Wednesday, January 3, 2018

A Detailed View of Sequence Stratigraphy

The components of depositional sequences are called system tracts. Systems tracts are divided into three groups according to relative sea level at the time of deposition -lowstand at low relative sea level, transgressive as the shoreline moves landward, and highstand at high relative sea level. Systems tracts are depositional groups that have a predictable stratigraphic order and predictable shapes and contents. A close look at system tracts, their geometries and lithologies, shows how sequence stratigraphy can be used to foretell reservoir location and quality. 

Each systems tracts exhibits a characteristic log response, seismic signature and paleontologic fingerprint, and performs a predictable role  in the oil and gas play -reservoir rock, source rock or seal. Gamma ray (GR) and spontaneous potential (SP) logs are expected to read low in sands and high in shales. Resistivity logs show the reverse, reading high in hydrocarbon-filled sands and low in shales. 

 Apparent layering interpreted on seismic sections-called stratal patterns -is determined by tracing seismic reflections to their terminations. The termination is categorized by its geometry and associated with a depositional style. Fossils are described by their abundance, diversity and first or last occurence, allowing dates to be determined based on correlation with global conditions. 

Starting with the lower lowstand systems tract at the bottom of a sequence, basin floor fans are typically isolated massive mounds of well-sorted grain flows or turbidite sands derived from alluvial valleys or nearshore sands. Log responses are blocky, with a sharp top and bottom bracketing clean sand. Seismic reflections curve down and terminate on the underlying sequence boundary-a feature called downlad - while the top may form a mound. The lowstand facies makes an execellent reservoir, with porosity often over 30% and permeability of several darcies. It may be overlain by a thin clay-rich layer that can act as a seal , but more often it is overlain directly by the next depositional unit. In these cases, the basin floor fan acts as a hydrocarbon migration pathway. Fossil content is minimal, since deposition rates are often very high. Basin floor fans derive their hydrocarbon from previous sequences.

In areas of high deposition , the major component of the lower lowstand systems tract is the slope fan complex. Slope fans can be extensive and can exhibit several depositonal styles , depending on the vertical gradient of the slope face and on the sediment source. The complex may include submarine channels with levees, overbank deposits, slumps and chaotic flows. Log responses commonly are crescent shaped. A sharp base within the crescent commonly indicates sand in a channel , with a bell shape indicating fining upward as the channel is abandoned. On the other hand, channels may fill with mud. On seismic sections, leveed channels in the fan show a characteristic mound with a slight depression in the top. Sand-filled channels make excellent exploration targets, but may be difficult to track. Sands flowing over channel levees may be deposited as overbank sheets and alternate with shales, createing subparallel reflectors.  Such sands can provide stacked reservoirs with porosities of  10 to 30% , but are usually very thin. Slumps from shelf edge deltas create a chaotic or jumbled pattern "hummocky" in interpreter vernacular - easily identifiable on seismic data. Hydrocarbon sources for channel and overbank reservoirs are deeper sequences. Seals are provided by a widespread "condensed" section of shale, a thin layer representing prolonged deposition at very low rates that comes with the rise in sea level. The sealing shale also contains abundant marine fossils used for dating. 

Part of the upper lowstand, the prograding wedge complex derives its name from shallowing-upward deltas that build basinward from the shelf edge and pinch out landward at the preceding shoreline. Log response shows more sand higher in the section and less sand basinward, indicating a coarsening upward. The seismic signature shows moderate to high amplitude continuous reflector that downlap onto the basin floor. This depositional unit often contains ample sand, especially near the sediment source. Updip seals are typically ppor, however, and structural trapping is required for hydrocarbon accumulation.

The transgressive systems tracts represents sedimentation during a rapid rise in sea level. The shoreline retreats landward, depriving the basin of sediment. SP and gamma ray logs show a fining upward. Retreat of the shoreline gives rise to seismic patterns that appear to truncate basinward. In practice, this systems tract is commonly thin, and such patterns are usually impercetible on typical seismic sections. Basal transgressive sands derived from reworked lowstand sands can be excellent reservoirs, except where shell fragments may later cement the sands. Shoreface sands will follow strike-oriented trends. 

The top of the transgressive systems tract is the limit of marine invasion and is called the maximum flooding surface. Widespread shale deposition results in a condensed section. Abundant fossils provide ages and well ties across the seismic section. This clay-rich layer shows low resistivity and high gamma ray readings. The seismic pattern of this surface is downlap, which becomes conformal-parallels adjacent reflectors-basinward , and disappears above the shelf. This surface is usually a very continous reflector. At the shelf edge, it can commonly be identified by changes in reflection patterns above and below.

 Layers deposited during highest relative sea level are known as the highstand systems tractk (above, "E). Early highstand sediments are usually shaly. The late highstand complex, deposited as the rise in sea level slows, contains silts and sands. Some late highstand sediments are deposited in the open air as fluvial deposits. Gamma ray and SP responses show a gradual decrease in gamma ray, indicating coarsening upward associated with decreasing water depth. Seismic reflections are characterized by sigmoidal -S shaped -stratal patterns ,similiar to prograding wedge reflections. There may be deltaic and shoreface sands at the top of the section, but in general, this systems tract has poor reservoir sands, and updip seals are uncommon. Fossil abundances diminish as the marine environment becomes restricted to the deeper parts of the shelf.


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