Henry Posamentier had success in a study he conducted using seismic geomorphology in the Java Sea while he was with Arco Indonesia.
The study used a combination of seismic reflection attributes, time slices and horizon slices extracted from 3-D seismic volumes to look at alluvial systems in the near surface Late Pleistocene sediments and the deeper Miocene of the southern Java Sea shelf, offshore northwest Java.
The plan view expression of these systems range from low sinuosity to high sinuosity and incised to unincised.
An incised valley develops when a river has cut into its floodplain sufficiently so that even when it is at flood stage, flow doesn't overtop the riverbanks. This leaves the formerly active floodplain abandoned and serving as interfluves. Incised valley systems are most commonly thought to have characterized extensive shelf settings each time a relative sea-level fall has occurred.
Late Pleistocene alluvial systems imaged on the shelf were active during periods of lowered sea level when vast shelf areas were exposed. Only a few of these systems are characterized by incision. The hallmark of an incised valley is the presence of numerous smaller tributary valleys that develop on the abandoned floodplain that feed the trunk valley.
These tributary valleys form in response to drainage of the floodplain and are characterized by longitudinal profiles anchored or adjusted to the level of the channel within the trunk incised valley, according to Posamentier.
Incised valley systems can be filled with a complex array of depositional systems, ranging from open marine to estuarine to fluvial. Incised valleys can be the sites of numerous stratigraphic discontinuities that punctuate the stratigraphic architecture, including:
- Tidal ravinement surfaces.
- Base of channel local erosional surfaces.
- Valley within valley widespread erosional surfaces.
It is not uncommon for incised valleys to be the sites of multiple incision events associated with successive sea-level falls.
Mistaken Identity
Incised valleys have been the subject of numerous studies in recent years, he said. Such systems have been thought to be the most common response to relative sea-level fall.
Studies of incised valleys largely have been based on outcrop, well log and core data. Incised valley recognition has been based largely on the presence of multi-story channel fill, the presence of a significant time break at the channel base or the absence of a transitional facies between the channel/valley fill and the underlying substrate.
However, the most unequivocal evidence for the existence of incised valleys -- the presence of small subordinate tributary valleys to the principal trunk valley -- has rarely been used.
That will change, Posamentier believes, now that 3-D seismic data has been introduced into the process. In past studies coeval incised tributary valleys were not commonly recognized due to the discontinuous nature of most forms of data other than 3-D seismic data.
Posamentier said lowstand unincised alluvial bypass systems have not been well documented. They are difficult to distinguish from high stand alluvial systems -- and in mid- to outer shelf environments, a niche commonly occupied by lowstand depositional elements, their presence has not been anticipated.
At such times, unincised alluvial systems are consequently mistaken for incised valleys.
Geomorphologic Evolution
The shallow Pleistocene section Posamentier studied in the Java Sea is characterized by a variety of alluvial systems that are clearly imaged on 3-D seismic data. The alluvial systems range from high to low sinuosity channels, to braided channels.
In some cases these alluvial systems are clearly associated with small tributary valleys.
"These time slices illustrate the geomorphologic evolution of the Late Pleistocene section under the southern Java Sea shelf," he said.
Several slices showed a prominent channel system that appears to lie within a valley -- which in turn appears to be associated with numerous subordinate tributary valleys.
The tributary valleys are characterized by a dendritic drainage pattern, and in most instances extend only short distances away from the trunk system.
The width of the principal trunk valley is about three to four kilometers in the north widening to five kilometers in the south. A single channel that bifurcates in places and is characterized by low sinuosity dominates the trunk system.
Also, lateral accretion with associated meander scroll bars can be recognized within the incised valley.
This system seems to be fully confined by valley walls. The trunk valley can be mapped across three 3-D seismic volumes for a total distance of 90 kilometers and lies approximately 110 kilometers from the closest shelf-slope break to the south-southwest, resulting in presumed total valley length of at least 200 kilometers.
"We gathered a great deal of details on how this incised valley formed in the near surface Pleistocene that we can export to datasets where the data quality might not be as good," Posamentier said. "We're not prospecting at this level that's just about 100 meters below the seafloor, but with the superior resolution at this level we are learning a great deal.
"Here we have a near modern system that shows us what has been preserved," he continued. "We can then extrapolate what we have learned here to deeper systems where we are prospecting for hydrocarbons.
"Clearly, the higher the resolution of the data the greater detail we can see," he said. "In near surface sections we can detect bed thicknesses of one to two meters. At deeper levers where the data resolution is not as good we might not be able to image anything less than 15 meters."
Down In the Valley
The study also looked at horizon slices through the deeper Miocene shelf section, which is interpreted as constituting an unincised lowstand alluvial bypass system, according to Posamentier.
Conspicuous by their absence are any incised tributary valleys, such as those observed within the Pleistocene section.
"This Miocene alluvial system was emplaced on the exposed seafloor during a rapid sea level fall and flowed towards the southeast as indicated by the truncation of meander scroll bars to the northwest," he said. "The high-sinuosity channel was established first, followed by infilling, abandonment, and eventual establishment of the low-sinuosity channel, which flowed towards the south."
No true incised valleys have been observed on the 3-D seismic data in the Miocene section. This could be a result of inadequate seismic coverage or the shelf in this basin was not fully emerged during the Miocene -- the amplitude of sea-level change was insufficient to expose the shelf relative to the highstand water depth at the outer shelf.
Posamentier said the seismic used in the study was high quality data, so it is likely the shelf was not fully exposed.
In light of the abundance of unincised lowstand alluvial deposits observed in this study, this play type should be relatively common in the rock record. However, review of the literature reveals a virtual absence of documented unincised lowstand alluvial deposits, he said.
"Whether these deposits have been overlooked or misinterpreted is not clear," he said. "In the least, their clear under-representation in the literature should prompt a re-evaluation of previously interpreted incised valley systems on the one hand and a closer look at lowstand deltaic or shoreface systems that seem to lack fluvial feeder channels on the other.
"Unincised lowstand alluvial deposits should be especially common in shallow marine basins with gentle rap margins such as intracratonic basins and shallow foreland basins," he said. "Such basins commonly lack a necessary high-gradient seafloor, such as encountered at the outer shelf or upper slope on passive continental margins, required to induce valley incision."