperature by translating the output of the aqueous geochemistry software to polynomial functions of temperature. Using this framework, we build models for two endmember (NaCl and MgSO₄) and two multi-ion “analog” endmember (chloride-dominated and sulfate-dominated) ice shell compositions. Motivated by the observation that a percolation threshold could ultimately govern the stable bulk salinity of a congelation ice shell, we use these models to study the efficiency of salt entrainment as a function of ocean salinity, composition, and the effective critical porosity, a new parameter introduced in this work. We find that the efficiency of salt entrainment in a congelation ice shell is minimally influenced by composition and ocean salinity and is ultimately governed by the effective critical porosity. Finally, we study the habitability of ice shell brine pockets and find that they are not geochemically prohibitive to life as we know it. Furthermore, we suggest that brine volume fraction, as a proxy for nutrient transport and recycling, may be a critical factor governing the habitability of Europa's ice shell and use it to define different classes of potential habitats within Europa’s ice shell.
Geophysical insights into the crater subsurface at the Chicxulub and Ries impact craters
(2022-12-02) McCall, Naoma; Gulick, Sean P. S.; Christeson, Gail; Hesse, Marc; Ketcham, Richard; Poelchau, Michael
Large impact events have shaped the evolution of life on Earth both during early Earth history and the K-Pg extinction event. They transport huge volumes of rock distances of kilometers in timescales ranging from seconds to minutes. Impacts are found on every rocky and icy planetary body and are the dominant form of crustal resurfacing on planets without plate tectonics. As impact cratering is an important geologic process, but one that is rarely observed in real time, our understanding relies on modeling and existing craters. On Earth, impact craters are often eroded, tectonically deformed, or buried in sediment. This dissertation uses drill core and seismic data at two craters largely obscured but preserved by sedimentation: the Chicxulub impact crater, Mexico and the Nördlingen Ries impact crater, Germany. My research ground-truthed impact cratering models and strengthened theories of faulting and acoustic fluidization at Chicxulub. At Ries, the data acquired, processed, and interpreted during this PhD shows that Ries is a transitional crater that exhibits neither a central peak nor peak ring and that the suevite deposition at the crater was emplaced via ground-hugging flows. I measured the permeability of peak ring rocks from the International Ocean Discovery Program (IODP) Expedition 364 core, an important parameter for determining the duration of the post impact hydrothermal system at Chicxulub, I found that the permeabilities were likely responsible for an isolated and heterogeneous post-impact hydrothermal system at Chicxulub.
Controls on terminus change of marine terminating glaciers in Greenland over the last 40+ years
(2022-11-17) Goliber, Sophie Ann; Catania, Ginny A.; Goudge, Timothy; Greenbaum, Jamin; Heimbach, Patrick; Spikes, Kyle
Since the 1980s, the Greenland ice sheet has been losing ice mass at an increased rate. Our current understanding of the complex physical processes that control dynamic mass loss is incomplete and, therefore, leads to a wide range of possible future contributions to sea level. Ice dynamics, or changes due to changes in ice flux, is dominated by the behavior of fast-moving outlet glaciers in Greenland. These glaciers are changing through melting of the terminus face and/or calving of icebergs; the combination of these processes and ice motion determines the position of a glacier terminus. In understanding how and why outlet glacier termini change over time compared to external forcing and internal glacier dynamics, we are able to move toward a better understanding of marine-terminating glaciers. In this dissertation, I use terminus traces to observe how and why marine-terminating glaciers change in order to better understand the mechanisms behind these complex heterogeneous changes in Greenland. I develop the largest database of manually-traced marine-terminating glacier terminus data for use in scientific and machine learning applications. These data have been collected, cleaned, assigned with appropriate metadata, including image scenes, and compiled so that they can be easily accessed by scientists. Then I use the location of the termini to identify features in the bed topography that inhibit the retreat of glaciers following the onset of ocean warming and widespread glacier retreat in the late 1990s. I find that the slope and lateral dimensions of bed features exhibit the strongest correlation to retreat and that the shape of the bed features allows different styles of terminus retreat, which may be indicative of how different ablation mechanisms are distributed across termini. Finally, I produce a time series of terminus morphological properties for four glaciers in western Greenland to identify the characteristics that are indicative of calving processes with the goal of categorizing glaciers by calving style. I find that a concave shape and low sinuosity are present at glaciers that calve via buoyant flexure, while the opposite is true at glaciers that are dominated by melt-induced calving via serac failure. I also find that glaciers do not persistently fit into single calving styles and may change over time. By studying how the terminus changes over time compared to external forcing and internal glacier dynamics, we are able to move toward a better understanding of marine-terminating glaciers.
