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- Volume 25, Issue 1, 2019
Petroleum Geoscience - Volume 25, Issue 1, 2019
Volume 25, Issue 1, 2019
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Oil and gas in the Vienna Basin: hydrocarbon generation and alteration in a classical hydrocarbon province
The Vienna Basin, a major petroleum province in Central Europe, hosts hydrocarbons in stacked carbonate and siliciclastic reservoirs. The study of 84 oil and 51 gas samples in the Austrian sector yields new insights into the petroleum system. The Upper Jurassic Mikulov and Falkenstein formations are the only significant source rocks, which generated oil between 0.7 and 1.0%Rr, and gas between 1.1 and 1.6%Rr. Microbial gas prevails in the southern part of the basin. Biodegradation affects oil down to a depth of 2000 m. Miocene reservoirs in the hanging wall of major faults and oils in transgressive sands are more prone to biodegradation than footwall flysch reservoirs and oils in turbiditic sands in structural-stratigraphic traps. Anaerobic biodegradation results in the formation of isotopically heavy CO2 and isotopically light (secondary) microbial methane. Hydrocarbons in deep carbonate reservoirs are affected by thermochemical sulphate reduction (TSR). While TSR-affected gas is rich in H2S and CO2, TSR-affected oil is characterized by increased dibenzothiophene/phenantrene (DBT/Ph) ratios. In clastic reservoirs, H2S is removed by pyrite precipitation, whereas DBT/Ph ratios remain high. Hence, high DBT/Ph ratios may be used as proxy for TSR. Stable sulphur isotopes signatures confirm Upper Triassic anhydrites as the main sulphur source for H2S.
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Analysing the limitations of the dual-porosity response during well tests in naturally fractured reservoirs
Authors D. O. Egya, S. Geiger, P. W. M. Corbett, R. March, K. Bisdom, G. Bertotti and F. H. BezerraGeological reservoirs can be extensively fractured but the well-test signatures observed in the wells may not show a pressure transient response that is representative of naturally fractured reservoirs (NFRs): for example, one that indicates two distinct pore systems (i.e. the mobile fractures and immobile matrix). Yet, the production behaviour may still be influenced by these fractures. To improve the exploitation of hydrocarbons from NFRs, we therefore need to improve our understanding of fluid-flow behaviour in fractures.
Multiple techniques are used to detect the presence and extent of fractures in a reservoir. Of particular interest to this work is the analysis of well-test data in order to interpret the flow behaviour in an NFR. An important concept for interpreting well-test data from an NFR is the theory of dual-porosity model. However, several studies pointed out that the dual-porosity model may not be appropriate for interpreting well tests from all fractured reservoirs.
This paper therefore uses geological well-testing insights to explore the limitations of the characteristic flow behaviour inherent to the dual-porosity model in interpreting well-test data from Type II and III NFRs of Nelson's classification. To achieve this, we apply a geoengineering workflow with discrete fracture matrix (DFM) modelling techniques and unstructured-grid reservoir simulations to generate synthetic pressure transient data in both idealized fracture geometries and real fracture networks mapped in an outcrop of the Jandaira Formation. We also present key reservoir features that account for the classic V-shape pressure derivative response in NFRs. These include effects of fracture skin, a very tight matrix permeability and wells intersecting a minor, unconnected fracture close to a large fracture or fracture network. Our findings apply to both connected and disconnected fracture networks.
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Petroleum occurrences in the carbonate lithologies of the Gohta and Alta discoveries in the Barents Sea, Arctic Norway
Authors Z. Matapour, D. A. Karlsen, B. Lerch and K. Backer-OweInvestigation of petroleum inclusions in carbonate samples from the Senilix well in the Barents Sea reveals petroleum entrapment in Paleozoic carbonates at reservoir temperatures from as low as 87.3°C to more than 130°C. Using corrected bottom hole temperatures, this corresponds to depths of 2800–4100 m, compared to the present-day depth of these samples of only 1965.9–2020.5 m. The oil in the Gohta and Alta discoveries is concluded to be of either Lower Triassic or Paleozoic origin based on the isomer distribution of triaromatic dimethylcholesteroids (TA-DMC). A potential source-rock candidate is the Ørret Formation, which is the time-equivalent to the Ravnefjeld Formation in Greenland. These oils are of a different origin compared to oils in the nearby Skrugard (renamed to Johan Castberg) discovery which contain oil sourced from the Upper Jurassic Hekkingen Formation. Evidence is presented to suggest that the Gohta and Alta oils represent blends of petroleum expelled at maturities ranging from about 1.0% calculated vitrinite reflectance (Rc) to more than 1.3%Rc, and this corroborates the inferences made from the petroleum inclusions. This emerging play is significant to exploration in the karst developed on the Barents Shelf and the Bjarmeland Platform during the Permo-Carboniferous. Karst reservoirs have been linked to eustatic sea-level changes, and analogous karst reservoirs may be present elsewhere in the Circum-Arctic: for example, in the Sverdrup Basin.
