Generation, Migration and Sequestration of Natural Gas

143

3.1 Subproject

Sub-Project 3.1

Generation, migration and sequestration of natural gas

during the post-breakup history of the

South African continental margin

Participants * Coordinators

Institution / Names / Email addresses
GFZ Potsdam
(GFZ) / Rolando di Primio *
Brian Horsfield /

Alfred Wegener Institute
(AWI) / Gabriele Uenzelmann-Neben /
University of Cape Town
(UCT) / George Smith * /
Petroleum Agency of South Africa
(PAS) / Ian McLachlan
David van der Spuy /

University of Western Cape
(UWC) / Charles Okujeni /

Requested Funding

Total for the 5-year duration project beginning in 2004: Euros 290 000

2004

2005

2006

2007

2008

GFZ

45000

72500

92500

50000

30000

Summary

Sedimentary basins constitute a quantitatively significant source and sink of greenhouse gases in both marine and terrestrial systems. Greenhouse gas emissions from underlying thermal sources and the deep biosphere make an extremely significant contribution to the Carbon-Cycle. At the same time, fossil energy resources continue to be the major foundation of the industrialised nations, and improved techniques for detecting sub-surface accumulations are sought.
GFZ will use its advanced basin modelling techniques to study source-transport-sequestration systems from the present day back in geological time through and beyond the Cenozoic into the Jurassic. This time range is needed in order to encapsulate all quantitatively significant processes of the Carbon-cycle impinging upon resources and climate. The formation and dissociation of gas hydrates are included. Episodic versus continuous gas flow will be studied, including quantification, utilising calibrations and observations from in-situ measurements wherever possible. In this regard, specific organic compounds called “Life Markers” (e.g. phospholipids) will also be used to delineate subsurface microbial ecosystems, some of which generate methane, i.e act as a source, and others oxidise it to carbon dioxide, i.e. act as a sink.
The teaching of organic geochemistry is an essential part of capacity building for South Africa. Organic geochemistry is the study of the distribution, composition, and fate of organic matter throughout earth history, from the present day to the beginning of life on earth. Combining aspects of geology, chemistry, and biology, the discipline is like forensic science on a colossal time scale, with pieces of evidence - from molecular to microscopic scales - employed to reconstruct past events and ecosystems. Research impacts the fields of Energy and The Environment, with organic geochemists actively involved in the exploration and exploitation of fossil energy resources (oil, gas, oil shales, tar sands, coal), studying pollution, and elucidating the earth's present and past climates.

Scientific Background

Various seafloor features, such as mud volcanoes, pock marks and carbonate mounds, have been postulated to be associated with active hydrocarbon leakage. In many cases gas hydrate dissociation was put forward as a possible hydrocarbon source, although discriminating gas-hydrate derived gas and deep thermogenic gas has proven to be difficult. In accretionary systems deep fluid sources and migration routes along faults seem to dominate in the formation of mud volcanoes, disregarding mud volcanoes associated with shale diapirism. In passive margin settings as well as in deep lake systems, a close association of seafloor gas hydrate subcrop and mud volcano occurrence is evident, but an association with deep-seated fault systems cannot be excluded.

Despite the vast amount of work already performed on varying aspects of natural gas seepage an integrated study involving detailed mapping of gas escape or sequestration features (mud volcanoes, pockmarks, gas chimneys and bottom simulating reflectors indicative of gas hydrates), their distribution through time, their association to heat flow anomalies or structural elements, their relationship to the hydrocarbon system as well as a characterisation of gas generation, migration and leakage dynamics through geologic time has as yet not been performed.

Mud volcanoes, pock marks and possibly cold-water corals have been observed in the distal part of the Orange River delta, the former in direct association to the BSR outcrop on the sea floor, but also linked to deeper seated structures (Ben Avraham et al., 2002). Those seismic lines cover an area between a set of seismic lines AWI shot there in 1996 in collaboration with GeoB, Bremen. In contrast to the observations of Ben Avraham et al. (2002) the AWI lines show no indications of gas hydrates. Three sites of ODP Leg 175, Sites 1085, 1086 and 1087, also show no evidence of gas hydrates. Hence, one goal of this project would be a careful mapping of the occurrence of gas hydrates.

The Orange River delta area, as well as any other occurrence of mud volcanoes, pock marks and possibly gas hydrates in the South African continental shelf, would be ideal for a regional programme of detailed investigation. Forrest Oil has expressed an interest in the research theme and is a potential technical advisor.

