Summary We have collected 3D tensor Controlled Source-and Radio Magnetotelluric data at a quick-clay site in southwestern Sweden with the aim to image the geological structures that have given rise to retrogressive landslides in the area. The 3D resistivity models show layering within the glacial and postglacial sediments including marine clay, quick-clay and coarser grain sands. Because of the smoothing regularization used in the inversion, the lateral and depth extents of the quick-clays cannot be determined just by using the resistivity model. The depth to the top of the resistive crystalline bedrock is to some extent uncertain. However, the geometry of the sediments and bedrock seen in the 3D resistivity model correlates reasonably well with the results from the high resolution reflection seismic data collected along the same lines in the site.
Summary Joint integration of seismic and electromagnetic (EM) data can improve the quality of characterization of hydrocarbon reservoirs because these two types of measurement are based on fundamentally different physics so they are sensitive to different reservoir properties. In this work, we extend ed this advantage in reservoir monitoring by jointly integrating time-lapse seismic and EM data. One of the possible ways of combining two data sources is statistical integration, where the joint and conditional probability distribution function (PDF) of seismic and EM data, as well as reservoir properties, need to be modeled. However, there is a critical issue of the scale differences between well logs, seismic and EM data since each of them represents different volumes of a reservoir. Our goal is to establish a workflow to statistically integrate time-lapse seismic and EM data by developing proper and efficient methods to upscale the joint PDFs considering the scale differences between well logs, seismic and EM data.
Scheiber-Enslin, Stephanie (Council for Geoscience, Pretoria) | Webb, Susan (University of the Witwatersrand) | Ebbing, Jörg (Geological Survey of Norway) | Eberle, Detlef (Council for Geoscience, Pretoria)
Recent interest in the Karoo basin of South Africa has been sparked by the possibility of extensive shale gas reserves. Here we present an integrated study of historical reflection and refraction seismic (industry and academic), borehole, MT and regional gravity and magnetic data, with the goal of evaluating the shale gas potential. The reflection seismic data, constrained by 5 deep boreholes and MT data are used to map the contact between the base of the Karoo (Dwyka Group) and the underlying basement. This provides an important constraint on the thickness of the Karoo Supergroup (~5.5 km). By constraining the gravity data with the structure known from the reflection seismic and density data from boreholes, this contact can be extended to regions where the seismic data are sparse and suggests that the basin in the region near 24° longitude is thinner than on either side. The flexure of the basin also suggests that at the time of deposition, the Kaapvaal Craton was stiff, preventing a deep basin from developing further northwards on-craton. Additionally, the magnetic and borehole data highlight pervasive Jurassic dolerite sills and dykes. These sills and dykes can be highly fractured at surface, but at depth are possibly impermeable, providing seals and traps. They are also an important source of heat and need to be accounted for when evaluating the gas potential. The borehole data indicate sills can occur at any depth throughout the basin. The near surface sills are easily mapped with the coarse national magnetic data and single flight line modeling suggests that the edges of deeper sills can be delineated with higher resolution magnetic data. This approach indicates the importance of integrated studies to evaluating the shale gas potential in the Karoo.
Summary Hydraulic fracturing is a process that generates (micro)seismic events due to fluid and stress interactions with the existing rock. In this paper, we examine different approaches to assessing the dynamics of the microseismicity in a reservoir during a fracture treatment: b-value analysis, stress release as estimated through apparent stress and static stress drop, and fracture complexity as assessed through Seismic Moment Tensor Inversion (SMTI). Introduction The generation of microseismic events during hydraulic fracturing has provided a number of avenues for interpretation of fracturing processes and the efficiency of the stimulation. For example, the length, width and height of the microseismic event cloud is frequently used to validate models of fracture propagation and the azimuth of the fracture is taken as an estimate of SHmax. These simple attributes of the microseismic events are merely touching on the detail and depth of analysis that microseismic events can provide.
Pre-salt targets in Brazil’s Santos Basin have become the focus of much exploration in recent years, creating a need for higher resolution images over these complex structures. Variable-depth streamer acquisition is an emerging technology in the Santos Basin that can increase the usable bandwidth in both the low and high frequency ranges, gain stronger low frequency penetration, and reduce acquisition related noise. We examine the benefits of variable-depth streamer data for producing high-resolution pre-salt images and the potential for higher resolution velocity models in the Santos Basin. We utilize a real field data set to demonstrate that variable-depth acquisition, combined with advanced processing techniques, provides improvements to the pre-salt imaging in this region.
