ALBERT

Description: GFZ German Research Centre for Geosciences

Description: Published

Description: 1T. Deformazione crostale attiva

Description: Delta–deepwater fold–thrust belts are linked systems of extension and compression. Margin-parallel maximum horizontal stresses (extension) on the delta top are generated by gravitational collapse of accumulating sediment, and drive downdip margin-normal maximum horizontal stresses (compression) in the deepwater fold–thrust belt (or delta toe). This maximum horizontal stress rotation has been observed in a number of delta systems. Maximum horizontal stress orientations, determined from 32 petroleum wells in the Gulf of Mexico, are broadly margin-parallel on the delta top with a mean orientation of 060 and a standard deviation of 49°. However, several orientations show up to 60° deflection from the regional margin-parallel orientation. Three-dimensional (3D) seismic data from the Gulf of Mexico delta top demonstrate the presence of salt diapirs piercing the overlying deltaic sediments. These salt diapirs are adjacent to wells (within 500 m) that demonstrate deflected stress orientations. The maximum horizontal stresses are deflected to become parallel to the interface between the salt and sediment. Two cases are presented that account for the alignment of maximum horizontal stresses parallel to this interface: (1) the contrast between geomechanical properties of the deltaic sediments and adjacent salt diapirs; and (2) gravitational collapse of deltaic sediments down the flanks of salt diapirs.

Description: This study examines present-day stress orientations from borehole breakout and drilling-induced fractures in 57 boreholes in the Nile Delta. A total of 588 breakouts and 68 drilling-induced fractures from 50 wells reveal sharply contrasting present-day maximum horizontal stress (SHmax) orientations across the Nile Delta. A typical deltaic margin-parallel SHmax exists in parts of the Nile Delta that are below or absent from evaporites (NNE–SSW in the west, east–west in the central Nile, ESE–WNW in the east). However, a largely margin-normal (NNE–SSW) SHmax is observed in sequences underlain by evaporites in the eastern Nile Delta. The margin-normal supra-salt SHmax orientations are often subperpendicular to the strike of nearby active extensional faults, rather than being parallel to the faults as predicted by Andersonian criteria. The high angle between SHmax and strike of these extensional faults represents a new type of non-Andersonian faulting that is even less-suitably oriented for shear failure than previously described anomalous faulting such as low-angle normal faults and highly oblique strike-slip faults (e.g. San Andreas). While the mechanics of these non-Andersonian faults remains uncertain, it is suggested that the margin-normal supra-salt orientation generated by basal forces imparted upon rafted blocks sliding down seawards-dipping evaporites.

Description: Shale dykes, diapirs and mud volcanoes are common in the onshore and offshore regions of Brunei Darussalam. Outcrop examples show that shale has intruded along both faults and tensile fractures. Conventional models of overpressure-induced brittle failure assume that pore pressure and total stresses are independent of one another. However, data worldwide and from Brunei show that changes in pore pressure are coupled with changes in total minimum horizontal stress. The pore pressure/stress-coupling ratio ({Delta}{sigma}h/{Delta}Pp) describes the rate of change of minimum horizontal stress magnitude with changing pore pressure. Minimum horizontal stress measurements for a major offshore field where undepleted pore pressures range from normal to highly overpressured show a pore pressure/stress-coupling ratio of 0.59. As a consequence of pore pressure/stress coupling, rocks can sustain a greater increase in pore pressure prior to failure than predicted by the prevailing values of pore pressure and stress. Pore pressure/stress-coupling may favour the formation of tensile fractures with increasing pore pressure rather than reactivation of pre-existing faults. Anthropogenically-induced tensile fracturing in offshore Brunei supports this hypothesis.

Description: Structural mapping, nearest neighbour and two-point azimuth statistical analysis of mud volcano vent distributions from nine examples in Azerbaijan and the Lusi mud volcano in east Java are described. Distributions are non-random, forming alignments subparallel to faults within anticlines, ring faults, conjugate faults and detachment faults; this finding confirms a spatial relationship and supports a model for subsurface flow along these features as well as showing fractionation at depth. As fracture and fault orientations are related to structures such as anticlines and the in situ stress state they are therefore predictable. We use vent distributions in Azerbaijan, where the structural geology is well constrained, to propose what controls the distribution of 169 vents at the Lusi mud volcano. This mud volcano system shows evidence for initial eruptions along a NE–SW trend, parallel to the Watukosek fault, changing to eruptions that follow east–west trends, subparallel to regional fold axes. Our analysis indicates that regions east and west of the Lusi mud volcano are more likely to be affected by new vents than those to the north and south, owing to probable onset of elongate caldera collapse within a 10 km diameter of the central vent.

