Erdem Idiz

Generation of nitrogen and methane from sedimentary organic matter: implications on the dynamics of natural gas accumulations

Nitrogen (N2) contents of natural gases in Rotliegend and Buntsandstein reservoirs of the North German basin regionally approach 100%. A review is given of the various hypotheses (primordial origin, volcanic or magmatic origin, radiogenic origin, atmospheric origin, organic origin, inorganic nitrogen in sedimentary rocks) presented to account for nitrogen anomalies in this area and other parts of the world. The objective of the present study was to investigate sedimentary organic matter, in particular coals, as a potential source of molecular nitrogen in the subsurface. Comparison of reservoir sizes and gas generation potentials indicates that Carboniferous coal measures, which are considered as the source of the natural gas in the North German basin, can readily account for the nitrogen quantities found in present-day reservoirs. Laboratory pyrolysis experiments were carried out to investigate the kinetics of generation of methane and molecular nitrogen from coals of different type and rank. Under experimental conditions nitrogen is formed at higher temperatures than methane supporting the concept of a ‘fractional generation’ of methane and nitrogen in natural systems. Based on the kinetic parameters derived from laboratory experiments methane and nitrogen generation rates from coals were calculated for geologic heating rates. Gas containing more than 50% nitrogen is generated under these conditions at temperatures in excess of 300°C. Nitrogen-rich gases are thus formed only in the final stage of gas generation after methane generation has practically ceased. It is concluded that the amounts of gas encountered in nitrogen-rich gas accumulations represent only a small fraction (possibly < 1 %) of the total gas generation potential of this area while the bulk of the generated gas has escaped to the atmosphere. The present-day composition of the reservoir gases reflects the composition of only the most recently generated gas (on a geologic time scale).

Molecular Nitrogen in Natural Gas Accumulations: Generation from Sedimentary Organic Matter at High Temperatures

The occurrence of natural gas accumulations with high percentages (up to 100%) of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk. In this context, a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen. The masses of nitrogen present in coals--if converted into molecular nitrogen--are sufficient to fill commercial gas reservoirs. A calculation for gas accumulations in northern Germany, where percentages of molecular nitrogen range from less than 5 to greater than 90%, reveals that the molecular nitrogen generated in underlying coal-bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds. In addition, much of the total nitrogen in clay-rich rock types, such as shales and mudstones, is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals. A kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane. Minimum temperatures for preferential nitrogen generation are in excess of 300°C for extremely low heating rates or isothermal conditions lasting over 300 m.y. Thus, nitrogen-rich gases are mainly formed in the final stage of gas generation, when sedimentary rocks grade into metamorphic rocks. Applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in Carboniferous sedimentary rocks are presented for part of the Northwest German basin. In this area, immense volumes of the methane-rich gas gener ted during the late Paleozoic and Mesozoic were lost to the atmosphere. The present gas composition mainly reflects the latest Cenozoic gas generation. This Cenozoic gas is nitrogen rich where the Carboniferous source rock sequences are highly mature and deeply buried.

Assessment of an Unusual European Shale Gas Play: The Cambro-Ordovician Alum Shale, Southern Sweden

In 2008 Shell obtained two licenses for unconventional gas exploration in the Skne region of southern Sweden, with a total size of 2500 km2 (600,000 ac). The objective was the Cambro-Ordovician Alum Shale, one of the thickest and richest marine source rocks in onshore northern Europe. The licenses covered the Hllviken Graben and the Colonus Shale Trough. In both areas the Alum Shale had been encountered in older wells, with a thickness of up to 90 m and TOC values up to 15%. Maturities of up to 2% Vre were considered encouraging for a shale gas play. Relative high quartz contents suggested good fraccability of the shales. All data was obtained through public sources. Identified risks were the uncertain timing of hydrocarbon generation and the position of the licenses adjacent to the Trans-European Suture Zone where several phases of fault movement have a risk for actually retaining the hydrocarbons. The derisking strategy for this opportunity was based on both technical and non-technical aspects. Aim was to collect geological and geophysical data to constrain depth and thickness of the shale and to identify potential dolerite dykes. In addition, well data were needed to establish rock properties and gas content. The external environment, especially concerns from the people in Skne regarding the visual impact of activities and potential impact of drilling activities on the aquifers and on the tourism industry have resulted in extensive engagements with stakeholders and specific requirements around seismic acquisition (low impact), site preparation and operations (e.g. small rig, different lighting). 80 km of 2D seismic was acquired in 2008 and three wells, with a final depth of around 1000 m, were drilled in 2009 to mid 2010. The Alum shale was fully cored and the well sites have been restored. Thickness, richness and maturity of the Alum were as predicted although the basin was shallower than previously anticipated. Canister desorption tests, however, indicated that the shales have only low gas saturation. This significantly increased the risk for a viable shale gas play and therefore the licenses were not renewed after the initial 3 year period.

