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Seismic evidence for hydrocarbon migration

Foschi, Martino 2014. Seismic evidence for hydrocarbon migration. PhD Thesis, Cardiff University.
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The migration of hydrocarbons on formation and accumulation on commercially viable oil and gas deposits/reserves/reservoirs. Reflection 2D/3D seismic data can be used to identify the position of these fluids in the subsurface and analyse their habitat and pathways in the context of the basin architecture. A multichannel reflection 2D seismic dataset from a large area offshore Falkland Islands (South American Plate) was analysed to interpret direct hydrocarbon indicators (DHIs) and basin architecture with the aim to find a link between these two elements. The first results of this research work have led to the definition of a new class of hydrocarbon indicator, here called vertical anomaly cluster (VAC) and observed in the east Falkland Basin. A VAC represents a group of individual hydrocarbon occurrences linked by a vertical migration of hydrocarbons within fine-grained sequences. VACs are interpreted to be low-impact fluid flow phenomena where the migration process exploits pre-existing permeability structures. In the second part of this research a suite of DHIs, including bottom simulating reflections (BSR), enhanced reflections, pipes, gas chimneys and flat spots have been analysed in the South Falkland Basin. The work demonstrates a significant link between the style of the migration and basin features. In particular it is observed that vertical migration phenomena such as pipes and gas chimneys are clearly linked to the secondary structural features of the basin while the gas hydrate system, composed of BSR and enhanced reflections, is more controlled by primary depositional features. The presence of a mature source rocks suggests that the fluids involved in the formation of the observed DHIs are of thermogenic origin. The third part of this research was focussed on the analysis of depression structures located in the North east Falkland Basin. The basin hosts other DHIs, such vi as VACs, gas chimneys and pipes. The depression structures propagate in depth and are associated with material withdrawal. These structures have therefore been interpreted as mud volcano conduits. The lack of mud edifices has been tentatively interpreted to be related with seabed erosion operated by bottom currents active in the study area. The fluids involved in the withdrawal of the material are interpreted to be related to hydrocarbon generation and expulsion. The comparison between the results of this research with the wider literature has been used to propose a classification where the style of the migration process and the DHIs are linked to the permeability architecture of the basin. Two styles of DHIs have been defined: (1) permeability-controlled DHIs, derived essentially from low impact migration processes acting in a pre-existing permeability structure (e.g.: VACs, enhanced reflection and BSR), and (2) fluid-controlled DHIs, derived from migration processes able to generate the migration pathways (e.g. pipes, mud volcanoes). Two speculative and, at the time of this research, not tested hypotheses have been proposed to justify the style of the migration process. The first hypothesis suggests that the style of the migration process is directly linked to the geometry of the deep structures where the hydrocarbon fluids are focussed in relatively small areas. These areas are suggested to be located at structural highs. Once the fluid is focussed in this region then the vertical migration is believed to not be sensitive to barrier and seals within the overlying lithologies. This hypothesis is consistent with the position of fluid-controlled DHIs (e.g. pipes and MVs) at the margin of the hydrocarbon plumbing systems and where the basement structures are relatively shallower. DHIs located in the inner regions are instead of the type sensitive to the permeability structures. vii The second hypothesis suggests that the style of the DHIs is function of the time. Hence in this hypothesis, assuming a given region of active fluid migration, the first DHIs to be generated are simple amplitude anomalies which then evolve to VACs. If the migration of hydrocarbons is still active the probable condition of high saturation may trigger some failure within the hosting lithologies and the formation of additional pathways. The newly generated permeability structure may finally be consistent with the one of more dramatic DHI style, such as mud volcanoes.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:56

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