INTEGRATED GEOPHYSICAL STUDIES OF THE FORT WORTH BASIN (TEXAS), HARNEY BASIN (OREGON), AND SNAKE RIVER PLAIN (IDAHO)

Abstract

Geophysical methods such as seismic, gravity, magnetics, electric, and electromagnetics are capable of identifying subsurface features but each has a different spatial resolution. Although, each of these methods are stand-alone tools and have produced wonderful and reliable results for decades to solve geological problems, integrating geophysical results from these different methods with geological and geospatial data, adds an extra dimension towards solving geological problems. Integration techniques also involve comparing and contrasting the structural and tectonic evolution of geological features from different tectonic and geographic provinces. I employed 3D and 2D seismic data, passive seismic data, and gravity and magnetic data in three studies and integrated these results with geological, and geospatial data. Seismic processing, and interpretation, as well as filtering techniques applied to the potential filed data produced many insightful results. Integrated forward models played an important role in the interpretation process.

The three chapters in this dissertation are stand-alone separate scientific papers. Each of these chapters used integrated geophysical methods to identify the subsurface features and tectonic evolution of the study areas. The study areas lie in the southeast Fort Worth Basin, Texas, Harney Basin, Oregon, and Snake River Plain, Idaho.

The Fort Worth Basin is one of the most fully developed shale gas fields in North America. With the shallow Barnett Shale play in place, the Precambrian basement remains largely unknown in many places with limited published work on the basement structures underlying the Lower Paleozoic strata. In this research, I show how the basement structures relate to overlying Paleozoic reservoirs in the Barnett Shale and Ellenburger Group. I used high quality, wide-azimuth, 3D seismic data near the southeast fringe of the Fort Worth Basin. The seismic results were integrated with gravity, magnetic, well log, and geospatial data to understand the basement and sub-basement structures in the study area. Major tectonic features including the Ouachita thrust-fold belt, Lampasas arch, Llano uplift, and Bend arch surround the southeast Fort Worth Basin. The effects of these tectonic units in the basement were imaged in form of faulted and folded basement and sub-basement layers. Euler deconvolution and integrated forward gravity modeling were employed to extend the interpretations beyond the 3D seismic survey into a regional context.

The Harney Basin is a relatively flat lying depression in the northeast portion of the enigmatic High Lava Plains volcanic province in eastern Oregon. In addition to the High Lava Plains active source seismic data, I also employed gravity, magnetic, digital elevation, geologic maps, and other geospatial data in this integrated study. I generated an upper crustal 3D seismic tomographic model of the Harney Basin and surrounding area using the active source seismic data. I then integrated it with gravity, magnetic, and geologic data to produce a geophysical model of the upper crustal structure, which reveals that the basin reaches as deep as 6 km in the central areas. I observed two major caldera shaped features within the basin. These calderas reveal seismic low velocity areas along with low gravity and magnetic anomalies. I interpreted the extent of these calderas with the help of integrated geophysical results. I propose a nested caldera complex in the northern Harney Basin and another caldera in the southern part.

The Snake River Plain is an arcuate-shaped topographic low that lies in southern Idaho. This rifted valley is filled by large volume of mafic magma with numerous exposures of silicic volcanic centers. The scientific discussion on the structural complexities and evolution of the Snake River Plain and the role of extension in its formation has been going on for decades. Similarly, high gravity and magnetic anomalies are associated with the Snake River Plains, and their possible causes are still the subject of many studies. Numerous recent passive seismic studies specifically focus on the deep mantle structures of the Eastern Snake River Plain. However, crustal scale studies in the Western Snake River Plains are limited. In this research, I used gravity and magnetic data in the area and integrated the results with seismic, geospatial data, and receiver function results. I identified the major differences and similarities in the structures and tectonics of the Western and Eastern Snake River Plain based on the gravity and magnetic anomalies. With the help of processed receiver function results, 2D seismic refraction and reflection data, interpreted well logs, and geospatial data, I generated 2D gravity models across the Western Snake River Plain. A mid-crustal mafic intrusion is the main reason for high gravity anomaly in the Western Snake River plain. Alternate gravity model along a profile showed underplating as a possible additional source for the gravity high along the Western Snake River Plain.