An integrated geophysical analysis of rifting evolution and gas hydrate occurrence in the Eastern Black Sea Basin

Abstract

One of the key elements in passive continental margins and basins research is the understanding of how extensional processes initiate and evolve from rifting to breakup stage. These processes may vary spatially and temporally as result of the complex trade-off between spreading rates, lithospheric compositional variations, and along-axis changes in melt supply. Thus, extension may focus over one conjugate margin resulting in an along-axis change in rifting style. Lower crust and/or mantle exhumation may occur at hyper-extended areas, and along-axis changes in melt supply during spreading may result in varying amounts of magmatic intrusions within a highly extended continental crust. The resulting crustal structure and composition at rifted margins may therefore present a geophysical signature differing from what is normally expected for continental and oceanic crust, making it difficult to define the transition from the late stages of continental rifting to initial oceanic accretion. Nevertheless, a clear understanding of the stages defining passive continental margins and extensional basins evolution is key to unravel the mechanisms driving lithospheric extension, as well as the present-day configuration at these settings. Sedimentary basins formed in continental rifts and rifted margins also contain some of Earth’s major hydrocarbon fields. Constraining the evolution and dynamics of these fields gives insight into the timing of basin formation, sedimentation, trap formation, and heat-flow evolution, which are crucial for efficient hydrocarbon exploration. Passive continental margins are also relevant to the study of gas hydrates deposits, which are of interest not only for their potential as a future energy resource, but also for their implications on seafloor stability and their contribution to climate change. However, the dynamics driving their formation and evolution along passive margins is yet to be fully understood. This study provides an integrated geophysical investigation at one of the currently most debated extensional regions, the Eastern Black Sea Basin (EBSB). Here, the timing and dynamics of rifting and breakup processes and the resulting crustal configuration are still poorly constrained. This is due to the deep water setting and thick sedimentary infill, limiting direct sampling of deep sedimentary units and the ability of imaging the deep basin structures using conventional seismic imaging. The EBSB is also a frontier for oil and gas exploration due to the presence of a regional source-rock, the Oligocene-lower Miocene Maykop Formation. Thus, untangling the stages in basin evolution and defining the extent of different crustal basements is essential for modelling the maturity of the hydrocarbon systems in place. This study uses high-resolution, 2D long-offset seismic reflection data by Geology Without Limits (GWL) to investigate the deep tectonostratigraphic elements of the basin and the morphological character of its basement. Magnetic anomaly data are integrated to complement and guide seismic interpretation, defining the magnetisation character of morphologically different crustal domains. The combined analysis of tectono-stratigraphic and magnetic anomaly results provides new insights on the present-day crustal configuration, and on the timing and kinematics of rifting and breakup processes across the EBSB. The Black Sea is characterised by thermo-dynamic conditions which make this area favourable to gas hydrate formation. Gas hydrates have long been known in this area, with related evidence for Bottom Simulating Reflectors (BSRs) predominantly found in the western part of the basin (WBSB). Seismic data used in this study reveal new evidence for multiple BSRs along the NE margin of the EBSB, thus providing a new case study to investigate the physical nature of the identified BSRs, the presence and distribution of gas hydrates and/or free gas zones, and the mechanism(s) driving multiple BSRs generation. The long-offset nature of the seismic acquisition provides both reflected and refracted travel-time information that can be used in a tomographic approach, aiming to define a 2D velocity model within the shallow sedimentary section. To improve refracted arrivals, downward continuation is also applied to the seismic data. Results help to constrain velocity changes in sediments across the BSRs, which are interpreted as indicative for the presence of free gas and gas hydrate in the area. Velocities defined form travel-time analysis are also used to provide estimates of free gas and gas hydrate saturation in sediments using a forward effective-medium modelling approach. Results from this thesis contribute to the understanding of rifting evolution on both small-scale basins, such as the Black Sea, and fully developed passive continental margins. Similarly, results also contribute to the general understanding of gas hydrate evolution in the Black Sea as well as along other well-known gas hydrate regions.

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Identifiers

ORCID for Vanessa Monteleone: orcid.org/0000-0002-5188-5879

ORCID for Timothy Minshull: orcid.org/0000-0002-8202-1379

ORCID for Héctor Marin-Moreno: orcid.org/0000-0002-3412-1359

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