Experimental Studies of Oxygen Isotope Fractionation in the Carbonic Acid System at 15^o, 25^o, and 40^oC. (August 2004)

William Cory Beck, B.S., University of Oklahoma

Co-Chairs of Advisory Committee: Dr. Ethan L. Grossman , Dr. John W. Morse

This abstract is in PDF form here.

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ABSTRACT

Acoustic Impedance Inversion of Lower Permian Carbonate Buildups

in the Permian Basin , Texas

August 2004

Pablo Buenafama Aleman

B.S., Simon Bolivar University

Chair of Advisory Committee: Dr. Richard L. Gibson

Carbonate reservoirs are usually difficult to map and identify in seismic sections due to their complex structure, lithology and diagenetic fabrics. The Midland Basin , located in the Permian Basin of West Texas , is an excellent example of these complex carbonate structures.

In order to obtain a better characterization and imaging of the carbonate buildups, an acoustic impedance inversion is proposed here. The resolution of the acoustic impedance is the same as the input seismic data, which is greatly improved with the addition of the low frequency content extracted from well data. From the broadband volume, high resolution maps of acoustic impedance distributions were obtained, and therefore the locations of carbonate buildups were easily determined. A correlation between acoustic impedance and porosity extracted from well data shows that areas with high acoustic impedance were correlated with low porosity values, whereas high porosities were located in areas of low acoustic impedance.

Theoretical analyses were performed using the time-average equation and the Gassmann equation. These theoretical models helped to understand how porosity distributions affect acoustic impedance. Both equations predicted a decrease in acoustic impedance as porosity increases. Inversion results showed that average porosity values are 5% ± 5%, typical for densely cemented rocks. Previous studies done in the study area indicate that grains are moderately to well-sorted. This suggests that time-average approximation will overestimate porosity values and the Gassmann approach better predicts the measured data.  A comparison between measured data and the Gassmann equation suggests that rocks with low porosities (less than 5%) tend to have high acoustic impedance values. On the other hand, rocks with higher porosities (5% to 10%) have lower acoustic impedance values. The inversion performed on well data also shows that the fluid bulk modulus for currently producing wells is lower than in non-productive wells, (wells with low production rates for brine and hydrocarbons), which is consistent with pore fluids containing a larger concentration of oil.

The acoustic impedance inversion was demonstrated to be a robust technique for mapping complex structures and estimating porosities as well. However, it is not capable of differentiating different types of carbonate buildups and their origin.

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ABSTRACT

Ajibola Olaoluwa David Owoyemi, B. Tech (Honors),

Federal University of Technology, Akure , Nigeria .

Chair of Advisory Committee: Dr. Brian J. Willis.

The Niger Delta clastic wedge formed along the West Africa passive margin. This wedge has been divided into three formations that reflect long-term progradation: 1) pro-delta shales of the Akata Formation (Paleocene to Recent), 2) deltaic and paralic facies of the Agbada Formation (Eocene to Recent) and 3) fluviatile facies of the Benin Formation (Oligocene- Recent). This study combines a three-dimensional seismic image with well log data from Delta field to describe lithic variations of the Agbada Formation and develop a sequence stratigraphic framework. The 5000-feet thick Agbada Formation in Delta Field is divided by five major sequence boundaries, each observed in seismic cross sections to significantly truncate underlying strata. Sequence boundaries developed as mass flows eroded slopes steepened by the structural collapse of the Niger Delta clastic wedge. Basal deposits directly overlying areas of deepest incision along sequence boundaries formed by the migration of large, sinuous turbidite channels. Upward-coarsening sets of inclined beds, hundreds of feet thick, record progradation of deltas into turbidite-carved canyons and onto down faulted blocks. Thinner, more continuous seismic reflections higher within sequences are associated with blocky and upward-fining well-log patterns interpreted to reflect deposition in shoreline, paralic, and fluvial environments.

Episodes of structural collapse of the Niger Delta clastic wedge appear to be associated with progradation of Agbada Formation sediments and the loading of underlying Akata Formation shales. Progradation may have been more rapid during third order eustatic sea level falls. Effects of syn-sedimentary deformation on patterns of sediment transport and deposition are more pronounced in lower sequences within the Agbada Formation, and include: 1) incision into foot walls of listric normal faults, 2) abrupt reorientation of channelized flow pathways across faults, and 3) thinning of deposits across crests of rollover anticlines on down thrown fault blocks. Structural controls on deposition are less pronounced within younger sequences and canyon incisions along sequence boundaries are more pronounced, suggesting that the locus of sediment accumulation and structural collapse of the clastic wedge moved farther basinward as accommodation was filled in the area of Delta field. Early sequences are thicker, contain more upward-coarsening 10-50 foot thick vertical log patterns and show significantly thicker growth strata. Later sequences are thinner, contain more blocky and fining-upward 10-50 foot thick vertical log patterns, are more deeply incised, and sediment transport pathways are straighter and apparently less influenced by local syn-depositional deformation.

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ABSTRACT

A finite element approach to the 3D CSEM modeling problem and applications to the study of the effect of target interaction and topography. (August 2004)

Jack Stalnaker, B.S., College of Charleston

Chair of Advisory Committee: Dr. Mark Everett

The solution of the secondary coupled-vector potential formulation of Maxwell’s equations governing the controlled-source electromagnetic (CSEM) response of an arbitrary, three- dimensional conductivity model must be calculated numerically. The finite element method is attractive, because it allows the model to be discretized into an unstructured mesh, permitting the specification of realistic irregular conductor geometries, and permitting the mesh to be refined locally, where finer resolution is needed. The calculated results for a series of simple test problems, ranging from one-dimensional scalar differential equations to three-dimensional coupled vector equations match the known analytic solutions well, with error values several orders of magnitude smaller than the calculated values. The electro- magnetic fields of a fully three-dimensional CSEM model, recovered from the potentials using the moving least squares interpolation numerical differentiation algorithm, compares well with published numerical modeling results, particularly when local refinement is applied. Multiple buried conductors in a conductive host interact via mutual induction and current flow through the host due to the dissipation of charge accumulated on the conductor boundary. The effect of this interaction varies with host conductivity, transmitter frequency, and conductor geometry, orientation, and conductivity. For three test models containing two highly conductive plate-like targets, oriented in various geometries (parallel, perpendicular, and horizontal), mutual coupling ranges as high as twenty times the total magnetic field. The effect of varying host conductivity is significant, especially at high fre- quencies. Numerical modeling also shows that the vorticity of the currents density induced in a vertically oriented plate-like conductor rotates from vertical at high frequencies, to horizontal at low frequencies, a phenomenon confirmed by comparison with time domain iv field data collected in Brazos County , Texas . Furthermore, the effect of the presence of a simple horst on the CSEM response of a homogeneous conductive earth is significant, even when the height of the horst is only a fraction of the skin depth of the model. When the transmitter is placed on top of the horst, the currents induced therein account for nearly all of the total magnetic field of the model, indicating that topography, like mutual coupling must be accounted for when interpreting CSEM data.