The >300 km long, northeast-trending dextral transpressive Norumbega fault system in Maine and New Brunswick is defined by a tens of kilometers wide zone of multiple dextral shear zones. Based on published 40 Ar/ 39 Ar hornblende data and U-Pb zircon and monazite ages, dextral shear along the fault system initiated by ~380 Ma. The location and nature of the southwestern termination of the Norumbega fault system was investigated in this study. Previously, no fault system in New Hampshire or eastern Massachusetts has been recognized with an orientation, timing, and motion sense consistent with those of the NFS. Detailed structural mapping was carried out along topographic lineaments and mapped shear zones in New Hampshire and Massachusetts to test whether the NFS extends into those areas. The Rye Complex in southeastern coastal NH was the only location investigated that presented field characteristics consistent with the NFS. The Nannie Island shear zone in southeastern NH is along strike with the NFS, but contains both sinistral and dextral shear sense indicators and has a lower metamorphic grade. The dextral ENEtrending Shirley mylonite zone in central-eastern MA is similar to the NFS, but the ENE trend is not consistent with that of the NFS, and no connecting shear zone between the Shirley mylonite zone and NFS has been found. U-Pb monazite laser ablation inductively coupled mass spectrometry (LA-ICPMS) dates dextral deformation along the NFS and its potential extensions to the southwest. The four sample locations dated were the NFS in ME, the Rye Complex, the Nannie Island shear zone, and the Shirley mylonite zone. The NFS contained monazite ages of ~390-370 Ma related to dextral deformation. The Rye Complex yielded ages of ~430 to ~370-360 Ma, ~380 Ma and ~370 Ma, consistent with NFS deformation and earlier Acadian metamorphism that started at ~430 Ma. The Nannie Island shear zone produced monazite populations of ~441, ~700, ~900-800, and ~1775iv Ma, which are all older than the late Silurian or earliest Devonian age of the rock and, therefore, interpreted as detrital. The Shirley mylonite zone in MA yielded an age of ~380 Ma which is consistent with the age of deformation along the NFS.Thus, the Rye Complex is the only location investigated in this study that matches the NFS in shear zone orientation, deformation style, shear sense, and age of deformation, and therefore could be the southernmost extension of the NFS. The NFS has been demonstrated not to extend farther to the southwest.
Many offshore wells and platforms are nearing abandonment, in the Gulf of Mexico, the North Sea, Offshore West Africa, and other areas. The current decommissioning process, however, is expensive. Instead of decommissioning, do these assets create an opportunity to decrease cost of decarbonization efforts using this existing infrastructure? Some platform repurposing options include geothermal energy generation (heat or electricity) using modular power generation units or pipelines back to the beach, carbon dioxide storage, hydrogen generation, and potentially others. Each option has both a technical probability of success and a financial outcome. In this study, we begin to answer the question "Where is geothermal energy an option for repurposing offshore wells and platforms?" Here, we examine the Corpus Christi Bay and the Galveston Bay for geothermal power potential as shallow water offshore platforms that will have the highest likelihood to sell power if a geothermal resource exists. Geothermal power potential is calculated and several end use scenarios for the geothermal energy, including cost of energy and project financials, are examined. Using these two regions as test cases, estimations on the potential shallow water offshore geothermal market is discussed. To perform this assessment, we examine published geothermal gradient maps and produce an industry standard geothermal resource assessment based on existing well production from the offshore platform. Using the thermal energy production, we estimate electrical power potential and total potential revenue, and then calculate high level financial metrics. Technical gaps and data gaps are assessed. CAPEX, OPEX, discounted cash flows, internal rates of return, and project lifetimes are part of the financial analysis. Indicative results are presented from the sample case studies in Corpus Christi Bay and Galveston Bay. There is no offshore geothermal energy production to date, although this is a topic of interest with significant publicity from Gulf of Mexico and North Sea operators. While there is interest, there are no public studies showing potential economics. This paper, to our knowledge, summarizes and publicly presents technical and financial parameters for shallow water offshore geothermal energy production with the specific goal to extend the life of offshore platforms and re-use existing wellbores.
The northern Gulf of Mexico basin contains geopressured zones ideal for geothermal energy production, still to be explored. These systems are defined by primarily Eocene to Miocene sands that are confined by shale beds, which facilitates the formation of anomalously high pressures and temperatures. The overpressure in these zones results in an increased geothermal gradient, which makes geopressured zones of interest for geothermal exploration. Resources are commonly found at 3 to 6 km depth and reservoir fluid temperatures can range from 90 to 200°C. There has been a substantial amount of work done to understand these geopressured reservoirs on the Gulf Coast for geothermal potential. Many of these geopressured zones extend and exist offshore in the Gulf of Mexico. The knowledge and technical success of wells completed in these geopressured zones onshore can be transferred to understand how to produce a high pressure high temperature offshore well for geothermal power production. This paper will provide a review of previous work on geopressured geothermal zones in the Gulf Coast, the challenges with these systems, how these were overcome, and the knowledge transfer of those findings for offshore geothermal opportunities in high pressure high temperature wells.
To create an efficient subsurface heat exchanger in an Enhanced geothermal System (EGS), it is essential to have a thorough understanding of the subsurface characteristics along with an effective real-time system to monitor and map the changes in induced and natural fractures during stimulation and production. The ability to monitor the variation in fracture network characteristics is necessary to evaluate reservoir performance over time. These continuous assessments provide valuable information and an opportunity to optimize stimulation schemes for maximum heat production. We developed a methodology for analysis of microseismic data through a novel “Time-Lapsed Microseismic” (TLM) technique combined with a Moment Tensor Inversion (MTI) method, to detect, locate and geomechanically characterize microseismic events in different time windows for monitoring the changes in the fracture network’s geometry and geomechanical status over time without requiring additional data acquisition or monitoring tools. 41 years of raw waveform data from multiple generations of seismic networks from the Geysers geothermal field were amalgamated to generate a catalog of 9,601 stable full-waveform MTI solutions in a rapid time frame. The resulting TLM data are the foundation for monitoring the temporal changes in fracture characteristics. Furthermore, as new microseismic data are collected, the TLM volume is updated, rendering itself to become a primary EGS monitoring tool.
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