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Copyright of Acta Petrologica et Mineralogica / Yanshi Kuangwuxue Zazhi is the property of Acta Petrologica et Mineralogica and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Radiogenic isotope dating of illitic clays has been widely used to reconstruct thermal and fluid flow events in siliciclastic sedimentary basins, the information of which is critical to investigate mechanisms of hydrocarbon maturation. This study carried out Rb−Sr and 40Ar−39Ar dating of authigenic illitic clay samples separated from the Palaeogene sandstone in the northern South China Sea. Our Rb−Sr data further confirm the previously reported three periods of fluid flow events (at 34.5 ± 0.9, 31.2 ± 0.6, and 23.6 ± 0.8 Ma, respectively) in the northern South China Sea, which are related to regional episodic tectonism. However, 40Ar−39Ar ages of illite obtained in this study are significantly younger than the corresponding Rb−Sr ages. The significantly younger 40Ar−39Ar ages were probably due to 40Ar loss caused by later dry heating events on the Hainan Island that have not affected the Rb−Sr isotopic systematics. The inconsistency between Rb−Sr and 40Ar−39Ar data should be attributed to different isotopic behaviors of K−Ar and Rb−Sr isotopic systematics in illite. Our results indicate that Rb−Sr isotopic dating method may be a preferential approach for clay dating in geological settings where exist younger dry heating events. [ABSTRACT FROM AUTHOR]

Abstract: By using thin section identification, cathodoluminescence, major and trace elements and fluid inclusion tests and authigenic illite dating, based on observation of core cracks, combined with the microscopic characteristics and imaging logging characteristics of fractures, the stages of the fractures in the Huagang Formation of the central reversal tectonic belt of the Xihu Sag in the East China Sea, and the matching relationship between the fracture development stages and the oil and gas charging stages are clarified. There are diagenetic fractures and tectonic fractures in the reservoirs of the Huagang Formation in the study area. The diagenetic fractures developed during the diagenetic stage of the reservoirs and have less effect on oil and gas migration and transport. The tectonic fractures are divided into three stages based on tectonic movements controlling the fractures and their relationships with hydrocarbon charging: The first stage of fractures was generated in the early stage of the Himalayan Movement–Longjing Movement (12–13 Ma ago), when the tectonic stress caused the sutures and shale strips to twist, deform, and break. Tectonic microfractures generated in this period had short extension, narrow width, and poor effectiveness, and had little effect on oil and gas migration and transport. The second stage of fractures came up during the middle-late period of Himalayan Movement–Longjing Movement (9–12 Ma ago), when tectonic movements caused the development of tectonic fractures in the central reversal tectonic belt, these fractures are of large scale, long extension, and good effectiveness, and matched with the first stage of large scale oil and gas charging (9–12 Ma ago), so they play an important role in oil and gas migration, transportation, and accumulation. The third stage of fractures were created from Himalayan Movement–Okinawa Trough movement to the present day (0–3 Ma ago), the fractures are tectonic ones developing successively; matching with the second stage (0–3 Ma ago) of large-scale oil and gas charging, they created conditions for continuous natural gas migration and transportation. All these prove that the development of reservoir fractures in the Huagang Formation of Xihu Sag can provide seepage space and continuous and effective channels for efficient migration and accumulation of oil and gas.

By using thin section identification, cathodoluminescence, major and trace elements and fluid inclusion tests and authigenic illite dating, based on observation of core cracks, combined with the microscopic characteristics and imaging logging characteristics of fractures, the stages of the fractures in the Huagang Formation of the central reversal tectonic belt of the Xihu Sag in the East China Sea, and the matching relationship between the fracture development stages and the oil and gas charging stages are clarified. There are diagenetic fractures and tectonic fractures in the reservoirs of the Huagang Formation in the study area. The diagenetic fractures developed during the diagenetic stage of the reservoirs and have less effect on oil and gas migration and transport. The tectonic fractures are divided into three stages based on tectonic movements controlling the fractures and their relationships with hydrocarbon charging: The first stage of fractures was generated in the early stage of the Himalayan Movement–Longjing Movement (12–13 Ma ago), when the tectonic stress caused the sutures and shale strips to twist, deform, and break. Tectonic microfractures generated in this period had short extension, narrow width, and poor effectiveness, and had little effect on oil and gas migration and transport. The second stage of fractures came up during the middle-late period of Himalayan Movement–Longjing Movement (9–12 Ma ago), when tectonic movements caused the development of tectonic fractures in the central reversal tectonic belt, these fractures are of large scale, long extension, and good effectiveness, and matched with the first stage of large scale oil and gas charging (9–12 Ma ago), so they play an important role in oil and gas migration, transportation, and accumulation. The third stage of fractures were created from Himalayan Movement–Okinawa Trough movement to the present day (0–3 Ma ago), the fractures are tectonic ones developing successively; matching with the second stage (0–3 Ma ago) of large-scale oil and gas charging, they created conditions for continuous natural gas migration and transportation. All these prove that the development of reservoir fractures in the Huagang Formation of Xihu Sag can provide seepage space and continuous and effective channels for efficient migration and accumulation of oil and gas. [ABSTRACT FROM AUTHOR]

