Lower limit of hydrocarbon accumulation in the Kuqa Depression, Tarim Basin, NW China

The lower limit of hydrocarbon accumulation in the Kuqa Depression is investigated. The Kuqa Depression in the Tarim Basin is one of the largest gas‐producing regions in China. High yields of hydrocarbons have been found in depths below 7000 m, which suggests that the deep strata have great potential for hydrocarbon exploration. However, for effective deep exploration, the effective limits such as maximum depth or minimum physical properties of reservoirs have not yet been established. The lower limit of hydrocarbon accumulation, i.e. the conditions at which hydrocarbon accumulation is no longer possible, is determined by analysing the characteristics of effective reservoir space, reservoir permeability, fluid potential differences inside and outside the reservoir, and drilling results of exploratory wells in actual geological conditions. Research shows that the lower limit of hydrocarbon accumulation is reached when irreducible water saturation reaches 100%, reservoir permeability reaches as low as 0.01 × 10−3 µm2, fluid potential ratio of outside to inside the reservoir (fluid potential ratio of the surrounding rock to sandstone reservoir) is less than 5.5, and exploratory wells reach reservoirs that are all dry layers. From these four aspects, the lower limit of hydrocarbon accumulation corresponds with reservoir porosity of approximately 2.5%, permeability of approximately 0.01 × 10−3 µm2, pore throat radius of approximately 0.003–0.004 µm, and burial depth of approximately 8200 m. Determining the lower limit of hydrocarbon accumulation in the Kuqa Depression will contribute to predicting favourable exploration areas and evaluating effective resources. Copyright © 2016 John Wiley & Sons, Ltd.

Coexistence of contractional and extensional tectonics during the northern Apennines orogeny: the late Miocene out‐of‐sequence thrust in the Elba Island nappe stack

In the northern Tyrrhenian Sea, the Elba Island is one of the westernmost portions of the northern Apennine inner belt. One of its noteworthy features is the anomalous tectonic repetition of continental‐derived (Tuscan Unit) and oceanic‐derived (Ligurian units) thrust sheets, lately intruded by late Miocene granitoids. Moreover, in detail, a slice of strongly deformed Ligurian peridotites results tectonically sandwiched between two thrust sheets of Tuscan units. This tectonic setting results from a middle Miocene folding and thrusting of the Apenninic nappe stack with development of large‐scale antiform and out‐of‐sequence thrust. In central‐eastern Elba Island, the folding of an imbricate stack is bracketed between Langhian (middle Miocene) and Messinian (late Miocene). Consequently, the anomalous repetition of Tuscan and Ligurian units thrust sheets gives evidence of middle‐late Miocene shortening deformation post‐dating nappe stack and pre‐dating late Miocene–Pliocene granite emplacement. We suggest that the architecture of the Elba Island nappe stack documents the coexistence of early‐middle Miocene contractional and extensional tectonics in an overall convergent tectonic setting in the westernmost zone of northern Apennines. Extensional tectonics in the upper portion of the wedge, balancing transient gravitational instabilities due to over‐thickened conditions, were followed by a renewal of contractional deformation leading to development of large‐scale out‐of‐sequence thrust responsible for inversion of the stack order. Copyright © 2016 John Wiley & Sons, Ltd.

A lower Viséan (Carboniferous) brachiopod fauna from the eastern Alborz Mountains, northern Iran, and its palaeobiogeographical implications

The brachiopods from the upper part of the Mobarak Formation at the Kiyasar section in the eastern Alborz Mountains, northern Iran, have been studied in detail. Fifteen species assigned to 13 genera of Strophomenida, Orthotetida, Orthida, Productida, Athyridida and Spiriferida are described. The fauna is determined as early Viséan in age based on the presence of Eomarginifera paucispinosa, Grandispirifer mylkensis and Unispirifer sp., and their associated foraminifers Uralodiscus, Paraarchaediscus, Glomodiscus and Endothyra, though taxa ranging through the Tournaisian–Viséan are also present. A comparison of its palaeogeographical affinities with other faunas show that the Alborz brachiopod fauna maintains strong Palaeotethyan affinities based on its general similarities with those of Eurasia, North America and Australia. This is suggestive of a prevailing oceanic current that circulated between the northern and southern shelf sea of the Palaeotethys Ocean, with which the warm‐water brachiopod taxa were widespread in the Palaeotethyan Realm through the early Viséan. Copyright © 2016 John Wiley & Sons, Ltd.

