Erosion Of Strata Research Articles

Multiple-Level Tectonic Control of Coalbed Methane Occurrence in the Huaibei Coalfield of Anhui Province, China

The Huaibei coalfield is an important coal base and one of the hot spots of coalbed methane development in China. Therefore, a detailed understanding of gas occurrence in the Huaibei coalfield is of great significance. This paper analyzes the gas occurrence from the perspective of multiple-level tectonic control, i.e., the regional tectonic level, the coalfield tectonic level, the mining area tectonic level, and the coal mine tectonic level. This study deduces that gas occurrence in the Huaibei coalfield is characterized by multiple-level tectonic control. At the regional level, the Huaibei coalfield is located in the southeast margin of the North China plate, affected by the tectonic evolution of the North China plate and by the evolution of the Dabie–Tanlu–Sulu orogenic belt. Therefore, the regional geological tectonic is complex, leading to the high gas content and serious gas hazard. At the coalfield level, gas occurrence in the Huaibei coalfield is controlled by east–west faults, NNE faults, and the Xuzhou–Suzhou arc nappe tectonic, which results in the highest gas occurrence in the Suxian mining area, followed by the Linhuan mining area and the Suixiao mining area, while the lowest amount of gas occurs in the Guoyang mining area. At the mining area level, considering the Suxian mining area as an example, the gas occurrence is controlled by the distance from the Tancheng–Lujiang fault zone and the intensity of tectonic compression, i.e., coal mine gas in the east is the highest, followed by coal mines in the south, while coal mine gas in the west is the lowest. At the coal mine level, gas occurrence is controlled by the buried depth of the coal seam, the tensional normal fault, magmatic activity, and uplift and erosion of strata. Finally, the findings of this study may help in the prevention of gas hazard and the exploration and development of coalbed methane in the Huaibei coalfield and other coalfields of similar geological characteristics.

The Use of Agricultural Waste Straw to Enhance Loess Shearing Behaviour: An Experimental Investigation

Strata erosion in northwest China has become an engineering concern as a result of overdevelopment of land. This issue is more distinct for loess soil than other soils since it is characterised by metastable microstructure, high porosity, and water sensitivity. This study explores the potential for the use of agricultural waste straw as a recycled reinforcement material to form the enhanced shearing behaviour towards preventing instability of the loess body. The stress‐strain relation and the pore pressure behaviour of Lantian loess and reinforced Lantian loess were studied using the conventional triaxial compression (CTC) stress path for three different confining pressures. Comparison with Jingyang loess and Delhi silt of similar relative fraction of silt to clay, sheared under the reduced triaxial compression (RTC) stress path and the reduced triaxial extension (RTE) stress paths, respectively, was conducted, with emphasis on strength uniqueness and critical state behaviour, to shed light on the effect of waste straw inclusions. The results indicate that the stress path in undrained compression and extension tests had a pronounced effect on the stress‐strain relation of the studied soils. Insertion of the waste straw in Lantian loess restrained the development of volumetric deformation, producing higher pore pressures than Lantian loess (unreinforced). This study explores an exciting potential for the use of agricultural waste straw to prevent instability of the loess body in hilly‐gullied regions of northwest China when subjected to quick surface thick fills.

Tectonic deformation features and petroleum geological significance in Yinggehai large strike-slip basin, South China Sea

Abstract The subtle strike-slip tectonic deformation and its relationship to deposition, overpressure and hydrocarbon migration were studied on the basis of systematic sorting of tectonic data. (1) The local T (tension) fractures derived from sinistral strike-slip process were formed before 10.5 Ma, large in number in the nose structure of the eastern slope, and reactivated episodically under the effect of fluid overpressure in the late stage, they served as dominant vertical hydrocarbon migration paths in the slope area of basin. (2) The dextral strike-slip extension was conducive to the increase of depositional rate and formation of regional under-compacted seal, and induced generation of local T fractures which triggered the development of diapirs; in turn, the development of diapirs made T fractures grow in size further. (3) The sinistral strike-slip process weakened after 10.5 Ma, causing tectonic movement characterized by compression in the north and rotational extension in the south, and the uplift and erosion of strata in Hanoi sag and a surge in clastics supply for south Yinggehai sag. Finally, migrating slope channelized submarine fans and superimposed basin floor fans were developed respectively on the asymmetrical east and west slopes of the Yinggehai sag.

