Environmental and Exploration Geophysics II

1. Environmental and Exploration Geophysics II Traps and Prospects / Conversion to Depth / Complete the 3D Interpretation Workshop tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Tom Wilson, Department of Geology and Geography

2. copy .... To begin with, please copy the folder Golden-3 from the class common drive to your G:\drive. We’ll all be doing the same exercise today. Tom Wilson, Department of Geology and Geography

3. Reflection seismology unveils the subsurface for our inspection and interpretation Tom Wilson, Department of Geology and Geography

4. Essential ingredients needed to form hydrocarbon rich zones - source, reservoir, trap and seal Tom Wilson, Department of Geology and Geography

5. The explorationist at work Tom Wilson, Department of Geology and Geography

6. Gulf Coast (Golden and BEG) Play Sediments shed from the uplifted Sierra Madre Mountains pile up in coastal areas of the Rio Grande Embayment. The pull of gravity on this large mass of sediments caused faults to develop that accommodated gradual sliding or creep of large sediment laden blocks out into the Gulf of Mexico. Tom Wilson, Department of Geology and Geography

7. Deltas load the shelf with sediments and gravity takes over Sediments pile up in the embayment which slopes off into the Gulf of Mexico. Mass wasting of the shelf proceeded under the pull of gravity Tom Wilson, Department of Geology and Geography

8. Faults rise to the surface in the landward direction as the sediments take a sled ride into the Gulf. These faults accommodate extension at a slow (creeping) but steady pace. Time is always available in excess for the geologist. Tom Wilson, Department of Geology and Geography

9. As extension faults develop, strata collapse back into the fault plane and additional sediments fill the resulting void and additional faults dipping toward and away from the direction of movement – the synthetic and antithetic faults, respectively. Tom Wilson, Department of Geology and Geography

10. From Seismic to reservoir image http://www.gcmwenergy.com/seismic_line.htm Tom Wilson, Department of Geology and Geography

11. Seismic acquisition to subsurface imaging http://www.gcmwenergy.com/seismic_survey.htm Tom Wilson, Department of Geology and Geography

12. Note the roll-over into the glide zone, synthetic and antithetic faults Tom Wilson, Department of Geology and Geography

13. Traps Tom Wilson, Department of Geology and Geography

14. Complex traps and cap rock Tom Wilson, Department of Geology and Geography

15. Converting times to depth requires that you have velocity information. There are three different ways to come up with the velocities Depth = velocity * time • In general you will have depths to formation tops derived from your log interpretations • You will have travel time data from your seismic horizon interpretations & well surveys (checkshot and vertical seismic profile (VSP)). • The checkshot and VSP data allow you to create a time-depth curve which can be used independently to convert any time to a depth or alternativel convert any depth to a time. Tom Wilson, Department of Geology and Geography

16. Conversion from time to depth Log picks Horizon time picks TD Curves Tom Wilson, Department of Geology and Geography

17. Average velocity approach Average Velocity = (2 * Depth) / Two-way Time • Three methods we’ll use: • Apparent > 2*formation top depth/time from seismic horizon pick • Time surface> 2* depth (from TD table)/time from seismic horizon pick (depth is determined from the TD chart for given horizon time). • Formation top > 2*formation top depth/time from TD chart Tom Wilson, Department of Geology and Geography

18. Apparent Velocity /Inverse Distance to Power From the compute average velocity map dialog help window. The depth in this approach is taken from the log picks In a 3D interpretation, you are likely to have horizon time picks and well formation top picks. This is just one approach Tom Wilson, Department of Geology and Geography

19. Apparent Velocity /Inverse Distance to Power The low in the southeast is anomalous. Bring up crossline 140 and have a look. The travel time to the interpreted C38 reflection is much higher than that to the well pick. The denominator is large and we have a small average velocity Tom Wilson, Department of Geology and Geography

20. Tom Wilson, Department of Geology and Geography

21. Time surface approach (with depth from TD curve) Well #13 is a deviated well. For this well, the total vertical depth (TVD) is erroneously high. The measured depth (MD) may have been used. Since velocity = depth/time, the resulting velocity is too high in this area. Tom Wilson, Department of Geology and Geography

22. Velocity map obtained without well #13 Tom Wilson, Department of Geology and Geography

23. This depth converted map was constructed from the using the apparent velocity approach Tom Wilson, Department of Geology and Geography

24. Formation top approach (time from the TD curve) Tom Wilson, Department of Geology and Geography

25. Depth from Apparent velocity and Formation Top approaches Tom Wilson, Department of Geology and Geography

26. Depth conversion using time-surface approach Tom Wilson, Department of Geology and Geography

27. Depth Contour – two versions Tom Wilson, Department of Geology and Geography

28. Isochron We may not have time for this …. • Create time grid for each horizon & include your polygon set (i.e. GreenT or C38Time grids) • Convert them to depth using your favorite velocity models • Associate polygon sets with your grids • Tools > Calculators > Math on two maps • fine tune parameters and select one or the other polygon set Tom Wilson, Department of Geology and Geography

29. In the end you have to ask yourself if the maps make reasonable geological sense and whether you can present a convincing argument in support of your interpretation. Tom Wilson, Department of Geology and Geography

30. Petroleum geology of the north sea: basic concepts and recent advances by Glennie (1998) Tom Wilson, Department of Geology and Geography

31. Times from seismic interpretations The seismic pick on the event interpreted as the Rodby is 2.056 seconds. Note that the autopicking on the Rodby shown here was performed with little guidence just to help show where interesting faults and structures might be located and to help uncover predominant structural trends. Tom Wilson, Department of Geology and Geography

32. Depth pick on the Rodby is 2311 Tom Wilson, Department of Geology and Geography

33. We can obtain two-way travel time to that depth using the TD function. Tom Wilson, Department of Geology and Geography

34. In the time-depth chart, there is a value for the time at a depth of 2312.83 feet of 2.0706 seconds. We interpolate to find the time corresponding to Rodby depth of 2311m From the TD function we estimate the time of 2.0698 for the Rodby Tom Wilson, Department of Geology and Geography

35. From the TD function we can also estimate a depth of 2281.4 from the horizon pick time of 2.056 Tom Wilson, Department of Geology and Geography

36. Average Velocity = (2 * Depth) / Two-way Time Three methods: Apparent > 2*formation top depth (2*2311.02)/time from seismic horizon pick (2.056) = 2248.1m/s Time surface> 2* depth (from TD table = 2281.4m)/time from seismic horizon pick (2.056) = 2219.2 m/s Formation top > 2*formation top depth (2*2311.02)/time from TD chart (2.0698) = 2233.1m/s Tom Wilson, Department of Geology and Geography

37. Average Velocity = (2 * Depth) / Two-way Time The three methods yield similar results in this case. Apparent > 2248.1 m/s Time surface> 2219.2 m/s Formation top > 2233.1 m/s Tom Wilson, Department of Geology and Geography

38. Another potential prospect Tom Wilson, Department of Geology and Geography

39. Tom Wilson, Department of Geology and Geography

40. Cutting loose the 3D Autopick Tom Wilson, Department of Geology and Geography

41. Autotracking fails at locations interrupted by local structure. These may be areas to explore further. Tom Wilson, Department of Geology and Geography

42. Complete this workshop exercise. Tom Wilson, Department of Geology and Geography