Well log sequence analysis

After seismic sections have been analyzed for sequences, well logs are analyzed for sequences and systems tracts. This involves interpreting depositional lithofacies on logs using cuttings and cores and then identifying sequences and systems tracts from the interpreted logs. Stacking patterns displayed in cross sections and individual logs show accommodation space changes which help us identify sequence and systems tracts.

Procedure

The list below, modified from Vail outlines a suggested procedure for well log sequence analysis.

1. Interpret depositional lithofacies on logs using cores and cuttings to calibrate the log.
2. Estimate sequences and systems tracts from the interpreted lithofacies using regional cross sections with well and outcrop data.
3. Determine accommodation space changes from parasequence stacking patterns seen in well log cross sections (see below).
4. Check estimates of sequences and systems tracts:
   • Correlate between wells that have biostratigraphic-time correlations, well log marker-bed correlations, and the global sea cycle chart.
   • Correlate with seismic profiles.

Parasequence stacking patterns

Figure 1 Three stacking patterns. From Hyne; courtesy Tulsa Geological Society. Modified from Van Wagoner et al.; courtesy AAPG.

Parasequences stack into three basic patterns as a result of the interaction of accommodation and rate of sediment supply:

• Progradational
• Retrogradational
• Aggradational

Figure 1 shows these three stacking patterns.

Stacking patterns for systems tracts

Systems tracts often have characteristic parasequence stacking patterns. Stacking patterns of the basin-floor fan and slope fan, contained within the lowstand systems tract, are difficult to identify. The table below summarizes typical stacking patterns for the three systems tracts.

Systems tract	Stacking pattern
Lowstand Basin-floor fan Slope fan Wedge	Difficult to recognize Difficult to recognize Progradational
Transgressive	Retrogradational
Highstand Early Late	Aggradational Progradational

Log patterns for systems tracts

Figure 2 Characteristic parasequence stacking patterns for the high-stand (HST), transgressive (TST), and lowstand (LST) systems tracts of a passive margin basin third-order depositional sequence. Modified. Copyright: Hyne; courtesy Tulsa Geological Society.

Figure 2 shows the characteristic parasequence stacking patterns for the high-stand (HST), transgressive (TST), and lowstand (LST) systems tracts of a passive margin basin third-order depositional sequence.

LST example

Figure 3 Cross section from the Late Cenozoic of the Gulf of Mexico. From Mitchum et al.; courtesy AAPG.

The cross section in Figure 3 is from the Late Cenozoic of the Gulf of Mexico. It shows the well log responses of a lowstand systems tract (labeled as PGC, or prograding complex, on the cross section). Log A is completely basinal with a slope fan overlain by shingled turbidites. Logs B and C have deltaic and delta front sands, and midslope turbidite sands. Log D has a “classic” coarsening-upward pattern. Log E has incised valley sands.

See also

• Geometrical analysis
• Procedure for geometrical analysis
• Other correlation features
• Seismic sequence analysis
• Combining well log with seismic sequence analysis
• Regional maps and cross sections
• Cross section

External links

find literature about Well log sequence analysis

• Original content in Datapages
• Find the book in the AAPG Store

highstand systems tract

true stratigraphic thickness, transgressive systems tract

lowstand systems tract