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Seismic
Stratigraphy-A Primer on Methodology
By
John W. Snedden1, and J. F. (Rick) Sarg2
Search and Discovey Article #40270 (2008)
Posted January 19, 2008
1ExxonMobil Upstream Research Company, Houston, Texas ([email protected])
2Colorado Energy Research Institute, Colorado School of Mines, Golden, Colorado ([email protected])
Seismic
stratigraphic methods allow one to interpret and map reservoir, source, and seal
facies from reflection
seismic
data
.
Seismic
stratigraphic methods have evolved
since the first publications in the late 1970’s. This document attempts to
provide an update of these elementary principles, written as a “how-to” series
of steps.
|
In this document, we discuss some guidelines for conducting a
However, these recommendations are meant to form a working approach
rather than a series of subjective directions. Methodologies must
always be adjusted to fit the
As regional
1. Plot regional base maps showing
shot points and posted wells. These should be at an appropriate
scale and size for later use in mapping. Bathymetry is also useful
to have in offshore datasets. Base maps serve several functions,
including places to mark
2. From the base map, select key 2D or 3D
3. Plot paper copies of selected
regional
Figures 1
and 2 illustrate the results of plotting a small portion of a
This also holds true for the
prospect or field scale at 1:25,000 (Figures
3 and 4). Variable density sections (as on
4. Avoid
5. Prepare well 1) Identifying a key reflection (typically a limestone/shale contact) with high acoustic impedance contrast and hanging the synthetic on it.
2) In some cases with limited or older velocity
Keep in mind that some bulk
time-shifting can still be required to match the 6. We highly recommend construction of a well-tie template for illustrating the relationship between seismically-defined surfaces, time-based well log, biostratigraphic calibration, and global chronostratigraphy. This template can be prepared once horizons have been identified and well-ties are made with general agreement among interpreters. It also useful for project presentations as it provides a clear documentation of the stratigraphic age model used.
Once Pencil-interpreted paper sections allow for some changes in correlation, especially when looping across other sections occurs. However, at some point the lead interpreter declares that the key horizons are “looped” and only limited significant subsequent alterations are allowed.
1. Identify areas of major
structural deformation and
2. In structurally complex terrains,
it may be useful to do an initial correlation of a few surfaces and
then cut, flatten, and tape together sections to see key tectonic
relationships. A few half-scale
3. Review key lines (especially dip
lines) to identify major (second-order) shelf margins, if present in
the region. Indicate by triangle or circular symbol. Get a feel for
the scale of the
4. Begin to identify major lapouts
with red pencil marks. Do this BEFORE making -angular truncation obvious erosional termination of dipping reflections up against a reflection of lesser dip) -onlap (stratal termination up against a reflection of greater dip) -downlap (stratal termination down against a reflection of lesser dip) -toplap (termination of successively younger reflections against a reflection, passing downdip to prograding clinoforms (in some cases))
5. Connect onlap and angular truncation terminations as a candidate sequence boundary. Connect the downlaps as a candidate maximum flooding surface (MFS), keeping in mind the caveats listed above. Toplaps remain unconnected temporarily. Be careful when interpreting onlaps and downlaps in strike sections or in tectonically rotated and growth fault sections. Please note that listric fault planes or glide planes can be misinterpreted as onlaps.
6. Keep in mind that the most
important 7. Look in basinal positions for double downlap as an indicator of LST-basin-floor thick or (in slope) slope thicks or channels. The sequence boundary on the basin floor is by definition a correlative conformity and may not necessarily show much associated erosion. However, in confined deepwater channel systems this surface will tie with significant erosion, collapse, or failure. 8. Look in shelf-margin position for LSW’s, which will often be indicated by detached, shingled toplap-downlap couplets. These should be colored separately from other systems tracts. The LSWpc (lowstand wedge prograding complex) is often identified where smaller clinoforms downlap the sequence boundary. 9. Carry through the correlations made by connecting stratal terminations marks. Loop-tie the sequence boundary (SB) and maximum flooding surface (MFS) in a progressively widening set of line ties, in order to gain confidence in the correlations. At least five or more surfaces need to be tied in multiple loops before correlations are considered more than “candidate” SB or MFS.
10. A good practice in 11. A general rule of thumb when correlating, either with pencil or with workstation cursor, is to stay low as possible without crossing reflections when correlating a SB in the basin. Conversely, it is wise to stay high when correlating on the shelf, without crossing reflections. A MFS surface may rise in the basin (due to sedimentation prior to downlap). As mentioned, low toplap is common and can be confused with a sequence boundary but may be an internal surface in the LSWpc. This is why it is so important to understand the type of surface that is being correlated and the basin position of the area being interpreted.
Integration
with Other
After key stratigraphic surfaces
have been identified and correlated, the next set of steps are
undertaken to integrate any available well 1. Integrate with logs, cores, and biostratigraphic information.
