用210Pb技术判断年轻石笋封闭性研究

通过测试采自桂林地区茅茅头大岩的两个相邻石笋DY-2和DY-3的现代滴水沉积物的²¹⁰Pb放射性活度发现: (1)处于同一洞穴相距约1.5 m, DY-2石笋初始210Pb放射性活度是DY-3石笋的3.0倍, 接取的现代滴水沉积物DY-2是DY-3的1.7倍, 210Pb放射性活度和现代滴水观测均证明DY-3石笋沉积速率大于DY-2石笋, 说明在相同地质背景和地理环境下, ²¹⁰Pb放射性活度可以作为沉积速率的一个判断工具; (2)两石笋²¹⁰Pb放射性活度剖面均呈现一定的波动性, 这与石笋沉积时存在晶间孔隙且与晶间孔隙的分布和大小、密集度有关, DY-3石笋²¹⁰Pb放射性活度剖面的紊乱与石笋晶间孔隙相互联通, 后来²¹⁰Pb与石笋纹层的²¹⁰Pb发生混合和交换有关; (3)处于非封闭系统的石笋氧同位素是否发生同位素的分馏还需进一步研究。     

A double-porosity model for a fractured aquifer with non-Darcian flow in fractures

Abstract

Non-Darcian flow in a finite fractured confined aquifer is studied. A stream bounds the aquifer at one side and an impervious stratum at the other. The aquifer consists of fractures capable of transmitting water rapidly, and porous blocks which mainly store water. Unsteady flow in the aquifer due to a sudden rise in the stream level is analysed by the double-porosity conceptual model. Governing equations for the flow in fractures and blocks are developed using the continuity equation. The fluid velocity in fractures is often too high for the linear Darcian flow so that the governing equation for fracture flow is modified by Forcheimer's equation, which incorporates a nonlinear term. Governing equations are coupled by an interaction term that controls the quasi-steady-state fracture—block interflow. Governing equations are solved numerically by the Crank-Nicolson implicit scheme. The numerical results are compared to the analytical results for the same problem which assumes Darcian flow in both fractures and blocks. Numerical and analytical solutions give the same results when the Reynolds number is less than 0.1. The effect of nonlinearity on the flow appears when the Reynolds number is greater than 0.1. The higher the rate of flow from the stream to the aquifer, the higher the degree of nonlinearity. The effect of aquifer parameters on the flow is also investigated. The proposed model and its numerical solution provide a useful application of nonlinear flow models to fractured aquifers. It is possible to extend the model to different types of aquifer, as well as boundary conditions at the stream side. Time-dependent flow rates in the analysis of recession hydrographs could also be evaluated by this model.     

INTERPLANETARY DUST AND CARBONACEOUS METEORITES: CONSTRAINTS ON POROSITY, MINERALOGY AND CHEMISTRY OF METEORS FROM RUBBLE-PILE PLANETESIMALS

This paper discusses measured textures, porosity, chemical compositions and the minerals of interplanetary dust and meteorites from primitive planetesimals and how this information can be used to explain some of the observed physical and chemical properties of meteor phenomena.     

A coupled transport and reaction model for long column experiments simulating bank filtration

Within the scope of the interdisciplinary Natural and Artificial Systems for Recharge and Infiltration research project dealing with riverbank filtration processes at the Berlin water works, a semi‐technical column experiment has been ongoing since January 2003. Here, a 30 m long soil column is infiltrated with surface water sampled from Lake Tegel (Berlin, Germany) under saturated flow conditions. Changes in pore water hydrochemistry sampled on 21 non‐equidistant distributed points are verified by coupled transport and reaction modelling. The objective of reactive transport modelling was to identify the main biogeochemical processes within the soil column during the flushing experiment as a conceptual model for riverbank filtration. Modelling was done with a combination of MATLAB and PHREEQC. The main processes identified are: (1) biogeochemical degradation due to interaction of natural surface water with the soil matrix; (2) continuous dissolution of refractory air bubbles from the soil column matrix. Copyright © 2006 John Wiley & Sons, Ltd.     

