沉积盆地地热史模拟方法及简要述评

沉积盆地的地热史模拟是目前盆地动态模拟的重要内容.盆地热史研究经历了定性、半定量及定量研究的过程,而且这一研究仍在继续深入.目前,盆地热史反演研究占据了主导地位.对目前一些常用的盆地地热史研究方法和手段作概要总结,并加以简要的评述,尤其着重介绍了近年发展起来的地热史反演方法——利用有机质镜质体反射率R_0资料和磷灰石裂变径迹资料反演沉积盆地的地热历史.     

Structural modifications of vitrinite and alginite concentrates during pyrolitic maturation at different heating rates. A combined infrared, C NMR and microscopical study

Immature vitrinite samples from a Miocene lignite seam of western Germany (H/C = 1.14, O/C = 0.41) and alginite concentrates from a Tasmanite deposit of Australia (H/C = 1.60, O/C = 0.10) were pyrolyzed in a stream of argon at heating rates of 0.1 and 2.0°C/min up to temperatures varying from 200 to 670°C. The solid maceral residues were subjected to elemental and microscopical analysis and studied by IR and ¹³C CP/MAS NMR spectroscopy with respect to structural modifications.The maximum pyrolytic weight loss amounts to 60% of the initial organic matter in the case of vitrinite and to 85% for alginite, the onset of degradation reactions being shifted to higher temperatures with increasing rate of heating. Both infrared and NMR spectra of the vitrinite samples indicate a rapid decomposition of the cellulose component upon heating whereas lignin related structures such as aromatic ether linkages remain remarkably stable. The main hydrocarbon release from vitrinite occurs at very early evolution stages (T_max = 296°C, R_m = 0.20% at 0.1°C/min; T_max = 337°C, R_m = 0.23 at 2.0°C/min). Hydrocarbon generation from alginite requires higher temperatures (T_max = 388 and 438°C) and is completed within a distinctly narrower temperature range.The pronounced increase of vitrinite reflectance between 350 and 670°C seems to be associated with a rather time-consuming reorganization of the residual organic material. The concomitant growth of polyaromatic units is illustrated by the increasing intensity ratio of the aromatic ring stretching vibration bands at 1600 and 1500 cm⁻¹. These reactions are moreover marked by increasing loss of phenolic oxygen and by increasing conversion of aliphatic carbon into fixed aromatic carbon.     

江汉盆地当阳向斜区主要不整合面剥蚀厚度

本文利用磷灰石裂变径迹、(U-Th Sm)/He及镜质体反射率Ro％等古温标方法综合分析了江汉盆地当阳向斜区主要不整合面剥蚀厚度.结果表明:发育于古近纪末期不整合面T1界面累积剥蚀厚度超过1000m,且局部正反转区域如谢家湾断褶带等则遭受更大规模的剥蚀,剥蚀厚度可能超过2000m,而发育于晚侏罗世一早白垩世的不整合面T11界面累积剥蚀厚度超过4000m,且主要是晚侏罗世早白垩世构造事件的结果,表明该期剥蚀量明显大于古近纪末T1界面剥蚀量;晚三叠世—侏罗纪期间,当阳地区发育前陆坳陷带,侏罗纪堆积体具有明显东厚西薄的楔形体特征,位于盆地东部的前渊区沉积厚度可超过5000m;包括现今三叠系和侏罗系出露区以及江陵凹陷局部断隆区在内的前白垩系在侏罗纪前陆坳陷带发育时期达到最大埋深和最高古温度,其Ro％主要是该期获得的;晚白垩世—古近纪发育的断陷盆地范围可能远比现今残留盆地分布广,江陵凹陷上白垩统—古近系厚度超过9000m,其中古近系可能超过7000m,而在河溶凹陷谢家湾断褶带古近系厚度可超过3300m.     

