A worldwide data set of more than 500 humic coals from the major coal-forming geological periods has been used to analyse the evolution in the remaining (Hydrogen Index, HI) and total (Quality Index, QI) generation potentials with increasing thermal maturity and the ‘effective oil window’ (‘oil expulsion window’). All samples describe HI and QI bands that are broad at low maturities and that gradually narrow with increasing maturity. The oil generation potential is completely exhausted at a vitrinite reflectance of 2.0–2.2%Ro or Tmax of 500–510 °C. The initial large variation in the generation potential is related to the original depositional conditions, particularly the degree of marine influence and the formation of hydrogen-enriched vitrinite, as suggested by increased sulphur and hydrogen contents. During initial thermal maturation the HI increases to a maximum value, HImax. Similarly, QI increases to a maximum value, QImax. This increase in HI and QI is related to the formation of an additional generation potential in the coal structure. The decline in QI with further maturation is indicating onset of initial oil expulsion, which precedes efficient expulsion. Liquid petroleum generation from humic coals is thus a complex, three-phase process: (i) onset of petroleum generation, (ii) petroleum build-up in the coal, and (iii) initial oil expulsion followed by efficient oil expulsion (corresponding to the effective oil window). Efficient oil expulsion is indicated by a decline in the Bitumen Index (BI) when plotted against vitrinite reflectance or Tmax. This means that in humic coals the vitrinite reflectance or Tmax values at which onset of petroleum generation occurs cannot be used to establish the start of the effective oil window. The start of the effective oil window occurs within the vitrinite reflectance range 0.85–1.05%Ro or Tmax range 440–455 °C and the oil window extends to 1.5–2.0%Ro or 470–510 °C. For general use, an effective oil window is proposed to occur from 0.85 to 1.7%Ro or from 440 to 490 °C. Specific ranges for HImax and the effective oil window can be defined for Cenozoic, Jurassic, Permian, and Carboniferous coals. Cenozoic coals reach the highest HImax values (220–370 mg HC/g TOC), and for the most oil-prone Cenozoic coals the effective oil window may possibly range from 0.65 to 2.0%Ro or 430 to 510 °C. In contrast, the most oil-prone Jurassic, Permian and Carboniferous coals reach the expulsion threshold at a vitrinite reflectance of 0.85–0.9%Ro or Tmax of 440–445 °C. 相似文献
The Bridport Sand Formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic‐carbonate shallow‐marine depositional system. At outcrop and in subsurface cores, conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces. The geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3D seismic data. Facies analysis suggests that the Bridport Sand Formation represents progradation of a low‐energy, siliciclastic shoreface dominated by storm‐event beds reworked by bioturbation. The shoreface sandstones form the upper part of a thick (up to 200 m), steep (2–3°), mud‐dominated slope that extends into the underlying Down Cliff Clay. Clinoform surfaces representing the shoreface‐slope system are grouped into progradational sets. Each set contains clinoform surfaces arranged in a downstepping, offlapping manner that indicates forced‐regressive progradation, which was punctuated by flooding surfaces that are expressed in core and well‐log data. In proximal locations, progradational shoreface sandstones (corresponding to a clinoform set) are truncated by conglomerate lags containing clasts of bored, reworked shoreface sandstones, which are interpreted as marking sequence boundaries. In medial locations, progradational clinoform sets are overlain across an erosion surface by thin (<5 m) bioclastic limestones that record siliciclastic‐sediment starvation during transgression. Near the basin margins, these limestones are locally thick (>10 m) and overlie conglomerate lags at sequence boundaries. Sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces, to form composite erosion surfaces. In distal locations, oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces. Over most of the Wessex Basin, clinoform sets (corresponding to high‐frequency sequences) are laterally offset, thus defining a low‐frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation. Aggradational stacking of high‐frequency sequences occurs in fault‐bounded depocentres which had higher rates of localized tectonic subsidence. 相似文献
We report here a multiphase mineral inclusion composed of quartz, plagioclase, K-feldspar, sapphirine, spinel, orthopyroxene, and biotite, in porphyroblastic garnet within a pelitic granulite from Rajapalaiyam in the Madurai Granulite Block, southern India. In this unique textural association, hitherto unreported in previous studies, sapphirine shows four occurrences: (1) as anhedral mineral between spinel and quartz (Spr-1), (2) subhedral to euhedral needles mantled by quartz (Spr-2), (3) subhedral to anhedral mineral in orthopyroxene, and (4) isolated inclusion with quartz (Spr-4). Spr-1, Spr-2, and Spr-4 show direct grain contact with quartz, providing evidence for ultrahigh-temperature (UHT) metamorphism at temperatures exceeding 1000 °C. Associated orthopyroxene shows high Mg/(Fe + Mg) ratio ( 0.75) and Al2O3 content (up to 9.6 wt.%), also suggesting T > 1050 °C and P > 10 kbar during peak metamorphism.
