ALBERT

Description: Temporary excavations during the construction of the Glendoe Hydro Scheme above Loch Ness in the Highlands of Scotland exposed a clay-rich fault gouge in Dalradian Supergroup psammite. The gouge coincides with the mapped trace of the subvertical Sronlairig Fault, a feature related in part to the Great Glen and Ericht–Laidon faults, which had been interpreted to result from brittle deformation during the Caledonian orogeny (c. 420–390 Ma). Exposure of this mica-rich gouge represented an exceptional opportunity to constrain the timing of the gouge-producing movement on the Sronlairig Fault using isotopic analysis to date the growth of authigenic (essentially synkinematic) clay mineralization. A series of fine-size separates was isolated prior to K–Ar analysis. Novel, capillary-encapsulated X-ray diffraction analysis was employed to ensure nearly perfect, random orientation and to facilitate the identification and quantification of mica polytypes. Coarser size fractions are composed of greater proportions of the 2M1 illite polytype. Finer size fractions show increasing proportions of the 1M illite polytype, with no evidence of 2M1 illite in the finest fractions. A series of Illite Age Analysis plots produced excellent R2 values with calculated mean ages of 296 ± 7 Ma (Late Carboniferous–Early Permian) for the oldest (2M1) illite and 145 ± 7 Ma (Late Jurassic–Early Cretaceous) for the youngest (1M) illite. The Late Carboniferous–Early Permian (Faulting event 1) age may represent resetting of earlier-formed micas or authigenesis during dextral displacement of the Great Glen Fault Zone (GGFZ). Contemporaneous WNW(NW)–ESE(SE) extension was important for basin development and hydrocarbon migration in the Pentland Firth and Moray Firth regions. The Late Jurassic–Early Cretaceous (Faulting event 2) age corresponds with Moray Firth Basin development and indicates that the GGFZ and related structures may have acted to partition the active extension in the Moray Firth region from relative inactivity in the Pentland Firth area at this time. These new age dates demonstrate the long-lived geological activity on the GGFZ, particularly so in post-Caledonian times where other isotopic evidence for younger tectonic overprints is lacking.

Description: Nanometer to micrometer mica and illite separates of indurated Cambrian and Ordovician oil-bearing sandstones from the Hassi Messaoud field (Algeria) were extracted, x-rayed, observed by scanning and transmission electron microscopy, and K-Ar dated. Electron microscope observations revealed typical euhedral shapes for the mica to illite particles of most size fractions; almost no odd-shaped detrital crystals were detected. The combined results document several generations of mineralogical and morphological identical mica to illite crystals that could not be differentiated by the traditional identification methods. Illite and mica genesis was multiphased with crystallization episodes at 340 ± 10 (ca. Middle Mississippian), 280 ± 10 Ma (ca. early Permian), and 170 ± 10 Ma (ca. Middle Jurassic). Younger than the stratigraphic age of the host rocks, which is incompatible with a detrital origin, the two older mica ages confirm that the hydrocarbon generation and emplacement had to start after the Variscan tectonothermal event and before exhumation of the meta-sediments. The younger K-Ar ages at 135 to 110 Ma (ca. Early Cretaceous) relate to further crystallization episodes, whereas those at circa 295, 265, and 210 Ma probably correspond to variable mixtures of the older and younger mica to illite end-members. Three average K-Ar values are statistically significant: the oldest at 340 ± 10 Ma corresponds to the start of the Variscan tectonic activity, and the intermediate at 280 ± 10 Ma sets its end, both episodes probably modifying the reservoir capacities of the potential hydrocarbon host rocks. The ages at 170 ± 10 Ma identify a further diagenetic activity characterized by illitization of dickite-type precursors in local reservoirs. These younger ages could correspond to the hydrocarbon charge into reservoirs, which stopped diagenetic illitization at a present-day depth of approximately 4000 m (∼13,000 ft).

