Reflectivity template,a quantitative intercept-gradient AVO analysis to study gas hydrate resources – A case study of Iranian deep sea sediments

Presence of gas hydrate and free gas in Iranian part of Makran accretionary prism changes the elastic properties of unconsolidated sediments and produces sharp bottom simulating reflectors (BSRs) which are observed on the 2-D seismic data. Different methods have been applied to estimate the gas hydrate and free gas saturations in marine sediments based on seismic measurements. Most of these methods are based on relating the elastic properties to the hydrate and free gas saturations and remotely estimating their concentration. In this regard, using the effective medium theory (EMT) which was developed for different modes of hydrate distribution is more considered among other rock physics theories. The main concern about saturation estimations based on EMT is that the velocities of the hydrate-bearing sediments primarily depend on how they are distributed within the pore space. Therefore, understanding the modes of hydrate distribution (at least cementing or non-cementing modes) is necessary to decrease the estimation uncertainties.The first intention of paper is to investigate amplitude variation versus offset (AVO) analysis of BSR to determine the hydrate distribution modes. The results from the probable saturation revealed that if the hydrate cements the sediment grains, BSR would show the AVO class IV and if hydrate does not cement the sediment grains, then BSR would show either the AVO class II or class III depending on the free gas saturation just beneath the BSR. The second intention of paper is to introduce some templates called reflectivity templates (RTs) for quantitative study of hydrate resources. These templates are provided based on the EMT to quantify the hydrate and free gas near the BSR. Validation of this approach by synthetic data showed that a reliable quantification could be achieved by intercept-gradient RTs, only if these attributes are determined with a high accuracy and good assumptions are made about the mineralogical composition and porosity of the unconsolidated host sediments. The results of this approach applied to a 2-D marine pre-stack time migrated seismic line showed that less than 10% of the gas hydrate accumulated near to the BSR in anticlinal-ridge type structure of Iranian deep sea sediments. The free gas saturation near to the BSR by assuming a homogeneous distribution was less than 3% and by assuming patchy distribution was about 3–10%.     

Monocyclic alkanes in Ordovician organic matter

The major compounds in the C₁₅₊ branched/cyclic alkane fractions of two Ordovician oils (Pine Unit and Midland Farms oils) and an immature Ordovician rock (Guttenberg Oil Rock) are monocyclics. One series of these compounds was identified as n-alkylcyclohexanes and another tentatively identified as methyl-n-alkylcyclohexanes. The carbon number distribution of these compounds resembled those of the n-alkanes found in these samples with an odd carbon number predominance. It is suggested that the monocyclic alkanes, in these Ordovician samples, may be principally derived from the cyclisation of straight-chain algal fatty acids, by mechanisms that involve decarboxylation. However, there is evidence from the m/z 97 fragmentograms of these samples, to indicate that methyl-n-alkylcyclohexanes can also be derived from fatty acids by a less preferred mechanism that does not involve decarboxylation or from other precursors.In addition, a sample of kerogen from the Guttenberg Oil Rock was hydrously pyrolysed. The saturated hydrocarbon products of this experiment showed very similar distributions (including monocyclics) to those observed in the Ordovician oils. This suggests that although the oils and the Guttenberg Oil Rock come from widely differing geographical locations, their precursor (algal derived) organic matter was very similar.     

Acoustic emissions associated with the formation of fracture sets in sandstone under polyaxial stress conditions‡

A polyaxial (true‐triaxial) stress‐loading system, developed originally for determining all nine components of P‐ and S‐wave velocities and attenuation and fluid permeability for 50.8 mm‐side cubic rock specimens tested to failure, has been modified to permit the measurement of acoustic emission events associated with the failure process. Results are reported for Crosland Hill sandstone tested to failure under loading conditions leading to the formation of sets of aligned microcracks, achieved by maintaining the minor principal stress at a low value while increasing the two other principal stresses until failure of the rock. An ultrasonic survey associated with the test has been employed to map the transversely‐isotropic velocity structure created by through‐going parallel fractures resulting from the sets of aligned microcracks. This velocity structure has then been employed to locate acoustic emission events recorded during the test by four acoustic emission sensors located in each of the six specimen loading platens. A selection of acoustic emission events associated with one of the fractures has been processed for moment tensor analysis information, in order to determine the source type and orientation of microcracking as the fracture grows. The mechanisms indicate tensile behaviour during initial fracture propagation. Shear failure, however, appears to dominate as the fracture finally approaches the opposite face of the cubic specimen. The work presented here has, in part, led to the development of new rock testing systems and geophysical monitoring and processing technologies that will enable laboratory study of rock behaviour under conditions better resembling those experienced in situ.     

