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- Volume 7, Issue 3, 2001
Petroleum Geoscience - Volume 7, Issue 3, 2001
Volume 7, Issue 3, 2001
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A review of palaeomagnetic data on the timing and origin of multiple fluid-flow events in the Arbuckle Mountains, southern Oklahoma
More LessA synthesis of palaeomagnetic/geochemical studies on Palaeozoic rocks in the Arbuckle Mountains, Oklahoma, indicates that basinal and meteoric fluids, as well as burial diagenetic processes, caused multiple remagnetization events. Basinal fluids migrated laterally through palaeoaquifers and vertically through faults/fractures, causing localized remagnetization and alteration in, and around, the fluid conduits. These fluids migrated during multiple flow events that spanned an approximately 60 Ma time interval in the late Palaeozoic. Based on the timing of remagnetization, most of the basinal fluids can be considered orogenic fluids. The palaeomagnetic results from the Arbuckle Mountains are consistent with migration of orogenic-type fluids during and after orogeny, although there were multiple flow events rather than one event as has been hypothesized for the central Appalachians. Meteoric fluids also caused localized alteration and remagnetization whereas a burial diagenetic mechanism (e.g. maturation of hydrocarbons) is a likely explanation for pervasive chemical remagnetization in an impermeable unit.
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A statistical method for correcting log-derived temperatures
Authors Douglas W. Waples and Mahadir RamlyStatistical analysis of subsurface temperatures in the Malay Basin has been carried out in order to (1) determine appropriate correction methods and correction factors for temperatures obtained from wireline logs and (2) investigate the confidence levels associated with those corrections. DST temperatures were taken as ‘ground truth’ for these corrections. For individual log-derived temperatures the correction factor f S, which is applied to the difference between the measured temperature and the surface temperature, is given by
Uncertainties in f S decrease markedly as TSC and depth increase. Uncertainties in f HP decrease as maximum TSC and the number of consistent temperature measurements at a given depth increase.
Although these correction factors were developed using data from a single basin, our experience suggests that they can be used with reasonable confidence in many or most other geological provinces. Additional local calibrations would help test and refine this hypothesis.
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Analysis of borehole temperature data in the Northeast German Basin: continuous logs versus bottom-hole temperatures
More LessTemperature recordings from exploration boreholes for hydrocarbons and geothermal energy are evaluated to select the best-suitable data for determining formation temperature and heat flow in the Northeast German Basin. Repeated temperature logs measured in several boreholes allow a classification into equilibrium logs and perturbed logs. For the latter, a simple empirical correction is proposed based on mean annual ground-surface temperature, the estimated amount of temperature correction at total depth of log, and the depth-dependent cross-over point of temperature perturbation due to drilling. Analysis of bottom-hole temperatures (BHTs) showed that the uncorrected values deviate, on average, by as much as 22.2±10.2°C (1σ) from the formation temperature. BHTs corrected using the Horner-plot method are systematically too low by 1–2°C compared to values corrected using the exact solution to the full line-source equation. Both correction methods yield values that underestimate formation temperatures by as much as 9°C and 8°C, respectively. The large standard deviation (1σ) of about ±8°C indicates that a corrected BHT reflects (with 96% confidence, 2σ) formation temperature not better than ±16°C. However, using temperature logs in conjunction with the corrected BHTs allowed a prediction of formation temperatures with an error less than ±10°C at somewhat deeper depths than the log in several boreholes. The availability of continuous temperature logs as a prerequisite for a reduction of error of BHTs significantly reduced the quantity of useable corrected BHTs in the basin.