Description: Isotopic dating of diagenetic processes in relation to hydrocarbon charge in buried sedimentary sequences is of importance to the oil industry. Yun et al. recently published in the AAPG Bulletin (v. 94, no. 6, p. 759–771) on that topic a study entitled, “Dating Petroleum Emplacement by Illite 40Ar/39Ar Laser Stepwise Heating Technique.” This study raises questions about basic aspects of 40Ar/39Ar dating fine-grained authigenic clay minerals, describes flawed methodology, and ignores important previous work. As isotopic dating of diagenetic processes may interest readers of the AAPG Bulletin that are not necessarily specialists in either isotopic geochronology or clay mineralogy, it appeared necessary to comment on both the technical aspects and the scientific interpretations of this study. We discuss especially the fact that the authors discarded 39Ar recoil followed by an evaluation of its impact on the results and the interpretations.

Description: Permian Rotliegende sandstone cores were collected from an area of about 14,800 km2 (∼5710 mi2) mostly to the east-southeast of the city of Bremen in northern Germany, at depths between 4596 and 5330 m (15,079–17,487 ft). The separated size fractions (〈0.2, 0.2–1.0, and 1.0–2.0 μm) consist of illite (90–100%) with small to minute amounts of chlorite and detectable quartz and feldspars in the coarser fractions. Scanning electron microscopic and transmission electron microscopic observations showed two types of illite morphologies: flakes coating detrital framework minerals and laths and fibers invading the pore space. The data points of most size fractions fit two isochrons, with slopes providing ages of 191 ± 8 and 178 ± 1 Ma, with initial 40Ar/36Ar ratios reasonably close to the atmospheric value. Microthermometric fluid-inclusion determinations in quartz and calcite characterize two types of percolating fluids: a highly saline (19% NaCl equivalent) fluid at variable temperatures depending on the reservoirs (185 to 150°C) and a slightly saline one (2.6% NaCl equivalent) again at varied temperatures (170 to 145°C), also depending on the locations. These temperatures are higher than paleotemperatures calculated on the basis of a present-day burial gradient of 30.5°C/km, therefore favoring hydrothermal illitization, with the oldest illite crystallizing at a generally higher temperature than that of the younger illite. However, if illite coincidently precipitated with quartz, which is supported by petrographic observation, then illite with ages of about 200 Ma in the eastern to central Rotliegende sandstones of the area formed at a lower temperature but in more saline fluids than that of about 155 Ma. Alternatively, illite crystallization temperature and salinity of the interacting fluids could have been activated by the same hydrothermal activity at different locations in the studied area and could have decreased and changed, respectively, when the fluids moved in the sandstone reservoirs. The potassium-argon data fit a regional model of recurrent thermal activity, with the oldest occurrence of authigenic illite in the eastern Imbrock-Ganderkesee area and a time-dependent trend of the hydrothermal activity toward the west. Previous results from northern Germany, central Netherlands, and the southern North Sea complete the model with an initial activity at the same 200-Ma time in the western offshore and central Netherlands and an activity trend toward the east.

Description: Diagenetic illite is one of two major porosity occluding cements in the Unayzah sandstone reservoirs (Permian–Carboniferous) of Saudi Arabia. The other is diagenetic quartz. This article focuses on the origin of illite and its timing. Illite has been formed by a reaction of detrital K-feldspar and early diagenetic kaolinite as temperatures increased due to burial. When either of the two reactants is exhausted, illite ceases to precipitate. There is no evidence that hydrocarbon emplacement, deep brine migration, or unique thermal events are factors in illite precipitation. Modeling of illite precipitation as a kinetically controlled reaction using burial histories of the samples studied generally yields a reasonable match between measured and modeled ages and amounts of illite. This lends further support to the gradual formation of illite over a time-temperature interval. Although quartz overgrowths and diagenetic illite may occur in the same thin section, they appear to be mutually exclusive locally. Quartz overgrowths do not occur on detrital quartz grains that are coated with diagenetic illite, and illite is rarely observed on quartz overgrowths. Therefore, it appears that not only does diagenetic illite inhibit nucleation of quartz overgrowths but quartz overgrowths may also inhibit precipitation of diagenetic illite. The two cements appear to compete for surface area on uncoated detrital quartz grains. 2nd revised manuscript received March 15, 2010 Stephen Franks received his Ph.D. in geology from Case Western Reserve University. He worked for Atlantic Richfield Corporation (ARCO) from 1974 to 1999, after which he formed a consulting company, RockFluid Systems, Inc., in Dallas, Texas. In 2001, he accepted a position with Saudi Aramco's Advanced Research Center. He retired from Saudi Aramco in April 2009 and now consults in Dallas. Horst Zwingmann is a principal research scientist at CSIRO and an associate professor at the University of Western Australia, Perth. He completed his Ph.D. at the University Louis Pasteur Strasbourg, France, in 1995. His research interests include isotopic dating and tracing of diagenetic processes in relation to petroleum exploration and clastic sedimentary systems.

