Integrated seismic geomorphological analysis of syn- and post-depositional fluid migration features in the Chalk Group of the Danish North Sea

Florian Walther Harald Smit

Research output: Book/ReportPh.D. thesis

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Abstract

Integrated seismic geomorphological analysis of the Upper Cretaceous and Lower Paleocene Chalk Group of the Danish Central Graben has led to critical new insights into seismic-scale heterogeneities related to syn-sedimentary fluid/gas and/or post-burial migration of fluids, and improved the understanding of the multi-phased fluid migration history. The thesis consists of one published article, two submitted manuscripts, and one manuscript in preparation for submission. A summary is given in the synopsis chapter.
Integration is achieved both in terms of scales and disciplines by combining seismic geomorphology, stratigraphy, petrophysics, petrography, and geochemistry (including clumped isotope analysis), to seismic chalk facies. Seismic geomorphology provides palaeoreconstructions of the seafloor from 3D seismic data, which then act as a spatial context for integration with well data. For the first time, clumped isotope analysis is applied to chalk from the North Sea Basin to study closed- and open-system diagenesis in relation to seismic-scale heterogeneities.
The aim of the integrated methodology is to arrive at a geological understanding of seismic-scale features observed on the palaeoseafloors. By interpreting a ‘stack’ of palaeoseafloor reconstructions, interlinkage between geological features might be present, which shines light on the geological evolution within the area of interest. This prepares for integration with other basin-scale trends, such as tectonic evolution and thermal maturity of major source rocks. We call this approach integrated seismic geomorphology, and have applied it to the Chalk Group. Though based on geophysical, petrophysical, petrographical, and geochemical data, the geological interpretation of the features forms the backbone of the methodology.
Key results of this thesis are (a) the revision of the ‘Seismic Chalk Paradigm’ in terms of the nature and origin of the seismic heterogeneities observed in the chalk, and (b) a much improved understanding of the phased migration of basin fluids and their impacts on the chalk deposits.
The seismic chalk paradigm is subdivided into two main categories: 1) syn-depositional features and processes; 2) post-depositional features and processes. The syn-depositional features include the previously known pelagic chalks (1.A), bottom current activity features (1.B), and slope failure (1.C). With this study, we add seismic-scale gas venting features that form as a result of explosive releases of gas-bearing fluids (giant and mega pockmarks, 1.D1), and from seepage (seep carbonates, 1.D2). These features are described in depth in Chapter III and IV (Papers 2 and 3). The gas venting features occur on top of inverted Campanian structures that formed the highest parts of the Late Cretaceous basin, are underlain by inverted deeply rooted Late Jurassic faults, which offset large successions of organic-rich mudstones. These features thus reflect syn-depositional fluid expulsion episodes during chalk deposition, and point to the interlinkage between basin morphology, faults, and source rock maturity.
The post-depositional features include known but refined faulted/fractured chalks (2.A), and the addition of seismic-scale diagenetic geobodies (2.B). These features are described in depth in Chapter II (Paper 1). A combination of recently developed methods of image processing of seismic data and swarm intelligence made it possible to visualize near sub-seismic faults and fractured zones. Secondly, by integrating petrographic and geochemical proxies, seismic-scale diagenetic geobodies have been recognized for the first time in the Chalk Group. The extent and 3D architecture of the geobodies are controlled by contrasts in
permeability/porosity and reflect the occurrence of formation tops, and feeder fault system. It therefore provides important new insights in fluid migration pathways through the Chalk Group.
Clumped isotope analysis (Chapter V, Paper 4) revealed that most chalks show a positive correlation between precipitation temperature and fluid δ18O value (increasing with temperature), which reflects the preferential uptake of the lighter 16O into the crystal lattice with increasing temperature, while the heavier 18O remains in the formation fluid. It is interpreted to reflect closed-system diagenetic behaviour (c.f. Swart et al. 2016). Outliers that are anomalously hot and have negative δ18O values, are interpreted to reflect open-system diagenetic behaviour, and occur in veins and in surrounding narrow zones of high permeability and porosity, such as in fault damage zones. These data, coupled to seismic geomorphological palaeoreconstructions, provide important insights in the fluid migration history and diagenesis within the Chalk Group. It shows that most of the chalk experienced normal burial diagenesis, even in areas where extraformational fluids migrated. Given the extremely low matrix permeability of chalk it is not a huge surprise that most diagenesis is in closed systems and opensystem diagenesis is limited to narrow zones along high permeability fairways and areas with high pressure gradients.
Apart from the stratigraphic variation, the geographical variation in the expression of these venting events has also been documented. A predictive model is proposed that explains the occurrence or absence of gas venting features and seismic-scale diagenesis, based on basin-wide source rock potential and maturity. Gas venting features occur on Late Cretaceous basinal highs (dictated by Campanian inversion) where thermogenic and/biogenic sources for gas-bearing fluids were present, as well as inverted Late Jurassic faults. Seismic-scale diagenesis occurred on highs with limited source rock potential, where instead pressurized basinal fluids migrated through the chalk when seal integrity was compromised.
The expulsion events predate thermal maturity of the main prolific source rock (e.g. Bo Member of the Farsund Formation), and therefore thermogenic sources consisted of Lower-Middle Jurassic mudstones and coals, and lower parts of the Farsund Formation. Only biogenic gas was generated from the upper parts of the Farsund Formation. The areas where these features occur show large overlap with present-day hydrocarbon accumulations (with expulsion from the Bo Member), and therefore indicate that fluid migration pathways have been reused and were long-lived.
The insights from this study have important applications in exploration and production of hydrocarbons, and for future well planning. Firstly, the occurrence of syn-depositional fluid expulsion features overlap with present-day hydrocarbon accumulations, and thus directly reflect both long-lived source rock maturity, and fluid migration pathways. Depending on the basin maturity history and emplacement of an integer seal, gas venting features point to areas that potentially hold hydrocarbon accumulations. Secondly, understanding the lithological variability within a pockmarked hydrocarbon reservoir is of key importance for increasing sweep and recovery, by optimizing reservoir models, and for planning future well trajectories.
In conclusion, integrated seismic geomorphology is a powerful tool that aims at understanding seismic reflectors as a result of a stack
of buried seafloors, rather than a pure geophysical signal. It provides a spatial framework to other types of data, and helps to more intuitively establish geological models for seismic-scale heterogeneities. The methodology integrates seismic data, petrography, and geochemistry, and has led to the first recognition of giant and mega pockmarks, seep carbonates, and importantly, also seismic-scale diagenesis in the Chalk Group.
We have shown that the basin-wide spatial distribution of syn-depositional and postdepositional processes occurring in the Chalk Group, to a large extend, is controlled by the tectonic evolution, basin morphology, and source rock maturity. These insights have thus important implications for other chalk reservoirs in the North Sea, and could lead to re-evaluation of established exploration ideas and reservoir models.
Original languageEnglish
PublisherTechnical University of Denmark, Department of Civil Engineering
Number of pages161
Publication statusPublished - 2018

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