Role of compressive tectonics on gas charging into the Ordovician sandstone reservoirs in the Sbaa Basin,Algeria: constrained by fluid inclusions and mineralogical data

Structure‐ and tectonic‐related gas migration into Ordovician sandstone reservoirs and its impact on diagenesis history were reconstructed in two gas fields in the Sbaa Basin, in SW Algeria. This was accomplished by petrographical observations, fluid inclusion microthermometry and stable isotope geochemistry on quartz, dickite and carbonate cements and veins. Two successive phases of quartz cementation (CQ1 and CQ2) occurred in the reservoirs. Two phase aqueous inclusions show an increase in temperatures and salinities from the first CQ1 diagenetic phase toward CQ2 in both fields. Microthermometric data on gas inclusions in quartz veins reveal the presence of an average of 92 ± 5 mole% of CH₄ considering a CH₄‐CO₂ system, which is similar to the present‐day gas composition in the reservoirs. The presence of primary methane inclusions in early quartz overgrowths and in quartz and calcite veins suggests that hydrocarbon migration into the reservoir occurred synchronically with early quartz cementation in the sandstones located near the contact with the Silurian gas source rock at 100–140°C during the Late Carboniferous period and the late Hercynian episode fracturing at temperatures between 117 and 185°C, which increased in the NW‐direction of the basin. During the fracture filling, three main types of fluids were identified with different salinities and formation temperatures. A supplementary phase of higher fluid temperature (up to 226°C) recorded in late quartz, and calcite veins is related to a Jurassic thermal event. The occurrence of dickite cements close to the Silurian base near the main fault areas in both fields is mainly correlated with the sandstones where the early gas was charged. It implies that dickite precipitation is related to acidic influx. Late carbonate cements and veins (calcite – siderite – ankerite and strontianite) occurred at the same depths resulting from the same groundwater precipitation. The absence of methane inclusions in calcite cements result from methane flushing by saline waters.     

Carbon dioxide controlled earthquake distribution pattern in the NW Bohemian swarm earthquake region,western Eger Rift,Czech Republic – gas migration in the crystalline basement

The ascent of magmatic carbon dioxide in the western Eger (Oh?e) Rift is interlinked with the fault systems of the Variscian basement. In the Cheb Basin, the minimum CO₂ flux is about 160 m³ h^?1, with a diminishing trend towards the north and ceasing in the main epicentral area of the Northwest Bohemian swarm earthquakes. The ascending CO₂ forms Ca‐Mg‐HCO₃ type waters by leaching of cations from the fault planes and creates clay minerals, such as kaolinite, as alteration products on affected fault planes. These mineral reactions result in fault weakness and in hydraulically interconnected fault network. This leads to a decrease in the friction coefficient of the Coulomb failure stress (CFS) and to fault creep as stress build‐up cannot occur in the weak segments. At the transition zone in the north of the Cheb Basin, between areas of weak, fluid conductive faults and areas of locked faults with frictional strength, fluid pressure can increase resulting in stress build‐up. This can trigger strike‐slip swarm earthquakes. Fault creep or movements in weak segments may support a stress build‐up in the transition area by transmitting fluid pressure pulses. Additionally to fluid‐driven triggering models, it is important to consider that fluids ascending along faults are CO₂‐supersaturated thus intensifying the effect of fluid flow. The enforced flow of CO₂‐supersaturated fluids in the transitional zone from high to low permeability segments through narrowings triggers gas exsolution and may generate pressure fluctuations. Phase separation starts according to the phase behaviour of CO₂‐H₂O systems in the seismically active depths of NW Bohemia and may explain the vertical distribution of the seismicity. Changes in the size of the fluid transport channels in the fault systems caused, or superimposed, by fault movements, can produce fluid pressure increases or pulses, which are the precondition for triggering fluid‐induced swarm earthquakes.     

楠溪江流域宗族与学术高峰

在地理环境相对封闭的楠溪江流域,以血缘宗族为纽带沿江分布的古村落群,在宋代已经形成了基本的格局。发展至今,多数古村落已绵延数百年,创造出了独具特色的宗族文化。在流域宗族文化形成的同时,学术文化也迅速发展,至宋时达到高峰,出现了文化繁荣、政治显赫的时代,形成永嘉事功学派。     

明清以降漓江上游地区的人类活动与环境变迁研究

漓江沙洲的形成,是自然环境和人类活动交互影响的结果。明清以降,外来人口不断进入漓江上游地区从事农业垦殖活动。垦殖活动在改造了上游山区自然环境的同时,其产生的泥沙在雨水作用下进入漓江,促进了桂林段漓江沙洲的发育,并改变了桂林段漓江河床的面貌。岸上居民也开始由岸上洲,这是漓江流域人地关系的重要变化。     

