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1、Well Log Interpretation:Petrophysical Techniques,Earth & Environmental Science University of Texas at Arlington,Interpretation Techniques,Most of the Interpretation techniques have already been introduced so this chapter is a summary and in introduction to the use of facies maps based on well loging
2、.,Interpretation Techniques:Lithology,The following logs and cross plots are used and modern logging software often computes and displays the results automatically:Gamma RayNeutron-DensityNeutron-SonicDensity-SonicM-Nrmaa Dtmaarmaa-Umaa,Interpretation Techniques: Lithology,The lithology cross plots
3、may give conflicting results especially when the sonic log is used in carbonates with vuggy porosity. The sonic log does not see widely spaced holes, so it underestimates porosity and the Dtmaa variable, and overestimates the “M” variable when vugs are present.,Interpretation Techniques: Lithology,E
4、xample Lithology Logs,Interpretation Techniques: Lithology,Example Lithology Logs,Interpretation Techniques,The previous examples show significant disparity in the lower interval, so vuggy porosity in carbonates is probably present. In those circumstances, the neutron-density and rmaa - Umaa data ar
5、e the most reliable.,Interpretation Techniques: Clean Zones (shale free),Both the SP and Gamma Ray log can be used to map “clean” sediments, which generally have higher permeability and porosity.,Interpretation Techniques: Clean Zones (shale free),The SP response is depressed by shale, thin beds and
6、 hydrocarbons. SP alpha mapping can be used to determine clean zones, assuming a lower response is produced only by shale.,Interpretation Techniques: Clean Zones (shale free),The first step is to compute the Static SP (SSP), which is the ideal SP response for clean, thick rocks.,Interpretation Techn
7、iques: Clean Zones (shale free),Using the shale baseline of the SP log, plot a line showing the value of SSP. In the example SSP=-130: left dashed line on log.,Interpretation Techniques: Clean Zones (shale free),Pick some % (alpha) of the SSP to be the shale cutoff and draw a line on the log (the ex
8、ample has two alpha values, 75% and 50%, or -98 and -65 respectively).,Interpretation Techniques: Clean Zones (shale free),Measure the thickness of the “clean” interval where the SP curve cuts the alpha line (either 27 for the 75% alpha or 45 for the 50% alpha),Interpretation Techniques: Clean Zones
9、 (shale free),Measure the alpha thickness in all the wells and contour. The idea is to look for trends to drill additional production wells.,Interpretation Techniques: Clean Zones (shale free),Of course thin beds and hydrocarbons depress the SP log so the thick alphas could be where hydrocarbons are
10、 absent and thin alphas where they are present.,Interpretation Techniques: Clean Zones (shale free),Gamma Ray logs are better than SP logs for shale determination.First determine GRmax (100% shale) and GRmin (0% shale).From formulas in Ch. 3 or using Figure 3.2, determine IGR for Vsh=5% (or whatever
11、 Vsh cutoff value you want). In this case IGR5% = 0.10,Interpretation Techniques: Clean Zones (shale free),If IGR5% = 0.10, then,In the example, GR5% = 40 API units.,Interpretation Techniques: Clean Zones (shale free),In the example a line is drawn at 40 API units and the thickness of the interval w
12、here the GR log reads less than 40 is the thickness of the “clean” interval: in this case about 44.,Interpretation Techniques: Clean Zones (shale free),The “clean” thicknesses from a number of wells can then be contoured to determine the trend of the most promising zone.,Interpretation Techniques: C
13、reate your own crossplot,If petrographic data is available from cores and cuttings, and sedimentary facies can be identified in several wells, then the log response to those various facies can be identified using various cross plots.,Interpretation Techniques: Create your own crossplot,In the exampl
14、e, the various carbonate facies determined from petrographic data plot in specific regions on a Rt-FN crossplot. Zones in wells without petrographic data can be assigned a facies from the logs.,Interpretation Techniques: Create your own crossplot,Facies maps can be made from the log data to determine where additional wells should be drilled (or not).,