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Arc magmatism and hydrocarbon generation in the northern Taranaki Basin, New Zealand
Authors Vaughan Stagpoole and Rob FunnellA Middle–Late Miocene andesitic arc named the Mohakatino Volcanic Centre is buried beneath younger sediments in the northern part of the Taranaki Basin, New Zealand’s primary oil production province. Volcanoes of the centre cover an area of about 3200 km2. An estimated 7000±3000 km3 of andesite were erupted from the centre and between 1000 to 2500 km3 of magma were intruded into the basement beneath the volcanic cones. The key element of the petroleum system altered by magmatism is the maturity of source rocks and the timing of expulsion, although volcanism also contributes to the formation of potential reservoirs and traps. In the northern Taranaki Basin, two periods of hydrocarbon expulsion occurred: following magmatism (14 Ma to about 8 Ma), and following renewed burial (since about 4 Ma). Thermal models indicate that Late Cretaceous terrestrial source rocks close to large magmatic intrusions became fully mature during magmatism. Overlying marine source rocks are modelled to still be generating and expelling hydrocarbons to the present day. Hence hydrocarbon expulsion and the charge history of this basin is partly governed by Miocene magmatism. Results are also relevant to many petroleum basins that contain similar andesitic arc volcanic rocks.
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Infiltration of Late Palaeozoic evaporative brines in the Reelfoot rift: a possible salt source for Illinois basin formation waters and MVT mineralizing fluids
Authors E. L. Rowan and G. de MarsilySalinities and homogenization temperatures of fluid inclusions in Mississippi Valley-type (MVT) deposits provide important insights into the regional hydrology of the Illinois basin/Reelfoot rift system in late Palaeozoic time. Although the thermal regime of this basin system has been plausibly explained, the origin of high salinities in the basin fluids remains enigmatic. Topographically driven flow appears to have been essential in forming these MVT districts, as well as many other districts worldwide. However, this type of flow is recharged by fresh water making it difficult to account for the high salinities of the mineralizing fluids over extended time periods. Results of numerical experiments carried out in this study provide a possible solution to the salinity problem presented by the MVT zinc–lead and fluorite districts at the margins of the basin system.
Evaporative concentration of surface water and subsequent infiltration into the subsurface are proposed to account for large volumes of brine that are ultimately responsible for mineralization of these districts. This study demonstrates that under a range of geologically reasonable conditions, brine infiltration into an aquifer in the deep subsurface can coexist with topographically driven flow. Infiltration combined with regional flow and local magmatic heat sources in the Reelfoot rift explain the brine concentrations as well as the temperatures observed in the Southern Illinois and Upper Mississippi Valley districts.
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The effect of allochthonous salt evolution and overpressure development on source rock thermal maturation: a two-dimensional transient study in the northern Gulf of Mexico Basin
Authors S. Chereé Stover, Paul Weimer and Shemin GeSequential two-dimensional forward modelling of source rock maturation along a north–south 600 km megaregional profile across the northern Gulf of Mexico Basin illustrates the coupled influence of allochthonous salt evolution and overpressure development on source rock maturation. Model results indicate that allochthonous salt evolution strongly affected local and regional source rock maturation, while overpressure development had a minor impact. Within the lower slope portion of the profile (south of the Sigsbee escarpment to the Tabular Salt/Minibasin province), localized thermal lows associated with salt stock development locally retarded the maturation of source rocks, while the slow development of overpressures effected little influence on the maturation. Within the centre of the profile (offshore Louisiana shelf), however, the evolution of allochthonous salt sheets and shallow salt structures locally and regionally lowered the oil generation window. The timing of overpressures in this region, related to the progradation of the Cenozoic marine depocentre, may have accelerated the maturation of middle Eocene source rocks, although the deeper Turonian source rocks had reached maturation at the time of greatest overpressure development. In the northern portion of the profile (southern Louisiana), the Terrebonne salt sheet lowered the oil generation window and retarded the maturation of the middle Eocene source rocks.
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Overpressure associated with a convergent plate margin: East Coast Basin, New Zealand
Authors David Darby and Robert H. FunnellThe East Coast Basin of New Zealand lies in the frontal arc of the Hikurangi subduction zone. Overpressure forms a major hazard for exploration in the basin. Fluid pressures of 12.8 MPa at 600 m depth have been encountered, equivalent to 90% of lithostatic pressure, and mud weights of up to 19.5 ppg are required to control formation fluids at subsurface depths of only 1400 m. The distribution and magnitude of overpressure shows no relation to present-day depth. Lithostratigraphy controls the distribution of overpressure and widespread low-permeability bathyal mudstones form seals to overpressure across the basin. The transition from normal pressure to overpressure is not associated with any single stratigraphic unit. The Neogene evolution of the plate boundary has given rise to rapid Miocene sedimentation (400 m Ma−1) in areas of the basin and has caused disequilibrium compaction. Late Neogene compression has subsequently uplifted the overpressured, undercompacted sediments by up to 3000 m at rates of 1000 m Ma−1. Lateral tectonic compression associated with the plate boundary has also caused undrained shear of thick mudstones, leading to extremely high overpressures in deformed sediments independent of depth.