论马家浜文化之前太湖地区的地理埋藏环境

近年来,地处东部沿海的浙江和山东地区陆续发现了距今11000～7000年间新石器时代中期和早期遗址,因此探讨江苏地区马家浜文化之前的地理埋藏问题也是必要的。本文重点探讨了长江下游太湖地区更新世晚期和早全新世时期的地理埋藏环境,并认为本区也应该存在新石器时代早中期的遗址。     

Recent Research on Chaco: Changing Views on Economy, Ritual, and Society

Current research on Chaco Canyon and its surrounding outlier communities is at an important juncture. Rather than trying to argue for the presence or absence of complexity, archaeologists working in the area are asking different questions, especially how Chacoan political, economic, ritual, and social organization were structured. These lines of inquiry do not attempt to pigeonhole Chaco into traditional neoevolutionary types, but instead seek to understand the historical trajectory that led to the construction of monumental architecture in Chaco Canyon and a large part of the northern Southwest in the 10th through 12th centuries. This review discusses the conclusions of current research at Chaco including definitions of the Chaco region, recent fieldwork, histories of Chaco archaeology, chronology, paleoenvironmental reconstruction, demography, political organization, outlier communities, economic organization, social organization, ritual, violence, and the post-Chacoan reorganization. Although many issues are hotly debated, there is a growing concensus that power was not based in a centralized political organization and that ritual organization was a key factor in the replication of Chacoan architecture across a vast regional landscape. Exactly how ritual, social, and political organization intersected is a central question for Chaco scholars. The resolution of this problem will prove to be of interest to all archaeologists working with intermediate societies across the globe.     

A new temnospondyl amphibian from the Lower Triassic of South America

A new temnospondyl from the Early Triassic Buena Vista Formation (Paraná Basin) in Uruguay is described. The specimen is represented by a skull table fragment which includes the left orbit, and, although fairly incomplete, shows close affinities with the basal temnospondyl clade (Dvinosaurus + Tupilakosauridae). Previously, members of this clade were only known from Upper Permian and Lower Triassic deposits from the Northern Hemisphere. The new specimen described herein together with a recent record from the Lower Triassic of South Africa (Karoo Basin), support the hypothesis of an invasion of members of the basal (Dvinosaurus + Tupilakosauridae) clade into Gondwana as early as the Late Permian.     

The first record of a Maastrichtian ammonite from the Perth Basin,Western Australia,and its biostratigraphical significance

The first known ammonite from the Maastrichtian of the Perth Basin, a single, incomplete specimen of Grossouvrites gemmatus (Huppé, 1854), is described from core material from the Lancelin No. 1 borehole. Foraminiferal and nannoplankton evidence date the ammonite as middle Maastrichtian; its age is also assessed on the basis of dinoflagellate assemblages from this section, but these give a less precise dating. The position of the ammonite with respect to a mid-Maastrichtian disconformity, which probably extended along much of the Western Australian margin is discussed.     

Overpressure in the Malay Basin and prediction methods

Predrill overpressure prediction is important for well planning and migration modeling for prospect evaluation. The Eaton (Journal of Petroleum Technology, 24 , 1972, 929) and Bowers (SPE Drilling & Completion, 10 , 1995, 89) methods are used worldwide for postdrill overpressure prediction using sonic log and predrill overpressure prediction using seismic interval velocity. In this research, these two methods were used for overpressure prediction using 3D anisotropic prestack depth‐migrated seismic interval velocity in a field of the Malay Basin. In the shallow overpressured zone, where the mechanism of overpressure is undercompaction, the onset of overpressure was predicted reasonably well using the Eaton and Bowers methods with their standard parameters (i.e., Eaton exponent 3 and Bowers loading curve) for seismic velocity. However, in the deep overpressured zone, where fluid expansion is the cause of overpressure generation, these methods underpredicted the high overpressure. In the deep overpressured zone, the overpressures were better predicted by applying a correction to the Eaton method. On the other hand, the Bowers unloading parameters for the fluid expansion mechanisms did not show any significant effect on overpressure prediction. Hence, in the study area, the Bowers method is not effective for 3D overpressure prediction using seismic velocity, whereas the Eaton method is more robust and can be used for 3D overpressure prediction from seismic velocity.     

The population of East Sicily during the second and first millennium BC: the problem of the Greek colonies

Knowledge of Sicilian biology during the past is very important in understanding the complex processes that characterized the population of the Mediterranean Basin. The problem of the first Greek settlement in Sicily is essential in understanding and reconstructing the indigenous biological tissue of an island that was and still is a fundamental crossroads for migratory strategies. In this research we studied ten Sicilian series chronologically attributed to the second and first millennium BC , using discrete cranial traits. The results show that the first biologically significant Greek presence in eastern Sicily could go back to the Bronze Age, while the cosmopolitan Hellenistic city of Syracuse showed the nature of the biological pattern during the first millennium BC . Copyright © 1999 John Wiley & Sons, Ltd.