Characteristics of Hydrogen-rich Coals in Southern China: Implications from Organic Geochemistry and Carbon Isotopic Compositions

The organic geochemical characteristics of hydrogen-rich coal in southern China were investigated synthetically through organic geochemistry and carbon isotope analyses.The results showed that the hydrogen contents of the test samples were more than 5.0% and the H/C atomic ratios were between 0.76-1.06.Samples were found to be composed mostly of Type Ⅱ-Ⅲ℃ kerogen,consistent with good hydrocarbon-generation potential.The R_o(0.54-1.10%)and T_max(430-453℃)values imply that the hydrogen-rich coals were in low maturity to mature stages.Stable carbon isotopic ratios(δ¹³C_org)of the samples used varied from ?24.5‰ to ?23.4‰,the barkinite content ranging from 13.9% to 83.3%,indicating a predominantly terrestrial origin with marine influence during coal formation.Some organic geochemical parameters showed corresponding changes as the hydrogen content increased from 5.0% to 7.0%,however,the source inputs changed significantly when hydrogen content was greater than 6.0%.Terrestrial higher plants gradually become predominant within the coal-forming materials,whereas this dominant position is not apparent at lower hydrogen contents,which is attributable to the strong seawater effect during the hydrogen-rich coal formation process.     

Molecular characterisation of vitrinite in relation to natural hydrogen enrichment and depositional environment

The Py-GC/MS results of the study carried out on two groups of vitrinites (perhydrous and non-perhydrous) of different age and properties and on a trimaceralic coal associated with one of the perhydrous group are discussed. Such a study provides information about the effect of natural hydrogen enrichment on vitrinite structure at the molecular level. Moreover, the influence of the different conditions in the sedimentary environment on the chemical structure of the vitrinite is also discussed. This influence is inferred through differences in the distribution and relative amount of phenolic compounds found in the pyrolysates from two samples of two different coal-beds in the same basin but formed under different paleoenvironmental conditions. For vitrinites with a high H/C atomic ratio, despite having a strong perhydrous character, their pyrolysates exhibit the highly phenolic signature typical of lignin-derived material with only minor aliphatic compounds. Thus, the major chemical structural elements in these vitrinites are simple phenols with a high contribution of para alkyl-substituted derivatives. However, there is no parallel relationship between the evolution of the oxygenated functionalities and the reflectance values. From the results obtained a coalification pathway where hydrogenation processes predominate over thermal ones is proposed. The presence of resin-like substances and/or oils (which are two of the causes of natural hydrogen enrichment) in the molecular structure of vitrinites have, therefore, affected the normal evolution of the lignin and contributed to the special properties of this type of materials.     

Coalification pattern and thermal modelling of the Permo-Carboniferous Saar Basin (SW-Germany)

Palaeo-heat flow values and thicknesses of eroded Permo-Carboniferous sediments in the Saar Basin were evaluated using one dimensional thermal modelling techniques. Thermal, burial and erosion histories for 16 wells were calibrated by comparing measured and calculated vitrinite reflectance using the kinetic EASY%R_o algorithm and by comparing measured and calculated temperature data. On the basis of 37 wells, coalification maps were constructed revealing a syn-kinematic coalification pattern. Thermal maturity of the sediments can only be explained by deep burial and moderate heat flows during time of maximum burial, i.e., in the Permo-Carboniferous. Calculated heat flow data range between 50 and 75 mW/m², which implies a crustal thickness between 30 and 40 km during the time of maximum burial. These values are in accordance with the geodynamic setting of the basin. The influence of the Permo-Carboniferous volcanism on the palaeo-temperature distribution was overwhelmed by the subsequent deep burial. During Permian times, between 1800 and 3000 m of Permo-Carboniferous sediments were eroded. Different sedimentation and erosion histories are characteristic for anticlines and synclines, respectively.     

贝尔凹陷布达特群垂直裂缝垂向封闭性演化特征

通过岩心观察和成像测井资料分析,发现贝尔凹陷布达特群垂直裂缝较发育,其形成与分布受断裂控制。利用SiO₂或CaCO₃沉淀胶结所需要的镜质体反射率值,对贝尔凹陷布达特群垂直裂缝目前垂向封闭性进行了评价。凹陷大部分地区开启,仅在凹陷少部分地区封闭,封闭区在凹陷中部呈环带分布,在凹陷边部呈条带分布。贝尔凹陷布达特群垂直裂缝现今垂向封闭性差于地质历史时期垂向封闭性,油气保存条件变差,造成油气大量散失,这可能是目前贝尔凹陷能在布达特群找到一批油气藏的重要原因之一。     