Synorogenic copper mineralization during the Alpine–Himalayan orogeny in the Zafarghand copper exploration district, Central Iran: petrogrography, geochemistry and alteration thermometry

The Zafarghand porphyry copper exploration district is located in the NE of Isfahan (Central Iran), about 15 km SW of the city of Ardestan. This area consists of Eocene extrusive and Miocene intrusive rocks. The Eocene (ca. 56 Ma) units include andesites, dacites and rhyodacites spatially associated with pyroclastic materials (ignimbrites and pyroclastic tuff breccias), which are intruded by a Miocene (26–19 Ma) microgranular I‐type granodiorite. The entire igneous complex is intersected by NW–SE to E–W‐trending andesitic and dacitic dykes and N–S‐trending andesitic basalt dykes. Both the Miocene granodiorite and Eocene extrusives have undergone several hydrothermal alteration stages: (i) potassic, (ii) phyllic, (iii) argillic and (iv) propylitic alteration. The geochemical data of these formations are consistent with (i) the modal mineralogy derived from petrographic analysis of the samples and (ii) the regional tectonic setting of the samples having been emplaced in an active subduction‐related continental volcanic arc setting during the Zagros Orogeny. Copper mineralization in the Zafarghand area shows an average grade of 0.4% Cu, 200 ppm Mo, 0.2 ppm Ag and 0.01 g/t Au and a maximum age of 21 Ma, which implies that copper mineralization was synorogenic, coeval with the Alpine–Himalayan orogeny in this region. Copper mineralization occurs as chalcopyrite and bornite minerals as disseminated and stockwork mineralization associated with remobilized late epithermal vein mineralization. Chlorite geothermometry for the potassic and propylitic alteration zones yields a temperature range of 361–471 and 196–266 °C, respectively, which implies copper mineralization in a cooling synorogenic magmatic arc environment. Chlorite grains in these alteration zones indicate a classification of chlorites mainly as ripidolite, pycnochlorite, brunsvigite and diabantite. Copyright © 2016 John Wiley & Sons, Ltd.

Cenozoic lacustrine stromatolites from the southern margin of the Junggar Basin, NW China and adjacent areas: indicators for palaeoclimatic and tectonic evolution

Lacustrine stromatolites provide high‐resolution and sensitive palaeoenvironmental and palaeoclimatic records of their respective drainage basins and beyond. A well‐developed stromatolite bed was discovered within the clastic deposits of the Early Miocene Taxihe Formation in the southern margin of the Junggar Basin, north of the Tian Shan Mountains. Their sedimentological features indicate a near‐shore stable environment of a shrinking lake. The study of the stable oxygen and carbon isotopes in this stromatolite bed revealed that δ18O V‐PDB values range from −11.3‰ to −5. 6‰ and those of δ13C V‐PDB from −1.8‰ to 1.7‰. Lower values −10. 2‰ and −8. 2‰, respectively, were determined from nearby lacustrine limestones of Oligocene age. The co‐variation and overall increasing trend of the δ18O and δ13C values indicate deposition in a closed basin during a period of aridification. This environmental change can be related to the major uplift of the Tian Shan Mountains, the southern border of the Junggar Basin, during the Early Miocene. Several δ18O and δ13C records of lacustrine stromatolites ranging in age from Oligocene to Middle Miocene were reported: from localities in the Northern Tibetan Plateau and western Qaidam Basin. The similarity of the individual stable isotope records in stromatolites from Northwest China, indicate an underlying regional cause. These stromatolites were deposited during the period of major uplift of the Tibetan Plateau and the Tian Shan Mountains. It is therefore concluded that the stromatolite records provide local snapshots of the effect of this major process on atmospheric precipitation and environmental conditions in Northwest China. Copyright © 2016 John Wiley & Sons, Ltd.