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Potential for subtle traps in the Permian Rotliegend of the UK Southern North Sea
Authors James R. Maynard and John P. GibsonThe Permian, Rotliegend Group of the UK Southern North Sea is shown to consist of periodic, cyclic stratal packages which can be used to aid regional correlation in the zone of playa–erg interaction colloquially known as the Rotliegend Feather Edge. In this area, the depositional topography controls the sedimentary facies, with a concentration of aeolian sandstone deposition in the topographic depressions, especially in the lee of hills. These units onlap the contemporaneous intra-basinal and marginal highs. The palaeotopographic lows rapidly became infilled, with later lacustrine deposition occurring over a smoother surface. This allowed the deposition of finer-grained lake deposits over the sands. Prediction of the palaeotopography is key to understanding potential reservoir distribution and seal juxtaposition within the Rotliegend. This paper demonstrates how pre-Permian subcrop maps, Bouguer anomaly maps and the Rotliegend isopach can be used to reconstruct this palaeotopography. Two areas are studied which demonstrate some of the subtleties of the model. The southern Quadrant 42–43 area had a palaeotopography with gentle gradients and a number of subtle intrabasinal highs that allowed for isolated reservoirs overlain by shales with high sealing potential. In contrast, the northern Quadrant 49 area, contained only intra-basinal relief, with high palaeotopographic gradients. Aeolian sediments banked up against this barrier and no intra-Rotliegend seal was deposited over these sands.
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Enhanced lithological description of a Jurassic mudrock sequence using geophysical wireline logs
Authors N. R. Brereton, R. W. Gallois and A. WhittakerMethods to combine individual wireline logs such as resistivity, sonic, bulk density, neutron porosity and natural gamma have been developed to enhance lithological interpretation, particularly of mudrock sequences. Multiple log data presentation to generate a synthetic, pseudo-lithology log is achieved by combining downhole geophysical log records using a colour cube. Well-documented lithological descriptions of borehole cores from the Kimmeridge Clay Formation (Jurassic) of Britain are compared with the newly processed combination geophysical log data. The colour cube logs confirm existing lithological descriptions and, importantly, enable considerable refinement of general lithological descriptions of ‘undifferentiated’ mudstone.
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Characterization of heterolithic deposits using electrofacies analysis in the tide-dominated Lower Jurassic Cook Formation (Gullfaks Field, offshore Norway)
Authors Renu Gupta and Howard D. JohnsonA range of heterolithic facies, comprising thinly interbedded (millimetre–decimetre scale) sandstones and mudstones, characterizes the heterogeneous Lower Jurassic Cook Formation, including the productive Cook-3 reservoir in the Gullfaks Field. These heterolithic facies were deposited in a tide-dominated estuarine to deltaic setting and show up as massive ‘shaly-sands’ on conventional wireline logs. This makes the recognition and discrimination of different heterolithic facies types virtually impossible, which severely limits detailed reservoir geological and petrophysical predictions. This problem has been addressed by undertaking a high resolution electrofacies analysis using core facies as a ‘training set’ and applying this, through multivariate statistical techniques, to the interpretation of the conventional logs.
An electrofacies database was created comprising five genetically linked rock types (ranging from lenticular-wavy bedding, through flaser bedding and into clean/massive sandstones). This electrofacies scheme was validated with reference to c. 125 m of cored section from five wells using gamma-ray, density and neutron logs. Multivariate statistical techniques have enabled probabilistic discrimination of the different types of heterolithic facies down to intervals of only 0.25 to 0.5 m thick, which is considerably greater than could be achieved using conventional well-log evaluation techniques alone.
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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)