The Khazzan gas accumulation, a giant combination trap in the Cambrian Barik Sandstone Member, Sultanate of Oman: Implications for Cambrian petroleum systems and reservoirs

Khazzan is a giant tight-gas accumulation located in northern Oman. The accumulation is associated with Cambrian Barik Sandstone Member reservoirs, in a semiregional combination (stratigraphic-structural) trap. This article outlines the background to the discovery of Khazzan and describes our understanding of its geology and petroleum systems and the study work used to support screening, appraisal, and development strategies for the Khazzan accumulation. The stratigraphic element of the trap is associated with a northerly thinning and transition of continental, fluvial braid-plain and shoreface sandstones into offshore mud rocks. The structural element of the trap formed as a result of the compactional drape of these reservoir units and the later structuring associated with an underlying, long-lived intrabasin high. The integration of multiple correlation techniques (well-log correlation, recognition of changes in core facies and ichnofacies and magnetostratigraphy) has helped to define an intrareservoir correlation framework for the Barik Sandstone Member, with a resolution of approximately less than or equal to 1 m.y., allowing detailed insights into stratigraphic and areal controls on hydrocarbon compositional variations, trapping mechanisms, reservoirs, and reservoir connectivity. Fluvial sandstones within the Khazzan stratigraphic pinch-out form better reservoirs. The stratigraphic framework developed indicates that these fluvial units are associated with discrete, geographically extensive, progradation events. The association of production with discrete reservoir levels and areas allows a robust semipredictive regional model for targeting better reservoirs and highlights a scope for an additional intra-Barik stratigraphic trapping potential regionally. Enhanced reservoir quality within the more productive reservoirs is attributed to depositional and possibly hydrocarbon charge controls on secondary porosity, and it is complex. Subtle crest-to-flank compositional variations in hydrocarbon type across the gas accumulation hint at complex hydrocarbon charge and mixing between two structural domains and petroleum systems before and during Barik deposition. John Millson holds a B.Sc. (honors geology) degree and a Ph.D. from Aston University in Birmingham, United Kingdom and the University of Wales, Abersytwyth. Joining the oil industry as a petroleum geologist in 1985, he has worked in the United Kingdom, Nigeria, Netherlands, and Oman, with some 11 years of working on various aspects of the geology of Oman. His main interests include tectonostratigraphy and unconventional hydrocarbon resources. Jamie Quin is a geologist for Statoil-Hydro. Since joining the oil industry in 2001, he has worked on assets in Oman (for Badley Ashton and Associates), Libya (for Repsol YPF), and Norway (Statoil). He holds a B.Sc. degree from the University of Glasgow, an M.Sc. degree from Royal Holloway, University of London, and a Ph.D. from Trinity College Dublin. Erdem Idiz is currently a global exploration adviser for Exploration New Ventures with Shell International Exploration and Production in the Netherlands. He received his M.Sc. degree in geology (1981) and Ph.D. in geochemistry (1987) from the University of California, Los Angeles (UCLA). After a postdoctorate at the Institut Francais du Petrole, he joined Shell in 1988, working in research and applications as a geochemist and basin modeler in the Netherlands, Germany, and Canada. His research interests are petroleum systems analysis, biomarkers, and stable isotopes. Peter Turner holds degrees from Cardiff and Leicester Universities and was a reader in earth sciences at the University of Birmingham in the United Kingdom from 1988 to 2006 where he was head of the Petroleum Geoscience Group. His research includes paleomagnetism, sedimentology, and hydrocarbon reservoirs, and since retiring from university life, he has worked as a full-time consultant focusing on exploration in north and west Africa and China. No details available.

Molybdenum-isotope chemostratigraphy and paleoceanography of the Toarcian Oceanic Anoxic Event (Early Jurassic)

Molybdenum (Mo)-isotope chemostratigraphy of organic-rich mudrocks has been a valuable tool for testing the hypothesis that the Toarcian Oceanic Anoxic Event (T-OAE, Early Jurassic, ~183 Ma) was characterized by the spread of marine euxinia (and organic-matter burial) at a global scale. However, the interpretation of existing Mo-isotope data for the T-OAE (from Yorkshire, Cleveland Basin, U.K.) is equivocal. In this study, three new Mo-isotope profiles are presented: from Dotternhausen Quarry (South German Basin, Germany), the Rijswijk core (West Netherlands Basin, Netherlands) and the Dogna core (Belluno Basin, northern Italy). Precise bio- and chemo-stratigraphic correlation between the three sites allows a direct comparison of the data, enabling some key conclusions to be reached: (i) The Mo-isotope composition of seawater during the peak of the T-OAE was probably close to ~1.45 ‰, implicating a greater removal flux of sulphides from seawater, and a larger extent of global seafloor euxinia compared to the present day; (ii) Mo-isotope cycles previously identified in the Yorkshire sedimentary succession are attributed to changes in the degree of local Mo drawdown from overlying Cleveland Basin seawater; (iii) The consistency of the new multi-site Mo-isotope dataset indicates a secular reduction in the burial of sulphides globally in the late stages of the T-OAE, implying a contraction in the extent of global marine euxinia; (iv) Subtle differences in the Mo-isotope composition of deposits formed in different euxinic sub-basins of the European epicontinental shelf were probably governed by local variations in basin hydrography and rates of water renewal.