Coarse spinel (Spl-1) with irregular grain morphology and adjacent quartz grains are separated by thin films of Spr-1 and K-feldspar, suggesting that Spl-1 and quartz were in equilibrium before the stability of Spr-1 + quartz. This texture implies that the P–T conditions of the rock shifted from the stability field of spinel + quartz to sapphirine + quartz. Petrogenetic grid considerations based on available data from the FMAS system favour exhumation along a counterclockwise P–T trajectory. The irregular shape of the inclusion and chemistry of the inclusion minerals are markedly different from the matrix phases suggesting the possibility that the inclusion minerals could have equilibrated from cordierite-bearing silicate-melt pockets during the garnet growth at extreme UHT conditions. 相似文献
Compressional and shear wave velocities and attenuation measurements have been carried out in some of the borehole samples
of acidic, basic and intermediate granulites of Mahabalipuram, Tamil Nadu, India. The results have been obtained at ambient
conditions using ‘time-of-flight’ pulse transmission technique at 1.0 MHz frequency. The results show linear relationships
between velocity and density, and velocity and attenuation properties of the rocks. The acidic granulites show lower velocities
and higher attenuation than the intermediate and basic granulites. The average values of the Poisson’s ratio of acidic, intermediate
and basic granulites have been found to be 0.210, 0.241 and 0.279 respectively. The variations in velocities and attenuation
in these low porosity crystalline rocks are found to be strongly influenced by their mineral composition. The laboratory velocity
data (extrapolated to high pressure) of the present study and the published field velocity data from deep seismic sounding
studies indicate that these granulite facies rocks may belong to mid-crustal depths only. 相似文献
This paper analyses the factors which influence the presence or absence of tributary-junction fans in the Iberian Range, northern Spain. Two valleys were selected, both characterised by wide variations in lithology, altitude, land use and plant cover. Two groups of factors were studied: those related to the internal characteristics of the drainage basins, which particularly control sediment generation; and those related to the characteristics of the depositional area which control accommodation space and main river power. Among the internal factors, the development of alluvial fans was related to: (i) the capacity of the basin to yield large volumes of sediment, (ii) the occurrence of intense human pressure until recent times, a good indicator of sediment yield, and (iii) the capacity of the basin to quickly increase discharge during rainstorms (discharge density and torrentiality). It is suggested that the areas that were intensively cultivated in the past, and have therefore been affected by intense erosion, have played a decisive role on the development of alluvial fans. This would imply that many of these alluvial fans have a relatively recent origin, perhaps related to the beginning of a widespread deforestation. The basins without alluvial fans are characterised by relatively steep hillslope gradients (that is, slopes that never were subjected to historical cultivation), low drainage densities and dense forest and shrub cover, mostly coinciding with high altitude basins composed of quartzite and shale bedrocks. Regarding the external factors, the shape, size and longitudinal gradient of the main river to which the fans are tributary are the most relevant conditioning factors determining the development of alluvial fans. 相似文献