Métodos de processamento de dados para a imagem latente de sísmica 3D da subsuperfície vulcões: aplicações para o basalto de inundação de Tarim

Resumo

Sísmica de reflexão (3D) tridimensionais é um método poderoso para imagens de vulcões de subsuperfície. Usando dados de Sismológico 3D industriais da bacia de Tarim, ilustramos como extrair os peitoris e os conduítes de vulcões subsuperficiais de cubos de dados sísmicos.

Resumo

A morfologia e estrutura dos sistemas de encanamento podem fornecer informações importantes sobre a taxa de erupção e estilo dos campos de lava basáltica. A maneira mais poderosa para estudar geo-corpos subsuperficiais é usar imagem Sismológico industrial reflexão 3D. No entanto, estratégias de vulcões subsuperficial de imagem são muito diferentes dos reservatórios de petróleo e gás. Neste estudo, nós processamos cubos de dados sísmicos da bacia de Tarim, China norte, para ilustrar como Visualizar peitoris através de técnicas de processamento de opacidade e como os conduítes de imagem por tempo de corte. No primeiro caso, isolamos sondas por horizontes sísmicos marcando os contactos entre peitoris e encerra estratos, aplicando técnicas de renderização de opacidade para extrair peitoris do cubo sísmico. A morfologia resultante do peitoril detalhada mostra que a direção do fluxo é do centro de cúpula para o rim. O segundo cubo sísmico, usamos frações de tempo para as canalizações, da imagem que corresponde ao marcado descontinuidades dentro das rochas encasing. Um conjunto de frações de tempo obtidos em diferentes profundidades mostram que os basaltos de inundação de Tarim entrou em erupção de vulcões centrais, alimentados por separado como tubos conduítes.

Introdução

O objetivo da maioria dos projectos de imagem sísmicas industriais nas bacias sedimentares é explorar para reservatórios de hidrocarbonetos. Nos últimos anos, exploração de hidrocarbonetos se expandiu para bacias que contém grandes quantidades de rochas ígneas, porque muitos das bacias volcanogenic têm considerável óleo e reservatórios de gás. No entanto, por causa da interface de rochas ígneas nas bacias volcanogenic, processamento de dados sísmicos apresenta uma série de desafios induzida por várias invasões, tais como a transmissão de energia reduzido, atenuação intrínseca, efeitos de interferência, refração e dispersão¹. Portanto, empresas de campo de petróleo estão focando seus esforços na redução de um "impacto negativo" sísmica de imagem^2,3,4.

Ígneas corpos dentro de bacias sedimentares são facilmente identificados por duas imagens tridimensionais ou 3D sísmica de reflexão devido ao contraste de impedância acústica grande com o encasing rochas^1,5,6. Esse método pode fornecer imagens espetaculares de estruturas verticais e horizontais do encanamento vulcânica sistemas^{7,8,9,10,11,12,13}. No entanto, as estratégias de imagem subsuperficiais vulcões são muito diferentes de petróleo e gás explorações^8,14,15. Isto tem limitado o uso de dados sísmicos industriais em estudos de vulcões subsuperficiais, além de alguns cases de sucesso^10,15,16. Neste trabalho, relatamos os procedimentos detalhados de processamento de dados sísmicos, que são personalizados para a interpretação dos vulcões de subsuperfície. Nós processamos dois cubos sísmicos, TZ47 e YM2 (Figura 1), para mostrar como visualizar os corpos enterrados ígneas na inundação de Tarim basalto¹⁷.

Protocolo

NOTE: The data processing procedures include: synthetic seismogram calculation, synthetic-real seismic trace correlation, and geo-body extraction. Below are the step-by-step details of each procedure.

1. Calculation of Synthetic Seismogram

1. Calculate the acoustic impedance at each interval of the down-well logging curve.
   NOTE: Acoustic impedance is the product of 'seismic wave velocities' and 'density' (ρ*ν)). The data are often averaged to sampling intervals larger than 1 ft, in order to reduce the computation time and aliasing.
2. Calculate the reflection coefficients (R₀) at each interface by using the acoustic impedance calculation:
   
   where ν₁ and ν₂ are the averaged velocities of the layers below and above the interface, respectively; ρ₁ and ρ₂ are the corresponding averaged densities.
   1. If the well does not intersect the igneous bodies, use nearby wells that have intersected the target rocks to obtain the parameters (velocity, density, etc.).
3. Chose a wavelet that has an amplitude and phase spectrum similar to that of the nearby seismic data.
4. Convolve the synthetic wavelet with the reflection series for the entire well survey and generate a synthetic seismic trace. The final simulated seismic trace T(t) can be described by the convolutional model as below:
   