Geological formation and geo-hazards during subway construction in Guangzhou

This paper summarises the stratigraphy, tectonic structure, geomorphology, hydrogeology and engineering geology in Guangzhou, China, and analyses the relevant geo-hazards and countermeasures during subway construction. The ages of the geological materials found in Guangzhou range from the Proterozoic era to the Cenozoic era. The bedrock is generally made up of sedimentary rock and weathered granite strata covered by soil with a thickness ranging from 2 to 20 m. The presence of underground karst caves, active faults and fissure water together with high piezometric levels leads to difficulties in shield tunnelling in Guangzhou. A case history, where two tunnels and one subway station on the Guangzhou Metro Line 7 were constructed, is referred to in order to analyse the mechanism between geological formations and geo-hazards. Three potential geo-hazards are listed: (1) water ingress and strata erosion, (2) ground surface settlement and (3) sinkholes; and their implications for shield tunnelling in Guangzhou are summarised and discussed. To mitigate the risk of these geo-hazards, a series of countermeasures, comprising (1) optimising TBM cutting tool design, and tunnelling parameters, (2) grouting to enhance watertightness and stiffness of strata and (3) injecting bentonite slurry and foam into the head chamber, are recommended in this paper.

Detrital muscovite geochronology and the Cretaceous tectonics of the inner Scotian Shelf, southeastern Canada 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin. 2Geological Survey of Canada Contribution 20120142.

Geochronology of detrital muscovite from the Lower Cretaceous of the central Scotian Basin has shown predominant supply of Meguma terrane detritus, including muscovite with Alleghanian (mid-Carboniferous to Permian) ages from the inner Scotian Shelf. In this study, 87 detrital muscovite grains from five wells in the eastern Scotian Basin, representing depositional ages from Tithonian to Early Albian, were dated by single-grain 40Ar/39Ar techniques. Previous work shows that the provenance of this sediment was different from wells in the central Scotian Basin. Compared with the central Scotian Basin, the muscovite age populations from the eastern Scotian Basin have more (20% versus 7%) pre-Devonian ages, derived from inboard terranes of the Appalachians. In both the central and eastern Scotian Basin, most old grains are found in the Tithonian and the Aptian–Albian, with fewer in the Valanginian and almost none in the Kimmeridgian and the Hauterivian–Barremian. These data suggest rapid unroofing of the Meguma terrane in the Kimmeridgian and Hauterivian–Barremian and greater supply of sediment from Newfoundland to the eastern Scotian Basin in the Tithonian and Aptian–Albian. Compared with the central Scotian Basin, which has most ages in the range 340–300 Ma, the muscovite age populations from the eastern Scotian Basin have a higher proportion of 420–340 Ma and in particular 300–260 Ma ages. Such late Alleghanian overprinting of argon ages is very rare on land in Nova Scotia, but in the Maritimes Basin, this was the time of uplift and erosion of strata.

Characteristics of Later Different Reformation and its Significance in Occurrence of Multi-Energy Deposits in Ordos Basin

Ordos Basin, rich in multi-energy resources of oil, gas and uranium, is a depression basin within the Late Triassic-to-early Cretaceous craton, which overlapps on the Paleozoic craton basin. The basin started its later reformation in the Late Cretaceous, which is of significant importance to energy deposits. Few studies have, however, been focused on ages of the reformation and its characteristics. Based on the analysis of the regional geological settings and Cenozoic sedimentary structures, combined with published and newly developed fission track ages, the reformation was divided into five stages with various characters, including 1) regional uniform uplift stage (Late Cretaceous to Paleocene); 2) different uplift and subsidence stage (Eocene to Oligocene); 3) different uplift stage of the east and west of the basin (Early-Middle Miocene); 4) structural reverses of the east and west of the basin (Late-Miocene to Pliocene); and 5) the formation of loessial plateau and the development of the Yellow River (Quaternary). These stages are consistent with development of the regional tectonic settings, which is closely related to the subduction of the Pacific Block to Euro-Asian Block and the collision of Indian Block to Euro-Asian Block. It has been indicated that there are good correlations among the petroleum formation, migration and the mineralization ages and the reformation ages. The later structural uplift, on the one hand, caused the strata erosion and the destruction of the petroleum pools and also the dissipation of the oil and gas. On the other hand, it decreased the pressure causing the escape of the gas dissolved in water, which is helpful to formation of the complementary gas resource and the secondary petroleum pools. Meanwhile, the later uplift and the petroleum migration to the northeast enabled the organic and inorganic interaction between reducing fluid and ore-bearing fluid, which is preferable for uranium deposits. The uplifted and eroded Mesozoic strata could be a new provenance, resulting in new mineralization and overlapped enrichment of uranium deposits.

Influence of tectonic uplift-erosion on formation pressure

The formation of abnormally low-pressure hydrocarbon reservoirs in petroliferous basins has a close relationship with tectonic uplift and the consequent erosion. In order to understand abnormally low-pressure reservoirs and to provide a scientific basis for exploration and development, we established, through numerical simulation and theoretical analysis, a set of equations for the formation pressure in a closed system influenced by uplift-erosion, discussed the relationship between the genesis of abnormal pressure and uplift-erosion, and put forward the concept of balance pressure (Pb). The results showed that abnormally high pressure coefficient may form when the current formation pressure was higher than Pb, and abnormally low pressure may form when the current formation pressure was lower than Pb. In the Santanghu Basin, the current formation pressure of abnormally low pressure reservoirs is lower than Pb, so tectonic uplift-erosion leads to the decrease of the pressure coefficient. There is a positive correlation between the pressure drop caused by the decrease of fluid temperature and the rebound of rock porosity and strata erosion. Calculation results indicated that the reservoir pressure of Jurassic strata in the Santanghu Basin was decreased by 11.6–17.1 MPa due to tectonic uplift-erosion during the Late Yanshanian period.