The Silurian bituminous sandstones (SBS) in the Tarim Basin, China are important basin-wide reservoirs with an estimated area of approximately 249000 km2. We investigated the ages of authigenic illites in the SBS reservoirs and constrained their formation timing by using the 40Ar/39Ar step wise heating method. The age spectra, 39Ar recoil loss and their controlling factors were investigated systematically. The 40Ar/39Ar ages were compared with the conventional K/Ar ages of identical clay fractions. The clay in the SBS reservoirs is dominated by orderly mixed-layer illite/smectite (I/S) with 5%-30% smectite layers. The I/S minerals morphology comprises primarily honeycomb, short filamentous and curved-lath particles, characteristic of authigenic illites. The unencapsulated 40Ar/39Ar total gas ages (UTGA) of the authigenic illites range from 188.56 ± 6.20 Ma to 491.86 ± 27.68 Ma, which are 7% to 103% older than the corresponding K/Ar ages of 124.87 ± 1.11 Ma to 383.45 ± 2.80 Ma, respectively. The K-Ar ages indicate multistage accumulations with distinct distribution patterns in the Tarim Basin: older (late Caledonian-early Hercynian) around the basin margin, younger (late Hercynian) in the basin centre, and the youngest (middle to late Yanshanian) in the Ha-6 well-block, central area of the North Uplift The age difference is believed to have been caused by the 39Ar recoil loss during the irradiation process. Compared with the K/Ar ages, the estimated 39Ar recoil losses in this study are in the range from 7% to 51%. The 39Ar recoil loss appears to increase not only with the decreasing particle sizes of the I/S, but also with increasing percentage of smectite layers (IR) of the I/S, and smectite layer content (SLC) of the samples. We conclude that due to significant 39Ar recoil losses, UTGA may not offer any meaningful geological ages of the authigenic illite formation in the SBS and thus can not be used to represent the hydrocarbon charge timing. 39Ar recoil losses during 40Ar/39Ar dating can not be neglected when dating fine authigenic illite, especially when the ordered mixed-layer I/S containing small amount of smectite layers (IR<30%) in the reservoir formations. Compared with the unencapsulated Ar-Ar method, the conventional K-Ar method is less complicated, more accurate and reliable in dating authigenic illites in petroleum reservoirs. [ABSTRACT FROM AUTHOR]

A belt of klippen and nappes was emplaced in the Longmen Shan thrust belt, mostly lying between the Yingxiu–Beichuan and the Pengguan fault zones. Timing of the allochthon emplacement is hotly debated, but quantitative age constraints are rarely documented. This study presents the first isotopic dating of authigenic illite isolated from a 30 m-wide fault gouge zone developed in reverse faults in the leading edge of the largest Baishi-Goujia klippe, a representative of the klippen and nappe belt in the Longmen Shan thrust belt. The results show that the authigenic illite formed 180–215 Ma ago. Combined with apatite and zircon fission track dating of the hanging wall of the Yingxiu–Beichuan fault zone, the emplacement of the Baishi-Goujia klippe is constrained at 171–180 Ma, in the Middle Jurassic. The thickness of a successive strata unit of the Baishi-Goujia klippe measures ~ 3 km and of the Tangwangzhai nappe is over 5 km. This implies that the elevation of the Longmen Shan was at least of 3–5 km a.s.l. during the Indosinian orogeny. [ABSTRACT FROM AUTHOR]

Abstract: Kaolin is a widespread authigenic clay mineral in the Permo-Triassic sandstones from the marginal areas of the SE Iberian Basin. However, relatively more extensive illitization of kaolin occurs in the SW basin margin compared with the slight occurrence in the NE margin. SEM analysis of sandstones reveals that illite replaced small kaolinite crystals while blocky dickite remained unaltered. Kaolin illitization is suggested to take place in relation to the maximum burial depth reached during Late Cretaceous post-rift stage. Vitrinite reflectance data denotes a maximum burial temperature of 118 °C and 144°C in the NE basin margin and in the SW basin margin, respectively. Thus, the extent of illitization in the SW margin is attributed to the higher T peak reached by the Permo-Triassic succession. Regarding the lack of K-feldspars observed in the sandstones and interbedded mudrocks, the source of K+ is mainly related to the alteration of detrital mica. [Copyright &y& Elsevier]

The history of thermal and fluid flow events is of crucial importance to understand the mechanism of hydrocarbon maturation. The northern South China Sea (SCS) contains abundant hydrocarbon resources with high heat flow anomalies, but the mechanisms responsible for modification of thermal regimes are poorly understood. Here we conducted the first high-precision Rb-Sr isochron dating and geochemical analyses of authigenic illites separated from the Palaeogene sandstones in the Beibuwan Basin, northern SCS. Eight high-precision Rb-Sr isochron ages reveal three major thermal and fluid flow events in the Beibuwan Basin, northern SCS. The 34.5 ± 0.9 Ma and 23.6 ± 0.8 Ma events occurred in the western area are probably related to the Eocene-Oligocene transition characterised by a significant regional unconformity resulted from intensive uplift of Hainan Uplift and Oligocene-Miocene transition marked by a post-rift unconformity, respectively. By contrast, the 31.2 ± 0.6 Ma event affecting the eastern area appears to be closely associated with the igneous intrusion dated by zircon U/Pb to 32.3 ± 0.7 Ma. REE and stable isotope characteristics indicate that the illites from the western area formed within a meteoric-hydrothermal system, whilst those from the eastern area were likely precipitated from fluids heated by the large-scale igneous intrusion at higher water/rock ratios. Moreover, our data in conjunction with palaeo-temperature reconstructions show that the fluid flow events had profound effects on the rapid organic maturation in the northern SCS. • Our work provides the first systematic chronology and characterization of the fluid flow history in the SCS. • 8 Rb-Sr isochron ages of illites reveal 3 fluid flow events related to the regional episodic tectonism in the SCS. • Fluid flow events played a significant role in the enhancement of hydrocarbon maturation. • It highlights the effective use of Rb-Sr isochron dating of illite to reconstruct multiphase fluid flow events. [ABSTRACT FROM AUTHOR]