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Controls on the distribution of volcanism and intra-basaltic sediments in the Cambo–Rosebank region, West of Shetland
Authors Jonathon Hardman, Nick Schofield, David Jolley, Adrian Hartley, Simon Holford and Douglas WatsonThe NE Atlantic margin is one of the last frontier areas of hydrocarbon exploration within the UK Continental Shelf. In 2004, a major oil and gas discovery (Rosebank) was made within Paleocene–Eocene-age lavas in the Faroe–Shetland Basin. The Rosebank Field consists of intra-basaltic terrestrial to marginal-marine reservoir sequences, separated by basaltic lava flows and volcaniclastics. Despite the identification of a major intra-lava-incised drainage system running parallel to the SW–NE-trending Rosebank Field, the controls on the distribution of both the volcanics and the intra-basaltic sediments were previously unclear, in part due to the difficulties that volcanic sequences pose to seismic acquisition, processing and interpretation. This has led to uncertainty in defining the wider intra-basaltic play. However, the examination of the recently acquired FSB 2011–12 MultiClient GeoStreamer® Survey has facilitated an increased definition of the geological units within and below the volcanic succession, and a reinterpretation of the Late Paleocene–Early Eocene stratigraphy. Through integration of 3D seismic data and well analysis, we present a regional overview of the volcanics and intra-basaltic sediments in the Rosebank region of the Faroe–Shetland Basin. We find that the structural setting of the Rosebank Field, in addition to lava-flow morphology, strongly influences the distribution of the intra-basaltic play fairway within the Paleocene–Eocene-aged Flett Formation. Restriction of fluvial siliciclastic sediments adjacent to the Corona Ridge extends the theorized Rosebank play fairway to the area SW of the Rosebank Field. Our analysis indicates that understanding the underlying basin structure is integral to the success of intra-basaltic plays.
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Hydrothermal vents and seismic anomalies: implications for the petroleum system NE of Shetland
Authors R. V. Hamilton and B. J. MinshellThe influence of igneous intrusions and their associated vent complexes on hydrocarbon migration is poorly understood, yet may be significant in the exploration of petroleum basins located on passive margins. A regional review of the northeasternmost frontier sector of the UK Continental Shelf (UKCS) encompassing the southern extent of the Møre Basin has revealed evidence of extensive vent formation during the Late Paleocene. Anomalous seismic amplitude events that have not been previously described in this area are identified above and clearly related to several of the vent complexes, and a number of scenarios are postulated to explain their presence. Integrating existing geological studies and regional seismic interpretation with a new 3D seismic dataset reveals a close relationship between sills, vents and seismic anomalies which may be related to the presence of gas/fluid. Recognition of the vent complexes as potential fluid conduits long after initial formation may be important in focusing migration to younger stratigraphic levels. Important observations are made regarding a link to possible hydrocarbon migration mechanisms within the study area and the associated implications for hydrocarbon exploration and risking within this petroleum basin are investigated.
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Subsurface expression of a salt weld, Gulf of Mexico
Authors Christopher A.-L. Jackson, Yue Zhang, Donald A. Herron and Peter J. R. FitchSalt welds form due to salt thinning by mechanical (e.g. salt-flow) and/or chemical (e.g. salt-dissolution) processes. Despite being ubiquitous in salt-bearing sedimentary basins, where they may trap large volumes of hydrocarbons, little is published on weld thickness and composition. We here use 3D seismic reflection, borehole and biostratigraphic data from the Atwater Valley protraction area of the northern Gulf of Mexico to constrain the thickness and composition of a salt weld. Seismic data image an ‘apparent weld’ at the base of a Plio-Pleistocene minibasin that subsided into allochthonous salt. Borehole data indicate the weld is actually ‘incomplete’, being c. 24 m thick, and containing an upper 5 m-thick halite and a lower 15 m-thick halite, separated by a 4 m-thick mudstone. The age and origin of the intra-weld mudstone is unclear, although we speculate it is either: (i) Late Jurassic, representing material transported upwards from the autochthonous level within a feeder, and subsequently trapped as allochthonous salt thinned and welded; or, perhaps, more likely (ii) Pliocene, representing a piece of salt carapace reworked from the top of and eventually trapped in the now locally welded sheet. We show that 3D seismic reflection data may not resolve weld thickness, with the presence of relatively thin remnant salt lending support to models of welding based on viscous flow. The halite-dominated character of the weld also supports the hypothesis that tectonic purification may occur during salt flow.
Supplementary material : uninterpreted seismic reflection profiles, raw biostratigraphic data and a BOEM biostratigraphic chart are available at https://doi.org/10.6084/m9.figshare.c.4183061
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South China Sea crustal thickness and oceanic lithosphere distribution from satellite gravity inversion
Authors Simon Gozzard, Nick Kusznir, Dieter Franke, Andrew Cullen, Paul Reemst and Gijs HenstraInversion of satellite-derived free-air gravity-anomaly data has been used to map crustal thickness and continental lithosphere thinning in the South China Sea. Using this, we determine the ocean–continent transition zone structure, the distal extent of continental crust, and the distribution of oceanic lithosphere and continental fragments in the South China Sea. We construct a set of regional crustal cross-sections, with Moho depth from gravity inversion, spanning the South China Sea from offshore China and Vietnam to offshore Malaysia, Brunei and the Philippines to examine variations in ocean–continent transition structure and ocean-basin width. Our analysis shows a highly asymmetrical conjugate margin structure. The Palawan margin shows a narrow transition from continental to oceanic crust. In contrast, the conjugate northern margin of the South China Sea shows a wide region of thinned continental crust and an isolated block of continental crust (the Macclesfield Bank) separated from the Chinese margin by a failed oceanic rift. The Dangerous Grounds are predicted to be underlain by fragmented blocks of thinned continental crust. We use maps of crustal thickness and continental lithosphere thinning from gravity inversion together with free-air gravity- and magnetic-anomaly data to identify structural trends and to show that rifting and the early seafloor-spreading axis had an ENE–WSW trend while the later seafloor-spreading axis had a NE–SW trend.
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)