Santa Margarita

Exercise 1: Stratigraphic-stacking patterns– Santa Margarita
Seismic Interpretation

The Santa Margarita exercise consists of a two-dimensional (2D) seismic line (Figure 6) and wireline-log and a synthetic seismic trace from one well (Figures 7 and 8). These data provide an excellent opportunity to interpret seismic-stratal patterns (i.e., downlap, toplap, onlap, and erosional truncation; Mitchum et al., 1977) and to identify unconformities formed by tectonically or eustatically induced base-level changes. Following the methods of Mitchum et al. (1977), Weimer and Sonnenberg (1989), Van Wagoner et al. (1990), and Posamentier and Allen (1999), the unconformities identified can be interpreted as sequence boundaries or lowstand-surfaces of erosion; and marine-flooding surfaces (Van Wagoner et al., 1990) or transgressive-surfaces of erosion (Weimer and Sonnenberg, 1989).

The quest is to complete a sequence-stratigraphic interpretation that is consistent between the seismic and wireline-log data.This is accomplished iteratively by using a synthetic seismogram to tie the seismic and well data. Thus, stratigraphic-stacking patterns as expressed by the wireline logs can be extended laterally (and mapped) using the seismic-stratal patterns.
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Figure 6:Uninterpreted west-to-east 2D seismic line from the San Joaquin Basin, California. Data for Well No. 1 are given in Figure 8. The stratigraphic interval of interest is denoted by the red arrow. 

Interpretation Methodology

1. On the 2D seismic line:
   a. Use a red colored pencil and mark the termination point of reflections within the interval defined    by the red arrow. See Figure 4: Mitchum et al. (1977) for an example.
   b. Use a yellow colored pencil to trace the lateral continuity of surfaces defined by the reflection terminations and identify:
      i.   Downlap surface;
      ii.  Toplap or erosional truncation (sequence boundary) surface;
      iii. Onlap surface 
   c. Characterize the internal reflection geometries between these surfaces as packages (or    systems tracts) of retrogradationally, progradationally, or aggradationally stacked parasequences    (i.e., Figure 5).
2.  Transfer the seismic stratal surfaces to the wireline-logs by matching surfaces with the SP and synthetic on Figures 7 and 8. Keep in mind that the synthetic and seismic data do not match exactly.
3.  Interpret the stacking patterns exhibited by the stratasets between the stratigraphic picks. That is, identify parasequences and parasequence sets. Draw arrows indicating upward-coarsening, (increased sandstone upward), upward-fining, or aggradational character of the parasequences. Note whether the stratasets are progradational, aggradational, or retrogradational in character as shown in Figure 4.
Figure 7: Uninterpreted Spontaneous Potential (SP) and induction (ILD) induction wireline logs from Well #1 tied to a synthetic seismogram


Images in the data set in this exercise section and for the exercise section that follows can be printed or imported into electronic media that include PC, Notebook, Tablet, or Pad. Using Power Point drawing tools in the electronic media is 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 in a separate tab.
Figure 8:.Uninterpreted Spontaneous Potential (SP) and induction (ILD) wireline logs from Well #1 tied to a synthetic seismogram.
4. On the wireline logs, interpret the stacking patterns exhibited by the stratasets between the stratigraphic picks. That is, identify parasequences and parasequence sets. Draw arrows on the wireline-logs indicating upward-coarsening, (increased sandstone upward), upward fining, or aggradational character of the parasequences and parasequence sets. Note whether the parasequence sets are progradational, aggradational, or retrogradational in character as shown in Figure 4.
5.  Interpret the type of stratigraphic surfaces (lowstand surface of erosion, transgressive surface of erosion, downlap surface, or channel diastem) using their seismic geometries, wireline-log character and stacking patterns, and interpreted facies associations. 
6. Interpret systems tracts on the seismic line and wireline logs. Color them by this convention:
   a.Highstand systems tract - light blue
   b.Transgressive systems tract - light green
   c.Lowstand systems tractorange
       Figures 9 and 10 show one interpretation.
7. Summarize your interpretation with a brief discussion of how this stratigraphic interval may have formed regarding the balance between base-level change and sedimentation rate, what type(s) of depositional systems might be present within each systems tract, and predict where in the section, away from the Well #1 location, reservoir-quality sediments, seals, and traps would likely occur.
8. Make recommendations as to whether the seismic-reflection character is indicative of lithology and/or porosity.
9.  Paleobathymetry Estimate: Measure the slope relief along a clinoform in the thickest part of the wedge. The slope relief provides an estimate of the minimum paleobathymetry for compacted sediments and helps in the prediction of the presence of a deepwaterlowstand system seaward of the slope. To convert from seismic two-way time to thickness, assume 100 milliseconds = 500 feet.

