>Say What??

I think we should break into groups :)

My general interpretation from the article is "we've never seen anything like this before"

LST, TST and HST are all part of what's known as the Exxon Sequence Model. Not to be confused with GST, PST and HST, which are part of the Canadian Government Taxation Model :)

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 seismic is often toplap/downlap couplets, with toe of clinoform debris wedges or sandstones. These are typically sand rich, although carbonates can also form (the downdip oolite play of the Permian basin).

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, interpretation, and mapping in these complex, hierarchical deepwater channel systems are well defined and described in documents at these chapters.

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