Exercise 2 - SEPM Strata

Well Log Interpretations > High Frequency Clastic Seq Str > Exercise 2

EXERCISE 2

Objectives
When you have finished this exercise you will be able to:

Correlate Spontaneous Potential and resistivity logs well logs using more well logs than in Exercise 1.
Identify an incised valley (clastic sequence with incised valley or click here for a smaller-version).
Identify a sequence boundary.
Produce an isopach map of sand thickness.

Data

You are provided with a basemap which locates 8 wells (W-1, W-2 , W-3, W-4, W-5, W-6, W-7, W-8) that can be correlated using Spontaneous Potential and Resistivity logs. A pdf file containing all wells and a basemap can viewed on electronic media that include PC, Notebook, Tablet, or Pad, Exercises can interpreted using Power Point drawing tools as an effective and easy way to handle the objectives of the exercises and a means for collective viewing of results in class. Click on red box for more details.or printed and taped (PRINTABLE-EX-2). A reduced pdf file version can be also be viewed/and or printed (VIEW-EX-2). 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.

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 (view the movie to understand this).
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 while 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.
Thus 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).
Utilize the spread sheet provided and net sands to contour the combined data. You should be able to establish the orientation of the regional trends in the depositional systems you identify.
Click on the thumbnial 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.

Well logs reveal evolving clastic sequence with valley incision subdivided into systems tracts.

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.

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
The right arrow will step it forward

To help with your interpretation of the well logs you should both view the above movies and read the earlier sections related to the geologic setting of the La Pascua Formation and the introduction to well logs and 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.