Intrusion-Related Hydrothermal Systems In Sedimentary Basins - Unlocking Subsurface Uncertainties Using Seismic Reflection Imaging

Abstract

Since the 1980’s, the concept of petroleum systems has proven success for understanding the fundamental elements and the essential processes that control the formation of oil and gas accumulations. Similarly, the introduction of a unifying concept to assess the economic potential of geothermal systems can help to de-risk prospects, and can increase the chances of finding valuable energy resources in association whit igneous activity. Key elements and processes for geothermal systems are: a source of heat (typically magma or hot igneous rocks), a reservoir body (which can be fractured or porous), water (vapour or liquid), and a permeable network to transfer fluids and heat through the system. Magma emplaced in sedimentary basins can provide these conditions and form hydrothermal systems prolific producers for geothermal energy. Understanding the distribution and impact of intrusions within the basin strata are important for assessing the fundamental elements and processes that determine if intrusion-related hydrothermal systems are to exist. However, intrusive bodies can form in a variety of depths, shapes, sizes and geometric relationships with the enclosing host rocks, creating a complex subsurface architecture that may be difficult to predict. Interpretation of high-quality 2D and 3D seismic reflection surveys across “fossil” magmatic systems buried in sedimentary basins provides an opportunity to observe how these intrusions vary systematically within the basin strata. Our studies in New Zealand sedimentary basins suggest that intrusion-related hydrothermal systems are likely to form in association with large saucer-shaped sills (Figure 1), and with dike-and-sill swarms. In Canterbury and Taranaki basins, saucer-sills occur in great number, typically widespread and emplaced at shallow depths (< 4 km), and with volumes up to 3 km3, which can provide a source of heat for hydrothermal systems. The emplacement of saucer-sills is commonly associated with a ring-shaped network of faults and fractures in the overburden strata, likely to form fractured reservoirs. These faults and fractures can produce a permeable interconnected network, merging fluids from near saucer-sills. The combination of these elements and processes have potential to form supercritical hydrothermal plumes located along the lateral edges of the sills. In contrast, dike-and-sill swarms are commonly related to sub-volcanic zones, typically occurring in association with both monogenetic fields and polygenetic volcanoes. These intrusive swarms, like the saucer-sills, can create intense fracturing of the host rocks, and provide the necessary elements to form intrusion- related hydrothermal systems. In both cases, the spatial distribution of intrusions is related with simultaneous rift faulting and/or with pre-existing faults and folds structures. If the intrusions are emplaced in organic-rich and carbonate rocks, the hydrothermal systems are likely to be enriched in CO2, CH4, and H2S, producing technological and environmental issues for exploitation of geothermal energy. Understanding the geological conditions that form “fossil” magmatic systems in sedimentary basins is important for assessing the exploration risks, and to improve the likelihood of finding commercially viable energy resources in active intrusion-related hydrothermal systems.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019