   where R₀(t) is the reflection coefficient, w(t) is the wavelet and n(t) is the noise.
5. If the frequency of the seismic data has large variations throughout the whole well, re-compute the synthetic seismic trace using a wavelet with a different phase and a dominant frequency at different depth intervals.
   1. Repeat the process if the match between the synthetic trace and the seismic data is not satisfactory.
6. Perform the calculation with the provided software (e.g., Petrel E&P Software Platform).
   1. Start the software. Select File | Open Project | and then select the demo research project tlm (users can select their own desired projects). The project should contain well data, wired log, well tops, seismic cube, and interpretation surface in the research area.
   2. Click on Home | Windows | 2D Windows | 3D Windows to open two display windows to show the data sets according to user's preference.
   3. In the "Wells Tree of Input Pane", right click the desired well. Open the Settings window of the well and select the Time tab to create a new time log. Select Velocity Function, then select DT data in the new time log. Click the OK button to close settings window. A new one-way time log is automatically created and will be shown in the "Wells Tree of Input Pane".
      NOTE: A one-way time log is a time-depth relationship of this well. Wired log domains can be transformed to time domains and be shown in the time domain window.
   4. Activate an existing 3D Window by clicking the displayed window. If there is no 3D Window displayed, create a new 3D Window by clicking Home | Windows | 3D Windows. Select TWT in the toolbar of the 3D Window to show the 3D Window in time domain.
   5. Select representative wired logs (such as 'GR', 'DT', or 'RT') in the Wells Tree to show them in the 3D Window; at the same time, select the seismic profile in the 'seismic' tree of the 'input' pane to show them in the same 3D Window.
   6. Use the Manipulate Plane tool in the toolbar of the 3D Window to adjust the location of the profile to intersect the well; the user will see that the wired log has been transformed to the time domain and displayed with the seismic profile in the same 3D window.
   7. Click Seismic Interpretation | Seismic Well Tie | Seismic Well Tie Process. Choose Integrated Seismic Well Tie in the type of study row, and add desired well in the Well row. Choose calibrated one-way time log as time-depth relationship in the TDR row of the input tab, choose seismic cube in the seismic row. Choose any log in the RC calculation method.
   8. Click Launch Wavelet Toolbox to create a Ricker wavelet to apply in this process. Click OK and a new well section window and synthetic seismogram display will be created.

2. Correlate the Synthetic Traces with the Real Seismic Reflectors

1. Use an automated correlation application, like Seismic Well Tie in the platform, to adapt the resulting synthetic trace to the vertical scale of the seismic section.
2. Adjust the synthetic seismogram to increase the overlapping of high amplitude reflectors of the synthetic trace and real trace.
3. Adjust the synthetic seismogram and the real trace repeatedly. When the overlapping trace reaches the maximum, the interpreter has reached the "best fits" between the obtained synthetic seismogram and real traces.
   1. Repeat the process until the correlations reach the desired level.
4. Perform the correlation with the provided software.
   1. Activate the window created in step 1.6.3, which is the one-way time log automatically created from the acoustic log.
      NOTE: This automatically created 'one-way time log' is not perfectly correlated with the real seismic reflectors. The users should calibrate the correlations between the one-way time log and the real seismic reflectors.
   2. To calibrate their correlations, choose a continual and representative reflector that is intersected by the well. Then manually adjust the depth of the well log. For example, to adjust the depth of the DT log, right click the One-Way Time Log in well tree | select the Calculator tool | then add a small time increment (for example, 10 ms) by typing 'DT=DT+10' in the input dialog of the Calculator tool.
   3. If the '10 ms' increment is too large or too small, change the increment to another time (can be negative value) in the 'calculator' tool. Check the correlation between the well log and the selected seismic horizon repeatedly and then adjust the time increment repeatedly, until the correlation is perfectly calibrated.