Autoencoders for seismic model upscaling and facies identification
(2022-08-12) DeFabry, Cameron Mark; Sen, Mrinal K.; Cardenas, Meinhard B; Spikes, Kyle T
The research presented here focuses on the resolution enhancement of inverted seismic volumes and geological facies identification. In the first section I utilize a regularized neural network in the form of an autoencoder to improve the resolution of seismic models which were inverted for compressional wave impedance. In the second section, I focus on the utilization of autoencoders to classify spatially small geologic facies with inverted seismic models and a computed facies map. In section one I focus on the processing and inversion of seismic data from the 3D Penobscot field on the Scotian shelf to produce models for compressional wave impedance. The dataset is inverted using two different approaches, first using a deterministic method, and secondly using a stochastic method. The stochastically derived models have higher frequency content than the deterministic models and are used as the target for the task of resolution enhancement. An autoencoder is trained to recreate the stochastic models with a set of randomly chosen starting weights. During training the network attempts to create a feature map that correlates the low-resolution deterministic model to the high-resolution stochastic model by using the deterministic models as input data and the stochastic models as target data. Once training is complete the network is given the deterministic model as an input and asked to predict an output with the convolution filters learned by recreating the stochastic models. The result is a model of higher resolution than the original deterministic model, but lower resolution than the original stochastic model. In section two I characterize a seismic volume from the Marco Polo field collected in the Gulf of Mexico and then classify five distinct facies, a shale member, an oil sand member, a gas sand member, and discrete brine sand members corresponding to the sand units. The brine sand members were simulated through fluid substitution and then have their probabilistic properties derived through a rock physics template. Bayesian classification is used to create an initial facies map with the brine sands predicted with rock physics templates, and the remaining units predicted directly from the distributions inherent to the well log. Geologic units less than 600 meters in any direction were specifically targeted by converting the standard facies map into a binary facies map. This binary facies map was used as an input in an autoencoder along with two seismic volumes inverted for compressional wave impedance and Vp/Vs. The result is a trained network that can take inverted model inputs and produce a probabilistic output predicting the location of a given facies. Additionally, when provided with smoothed inputs, the autoencoder can produce outputs of a similar resolution to that of the original data, with a loss of performance noted in the probabilities displayed. This result, along with the result from section one are used to justify the claim that autoencoders can be effectively used for the tasks of seismic model upscaling and facies identification without the direct use of well log data as a network input. Convolutional layers provide a way of processing these data in a manner often seen in image recognition and enhancement problems. These networks are limited to the data they were trained on, so additional training would be required for use on separate datasets. The utilized methods though, should maintain their efficacy provided the appropriate training has taken place.
Dynamic development of the Athabasca Valles outflow system from volcanic facies and 15m scale roughness
(2022-08-05) Miller, Russell Cremean; Grima, Cyril; Gulick, Sean P. S.; Goudge, Timothy A.
The characterization of surface roughness properties provide context to the nature of geologic terrains, commonly partnered with field-based mapping on Earth and satellite-based photogeologic mapping in planetary sciences. In this study, we combine morphology-based facies mapping and perform quantitative roughness analysis and characterization of facies for the Athabasca Valles lava flow-field on Mars to identify flow features, provide insight into eruption conditions, and link roughness patterns throughout the flow to emplacement conditions. The root-mean-square (RMS) height and effective slope were acquired at 15m wavelength for 14 unique lava facies using statistically derived components from the Shallow Radar (SHARAD) surface echo strength. Quantitative RMS height surface roughness of Athabasca lava features range from 1.09m to 1.76m. We show that the RMS height response is generally consistent with facies transitions confirming the linkage between surficial morphologies and lava flow roughness, including the ability to constrain the relative spatial and temporal evolution of emplacement processes. Roughness patterns and facies localities suggest that the emplacement of Athabasca lava experienced a dynamic progression of local discharge surges and substrate influence on morphology. Additionally, we consider surface roughness derived from nadir-looking radar tracks, a superior tool for distinguishability between transitional lava types.