Near-salt deformation in La Popa basin, Mexico, and the northern Gulf of Mexico: A general model for passive diapirism

Strata adjacent to exposed diapirs in La Popa basin, northeastern Mexico, comprise stacked halokinetic sequences consisting of unconformity-bounded packages of thinned and rotated strata cut by radial faults. Deformation results from shallow drape folding over the flanks of the rising diapirs and not from deep drag folding in diapir-peripheral shear zones. Subsurface analogs from the Gulf of Mexico have diapir-flanking geometries ranging from similar, wide zones of upturned and thinned strata to undeformed, constant-thickness strata. Subhorizontal salt tongues display little subsalt deformation and thinning.We propose a general model for passive diapirism and flank deformation that includes (1) gradually varying salt-flow rates, (2) superposed episodic sedimentation that results in changing bathymetric relief, (3) rotation of near-surface strata as salt inflates relative to the adjacent basin, (4) failure and erosion of strata in the steepening bathymetric halo, and (5) bedding-parallel slip surfaces that converge on unconformities and onlap surfaces. A primary factor influencing flank geometries is the width of the bathymetric high extending beyond the diapir edge. This is largely dependent on the thickness of the halokinetic sequence onlapping the diapir, which in turn is controlled mostly by the interplay between salt inflation/deflation rates and sedimentation rates. Other factors include the amount of concurrent shortening, which produces a wider but less intense zone of deformation, and the position on the scarp of salt breakout and extrusion.Our model is important for exploration and production in diapir-flank and subsalt settings because of its implications for trap size and geometry, reservoir distribution, trap compartmentalization and pressure seals, and hydrocarbon charge. It can help in explaining complex and enigmatic well data and in better assessing risk in areas of poor seismic imaging.

Material transport from the near shore to the basinal environment in the southern Baltic Sea: II: Synthesis of data on origin and properties of material

The Pomeranian Bight (southern Baltic Sea) is a mixing zone between waters of the Baltic Proper and the river Oder, which drains a densely populated and highly industrialised catchment of central Europe. The bight is a nondepositional area, and all material produced in its water column, from erosion of strata at the seafloor and cliffs, and delivered by rivers, is transported near the seafloor to the depositional areas of the Arkona, Bornholm and Gdansk basins. In this contribution, we assess the origin, transformation and mass fluxes of material through the bight based on an integrated field study conducted in the period 1996–1998. The transport mechanism is by wave- and current-induced resuspension and settling cycles, which effectively enrich organic-rich material and associated substances (organic pollutants, heavy metals) in deeper water; the estimated transport time is less than 6 months. The phases in which the material is transported are suspended matter in the water column, a particle- and aggregate-rich benthic boundary layer of <1 m above the seafloor and a layer of fluffy material fed from the two other sources that covers the sandy near-shore sediments as a discrete phase; it collects up to 130 g m −2 of particulate material after quiescent periods lasting several days. It is easily resuspended at shear velocities around 5 cm s −1 and is recycled into the suspended matter and benthic boundary layer pools of material. In deeper waters (>20 m water depth), the fluffy layer is not readily distinguished from the underlying soft, organic-rich sediment and the change in physical and chemical properties is gradual. The organic matter passing through the coastal zone in the southern Baltic is unaffected by biological or chemical modifications in composition. We find no evidence for a preferential removal of nitrogen or phosphorus, even if the speciation of phosphorus changes from biological compounds to minerals. The compositional changes which we see, i.e., in the nitrogen isotopic composition and in trace metal concentrations, are mainly caused by dilution of the river signal. In the case of polyaromatic hydrocarbons (PAH), different solubilities and compound stabilities affect the concentrations as well and result in the enhanced accumulation of stable compounds in Arkona Basin. Seasonal changes are pronounced in the amount of freshly produced biomass, as is seen in phytoplankton pigments and their degradation products, but significant amounts of fresh biomass are swept out of the bay and supply the Arkona Basin benthic community with additional nutrition. An imbalance in carbon import and export emerges from mass balance calculations: 50,000 t of organic carbon per year may be exported, which cannot be accounted for by known sources in the river or the bight. We may exclude erosion of early Holocene peat exposures at the seafloor as a possible source, but cannot exclude either errors in our export estimate, or large-scale erosion of other organic carbon pools, for example, the stock of seaweed and its substrate in the Greifswalder Bodden.