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EXERCISE
3
Objectives
When you have finished this exercise you will be able to:
Data
- You are provided
with a basemap 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 can 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).
- You can also view
and/or print a smaller version of these files as .jpg 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 (VIEWABLE-PHANTOM,
PRINTABLE-PHANTOM,
or EXCEL-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!
Don't forget to use the left and right
keyboard arrows to control the forward and backward motion
of the movie so you can review this as you view it!
- 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).
- 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 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.
Don't forget to use the left and right keyboard
arrows to control the forward and backward motion of the movie
so you can review this as you view it!
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.
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