A modern distribution of foraminifera to reconstruct environmental change offshore Galveston Bay, Texas
(2022-08-11) Schilling, Solveig H.; Lowery, Christopher M.; Goff, John A.; Martindale, Rowan
Sand is an important resource for coastal engineering. In Texas, many offshore sand deposits are found buried in Holocene fluvial sediments or estuarine environments like tidal channels or bayhead, flood, and ebb tide deltas. Exploring for these resources requires a comprehensive understanding of the depositional system in which they are buried. The distribution of benthic foraminifera can be used to reconstruct the paleoenvironment of sand-bearing Holocene estuary deposits. The Gulf of Mexico foraminifera have long been tied to specific environments, and facies in which one genera of benthic foraminifera is most abundant (i.e., the predominance facies) represent a promising technique to reconstruct paleoenvironment in Holocene cores. Although predominance facies have been well documented in Galveston Bay, to date, there are no equivalent records directly offshore for comparison, leaving the inner shelf assemblage unconstrained. Additionally, no direct comparisons have been made between ancient estuary assemblages and the modern living assemblages in Galveston Bay. To address this knowledge gap, this project examines the environmental evolution of the Trinity River estuary using benthic foraminifera and core data. The foraminifera trends offshore Galveston Bay show a greater diversity compared to estuary samples, which is likely driven by increased salinity on the inner shelf compared to Galveston Bay. The increased diversity of inner shelf assemblages compared to those of the bay can be used for recognizing offshore assemblages as distinct from estuarine samples in core data. Additionally, the comparison of living and Holocene estuary populations shows ancient samples with much higher dominance of Elphidium than is observed in the modern bay. This non-analogue population suggests environmental conditions (likely salinity) in the Holocene Trinity River estuary varied significantly from modern conditions implying a recent environmental change in the bay.
Tracking barrier island reponse to early Holocene sea-level rise : high resolution study of estuarine sediments in the Trinity River Paleovalley
(2021-08-13) Burstein, Jacob Thomas; Goff, John A.; Gulick, Sean P.S.; Lowery, Christopher M
Understanding how barrier islands respond to factors such as variations in sediment supply, relative sea-level rise, and accommodation is valuable for preparing coastal communities for future impacts of climate change. Increasingly, the underlying antecedent topography has been observed to have a significant control on the evolution of the barrier island system by providing increased elevation, decreased accommodation, and sediment supply for the barrier to rework and anchor upon. However, less attention has been focused on how back barrier sediments respond to this decreased accommodation, and how this may affect barrier island evolution. Additionally, the control in which the geometry of the underlying valley itself has on the initiation of barrier islands is poorly understood. Here we examine the stratigraphic framework of the Trinity River incised valley, offshore Galveston, Texas in order to investigate the role of antecedent topography in the evolution of an ancient barrier island system. We present high-resolution imaging of the Trinity incised valley fill using over 1200 km² of 3D seismic, <700 km of 2D envelope and full waveform chirp data, along with 2 piston cores, 4 gravity cores, 1 platform boring, with associated grain size, foraminiferal, and radiocarbon data. We find that the geometry and elevation of the underlying antecedent topography plays a central role in the evolution of the barrier island system, promoting both initiation and stabilization. This study provides a methodology to investigate the evolution of a relict barrier island system where little to none of the barrier is preserved. With this methodology, we revise the established Holocene paleoshoreline model for the Trinity incised valley.
Late Pleistocene fluvial-deltaic deposition, Texas coastal plain and shelf
(1979) Winker, Charles David; Baker, Victor R.