The purpose of this work is to evaluate the geo-potentials of graptolitic argillite in northern Estonia from the point of geology and even more from economic perspective, i.e. to understand spatial features of the shale layers including changes in thickness and compositional dynamics. The Late Cambrian to Ordovician crustal section of Estonia contains kukersite oil shale as well as graptolite argillite. Kukersite has been predominantly mined because of its higher calorific value (9–11 MJ/kg). On the other hand, graptolite argillite has a lower calorific value (4.2–6.7 MJ/kg); consequently it has not been mined as a source of fuel. However, graptolite argillite is characterized by significant amounts of U, Mo, Pb and other metals; thus can be treated as metal ore and two-fold energy source including U and shale oil. Sequel to the identified objectives, spatial analysis tools in ArcGIS have been used to integrate geologic, geochemical and environmental information to evaluate, better understand and spatially analyse the graptolite argillite in northern Estonia. A relatively easy pancake geology as induced from the bedrock geological map of Estonia indicates more or less even pattern of resource distribution but spatial analysis of available well data revealed that relatively high contents of microelements: U (>200 ppm), Pb (> 200 ppm), Mo (> 300 ppm), Zn (> 200 ppm), V (> 1000 ppm), Th (> 11 ppm) and Ag (> 0.7 ppm) occur in northeastern Estonia. More so, thickness of graptolitic argillite ranges from 0–2 m in northeastern Estonia and reaches up to 8 m in the northwestern area. Depth of the upper and lower surfaces of graptolitic argillite in northern Estonia increases from the northeastern area with values from 0.1 m and 0.4 m, respectively to the northwestern area where it reaches 266.2 m. From the results of cost-benefit analysis, it can be adduced that the microelements in graptolitic argillite hold highly scientific as well as significant economic value and possibly form future energy/ore resources.
Water velocity is neither invariant in space nor in time. Therefore for surveys that last from days to months, the estimation of the node position is coupled with the inherent varying water velocities during that period. In this paper we describe a method to solve jointly for water velocities (varying with time), and position of all the nodes simultaneously.
We develop an efficient scheme of illumination analysis along a target horizon. With this scheme, we can calculate the Directional Illumination (DI) from the sources and the Acquisition Dip Response (ADR) along a target horizon in very short turnaround time. Therefore, it can be a useful tool to study the influence of the model (e.g. salt body) and the acquisition system (e.g. shot distribution and aperture size). The result can be a guide for acquisition design and model building. With the illumination map along the target horizon, it also is helpful for the interpretation in areas where the image amplitudes are not reliable. Here, we use the wave-equation based migration and local plane wave decomposition method to get the frequency domain illumination in the local angle domain. We pre-calculated and saved the angle domain Green’s function along the target horizon. These Green’s functions are reusable so that we can save a lot of computational and I/O cost. We use the 3D SEG/EAGE salt model and a real model example to demonstrate the validity of our method.
Yubao Liu*, Sam Zandong Sun and Di Wang, Lab for Integration of Geology and Geophysics (LIGG), State Key Lab for Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Bing Jing, Tarim Oilfield Company, CNPC Summary In Tarim Basin, diffraction waves caused by discontinuous geological objects such as dissolved caves and faults, are much developed, which sheds some light on a new way to detect favorable carbonate reservoirs. However, it's really challenging to separate diffraction from reflection. In this paper, a new 3D diffraction wavefield separation method in dip-angle domain based on Radon spectrum is proposed, which can effectively discriminate diffraction from reflection. This method is applied to 3D field seismic data in Tarim Basin, China. The result demonstrates that, using separated diffraction imaging, more accurate and precise information of dissolved caves and faults can be detected, which helps a lot to discover some potential reservoirs.
More interest is now paid on development of new methods or modification of traditional methods (Zhang and Claerbout, 2010; Wang and Peng, 2012; Zhou et al., 2012). Joint deconvolution is a receiver deghosting method especially suitable for variable-depth streamers. It uses both the normal image and the mirror image, which was generated using velocity information. The pioneer work by Soubaras already has exhibited great success when it was used for variable-depth streamers (Soubaras, 2010; Soubaras, 2012). In this paper we will show it could also be used for conventional fixed-depth streamer data, when combined with L1 norm optimization. We will discuss the causes of artifacts when applying joint convolution to fixed-depth streamer, and how we can minimize them. Two field data examples are given. One is an application on post-stack images; the other is an example on pre-migration gathers.