--Biostratigraphic
--Logs:
Stacking patterns, log motifs, and lithology are keys to the
intermediate scale of correlation which should support the
Stacking patterns seen on logs (and
outcrops sections) are often indicative of key stratigraphic
surfaces. For example, the change from retrogradational to
progradational stacking often is associated with a maximum flooding
surface, which can be checked against both
Log motif --Lithologic relationships can help identify systems tracts: 1) in mixed siliciclastic/carbonate systems, HST's are often dominated by carbonate rocks while sandstones are often found in the LSW’s and TST (e.g., Guadalupian strata of the Permian Basin; Sarg and Lehman, 1986). 2) In some LST’s, the carbonates can dominate the LSWpc, but sandstones onlap as basin-floor thicks. In-situ coals often reside in the HST’s and/or TST’s while transported terrestrial organic matter and coal spar (clasts) occur in the LST’s (e.g., North Sea Tertiary; Armentrout et al., 1993). Juxtaposition of contrasting lithologies and unlike facies types often signals a major basinward facies shifts (SB) or major transgressive event (parasequence set boundary (PSSB)). --Cores: The best evidence for identification and validation of important stratigraphic surfaces often comes from cores. Sequence boundaries can be associated with basal lags or paleosols (on the interfluves of incised valley-fills (ivf’s)). Parasequence boundaries (PSB’s) can be associated with burrowed, wave rippled surfaces. The Glossifungites trace fossil assemblage is a firm or hard ground indicator and this can be associated with PSSB or PSB’s.
At this point, it is often helpful
to take some of the sequence boundaries and maximum flood surfaces
from the sequence stratigraphically interpreted Once surfaces are established, it is relatively easy to compute statistics like net/gross, etc., used in map overlays described below. Multiple datums may be necessary, particularly with long regional cross-sections, but many computer cross-section programs have some difficulty with this.
2.
Color systems tracts: green = TST, Blue=HST, terra cotta (brown)
= LST. Coloring lightly with pencil is particularly good for 3. Use biostratigraphic information to date the sequence boundaries and MFS. It is very important to establish sequence boundaries ages at the narrowest lacuna (smallest hiatus). This is particularly critical for major angular or structural unconformities (e.g., Middle Miocene Unconformity (MMU) of SE Asia, Base Cretaceous in Northern Viking Graben). Figure 6 illustrates how the MMU of Malaysia was definitively dated at 15.5 ma (Haq et al., 1987 terminology) by using biostratigraphic age dates where the gap between the oldest strata above and the youngest strata below the MMU was identified. 4. Compare to global chronostratigraphy: a) assign age and b) appropriate surface nomenclature. We recommend use of terminology following the European Basins Cenozoic and Mesozoic Chronostratigraphy (de Graciansky et al., 1998). This system and associated charts are gaining industry acceptance as a global reference standard. The surface is named using the European Basins nomenclature; e.g.: Tor1_sb Tortonian-1 sequence boundary (3rd order) Tor1_200fs Tortonian-1 200 flooding surface (4th order) MioX1_100mfs Miocene 4th order surface, unknown stage or depositional sequence
Once a preliminary stratigraphic
framework has been established, mapping based upon sequence and
Workstation- and some PC-based
External Form and Internal Geometry-A-B-C Mapping
The three categories (A-B-C) of Ramsayer's (1979)
Figures 8 and 9
illustrate use of the Ramsayer (1979) A-B-C
Four
When placed in a map view, the interpreter infers patterns of
similar
In addition to A-B-C
Rather than mapping the entire sequence, it is recommended that
individual maps be constructed for each depositional systems tract (Figure
10). These often have different Comparing these maps, one can see the variations in map pattern through one eustatic sea level cycle (Figure 8). Stacking all the systems tracts for one cycle, by contrast, leaves a very complicated map (Figure 10, inset).
Since Ramsayer’s seminal paper in 1979,
Recent innovations in
Combining
Confidence in
Isochron/Isochore Maps:
These maps provide more quantitative information on the gross
thickness of sequences or systems tracts and are particularly
powerful when combined with overlays showing net sand, net/gross
reservoir, etc. (e.g., Snedden et al., 1996). Conventional methods
for isochron (
Paleogeographic Maps:
Traditionally, paleogeographic maps have been based on
paleoenvironmental trends inferred from depositional systems
analysis. Paleogeographic maps based on sequence stratigraphic
correlations are truer representations of the paleogeography as they
are based on stratal "timelines" observed in
Paleogeographic maps are
particularly useful when they represent the sum of other
Application to Petroleum Exploration and Exploitation
The major reason for developing
Highstand Systems Tract (HST) In many hydrocarbon exploration plays, many of the earliest discoveries are found in updip structural traps, which tend to be dominated by reservoirs of the HST or highstand sequence set (Figure 16; Snedden et al., 2002). In some high accommodation basins like West Africa or Gulf of Mexico, this scales up to the highstand sequence set level (Figure 14). Stratigraphic traps are less common in HSTs as strata often continue updip without significant barriers and hence are regionally "leaky" (Figure 17). Structural closure (anticlinal or fault-type) can provide the potential for entrapment, especially if sealed by overlying shaly TST's.
