湖南弗拉阶地层的分布特征和页岩气有利区

湖南上泥盆统弗拉阶佘田桥组泥页岩是重要的页岩气勘探目的层。由于区域上具有岩相分异显著、变化快的特征,佘田桥组存在多个同期地层,容易造成引用和野外识别困难,严重制约其页岩气调查工作进程。通过梳理佘田桥组及其同期地层的历史沿革,明确和圈定了它们的分布区域和使用范围。基于佘田桥组的岩石组合特征、有机碳含量的区域变化和已有调查成果,提出受陆缘碎屑输入影响较强的新化-涟源、邵东地区为页岩气有利勘探区的设想,同时认为找准台盆范围是页岩气调查发现的关键。     

Effects of composition and pore structure on the reservoir gas capacity of Carboniferous shale from Qaidam Basin,China

Shale adsorption and breakthrough pressure are important indicators of shale gas development and key factors in evaluating the reservoir capacities of shales. In this study, geochemical tests, pore-structure tests, methane adsorption tests, and breakthrough-pressure tests were conducted on shales from the Carboniferous Hurleg Formation in eastern Qaidam Basin. The effects of the shale compositions and pore structures on the adsorption and breakthrough pressures were studied, and the reservoir capacities of the shales were evaluated by analyzing the shale adsorptions and sealing effects. The results indicate that the organic carbon content was only one of factors in affecting the adsorption capacity of the shale samples while the effect of the clay minerals was limited. Based on the positive correlation between the adsorption capacity and specific surface area of the shale, the specific surface area of the micropores can be used as an indicator to determine the adsorption capacity of shale. The micro-fracturing of brittle minerals, such as quartz, create a primary path for shale gas breakthrough, whereas the expansion of clay minerals with water greatly increases the breakthrough pressure in the shale samples. Methane adsorption tests showed that maximum methane adsorption for shale samples Z045 and S039 WAS 0.107 and 0.09655 mmol/g, respectively. The breakthrough pressure was 39.36 MPa for sample S039, maintained for 13 days throughout the experiment; however, no breakthrough was observed in sample Z045 when subjected to an injected pressure of 40 MPa for 26 days. This indicates that sample Z045, corresponding to a depth of 846.24 m, exhibited higher adsorption capacity and a better reservoir-sealing effect than sample S039 (498.4 m depth). This study provides useful information for future studies of Qaidam Basin shale gas exploration and development and for evaluation of shale quality.     

Urban seagrass: status of Posidonia oceanica facing the Genoa city waterfront (Italy) and implications for management

A system of five adjacent Posidonia oceanica meadows facing the waterfront of Genoa city (Ligurian Sea, NW Mediterranean) was investigated over different spatial scales (meters-kilometers) using three environmental indices: conservation index (CI), substitution index (SI) and phase-shift index (PSI). CI revealed differences mostly at large spatial scale, distinguishing the poor condition of the meadows closest to Genoa centre and harbour from the comparatively healthy condition of the farthest meadows. SI showed differences mostly at small spatial scale (i.e., within meadows), suggesting the influence of local factors in the re-colonisation of regressed meadows by the seagrass Cymodocea nodosa and/or the invasive alga Caulerpa racemosa. Mapping of PSI showed that the meadows closest to Genoa centre and harbour have undergone a nearly total phase shift and have no real potential for recovery: attempts to re-establish P. oceanica there might be a waste of time and money. On the contrary, the meadows farthest from Genoa centre and harbour showed a comparatively low level of phase shift and could still fully recover given specific management actions.     

Neogastroplites meets Metengonoceras: morphological response of an endemic hoplitid ammonite to a new invader in the mid-Cretaceous Mowry Sea of North America

High levels of morphological variability have been attributed to various ecological and developmental mechanisms, including selection for ecological generalists in an unstable environment, inherent developmental plasticity, and the absence of competitors in an empty ecosystem. The late Albian–early Cenomanian hoplitid ammonite Neogastroplites of the Mowry Sea of North America, notorious for its extreme levels of intraspecific variability, is used as a test case of the idea that a lack of competitors promotes variability. The group responded to the invasion of a second ammonite, the engonoceratid Metengonoceras, into the Mowry Sea by spreading into a previously unoccupied region of morphospace, suggesting possible ecological character displacement and a release from constructional constraints. However, morphological variability did not decrease, as would be predicted by the competition model. The variability of Neogastroplites might instead result from environmental instability or developmental flexibility.     

Relation between source rock properties and wireline log parameters: An example from Lower Jurassic Posidonia Shale, NW-Germany

The relationships between source rock quality and wireline log parameters were established for the Lower Jurassic Posidonia Shale, the most prolific petroleum source rock of Central Europe. Wireline logs used are the gamma ray spectrometry, density, sonic and various resistivity logs. The organic geochemical parameters considered include the organic carbon content, hydrogen index, and the hydrocarbon potential expressed by pyrolysis yield. The case history presented is based on a continuous core through the entire Posidonia Shale interval from a location where this source rock is immature. Care was taken to detect and to avoid samples and log values influenced by bore hole roughness and fractures. Fair correlations were observed only between uranium content from gamma ray spectrometry log and organic carbon content. No correlation was recognized between uranium content and kerogen type as defined by hydrogen indices. For various reasons density, sonic, and resistivity logs did not provide useful data in this study.Three rock units with irregular, high and low uranium contents could be differentiated. Possible causes for this tripartition are different environments during deposition and postdepositional redistribution of uranium during early diagenesis and in association with fractures.     