Deposition on the Texas coastal plain and shelf during the last Pleistocene glacial cycle has been interpreted from topographic and bathymetric maps, from borehole data including results of a detailed drilling study in Brazoria County, and from offshore sparker-profiles. The stratigraphic unit deposited during the last glacial cycle is bounded above by a largely undissected topographic surface (Beaumont), and below by a buried paleosol of stiff gray clay that correlates offshore with a persistent seismic reflector. The lower Texas coastal plain is essentially a clay-rich alluvial plain made up of coalescing low-gradient fans. The Beaumont alluvial plain onlaps an older surface (Lissie) which was tilted seaward prior to Beaumont deposition; the Lissie in turn onlaps remnants of older surfaces. During Beaumont deposition, each major coastal river deposited a branching network of meanderbelt sand-bodies by repeated avulsions. Borehole data for a meanderbelt of the ancestral Brazos River indicate that the channel was 5 to 7 m deep but that substantially greater sand thicknesses developed by stacking of point-bar sequences during fluvial aggradation. Downdip transition of fluvial deposits into deltaic and paralic sediments is inferred from shell beds, strike-oriented sand bodies, beach ridges, changes in clay color, and clinoform reflectors on sparker profiles. The updip limit of marine influence is delineated by the distribution of shell beds; the downdip limit of deltaic pro gradation is indicated by a paleobathymetric break in slope. Strike-oriented sands (Ingleside), including barrier islands and strandplains, developed contemporaneously with Beaumont fluvial aggradation. Sand thicknesses and multiple levels of sand suggest that beach-shoreface sequences are multistoried, similar to the fluvial sands. The thickest and widest strike sands formed in bights between the larger, more prominent deltaic systems. In response to falling sea level, deltas prograded from the Ingleside shoreline to the shelf edge. Sparker profiles show that deltaic thicknesses and offshore slopes increased gradually during progradation, then rapidly near the shelf edge, where deltaic sequences became stacked or imbricated. Major growth faulting and salt flowage near the shelf break were associated with the thickest deposits. Between the large Colorado and Rio Grande delta systems, reefs grew near the shelf edge. Late Pleistocene sea-level fluctuations resulted in three depositional phases: an aggradational phase (ca. 120,000 B.P.) during late rise and stillstand, dominated by fluvial and strike systems; a progradational phase (100,000 to 20,000 B.P.) during a gradual fall, dominated by deltaic systems, and a rapid transgressive phase (20,000 to 4000 B.P.). The Texas coast is now in another aggradational phase. Average rates of late Pleistocene sediment influx were similar to historic rates, and show a decrease in sediment production toward the arid southwest. Post-depositional deformation of the Beaumont and Lissie alluvial plains and Ingleside shoreline can be explained largely as an isostatic response to sedimentary loading
Organic geochemistry of an oil and gas seep in northern Gulf of Mexico sediments
(1984) Anderson, Richard Kent; Parker, Patrick L.
During an extensive geochemical and geophysical survey of the outer slope of the northwestern Gulf of Mexico nine piston cores were recovered which had visible liquid organic deposits. In three of the cores deposits were observed concentrated in oblique fracture planes. Other occurrences included large deposits as liquid veins and smaller disseminated pockets in gassy and non-gassy sediments. The benzene soluble material (bitumen) was extracted and chemically and isotopically characterized. Bitumen content ranged as high as 8.6 percent in sediment samples. Gas chromatographic analyses of silica gel fractions showed that both the saturated and aromatic hydrocarbon components are highly biodegraded. The δ¹³C values for the whole oil and fractions were between -26.2 and -26.7 per mil on the PDB scale which closely resembles other Gulf coast oils. The δD values of the oil averaged -104 per mil relative to SMOW. Carbonate nodules found in the oil-rich zones were ¹³C depleted, indicating oxidized organic matter to be the source of the inorganic carbon. Several cores contained natural gas in concentrations high enough to result in large expansion gaps under the reduced ambient pressure at sea level. Hydrocarbon gases from methane through pentanes were sampled in nine cores. Chemical composition and δ¹³C values for methane, ethane, propane, and butanes (-30.5 to -61.9, -28.5, -24.5, -25.7 per mil) indicated that the gas has a major petrogenic component. δD values for methane, ethane, propane and butanes were -172, -101, -104, -101 per mil. Compositional variability of C₂⁺ gases among cores suggests the possible regional influence of gas hydrate formation. Compositional and isotopic variability of methane within and between cores does not conform to a two component mixing model (e.g. biogenic plus petrogenic methane). Instead, highly localized processes, possibly microbial, are implicated
Subsurface structure, stratigraphy, and regional tectonic controls of the Guajira margin of northern Colombia
(2008-05) Vence, Eleine Melisa; Mann, Paul, 1956-
I combine previous data from Mesozoic outcrops in the Guajira Peninsula of northern Colombia with new regional gravity, bathymetric, and seismic interpretations to demonstrate the existence of a 280-km-long, western extension of the Great Arc of the Caribbean (GAC) along the continental margin of Colombia. Seismic data reveal a 80 to 100 km-wide domal-shape basement high lacking seismic reflectivity and mappable for 1800 km from the Aves Ridge to the study area in offshore Colombia. The western extension of the GAC in Colombia and western Venezuela is buried by 700 to 3000 m of continental margin sediments because it collided earlier (Cretaceous-early Paleogene collision) than its subaerially exposed eastern part on the Leeward Antilles Ridge (late Paleogene-Neogene collision). Compilation of radiometric age dates and geologic information from the entire GAC shows that arc magmatism ocurred from 128 to 74 Ma with a general pattern of younging from west to east. I mapped six Upper Eocene to Recent marine seismic sequences that overlie the domal basement high of the GAC using 2400 km of seismic reflection data and 12 wells. Three deformation events affecting the sequences include: 1) late Eocene rifting, in an east-west direction produced half-grabens in the northern part of the area; 2) Oligocene transtension, in the southern part of the area expressed by right-lateral Oligocene strike-slip faulting and extensional basin formation; 2) early - middle Miocene transtension; and 3) late Miocene - early Pliocene Andean uplift and clastic infilling of offshore basins
Effective porosity estimation from 3D seismic reflection data : Marco Polo Field, Gulf of Mexico
(2007-05) Young, Gregory Russell; Sen, Mrinal K.