Log Interpretation

Four upward-sandier parasequences in the lower part of the log form a progradational parasequence set.This is overlain by an aggradational sand-rich interval.The top of the log consists of upward-thinning parasequences interpreted as a retrogradational parasequence set.Candidates for sequence stratigraphic surfaces include:

1.  A maximum flooding surface at the base of progradationalparasequence set(i.e., at the base of the lowest parasequence);

2.  A flooding surface near the base of the retrogradational parasequence set;

3.  A maximum flooding surface at the top of the retrogradational parasequence set;

4. A sequence boundary is more difficult to interpret from the log patterns alone; see the next section.

Figure 9: Interpreted Spontaneous Potential (SP) and induction (ILD) induction wireline logs from Well #1 tied to a synthetic seismogram showing stratigraphic-stacking patterns, interpreted key (i.e., sequence stratigraphic) stratigraphic surfaces, and systems tracts.


Highstand progradational wedge

A series of seismic clinoforms uniformly converge onto a basal downlap surface and toplap and/or are erosionally truncated against an upper surface.The two surfaces define a progradational wedge with a distinct shelf-slope morphology. Slope relief is about 1500 feet.clinoform relief increases to the west suggesting the system built into ever-increasing water depths.

Transgressive or retrogradational systems tract (TST)

A sequence boundary is interpreted at the top of the wedge based on:

1.  Consistent toplap/truncation of clinoforms;

2.  A continuous reflection immediately overlying the toplap/truncation surface suggesting regional erosion and subsequent marine transgression.The overlying transgressive systems tract is thin (a single reflection) likely due to abundance of lateral (rather than vertical) accommodation.

3.  Concave-upward features near the top of the wedge and landward of the shelf edge suggest base-level lowering and fluvial-valley incision

Seismic-Log Integration

The seismic downlap surface is correlated to the base of the oldest parasequence and is interpreted as a maximum-flooding surface or condensed section.Seaward of this area, the condensed section is the stratigraphic equivalent of chert-rich deepwater suspension sediments (i.e., “O-chertmarker”) of the Monterey formation.Four parasequences defined on the log correlate to approximately four clinoforms withinthe progradational-highstand wedge.The toplap/truncation surface (i.e., sequence boundary) ties within a sand-rich interval.This significant surface is not an obvious surface picked from the well data alone.Seaward of the depositional shelf break and slope is the time equivalent Bellevue Member of the Miocene Stevens turbidite system (Figures 11 and 12). The Bellevue thins and onlaps landward onto the slope of a progradational wedge in this exercise. See Figures10, 11, and 12. This Upper Miocene petroleum system is summarized in Figure 13.

Figure 10: Interpreted west-to-east 2D seismic line from the San Joaquin Basin, California. The stratigraphic interpretation from Well No. 1 is imposed on the seismic data. Stratal terminations and their geometries on the 2D line enable the interpretation of systems tracts within the depositional sequence.
 Figure 11: Paleogeographic reconstruction of the progradational shelf, incised valleys, turbidite channels and turbidite lobes of the Bellevue Member of the Stevens formation. Well 1 of the Santa Margarita exercise and its corresponding seismic line are shown. In addition, inverted L-shaped wireline-log and seismic cross sections traverse five oil fields and display six wells tied to seismic data.
 Figure 12:  Wireline-log and 2D seismic sections showing stacked Stevens Sand submarine-fan lobes. chert beds deposited during periods of no- to minmal-sedementation (i.e., highstand) constitute condensed sections separating successive lobes. Sections pass through five oil fields. Seismic line and well locations shown in Figure 11.
Figure 13: Summary of the Upper Miocene petroleum system; southern San Joaquin Basin, California.
Contributed by James S. Hewlett and Robert S. Tye, PhD., P.G. 
Friday, March 20, 2015
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