济阳坳陷陡坡带深层砂砾岩体次生孔隙成因机制探讨

从岩芯、薄片、扫描电镜以及FMI成像测井等资料分析发现,济阳坳陷陡坡带深层砂砾岩体储层次生孔隙发育,大量发育的次生孔隙不仅增大了储层的有效储集空间,更重要的是改变了储层的孔隙结构,使储层孔喉增大,大大提高了储层的渗透性能。研究认为长石、岩屑、碳酸盐和硫酸盐等酸性组分的溶蚀以及构造运动是深层次生孔隙形成的主导因素。垂向上存在两个次生孔隙发育带,对应深度范围从上至下依次为3450～3850 m和4075～4550 m。其主要形成机理有：①有机质热演化过程中产生的酸性流体的溶蚀作用;②去石膏化和去白云化以及硫酸盐热化学作用;③异常高压加速有机质热演化和对已形成孔隙的保护作用;④构造作用、脱水收缩作用等。     

Shallow-burial dolomite cement: a major component of many ancient sucrosic dolomites

Dolomite cement is a significant and widespread component of Phanerozoic sucrosic dolomites. Cements in dolomites that were never deeply buried are limpid, have planar faces (non‐saddle forms), often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores. Five samples of sucrosic dolomites, interpreted as having had mostly lime‐mudstone or wackestone precursors in four carbonate aquifers, provide insights into the abundance of planar cements in sucrosic dolomites. Such cement comprises 11% to 45% (32% mean) of peritidal to sub‐tidal dolomites on an outcrop in the Edwards aquifer (Early Cretaceous) of central Texas; 19% to 33% (25% mean) of ramp dolomites in the Hawthorn Group (Oligo‐Miocene) and 50% to 70% in shelf dolomites of the Avon Park Formation (Eocene) in the Upper Floridan aquifer of sub‐surface peninsular Florida; 18% to 45% (32+% mean) of sub‐tidal shelf dolomites in quarry sections of the Burlington‐Keokuk Formation (Early Mississippian) in south‐eastern Iowa; and 18% to 76% (50% mean) in shallow cores and outcrops of outer‐shelf dolomites from the Gambier Limestone (Oligo‐Miocene) of South Australia. Backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening, pore‐space reduction, induration and general ‘maturation’ of these dolomites. Most pre‐Holocene dolomites are multiphase crystalline rocks composed of: (i) seed crystals or ‘cores’; (ii) crystal cortices that concentrically enlarged the cores; and (iii) free‐space, syntaxial precipitates of limpid cement around the crystals. Remaining CaCO₃ grains and micrite can be replaced by dolomite, but typically they are dissolved between stages (ii) and (iii), creating systems of intercrystal and mouldic pores typical of sucrosic dolomites. Networks of cement overgrowths, aided by water‐filled pore systems under hydrostatic to lithostatic pressure, are judged to slow or prevent compaction in sucrosic dolomites. It can be argued that cortex growth involves both replacement of CaCO₃ particles and microcementation of their interparticle pores. This interpretation, and the abundance of cements in so many dolomites, would obviate the controversy over the volumetrics of ‘replacement dolomitization’. Limpid, planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters, at low temperatures (<30 to 35 °C) and shallow burial depths (<100 m), in water‐saturated networks of dolomite ‘silt’ and ‘sand’. Cements in many dolomites in island and continental–aquifer systems appear to result from event‐driven processes related to sea‐level highstands. Cementation events can follow ‘replacement dolomitization’ events by time intervals ranging from geologically ‘instantaneous’ to tens of million years.     

克拉苏构造带盐下超深层储层裂缝组合模式及分布规律

克拉苏构造带盐下超深层储层属于裂缝性低孔砂岩储层,构造裂缝普遍发育,分布复杂且对单井产量影响较大。目前针对该地区储层裂缝的研究已投入大量的工作,主要体现在裂缝的描述、表征和预测等方面,但由于该地区地震资料分辨率低、岩心资料少、成像测井多解性强,裂缝的研究受到一定程度的限制,生产中仅应用了裂缝密度、开度等统计数据。笔者综合野外露头、岩心和测井等资料,从裂缝组合的角度出发,系统开展了该地区储层裂缝及其组合特征研究。发现构造裂缝存在主次之分,主裂缝规模相对较大,一般垂直于地层,多为张性或张剪性,次级裂缝一般是主裂缝的伴生缝。裂缝之间存在多种接触关系,主要表现为"T"、"λ"、"y"、"τ"字等形状。依据裂缝之间的排列接触关系,建立了该地区储层裂缝四种组合模式,即"平行状"、"扫帚状"、"调节状"、"雁列状"。结合各裂缝组合成因的差异和断背斜形成过程中的构造演化过程,划分了裂缝组合平面三类成因区,即调节成因区、褶皱成因区、断裂成因区,与之对应的是高产井部署的一类区、二类区、三类区。     