Coal as a source rock for oil: a review

The geological debate about whether, and to what extent, humic coals have sourced oil is likely to continue for some time, despite some important advances in our knowledge of the processes involved. It is clear that not only liptinites, but also perhydrous vitrinites have the potential to generate hydrocarbon liquids in the course of natural coalification. Some liptinites, especially alginite, cutinite, and suberinite, contain a higher proportion of aliphatic moieties in their structure than other liptinites such as sporinite and resinite and are, therefore, more oil-prone. It is of potential value to be able to predict the several environments of deposition in which coals with high liptinite contents or containing perhydrous vitrinites may have been formed. Review of the distribution of oil-prone coals in time and space reveals that most are Jurassic–Tertiary with key examples from Australia, New Zealand, and Indonesia. Methods based both on experimental simulations and the examination of naturally matured samples have been used to determine the order of generation of hydrocarbons from different macerals. Results are not entirely consistent among the different approaches, and there is much overlap in the ranges of degradation, but it seems probable that in the natural environment vitrinites begin to generate early, followed by labile liptinites such as suberinite, then cutinite, sporinite, and, finally, alginite.Petroleum potential may be determined by experimental simulation of natural coalification or inferred through various micro-techniques, especially fluorescence and infrared (IR) spectroscopy, or bulk techniques such as elemental analysis and ¹³C NMR spectroscopy. The latter three techniques enable a measure of the polymethylene component of the coal, which now appears to be one of the best available approaches for determining petroleum potential. No method of experimental simulation of petroleum generation from coals is without criticism, and comparative results are highly variable. However, hydrous pyrolysis, confined pyrolysis, and forms of open-system hydrous pyrolysis approach acceptable simulations.Whether, and to what degree generated liquid hydrocarbons are expelled, has long been the central problem in ‘oil from coal’ studies. The structure of vitrinite was believed until recently to contain an interconnected microporous network in which generated oil would be contained until an expulsion threshold was attained. Recent studies show the pores are not interconnected. Combined with a dynamic model of pore generation, it now seems that expulsion of hydrocarbons is best explained by activated diffusion of molecules to maceral boundaries and ultimately by cleats and fractures to coal seam boundaries. The main reason for poor expulsion is the adsorption of oil on the organic macromolecule, which may be overcome (1) if coals are thin and interbedded with clastic sediments, or (2) if the coals are very hydrogen-rich and generate large quantities of oil.The existence of oil in vitrinite is attested to by solvent extractions, fluorescence properties, and by microscopic observations of oil and bitumen. Experimental simulation of expulsion of oil from coals has only recently been attempted. The relative timing of release of generated CO₂ and CH₄ could have considerable importance in promoting the expulsion of liquid hydrocarbons but the mechanism is unclear. As it is universally agreed that dispersed organic matter (DOM) in some shales readily generates and expels petroleum, it is curious that few consistent geochemical differences have been found between coal macerals and DOM in interbedded shales.Unambiguous evidence of expulsion from coals is limited, and in particular only a few commercial oil discoveries can be confidently correlated to coals. These include Upper Cretaceous Fruitland Formation coals in the USA, from which oil is produced; New Zealand Tertiary coals; and Middle Jurassic coals from the Danish North Sea. It is likely that coals have at least contributed to significant oil discoveries in the Gippsland Basin, Australia; in the Turpan Basin, China; and in the Kutei and Ardjuna basins in Indonesia, but this remains unproven. Early reports that early Jurassic coals in mid-Norway were a major source of the reservoired oils have been shown to be inaccurate.None of the proposed ‘rules of thumb’ for generation or expulsion of petroleum from coals seem particularly robust. Decisions on whether a particular coal is likely to have been an active source for oil should consider all available geological and geochemical information. The assumptions made in computational models should be well understood as it is likely with new understandings of processes involved that some of these assumptions will be difficult to sustain.     

Ellipsoid estimation in coal reflectance anisotropy

The procedure of fitting an ellipsoid to vitrinite reflectance anisotropy is described. Several authors on this subject use incorrect fitting formulae. The correct formulae are given and a least- squares procedure is developed to give confidence regions for the principal reflectances and their orientations. A FORTRAN program is offered that computes the principal reflectances using the correct formulae. A test is constructed for whether the principal reflectance orientations are the same at two locations. Because of the ellipsoid shape, the reflectance anisotropy can be compared to the strain ellipsoid. As an example, the methods are used to compare two coal blocks from an openpit mine in Alberta, Canada. The test shows that the principal reflectances of the two blocks have different orientations, indicating that the orientation of the principal strain axes is different at the two sites.