Seismic inversion is a geophysical tool that uses geologically constrained physical models to infer elastic interval properties (e.g., P-velocity, S-velocity, and density) from seismic interface data (e.g., reflection amplitude and moveout). This thesis focuses on one such application to pre-stack seismic data from the Marco Polo Field, deepwater Gulf of Mexico. I compare the usefulness of post-, partial-, and pre-stack seismic inversion methods for estimating effective porosity by 1) inverting the Marco Polo Field seismic data with post-, partial-, and pre-stack seismic inversion methods, 2) transforming the estimated elastic parameters into effective porosity via fluid-dependent transformations derived from borehole data, and 3) interpreting all results in terms of inverse theory, rock physics, and the Marco Polo Field geology. I derive fluid-dependent transformations that are calibrated to the Marco Polo Field by cross-plotting measured elastic parameters (i.e., well logs) against petrophysical logs for gas-, oil-, and brine-saturated intervals in the wells. Cross-plot analysis indicates that density is best-suited for estimating effective porosity because the steep gradient of the density-to-effective porosity transformation implies minimal error magnification during mapping from the elastic parameter domain to the reservoir characteristic domain. The shallow gradient of the P-impedance-to-effective porosity transformation gradient results in substantial error magnification. Although density is the choice parameter for estimating effective porosity, deterministic linear seismic inversion methods have historically failed to resolve density. Post-stack inversion methods, which parameterize the model space with the single parameter P-impedance, provide no information about density. Partial-stack inversion methods fail to resolve density from angle-dependent amplitude variations because the NMO corrected gathers, upon which partial-stack inversions rely, have a separate null space that contains the necessary information to separate density from velocity within the impedance estimates. Pre-stack seismic inversion methods utilize the full waveform data (i.e., amplitude and moveout) and show great potential for estimating mass density. However, pre-stack seismic inversions are viewed as impractical due to high computation costs, and they are typically only applied to a few CMP's or over small time windows. Using the Miocene submarine fan system in the Marco Polo Field as a case study, I invert the 3D pre-stack seismic volume with a computationally efficient pre-stack seismic inversion algorithm, and I demonstrate that only full-waveform nonlinear pre-stack inversion accurately resolves density and estimates effective porosity within the deepwater system. The density and effective porosity estimates accurately tie with a type well with VSP, and they both correctly model the a priori structural and stratigraphic information in the Marco Polo Field. Moreover, the pre-stack inversion algorithm is able to resolve all model parameters from nearly flat initial models.
Stratigratigraphic architecture and basin fill evolution of a plate margin basin, eastern offshore Trinidad and Venezuela
(2006-05) Garciacaro, Emilio José; Mann, Paul, 1956-; Wood, Lesli J.