DEM modelling of cone penetration tests in a double-porosity crushable granular material

A three-dimensional discrete element model is used to investigate the effect of grain crushing on the tip resistance measured by cone penetration tests (CPT) in calibration chambers. To do that a discrete analogue of pumice sand, a very crushable microporous granular material, is created. The particles of the discrete model are endowed with size-dependent internal porosity and crushing resistance. A simplified Hertz–Mindlin elasto-frictional model is used for contact interaction. The model has 6 material parameters that are calibrated using one oedometer test and analogies with similar geomaterials. The calibration is validated reproducing other element tests. To fill a calibration chamber capable of containing a realistic sized CPT the discrete analogue is up-scaled by a factor of 25. CPT is then performed at two different densities and three different confinement pressures. Cone tip resistance in the crushable material is practically insensitive to initial density, as had been observed in previous physical experiments. The same CPT series is repeated but now particle crushing is disabled. The ratios of cone tip resistance between the two types of simulation are in good agreement with previous experimental comparisons of hard and crushable soils. Microscale exploration of the models indicates that crushing disrupts the buttressing effect of chamber walls on the cone.     

Diagenesis of the Triassic Yanchang Formation Tight Sandstone Reservoir in the Xifeng–Ansai Area of Ordos Basin and its Porosity Evolution

This work aims to reveal the evolution of the porosity in the Triassic Yanchang Formation tight sandstone reservoir of the Xifeng–Ansai area of Ordos Basin. Based on destructive diagenesis(compaction and cementation) and constructive diagenesis(dissolution) of sandstone reservoirs, this study analyzed the diagenesis characteristics of the tight sandstone reservoirs in this area, and discussed the relationship between sandstone diagenesis and porosity evolution in combination with present porosity profile characteristics of sandstone reservoir. The effect simulation principle was employed for the mathematical derivation and simulation of the evolution of porosity in the Yanchang Formation tight sandstone reservoirs. The result shows that compaction always occurs in tight sandstone reservoirs in the Yanchang Formation, and cementation occurs when the burial depth increases to a certain value and remains ever since. Dissolution occurs only at a certain stage of the evolution with window features. In the corresponding present porosity profile, diagenesis is characterized by segmentation. From the shallow to the deep, compaction, compaction, cementation and dissolution, compaction and cementation occur successively. Therefore, the evolution of sandstone porosity can be divided into normal compaction section, acidification and incremental porosity section, and normal compaction section after dissolution. The results show that the evolution of sandstone porosity can be decomposed into porosity reduction model and porosity increase model. The superposition of the two models at the same depth in the three stages or in the same geological time can constitute the evolution simulation of the total porosity in sandstone reservoirs. By simulating the evolution of sandstone reservoir porosity of the eighth member in Xifeng area and the sixth member in Ansai area, it shows that they are similar in the evolution process and trend. The difference is caused by the regional uplift or subsidence and burial depth.     

Determination of groundwater solute transport parameters in finite element modelling using tracer injection and withdrawal testing data

The groundwater tracer injection and withdrawal tests are often carried out for the determination of aquifer solute transport parameters. However, the parameter analyses encounter a great difficulty due to the radial flow nature and the variability of the temporal boundary conditions. An adaptive methodology for the determination of groundwater solute transport parameters using tracer injection and withdrawal test data had been developed and illustrated through an actual case. The methodology includes the treatment of the tracer boundary condition at the tracer injection well, the normalization of tracer concentration, the groundwater solute transport finite element modelling and the method of least squares to optimize the parameters. An application of this methodology was carried out in a field test in the South of Hanoi city. The tested aquifer is Pleistocene aquifer, which is a main aquifer and has been providing domestic water supply to the city since the French time. Effective porosity of 0.31, longitudinal dispersivity of 2.2 m, and hydrodynamic dispersion coefficients from D = 220 m²/d right outside the pumping well screen to D =15.8 m²/d right outside the tracer injection well screen have been obtained for the aquifer at the test site. The minimal sum of squares of the differences between the observed and model normalized tracer concentration is 0.00119, which is corresponding to the average absolute difference between observed and model normalized concentrations of 0.035 5 (while 1 is the worst and 0 is the best fit).