Transgressive Systems Tract (TST) Transgressive systems tract (TST) and in high accommodation settings, the transgressive sequence sets (TSS), are the most overlooked hydrocarbon-bearing component of the sequence stratigraphic model (Posamentier, 2002b). TST's often provide lateral and top seal for LST reservoirs in the basin, when they are shale-prone, and for highstands on the shelf, when they comprise 2nd-order transgressive mudrocks. They also can contain significant source rocks facies, particularly at the second-order (Duval et al., 1998; green strata in Figure 14). When reservoirs are present, they tend to be more marine than those of the HST or LST, and thus more laterally continuous. Development of thick TST’s usually involves high local subsidence (e.g., growth fault wedges).
Lowstand Systems Tract (LST) The lowstand systems tract (LST; Figure 15) and lowstand sequence sets (LSS; Figure 14) are the most controversial and yet often the most economically important elements of any sequence (Posamentier et al. 1992). Much attention has been devoted to LSTs as the greatest remaining potential in many plays lies in deeper and depositional downdip areas (Figure 16), where LST/LSSs are more common than HST/HSS’s and TST/TSS's (Snedden et al., 2002). The potential for stratigraphic entrapment is also greater, as strata do not generally continue updip (Figure 17).
The presence of a significant
relative sealevel fall causes a major basinward shift in onlap,
particularly when shifted seaward of the offlap break. Mid-shelf
LST's can also occur (incised valley-fill of Van Wagoner et al.,
1990). A common motif on The vertical succession in a LST prograding complex is (bottom to top): downlap, progradation, toplap, aggradation, and floodback (Figure 17). Earlier models for deepwater settings suggested that there may be three parts to the LST: the basin-floor systems (distributary channel and sheet), slope channel systems (confined to weakly confined), and the prograding complex (LSWpc; Mitchum et al., 1994). Basin-floor systems sometimes show double downlap while the prograding complex shows toplap/downlap lapouts. Slope systems exhibit incision, lateral truncation of reflections, and complex filling geometries. These can greatly impact the internal fluid connectivity of a deepwater reservoir within the LST.
More recent work suggests that
deepwater systems are very complex arrangements reflecting shelf
margin evolution, sediment load, climate, eustacy, and other
factors. The methodologies for stratigraphic correlation,
The lowstand systems tract prograding complex (LSWpc) can be confused with the highstand systems tract, as both are progradational. However, there are ways to differentiate the two systems, which have important implications for hydrocarbon entrapment (Figure 17). The LSWpc typically is dip-restricted, with strata not continuing updip vs. the more continuous HST. As a result, all other factors being equal, the HST’s tend to have less potential for lateral sealing than the LSWpc. Stratal terminations at the top of a HST tend to be tangential to non-terminated, versus toplap patterns in LSWpc's. The stacking patterns also differ, as LSWpc show early progradational and late aggradational patterns on logs, versus HST's with early aggradation and late progradational motifs
One measure of the value of a
a) Is the petroleum system complete? Is there a critical missing element which will fatally flaw the petroleum system and prevent discoveries in un- or under-explored basin? It is recommended to use the resulting products (cross-sections and maps) to identify source and seals, not just reservoir rocks. For marine source rock mapping, recognition of the large scale, major downlaps (maximum flood) of major continental encroachment cycles is a good starting place (for more detail, see Duval et al., 1998). It is also useful to relate to worldwide eustatic charts and known source bed events. For example, Klemme and Ulmishek (1991) determined that six stratigraphic intervals have provided 90% of the world's discovered original reserves of oil and gas (Silurian-9%, U. Devonian-Tournasian-8%, Pennsylvanian/Lower Permian-8%, Upper Jurassic (25%), Mid-Cretaceous-29%, Oligo-Miocene (12.5%)). b) Are certain systems tracts under- or unexplored? In a recent survey of Texas onland plays, it was determined that nearly one-third of the plays produced from only one systems tract, with the highstand systems tract containing nearly 70% of the produced hydrocarbons (Snedden et al., 2002). It is evident that in many plays, the lowstand systems tract is underexplored c) Can the sequence stratigraphic model built here explain the present distribution of fields and dry holes? Do the downdip dry-holes define a poorly developed lowstand systems tract, or just the distal limits of the highstand systems tract? In some basins, there is a zone of bypass between the HST and LST, which can be misinterpreted. d) If the lowstand system tract play corridor can be identified, are downdip prospects located in the major deltaic fairway or marginal to it? Even the world's greatest basinward shift will fail to send sand into a basinal area of interest if no updip deltaic source is present or an appropriate conduit for sand delivery is not in proximity. It is critical to be in the sand "fairway"!! e) Finally, identify possible play types for prospectors: e.g., pre-orogenic HST, if sealed by syn-orogenic shales; LST, if detached and sealed. TST, if sealed by MFS and sourced by 2nd-order TST shales. Summary and Conclusions
This document is meant to be used as
a working guide to
Much of the methodology described
here and in this volume involves
Acknowledgements
The authors appreciate the
assistance of Kurt Johnston (EMEC) in preparing
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