EXERCISE 3
Objectives
When you have finished this exercise you will be able to:
- Correlate well logs using logs for more Spontaneous Potential and Resistivity wells than in Exercise 2 .
- Subdivide the sedimentary section characterized by the well logs on the basis of the prominent correlatable surfaces (TS, mfs and SB) you can identify.
- Using these surfaces identify the parasequences within the sedimentary section characterized by the well logs and interpret the history of their evolving depositional settings.
- Describe the role of relative sea level in forming the resulting depositional settings utilizing the report on the geologic setting of the La Pascua Formation, the introduction to the sequence stratigraphy of the La Pascua Formation and the eustatic chart.
Data
- A basemap is provided containing 22 wells (W-1 - W-22) which are to be correlated using the Spontaneous Potential and Resistivity logs. These well logs are flattened on the top of a major transgressive surface (TS) on the silt marking the contact between the La Pascua below and the overlying Roblecito Formation.
-
The wells are divided into three cross sections (X-SEC-A, X-SEC-B, X-SEC-C). Each of these cross sections and the basemap can viewed on electronic media that include PC, Notebook, Tablet, or Pad. Power Point drawing tools are an effective and easy way to handle the objectives of the exercise and a means for collective viewing of results in class. Click on red box for more details. Exercises can also be printed from the .pdf format, reassembled and taped to aid in the correlation (PRINTABLE-X-SEC-A, PRINTABLE-X-SEC-B, PRINTABLE-X-SEC-C).
- View and/or print a smaller version of these files (VIEW-X-SEC-A, VIEW-X-SEC-B, VIEW-X-SEC-C).
- A report describing the high-frequency (fourth-order cycles) sequence stratigraphy and regional geology of this region is provided. A sea-level curve for this region is also provided.
- A set of 18 previously interpreted wells (the phantom well of the maps and the data sheet) is also provided to enable the construction of sand thickness isopachs for each of the intervals (PRINTABLE-PHANTOM). Please refer to the second basemap for the location of all the wells.
Methods
- Utilize the Gulf Coast slip-slide method to help you match the log sections and identify the same geologic events on the various wells.
- Color sands yellow and shales green.
- Correlate using the upper common (dashed) marker as your basic datum.
- For each parasequence identify the TSs (transgressive surfaces) and then correlate these on all the well logs provided. These transgressive correlation surfaces used in all the sets of exercises extend across the area penetrated by the wells, except where they are interrupted by the incision of local channels. These surfaces cap silty horizons that are equated with surfaces of transgression (TS) that formed at wave base, and above, when the sea floor was reworked just following a sea level low.
- Click on the thumbnail below to view the movie that demonstrates the technique that first uses the transgressive surfaces to build a framework of parasequences from these correlated surfaces and then infills the other surfaces between these as outlined below!
| Technique tying well logs to interpret clastic sedimentation on coast with valley incision. |
 |
 |
 |
Click on links above to view YouTube and QuickTime movies of the output of the identification of log character to correlated and interpret the logs. The best way to view the YouTube movie frames is to first click on the pause symbol on the control bar and then use the mouse to grab the thumb (red circle) on the red seekbar (this tracks the progress of the movie) and move the thumb back and forth to chosen frames and so gain a better understanding of the evolving sedimentary geometries.
In contrast the best way to view the QuickTime movies is to control their motion using the arrow keys on your keyboard. The left arrow will step the film back and the right arrow will step it forward
- Normally radioactive peaks on gamma ray logs, associated with mfs, are more extensive and would have been better correlation surfaces than the transgressive surfaces but gamma ray logs were not available for much of the Guarico Sub basin.
- As in the movie above you should use geologic logic to infer the location of the maximum flooding surface (mfs) and correlate these on all the well logs.
- As shown in the film make a cross-section first correlating the prominent silts for all the wells and then correlating the sands.
- Pick base of massive sands and correlate these from well to well when they occur on other logs in other wells. Identify the sequence boundaries (SB) at base of the massive sands and correlate these from well to well. Identify all system tracts, including the incised valley, on all well logs.
- For each parasequence (bounded by mfs's) estimate and record the net sand (in feet). Correlations should be made across the entire area delineated by all the cross sections.
- Tie the stratigraphic dip and two stratigraphic strike sections and build a the correlation framework for the E-logs.
- These correlations have been tied into existing biostratigraphic and palynologic markers to bracket the age of the intervals and help interpret the depositional setting of the sediments and the frequency of their cyclicity.
- Click on the thumbnail below to view the movie that tracks an interpretation of the character of the sedimentary fill of the Guarico basin during the deposition of the La Pascua Formation.
Click on links above to view YouTube and QuickTime movies of the output of the movie capturing the 3D perspective of the interpretation of the depositional setting from the well logs.
The best way to view the YouTube movie frames is to first click on the pause symbol on the control bar and then use the mouse to grab the thumb (red circle) on the red seekbar (this tracks the progress of the movie) and move the thumb back and forth to chosen frames and so gain a better understanding of the evolving sedimentary geometries.
In contrast the best way to view the QuickTime movies is to control their motion using the arrow keys on your keyboard and the left arrow will step the film back and the right arrow will step it forward.
- Contour the combined sand thicknesses from the spread sheet provided and the net sands you measured. You should be able to establish the orientation of the regional trends in the depositional systems you identify.
- Provide an overall analysis that describes your conclusions.
As with the earlier two exercises (Exercise 1 and Exercise 2) to help with your interpretation of the well logs you should view the movies and read the earlier sections related to the geologic setting of the La Pascua Formation and the introduction to the sequence stratigraphy of the La Pascua Formation. From these you will know something of the depositional setting of these rocks. You should combine the techniques outlined above with this understanding of the regional geology and your understanding of vertical and lateral facies relationships in near shore clastic settings (eg. shoreline, beach, stacked beaches, tidal flats, deltas) and Walther's Law. You should use these to build a depositional model and a sequence stratigraphic interpretation of the well log section.