Kerogen of Toarcian shales of the Paris Basin. A study of its maturation by flash pyrolysis techniques

A set of 14 samples—both extracted and unextracted from the Toarcian of the Paris Basin have been investigated using Curie-point pyrolysis-mass spectrometry and Curie-point pyrolysis gas chromatography mass spectrometry. The relative amount of n-alkenes and n-alkanes in the pyrolyzates increases with increasing maximum burial depth of the samples. Comparison of the pyrolysis data of extracted and unextracted samples shows that generation of hydrocarbons from the kerogen starts at a maximum burial depth of ~ 1000m. The increase of pristane and phytane in the extracts of the deeper samples is correlated with the gradual decrease of the characteristic pyrolysis product prist-1-ene. Three samples yield pyrolyzates with high relative amounts of aromatic compounds. This phenomenon probably reflects a different type of contributing organic matter and/or a different environment of sedimentation.     

Characterisation of ground motion recording stations in the Groningen gas field

The seismic hazard and risk analysis for the onshore Groningen gas field requires information about local soil properties, in particular shear-wave velocity (V_S). A fieldwork campaign was conducted at 18 surface accelerograph stations of the monitoring network. The subsurface in the region consists of unconsolidated sediments and is heterogeneous in composition and properties. A range of different methods was applied to acquire in situ V_S values to a target depth of at least 30 m. The techniques include seismic cone penetration tests (SCPT) with varying source offsets, multichannel analysis of surface waves (MASW) on Rayleigh waves with different processing approaches, microtremor array, cross-hole tomography and suspension P-S logging. The offset SCPT, cross-hole tomography and common midpoint cross-correlation (CMPcc) processing of MASW data all revealed lateral variations on length scales of several to tens of metres in this geological setting. SCPTs resulted in very detailed V_S profiles with depth, but represent point measurements in a heterogeneous environment. The MASW results represent V_S information on a larger spatial scale and smooth some of the heterogeneity encountered at the sites. The combination of MASW and SCPT proved to be a powerful and cost-effective approach in determining representative V_S profiles at the accelerograph station sites. The measured V_S profiles correspond well with the modelled profiles and they significantly enhance the ground motion model derivation. The similarity between the theoretical transfer function from the V_S profile and the observed amplification from vertical array stations is also excellent.     

Towards spatial geochemical modelling: Use of geographically weighted regression for mapping soil organic carbon contents in Ireland

It is challenging to perform spatial geochemical modelling due to the spatial heterogeneity features of geochemical variables. Meanwhile, high quality geochemical maps are needed for better environmental management. Soil organic C (SOC) distribution maps are required for improvements in soil management and for the estimation of C stocks at regional scales. This study investigates the use of a geographically weighted regression (GWR) method for the spatial modelling of SOC in Ireland. A total of 1310 samples of SOC data were extracted from the National Soil Database of Ireland. Environmental factors of rainfall, land cover and soil type were investigated and included as the independent variables to establish the GWR model. The GWR provided comparable and reasonable results with the other chosen methods of ordinary kriging (OK), inverse distance weighted (IDW) and multiple linear regression (MLR). The SOC map produced using the GWR model showed clear spatial patterns influenced by environmental factors and the smoothing effect of spatial interpolation was reduced. This study has demonstrated that GWR provides a promising method for spatial geochemical modelling of SOC and potentially other geochemical parameters.     

Mass wasting along the Labrador shelf margin: Submersible observations

A dissected valley system along the central portion of the Labrador upper slope has been investigated using geophysical and submersible observations. The complex slope is a result of mass wasting on a steep slope gradient which appears to be independent of processes on the bank top. Small scale slumping and creep, a result of biological activity and iceberg rafting, and large-scale debris flows occur presently below 750 m water depth. A recent debris flow in the valley was examined in detail. Degraded iceberg scour marks are the principal sea bed feature above 750 m water depth.     

Does quantifying antecedent flow conditions improve stream phosphorus export estimation?