Description: The Tarim Basin is the largest inland basin in China. The hydrocarbon charge history of the Silurian bituminous sandstone reservoirs in the Tazhong uplift was investigated using an integrated approach, combining detailed petrographic analysis (thin section, scanning electron microscopy, and transmission electron microscopy), clay mineral x-ray diffraction characterization, and modeling of authigenic clay minerals and subsequent potassium-argon (K-Ar) dating of authigenic illite. The timing of the hydrocarbon charges of the reservoirs in the Tazhong uplift area is compared with that of the Kongquehe area within the basin. The authigenic illite ages in the Tazhong uplift area and the Kongquehe area range from 383 Ma to 204 Ma, indicating a unique age distribution pattern in the basin. The Silurian oil accumulations were primarily formed in the late Caledonian through the late Hercynian. The reservoirs within wells Qiao-1, Kongque-1, Longkou-1, and Yingnan-2, located in the western area and the eastern area of the basin, were charged around 383 to 271 Ma in the late Caledonian to the early Hercynian. The reservoirs in wells Tazhong-37, Tazhong-67, Tazhong-12, and Tazhong-32, located in the center of the basin, were charged at around 235 to 204 Ma in the late Hercynian. A correlation between the authigenic illite ages and the thickness of the Silurian bituminous sandstones suggests that the paleostructural framework is a key controlling factor. The paleo-oil pool located in and around the sedimentary centers, such as wells Qiao-1 and Kongque-1, was formed relatively early (383 Ma). The K-Ar dating results were consistent with the results of conventional hydrocarbon accumulation history analysis and highlight the differences in the accumulation timing between various oil pools in different parts of the basin. Youyu Zhang received his M.S. degree from the China University of Geosciences, Beijing. His current research interest includes authigenic illite dating and hydrocarbon accumulation, clay minerals in sandstone reservoirs, He and Ar isotopic analyses, and K-Ar and Ar-Ar dating. He is currently a research professor at the Research Institute of Petroleum Exploration and Development, PetroChina, Beijing. Horst Zwingmann is a principal research scientist at the Commonwealth Scientific and Industrial Research Organisation Earth Science and Resource Engineering and associate professor at the University of Western Australia, Perth. He completed his Ph.D. at the University of Louis Pasteur Strasbourg, France, in 1995. His research interests include isotopic dating and tracing of diagenetic processes in relation to petroleum exploration and application of radiogenic isotopes and geochemistry to clastic sedimentary systems. Keyu Liu is a principal research scientist at the Commonwealth Scientific and Industrial Research Organisation Earth Science and Resource Engineering. He received his M.S. degree from Sydney University and his Ph.D. from the Australian National University. Before his present employment, he held various research positions in Australia at CSIRO Petroleum, Adelaide University, and James Cook University. His current research interest includes clastic sedimentology, filling history analysis of petroleum reservoirs, and laboratory investigation of oil migration and enhanced oil recovery. Andrew Todd is the potassium-argon laboratory manager at Commonwealth Scientific and Industrial Research Organisation Earth Science and Resource Engineering (CSIRO ESRE) and the John deLaeter Centre of Excellence in Mass Spectroscopy at Curtin University, Perth. He completed his B.S. degree in 1985 at Sydney University. He worked in stable isotope geochemistry before his current position at CSIRO ESRE. Xiuquan Luo received his M.S. degree from the China University of Geosciences, Beijing. Before his present employment, he was a research professor at the Research Institute of Geology, Chinese Academy of Geological Sciences, Beijing, and was engaged in K-Ar and Ar-Ar dating and related scientific research. His current research interest includes He and Ar isotopic analysis and K-Ar and Ar-Ar dating.