3. Extraction of Basaltic Sills

1. Pick 2 high-amplitude reflectors encasing the target sills.
   NOTE: Most intrusions are expressed in seismic data as tuned reflection packages, whereby the reflections from the upper and lower intrusion contacts cannot be distinguished. Tuning occurs when the vertical intrusion thickness is between λ/4 and λ/8 (λ is seismic wavelength)¹⁹. Therefore, sills are shown as a set of strong reflections in the seismic section, and their apparent thickness is false.
2. Extract probes between the horizons corresponding to the two high-amplitude reflectors.
   NOTE: There are different tools based on the rendering technique that can help the interpreters better visualize the targets, such as "box probes", "surfaces probes" and "well probes". However, for identification of contacts between the sills and encasing strata, the best tool is "surface probe". ("Surface probe, etc." are terms in 'Petrel' software. The software users should be familiar with these terms).
3. Remove the areas surrounding the geological objects of interest by changing the Voxel connectivity opacity threshold value. Set the default threshold value to 20%. The visualization method of "opacity rendering" is used here to display the result of the extraction of basaltic sills ( Figure 2C).
   NOTE: There are high amplitude reflections along the surface between igneous rock and sedimentary rock because of their significant difference in acoustic impedance. Make the low amplitude parts transparent to highlight the shape of the igneous bodies.
4. As the value for isolation can be higher than 20 - 30%, change the value with small increments to make sure all important igneous bodies are not lost; the larger the value, the higher the risk of losing the volume of the real igneous bodies.
5. Perform the operation with the provided software.
   1. Click the Seismic Interpretation pane, click Insert a Horizon Probe. A probe will be added in the geobody interpretation probes tree of input pane. Double click the added horizon probe and a pop-up window will appear.
   2. Click the Horizons tab in the pop-up window and choose two seismic surfaces that isolate the zone of sills. Click OK to apply the operation.
   3. Check the newly added probe in the geobody interpretation probes tree shown in the input pane. A seismic cube will then appear in the 3D window.
   4. Double click the probe and choose the Opacity tab. A histogram of seismic amplitude will be shown in the tab. Use the left mouse button to draw a line in the histogram to control the opacity of the seismic cube. The low amplitude parts of the tube should be invisible and the high amplitude parts will be left.
   5. Adjust the histogram repeatedly until the desired shape of the interested geobody is achieved.

4. Extraction of the Feeding Conduits

1. Choose continuous and high energy reflection horizons at different depths beneath the surface lava flow.
2. Do time slicing along the selected horizons, to find out discontinuities corresponding to the vertical conduits.
3. Adjust the Two-Way Time (TWT) repeatedly, to achieve the best imaging of the discontinuities of the conduits.
   NOTE: Seismic data cannot image vertical structures well, so better images from amplitude volumes and variance volumes are chosen by comparing clearness at different travel times.
4. Try different slicing techniques, and then choose which can better image the discontinuities.
   NOTE: Different tools can be used here, such as variance body slicing. Its theoretical basis is the similarity between each seismic section and adjacent seismic traces in the seismic data. Another tool, the variance cube, is a new data body processed by the conventional seismic data, which is helpful for the identification of changes in the structure and lithology, plane combination of the fault, etc.²⁰
5. Plot the slices at different travel times or depths into a 3D space.
6. Perform the operation with the provided software.
   1. Double click Volume Attributes in the geophysics tree of the processes pane. Check Structural Methods in the category column and Variance in the attribute column. Select the seismic cube to input box and adjust the other parameter in the parameter tab. For better reading performance, check the box in the realize column. A variance cube is created in the seismic tree of the input pane.
   2. Right click the variance cube and click Insert Time Slice Intersection to show more horizontal intersections in the 3D window. Use the Manipulate Plane tool in the toolbar of the 3D window to adjust the location of the slices to optimize the display of conduits.
   3. Right click the seismic amplitude cube and click Insert Time Slice Intersection to show more horizontal intersections in 3D window. Do the same operation as step 4.6.2 to adjust the location of the slices to optimize the display of conduits.

Resultados

Vamos demonstrar a utilidade das técnicas descritas acima, aplicando-lhes a 2 tipos de corpos de rochas ígneas, peitoris horizontais e verticais condutos vulcânicos. Extração dos peitoris é realizada usando a técnica de processamento opaco, e interpretação do conduto vulcânico é realizada usando a técnica de fatiamento.

Extração de peitoris

Poços de perfura...

Discussão

Aqui demonstramos 2 métodos para ilustrar a morfologia e estrutura do sistema de encanamento de vulcões basálticas enterrados; um processamento de opacidade, o outro é a hora de corte.

O método de renderização de opacidade é apropriado para geo-corpos que possuem contínua e perto de interfaces horizontais com os estratos encasing. Com este método, pode-se extrair a morfologia 3D de lóbulos de magma. Normalmente, as direções de fluxo devem estar no eixo longo dos lóbulos magma. Ta...

Materiais

Name	Company	Catalog Number	Comments
The Petrel E&P software platform	Schlumberger		software version:2014