Eastward migration of the Caribbean plate relative to the South American plate is recorded by a 1100-km-long foreland basin which is oldest in the west (Maracaibo basin, 65-55 Ma) and youngest in the east (Columbus basin, eastern offshore Trinidad, 15-0 Ma). Regional transpression has caused lithospheric loading and flexure along the northern margin of South America creating a large foreland basin area which propagated from west to east as the Caribbean plate moved eastward relative to the South American plate. I have integrated 775 km of deep-penetration 2D seismic lines acquired by the 2004 BOLIVAR survey, 325 km of 1975 GULFREX seismic data, 8,000 km2 of industry 3-D seismic data, and published industry well data from offshore eastern Trinidad. Interpretation of seismic sections tied to wells reveals the following fault chronology: 1) middle Miocene thrusting along the Darien ridge related to highly oblique convergence between the Caribbean plate and the passive margin of northern South America; continuing thrusting and transpression in an oblique foreland basin setting through the early Pleistocene; 2) early Pliocene-recent low-angle normal faults along the top of the Cretaceous passive margin; these faults were triggered by oversteepening related to formation of the downdip, structurally and bathymetrically deeper, and more seaward Columbus basin; large transfer faults with dominantly strike-slip displacements connect gravity-driven normal faults that cluster near the modern shelf-slope break and trend in the downslope direction; to the south no normal faults are present because the top Cretaceous horizon has not been oversteepened as it is adjacent to the foreland basin; 3) early Pliocene-Recent strike-slip faults parallel to the trend of the Darien ridge and accommodate present-day plate motions. Active mud diapirism in the Columbus basin is widespread and is related to overthrusting and loading of upper Miocene-lower Pliocene age mud. Analysis of the 3-D seismic data reveals the presence of extensive gravity-flow depositional elements on the Columbus basin deepwater area, characterized by mass-transport deposits at the base, turbidite frontal-splay deposits, leveed-channel deposits, and capped by fine-grained condensed-section deposits. Deep basin wells drilled in recent years have proven that turbidites were transported into the Columbus basin deepwater during the Plio-Pleistocene. Analysis of these well results suggest that a deeper oil charge is present within the Columbus basin deepwater area. The primary uncertainty for this variable hydrocarbon system is whether fault or diapiric pathways connect the petroleum charge at depth with shallower reservoir rocks.
Integrated stratigraphic and petrophysical analysis of the Wolfcamp at Delaware Basin, West Texas, USA
(2022-04-12) Ramiro-Ramirez, Sebastian; Flemings, Peter Barry, 1960-; Bhandari, Athma R; Daigle, Hugh C; Kerans, Charles; Tisato, Nicola
Hydrocarbons stored in low-permeability reservoirs, also known as ‘unconventional reservoirs’, represent important energy resources worldwide. Although current technology allows their production at economic rates, there still are numerous production challenges and unknowns regarding their flow behavior. A better understanding on how fluids stored in these reservoirs are drained by the hydraulic fractures after stimulation may help to optimize completion designs and field development plans. This research is an attempt to describe such drainage behavior in the largest oil producing unconventional formation in the World. I investigated the drainage behavior in Wolfcamp reservoirs at the completion scale by integrating stratigraphic and petrophysical analyses with flow modeling. I interpreted the depositional and diagenetic processes that generated three Wolfcamp cores recovered in the central-eastern Delaware Basin, measured the porosity and permeability of distinct lithofacies, and developed simple models to describe flow in these strata. I found that most fluids (~95% of the pore volume) are stored in low-permeability (e.g., < 60 nD) mudstones that I interpreted as hemipelagics and siliciclastics turbidites. Interbedded with these deposits are the low-porosity (~5% of the pore volume) and low-permeability (e.g., < 50 nD) carbonate lithofacies that I interpreted as gravity flow deposits and diagenetic dolomudstones. The carbonate gravity flow deposits, when dolomitized, are up to 2000 times more permeable than the other deposits and represent preferential flow pathways that drain fluids from the low-permeability strata during production. This drainage behavior increases the reservoir upscaled permeability, and therefore production rates, multiple times higher compared to a reservoir consisting of only low-permeability strata. Hence, the presence of these permeable, dolomitized, gravity flow deposits plays a critical role when producing from Wolfcamp reservoirs as they accelerate drainage. These findings are also applicable to other low-permeability formations exhibiting significant permeability heterogeneity
Cost effective strategies for problems in computational geophysics : seismic modeling and imaging
(2020-02-03) Vamaraju, Janaki; Sen, Mrinal K.