A reliable and economical method for the estimation of nutrient export (e.g. phosphorus) in stream flow from catchments is necessary to quantify the impact of land use or land use change upon aquatic systems. The transport of phosphorus (P) from soil to water is known to impact negatively on water quality. A key observation from studies is that most P export occurs during high stream flow. However, it is not yet clear how flood-antecedent conditions affect the P export during flood events. In this study, the P loss from soil to water as represented by soluble reactive phosphorus (SRP) in stream waters from three different catchments, varying in land use, scale and location in Ireland was monitored over 1 year. This study examined the role of antecedent stream flow conditions on SRP export and identifies a catchment-specific relationship between SRP flood event load (EL) and a flow ratio (FR). The FR is defined as the ratio of the flood event volume (EV) to the pre-event volume (PEV). The latter is the cumulative flow volume for a number of days preceding the event. This PEV period was found to be longer (average 81 days) in the grassland catchments which were known to be saturated with soil P than in the forested catchments (average 21 days) with minimal soil P. This FR ratio is a measure of the antecedent hydrological state (wet or dry) of the catchment. For SRP for each catchment, a specific relationship between SRP EL and FR was identified. The annual SRP export was estimated, using this ratio and compared with the concentration/discharge (C/Q) method. The new flow ratio method was used with data from 12 flood events during the year to estimate an annual export of SRP. For the two grassland catchments in the study, using the FR method, we estimated an SRP export of 1.77 and 0.41 kg ha⁻¹ yr⁻¹. Using the C/Q method, for the same sites, our estimate of SRP export was 1.70 and 0.50 kg ha⁻¹ yr⁻¹ respectively. The C/Q method used SRP concentrations covering 40% of the year while the FR method used only 12 flood events covering less than 2% of the year. This new method which takes account of the antecedent flow state of the river is an alternative to and may be more promising than the traditional C/Q method, particularly when short duration or flood sampling of water quality is carried out.     

Reservoir induced seismicity—a new model

Deficiencies of different existing models on ‘Reservoir Induced Seismicity’ have been discussed and a new mathematical model, which enhances a better understanding of triggering mechanism in terms of changes in effective stresses, in situ stresses and water level variations, has been discussed in this paper. In the model fractured rock is simulated by a fluid-filled elastic material subject to Mohr–Coulomb failure criterion. The model has been found to be capable of responding effectively to site specific attributes. It can recognize and explain the phenomenon of time lag observed in several actual cases. It is also capable of simulating stabilization of rock–reservoir system after a period of activities that follow the initial stage of filling. One dimensional and two dimensional, isotropic and anisotropic cases have been analysed and the model predictions have been found to agree qualitatively with the field observations.     

北美典型页岩气藏岩石学特征、沉积环境和沉积模式及启示

北美典型页岩气藏赋存的泥页岩主要为细颗粒沉积,呈暗色或黑色薄层状或块状产出。页岩气储层无机矿物成分中硅质含量较高,含有黄铁矿、磷酸盐矿物(磷灰石)、钙质和黏土矿物。具有相对高有机质质量分数,代表了富有机质的缺氧的沉积环境。不含或者含较少的陆源碎屑输入。有机质类型以Ⅰ和Ⅱ型干酪根较为常见。生物化石碎片在页岩层中比较常见,化石碎屑的类型多样化。重点剖析了福特沃斯盆地Barnett页岩的沉积发育模式,福特沃斯盆地是一狭长的前陆盆地,主要沉积区离物源区较远,Barnett页岩沉积于较深的静水缺氧环境,沉积速度缓慢(饥饿性沉积),最终形成富含有机质的Barnett页岩。常见生物化石碎片,但缺少生物扰动遗迹,推测盆地中大部分的生物化石为外部输入的结果。上升流作用致使磷酸盐矿物(磷灰石)发育。北美典型页岩气藏的岩石学特征、沉积环境和福特沃斯盆地Barnett页岩沉积发育模式可以用来指导我国页岩气勘探,黔南坳陷下寒武统黑色高碳质页岩系、二叠系吴家坪组和四川广元—绵竹地区下寒武统泥页岩具有和北美典型页岩气藏可类比的岩石学特征、沉积环境和沉积模式,可作为页岩气勘探的优选区域。