Organic geochemical characterization of the New Albany Shale group in the Illinois Basin

Benzene extractable aliphatic hydrocarbons from the New Albany Shale in the Illinois Basin were characterized by gas chromatography and mass spectrometry, and the total organic matter of the shale was characterized by solid state carbon-13 cross polarization magic angle spinning nuclear magnetic resonance. Core samples from a northwest-trending cross-section of the Illinois Basin were studied. Gas chromatography (GC) and gas chromatography-mass spectrometric analysis (GC/MS) data indicate a regional variation of the aliphatic composition of the shale extracts. A positive, linear relationship between the two ratios, pristane/n-C₁₇ and phytane/n-C₁₈, is indicated. The NMR results indicated that organic matter deposited in northwestern Illinois shale is relatively high in aliphatic hydrocarbon content while, in contrast, organic matter found in southeastern Illinois shale is relatively low in aliphatic hydrocarbon content. Our findings suggest that the organic variation of the shale is mainly due to the differences in thermal maturity of the shale organic matter and the use of pristane/n-C₁₇ ratio as a thermal parameter in the study of oil may be extended to the study of the ancient sediments.     

Biogeomorphology of the Mediterranean Posidonia oceanica seagrass meadows

Here we review the multiple interactions between the endemic Mediterranean seagrass, Posidonia oceanica, and coastal geomorphologic processes as an outstanding example of biogeomorphology, taking into account recent advances in the field. Seagrass meadows are among the most important elements for the functioning of marine coastal ecosystems, and represent a major focus for research and conservation. Being considered a priority habitat, P. oceanica meadows are protected by several European Union directives and national laws. In this paper we examine: the role of sedimentary features in controlling the development of the meadows; the interplay between P. oceanica leaf litter (i.e. beached necromass) cast ashore and erosional‐depositional processes on the beaches; the interactions between meadows and nearshore hydrodynamics, and; possible linkages between geomorphological features of the seafloor and the architecture of meadows. Finally, we provide perspectives for future research on P. oceanica and other Mediterranean seagrass meadows in a biogeomorphological context with specific reference to climate change. Copyright © 2016 John Wiley & Sons, Ltd.     

From marine bands to hybrid flows: Sedimentology of a Mississippian black shale

Organic-rich mudstones have long been of interest as conventional and unconventional source rocks and are an important organic carbon sink. Yet the processes that deposited organic-rich muds in epicontinental seaways are poorly understood, partly because few modern analogues exist. This study investigates the processes that transported and deposited sediment and organic matter through part of the Bowland Shale Formation, from the Mississippian Rheic–Tethys seaway. Field to micron-scale sedimentological analysis reveals a heterogeneous succession of carbonate-rich, siliceous, and siliciclastic, argillaceous muds. Deposition of these facies at basinal and slope locations was moderated by progradation of the nearby Pendle delta system, fourth-order eustatic sea-level fluctuation and localized block and basin tectonism. Marine transgressions deposited bioclastic ‘marine band’ (hemi)pelagic packages. These include abundant euhaline macrofaunal tests, and phosphatic concretions of organic matter and radiolarian tests interpreted as faecal pellets sourced from a productive water column. Lens-rich (lenticular) mudstones, hybrid, debrite and turbidite beds successively overlie marine band packages and suggest reducing basin accommodation promoted sediment deposition via laminar and hybrid flows sourced from the basin margins. Mud lenses in lenticular mudstones lack organic linings and bioclasts and are equant in early-cemented lenses and in plan-view, and are largest and most abundant in mudstones overlying marine band packages. Thus, lenses likely represent partially consolidated mud clasts that were scoured and transported in bedload from the shelf or proximal slope, as a ‘shelf to basin’ conveyor, during periods of reduced basin accommodation. Candidate in situ microbial mats in strongly lenticular mudstones, and as rip-up fragments in the down-dip hybrid beds, suggest that these were potentially key biostabilizers of mud. Deltaic mud export was fast, despite the intrabasinal complexity, likely an order of magnitude higher than similar successions deposited in North America. Epicontinental basins remotely linked to delta systems were therefore capable of rapidly accumulating both sediment and organic matter.     

Seagrass meadows at the extreme of environmental tolerance: the case of Posidonia oceanica in a semi-enclosed coastal lagoon

Atoll‐like structures of the endemic Mediterranean seagrass Posidonia oceanica were encountered in the innermost area of the Stagnone di Marsala, a semi‐enclosed coastal lagoon along the western coasts of Sicily. The area is characterized by limited water exchange with the open sea and by a marked seasonal variation of water salinity and temperature, reaching beyond the theoretical tolerance limits of the species. In the present study we determined the genetic composition of the atoll‐like structures, as well as the growth performance and flowering rate of these stands. We also assessed whether and to what extent the atoll‐like structures are genetically isolated from plants growing in meadows outside the lagoon. For this purpose we utilized 13 microsatellite markers to genotype single shoots sampled inside and outside the lagoon. Lepidochronological analyses were performed on the same shoots to determine the annual rhizome growth rate, the number of leaves and the inflorescences formed as an estimate of growth‐ and reproductive performance over the years. The innermost area of the lagoon showed a lower number of alleles, a lower percentage of polymorphic loci, a lower clonal diversity, but higher heterozygosis excess with respect to the other areas analysed. Spatial autocorrelation was here significant, up to slightly below 300 m. Shoots collected in the atolls exhibited a 25% lower vertical growth rate and 16% lower leaf formation in comparison to those in open‐sea meadows. No flowering events were recorded during 24 years of investigation, whereas inflorescences were observed frequently in meadows outside the lagoon. Results from F_st and factorial correspondence analysis confirmed the expected genetic isolation of the confined atolls with respect to the meadows outside the lagoon and revealed limited gene flow within the lagoon itself. Apparently, the enclosed system of the Stagnone lagoon is genetically isolated, with a possible selection of genotypes adapted to persistent stressful conditions, consistent with reduced growth and lack of flowering events.