The first part of my thesis focuses on seismic modeling in fractured media. Several recent developments in finite elements such as usage of high degree polynomials to approximate the wavefield, diagonalization of mass matrices to be inverted through mass lumping techniques and usage of high order time-stepping schemes, have made these methods (along with their classical advantages) more attractive when compared to the finite difference methods (FDM). Discontinuous Galerkin finite element method (DGM) and spectral element method (SEM) have particularly attracted researchers in the field of numerical wave propagation. SEM uses continuous basis functions, which do not allow for discontinuities in the displacement field. Hence it can be used to simulate wave propagation only in non-fractured media. On the other hand, DGM allows for discontinuities in the displacement field to simulate fractures or faults but with a significant increase in computation cost and memory requirement. Here, I formulate and analyze two new, improved finite element techniques (FEM) for the numerical solution of elastic wave propagation in fractured and non-fractured media. Enriched Galerkin (EGM) and hybrid Galerkin (HGM) formulations are proposed for solving elastic wave propagation that has advantages similar to those of DGM but with a computational cost comparable to that of SEM. EGM uses the same bilinear form as DGM, and discontinuous piecewise constants or bilinear functions enrich the continuous Galerkin finite element spaces. EGM satisfies local equilibrium while reducing the degrees of freedom in DGM formulations. HGM employs DGM in areas containing fractures and SEM in regions without fractures. The coupling between the domains at the interfaces is satisfied through interface conditions. The degree of reduction in computation time depends primarily on the density of fractures in the medium. I apply these methods to model wave propagation in 2D/3D fractured media and validate their efficiency with numerical examples. Fractured reservoirs are more complicated due to the presence of fractures and pores. Biot’s fundamental theory on wave propagation in fluid-saturated porous media is still well accepted and forms the basis of this work. To examine the effects of fluid-filled cracks and fractures, I next propose to combine poroelasticity with the linear slip theory for simulating wave propagation in fractured porous media. This study provides an equivalent anisotropic medium model for the description of porous rock with fractures in the seismic frequency band. I solve Biot’s poroelastic wave equations using a velocity-stress staggered grid finite difference algorithm. Through numerical examples, I show that fractures and pores strongly influence wave propagation, induce anisotropy, and poroelastic behavior in wavefields. I also validate the presence of two compressional waves as predicted by Biot’s theory along with the converted waves due to faults. Compared to elastic methods, this approach provides a considerably concise and more accurate model for fractured reservoirs. The second part of my thesis centers on developing cost-effective solutions for seismic migrations and anisotropic moveout corrections. Least-squares migration (LSM) is a linearized inversion problem that iteratively minimizes a misfit functional as a function of the model perturbation. The success of the inversion largely depends on our ability to handle large systems of equations given the massive computation costs. I propose a suite of unsupervised machine learning (ML) approaches that leverage the existing physics-based models and machine learning optimizers to achieve more accurate and cheaper solutions. First, I use a special kind of unsupervised recurrent neural network and its variant, Hopfield neural networks, and the Boltzmann machine, to solve the problems of Kirchhoff and post-stack reverse time migrations. Physics-based forward models can be used to derive the weights and biases of the neural network. The optimal configuration of the neural network after training corresponds to the minimum energy of the network and thus gives the reflectivity solution of the migration problem. I next implement a fast image-domain target-oriented least-squares reverse time migration (LSRTM) workflow using a conjugate Hopfield network. The method computes a low-cost target-oriented Hessian matrix using plane-wave Green’s functions. I recover a more accurate image in the presence of a truncated Hessian matrix. I further implement pre-stack LSRTM in a deep learning framework and adopt various machine learning optimizers to achieve more accurate and cheaper solutions than conventional methods. Using a time-domain formulation, I show that mini-batch gradients can reduce the computation cost by using a subset of total shots for each iteration. Mini-batches not only reduce source cross-talk but are also less memory intensive. Combining mini-batch gradients with Adam optimizer and Huber loss function can improve the efficiency of pre-stack LSRTM. I demonstrate high accuracy predictions on complex synthetic models that can generate noisy data. Finally, I develop a Hough transform neural network based technique for normal moveout correction in vertically transverse isotropic (VTI) media. This technique offers advantages when compared to the time and computational costs required in a conventional anisotropic normal moveout correction. Using a Hough transform based neural network, I simultaneously fit all the non-hyperbolic reflection moveout curves using intermediate to long offsets. I apply the network to synthetic VTI datasets and demonstrate the practical feasibility of anisotropic moveout correction that is independent of travel-time picking and velocity analysis

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