Scientific Goals and Societal Benefits

· To elucidate source-transport-sequestration systems in the Carbon-Cycle from the present day back in geological time through to the Jurassic

· To understand the controls on the location, initiation and evolution of surface and buried features as well as their potential significance for hydrocarbon exploration

· To quantify the emission of greenhouse gases into the atmosphere now and in the past

· To train South Africans in organic geochemistry, beginning at the undergraduate level via short courses and culminating at the end of the period via Ph.D students working on organic geochemical research issues

Implementation and Work Schedule

A postdoc will be given the responsibility of leading the research project, helping to present short courses in South African universities and supporting the supervision of Ph.D students.

Because it is important that this sequential development starts as soon as possible, GFZ has taken the initiative of funding the postdoc position. The two Ph.D positions will be offered preferentially to South African students who show talent and a will to learn during the compact courses.

The methodology to be used will be based on the successful investigation of the Porcupine Basin carbonate mounds. Deep-water carbonate mounds occurring in the Porcupine Basin, have been linked to preferred sites of present and palaeo hydrocarbon leakage by basin and fluid flow modelling and serve, hence, as indicators of an active petroleum system. In this study map-based characterisation of drainage areas and closures proved to be instrumental in relating surface expressions of leakage to deeper structural or stratigraphic features. While the present day active seepage of natural gas at these mound locations could not be proven, phase behaviour modelling indicated the likelihood of enhanced leakage at times of low sea level, which coincided with the timing of mound formation during the Pliocene lowstand.

Analysis of gas, oil and source rock samples will provide the geochemical framework for the petroleum systems study. Availability of hydrate, mud volcano or sea floor sediment samples must be discussed. Detailed geochemical analysis of such samples could aid in identifying the source of gas leading to their formation (biogenic vs thermogenic). Basin evolution will be reconstructed using basin modelling (Petromod) based on interpreted 2D or 3D seismics, and calibrated to available well data. Compositional kinetic modelling (based on source rock analysis or on available PVT data from reservoirs) will allow the characterisation of fluid phase behaviour during secondary migration, entrapment and leakage. Compositional characterisation of the migrating gas may allow the determination of hydrate stability zones in the model (module currently under development), as well as their evolution through time. Preferred leakage sites will be identified based on the flow modelling results (calibration of cap rock lithologic properties will be performed using observed gas chimneys).

This approach will allow a much better constraint on the evolution of the petroleum system in the study area, as well as the identification of the main hydrocarbon migration routes. The question whether the present day sea floor features observed in the high resolution seismics are related to hydrate dissociation or focussed hydrocarbon leakage (or a combination of both via updip focussing of gas flow under the hydrate layer) will be addressed.

Recognition of similar buried features in the Tertiary sequences could indicate the occurrence of similar dissociation/fluid leakage events (gas) staggered through time and related to sea-level changes, uplift or tectonic events, allowing thus a quantitative assessment of gas generation, migration and sequestration dynamics through time. In order to be able to identify these subtle expressions of gas leakage the reprocessing of selected 2D seismic lines could be necessary. Reprocessing would be performed by AWI and would focuss on better imaging the shallow sedimentary sequence (upper 1 km). Access to the available 3D surveys would be of great benefit, as detailed mapping of the Tertiary reflectors would significantly aid in recognising burried pock marks, mud volcanoes or carbonate mounds, and relating their occurrence to specific leakage events.

A successful completion of the proposed project would significantly help in the identification of the main carrier systems in the area and, hence, aid in enhancing the prospectivity of associated hydrocarbon prospects and leads.

Expected Outcome

· Regional database of recognised gas leakage expressions in the Orange Basin

· Identification of preferred leakage sites in time and space and related dominant carrier systems

· High resolution 3D seismic interpretation of the shallow Tertiary levels

· Estimation of methane fluxes into the atmosphere during rifting and drifting

· 2D and 3D models of petroleum system evolution, including migration modelling

Time chart and work plan

Funding Requirements

A three year postdoc position has been provided by GFZ starting in October 2003. Two Ph.D students will be employed beginning in 2005 and 2006 respectively, from funding agencies and industry. Total costs are 240 000 €, half of which has already been committed by GFZ.

A close cooperation between the institutions is of paramount importance for the successful running of the work programme. This is particularly the case for the teaching of short courses, where an unwavering commitment from both sides is required. Total travel costs, which includes accommodation and living expenses, have been calculated at 50 000 € over five years.

Year /

2004

2005

2006

2007

2008

GFZ-funded Postdoc

40000

/ /
Ph.D positions / /

20000

40000

20000

Travel costs /

5000

12500

10000

Reference

Ben Avraham, Z., Smith, G., Reshef, M., and Jungslager, E. (2002). Gas hydrate and mud volcanoes on the southwest African continental margin off Sough Africa. Geology (Boulder) 30 (10), 927-930.