砂泥岩储层孔隙度和含水饱和度同步反演

基于岩石物理和地震反演理论,提出了一种同步反演储层孔隙度和含水饱和度的方法.以岩石物理为基础,建立了砂泥岩储层物性和弹性参数之间定量的关系-Simon模型,以贝叶斯理论为手段,结合不同类型的砂泥岩储层,建立了多信息联合约束的物性参数反演目标函数,并通过蒙特卡罗和遗传算法相结合的思路求解该目标函数,最终得到孔隙度和含水饱和度的同步反演结果.将该方法应用于河道砂和砂砾岩两种不同的砂泥岩储层中,孔隙度和含水饱和度数据的联合应用,进一步减少了储层预测的多解性,为石油地质综合研究提供了更加丰富准确的基础数据.

     

第二次全国土地调查底图生产项目质量控制的设计与研究

本文通过对第二次全国土地调查底图生产项目特点的分析和研究,根据工序质量管理的理论,为该项目设计了切实可行的质量控制方案。并对该方案的优缺点进行了分析,为该项目的质量控制工作提出了新思路和新方法。     

岩石构造对声发射统计特征的影响

利用一套最新型的全波形、宽动态声发射观测系统,对两种不同构造的花岗岩在三轴压缩变形实验中的声发射活动进行了对比研究。二者在声发射空间分布,声发射与体积扩容的对应关系,声发射频谱特征及频度能级统计参数等方面均有明显差异。这表明,岩石构造对声发射的基本统计特征有显著的控制作用。其原因在于岩石构造的差异造成变形方式和变形过程的不同     

The Time-Dependent Reduction of Sliding Cohesion due to Rock Bridges Along Discontinuities: A Fracture Mechanics Approach

Summary In this paper, a fracture mechanics model is developed to illustrate the importance of time-dependence for brittle fractured rock. In particular a model is developed for the time-dependent degradation of rock joint cohesion. Degradation of joint cohesion is modeled as the time-dependent breaking of intact patches or rock bridges along the joint surface. A fracture mechanics model is developed utilizing subcritical crack growth, which results in a closed-form solution for joint cohesion as a function of time. As an example, a rock block containing rock bridges subjected to plane sliding is analyzed. The cohesion is found to continually decrease, at first slowly and then more rapidly. At a particular value of time the cohesion reduces to value that results in slope instability. A second example is given where variations in some of the material parameters are assumed. A probabilistic slope analysis is conducted, and the probability of failure as a function of time is predicted. The probability of failure is found to increase with time, from an initial value of 5% to a value at 100 years of over 40%. These examples show the importance of being able to predict the time-dependent behavior of a rock mass containing discontinuities, even for relatively short-term rock structures. Received November 5, 2001; accepted July 24, 2002; Published online November 19, 2002     

谈地震科技期刊与读者需求的关系

为使从事地震科技期刊的编辑能正确地把握读者的心理，并实施有效的办刊对策，对地震科技期刊与读者的要求进行了研究，指出，作为地震科技期刊工作者，只有懂得读者的各种的心理现象和各种心理趋向，才能保证地震刊物的质量、生命力并得到读者的认同。而只有真正了解、认识和掌握各种读者的心理现象各心理趋向，才能更准确、更完美地扣住读者的心弦，使得编者与读者产生强烈的共鸣。     

Seismic tomography imaging of an unstable embankment

Seismic tomography imaging was employed to make a diagnosis and choose a remedy for an embankment supported by a retaining wall showing clear evidence of structural instability. The geometry and structural characteristics of the wall, the inside geometry of cracks, and the physical parameters of the underground materials were the primary objectives of the geophysical survey. Seismic data were acquired along two vertical sections each one delimited by the lines of the sources within an inclined borehole and of the receivers lying on the ground. For each section, a total of 744 travel times were inverted to obtain compressional-wave velocities on a regular rectangular grid of squared cells (1 m × 1 m) using an inversion algorithm which is based on the simultaneous iterative reconstruction technique (SIRT). Ultrasonic tests carried out in laboratory on intact specimens, together with other supplemental site information, improved tomography resolution, allowing global and node constraints that forced the solution of the inverse problem to match known boundary values. Tomography imaged two high-velocity zones separated by a curved-shape low-velocity zone. The first ones were found to be compatible with the concrete retaining wall and with the schist materials, ranging from highly weathered to intact schist, while the low-velocity zone was interpreted as filling materials and/or completely decomposed rock schist. The combination of seismic tomography and laboratory measurements allowed extrapolation of important parameters over a large volume of rock mass, otherwise only representative of small rock samples near boreholes. It also enabled engineering characterization of subsurface rock mass, providing useful and accurate information to design a remedy for the embankment.