Description: Evaporitic horizons are routinely interpreted to act as mechanical detachment sequences and thus significantly influence the structural evolution of sedimentary basins and fold-thrust belts. However, over 30 years of global in situ stress analysis have provided only poor evidence to support this widespread assumption. This study examines present-day stress orientations inferred from borehole breakout and drilling-induced fractures in 44 boreholes in the offshore Nile Delta. A total of 446 breakouts and 19 drilling-induced fractures from 37 wells reveal sharply contrasting present-day maximum horizontal stress (SHmax) orientations in sequences above and below the extensive Messinian evaporites of the eastern Nile Delta. A typical deltaic margin-parallel SHmax (E-W in the central and ESE-WNW in the eastern Nile Delta) is observed in parts of the Nile Delta that are below or do not contain evaporites. However, a scattered but largely margin-normal (NNE-SSW) SHmax is observed in sequences underlain by evaporites. The [~]90{degrees} variation in present-day SHmax orientation above and below the Messinian salts provides the first convincing in situ evidence that evaporite sequences can act as major mechanical detachment horizons. In addition, the margin-normal SHmax orientation is subperpendicular to the strike of nearby active extensional faults, indicating the existence of non-Andersonian faulting in the suprasalt region. Furthermore, the evidence that the Messinian evaporites act as an effective mechanical detachment suggests that suprasalt faulting in the eastern Nile Delta is not the result of basement-related deformation and thus raises doubts about the often postulated extension of the Suez fault zone into the eastern Mediterranean.

Description: Twitter is an established social media platform valued by scholars as an open way to disseminate scientific information and to publicly discuss research results. Scientific discussions on Twitter are viewed by the media, who can then pass on information to the wider public. Social media is used widely by geoscientists, but there is little documentation currently available regarding the benefits or limitations of this for the scientist or the public. Here, we use the example of two 2018 earthquake-related events that were widely commented on by geoscientists on Twitter: the Palu Mw 7.5 earthquake and related tsunami in Indonesia and the long-duration Mayotte island seismovolcanic crisis in the Indian Ocean. We built our study on a content and contextual analysis of selected Twitter threads about the geophysical characteristics of these events. From the analysis of these two examples, we show that Twitter promotes a very rapid building of knowledge in the minutes to hours and days following an event via an efficient exchange of information and active discussion between the scientists themselves and the public. We discuss the advantages and potential pitfalls of this relatively novel way of making scientific information accessible to scholarly peers and lay people. We argue that scientific discussion on Twitter breaks down the traditional “ivory tower” of academia, contributes to the growing trend towards open science, and may help people to understand how science is developed and, in turn, to better understand the risks related to natural/environmental hazards.

Description: Accurate pore-pressure prediction is critical in hydrocarbon exploration and is especially important in the rapidly deposited Tertiary Baram Delta province where all economic fields exhibit overpressures, commonly of high magnitude and with narrow transition zones. A pore-pressure database was compiled using wireline formation interval tests, drillstem tests, and mud weights from 157 wells in 61 fields throughout Brunei. Overpressures are observed in 54 fields both in the inner-shelf deltaic sequences and in the underlying prodelta shales. Porosity vs. vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the prodelta shales but have been generated by fluid expansion in the inner-shelf deltaic sequences. However, the geology of Brunei precludes overpressures in the inner-shelf deltaics being generated by any conventional fluid expansion mechanism (e.g., kerogen-to-gas maturation), and we propose that these overpressures have been vertically transferred into reservoir units, via faults, from the prodelta shales. Sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer, and hence, different pore-pressure prediction strategies need to be applied in the prodelta shales and inner-shelf deltaic sequences. Sonic and density log data detect overpressures generated by disequilibrium compaction, and pore pressures are accurately predicted using an Eaton exponent of 3.0. Sonic log data detect vertically transferred overpressures even in the absence of a porosity anomaly, and pore pressures are reasonably predicted using an Eaton exponent of 6.5. Mark Tingay is currently an Australian postdoctoral fellow at Curtin University, where he works on stress, overpressure, and the tectonic evolution of Southeast Asia. He received his Ph.D. in 2003 from the Australian School of Petroleum. He then became a petroleum geomechanics researcher at the World Stress Map Project, where he worked on projects in 11 countries, including the United States, Egypt, Azerbaijan, and Thailand. Richard Hillis is the head of the Australian School of Petroleum and a state of South Australia professor of petroleum geology at the University of Adelaide. He received his B.Sc. (hons) degree from Imperial College and Ph.D. from the University of Edinburgh. He is a director of JRS Petroleum Research, an image log and geomechanics consulting company, and of Petratherm, a geothermal exploration company. Richard commenced his career in 1979 when he joined Mobil with assignments in the United Kingdom and the United States. He joined Durham University in 1989 and was a principal investigator for a multidisciplinary research group funded by 17 oil and gas companies. Over that period, he developed training courses in subsurface pressures and founded the company GeoPressure Technology. He is an honorary professor at Durham University and has been an AAPG member since 1982. Chris received his Ph.D. in 1983 before working for Amoco and Elf Aquitaine and as a professor at the University of Brunei Darussalam. He is currently working for PTT Exploration and Production as a senior geophysicist. He has worked as an exploration geologist and as a structural geologist in east Africa, Morocco, the Norwegian Caledonides, the Carpathians, northwest Borneo, and Thailand. Abdul Razak Damit is currently the chief geologist with the National Oil Company of Brunei (PetroleumBRUNEI). He obtained his Ph.D. at Aberdeen University and has 20 years of industry experience, primarily in Shell where he worked on both reservoir and regional evaluation. His main interests are in the geology of northwest Borneo and in raising public awareness of the natural and social history of Brunei.