Detailed stratigraphic evaluation of 3D seismic volumes calibrated with well-log and core data from the Miocene section of the offshore northwest Java shelf reveals the extensive presence of preserved shelf ridge deposits.
These features are thought to have formed as a result of shoreline transgression, followed by erosion and reworking of sand-prone deltaic and/or coastal-plain deposits by shelf tidal currents, which became active immediately after shoreline transgression.
Individual shelf ridges appear to have migrated across the ancient sea floor and represent a significant component of the transgressive systems tract.
These deposits can have significant exploration potential insofar as they are commonly sand-prone and they tend to be encased in shelf mudstone seal facies.
Introduction:
Recently deposited sediments are common in most modern shelf environments. Most of the coarser-grained sediments were transported there by alluvial processes during Pleistocene sea-level low stands.
During subsequent sea-level rise, shoreline transgression occurred, and the ocean once again flooded shelf areas; earlier-deposited sediments were subjected to and modified by shelf currents of various origins.
The effect of these shelf currents was to rework the earlier-deposited sediments, winnowing and streamlining them to form shelf ridges in some instances.
With the advent of 3D seismic data, identification of subsurface depositional elements has become less equivocal. Direct imaging of these elements in map view has led to the development of a new discipline called seismic geomorphology.
Geomorphologic analyses can yield key insights as to basin fill evolution and stratigraphic architecture. This study documents one such example: shelf ridges from the Miocene section offshore northwest Java.
These examples clearly illustrate the power of seismic attribute analysis in the development of depositional models. In this regard, there is no substitute for direct imaging of depositional elements!
Method of Analysis:
The establishment of a depositional model for the Miocene section offshore Java was accomplished through an integrated analysis of seismic data calibrated by borehole information.
Reconnaissance horizon slicing was accomplished using multiple reference horizons, each having been chosen for its reflection continuity.
Successive reference horizons generally were spaced at 100 msec intervals. Horizon slices were generated at 2 msec intervals using Flagship Geoscience’s StratimagicTM seismic interpretation software.
Where features were observed that warranted closer inspection, a variety of additional attributes were generated and analyzed.
At a number of stratigraphic levels in the Miocene section, long linear features were observed on reconnaissance seismic horizon slices.These linear features are expressed as bands of alternating light and dark amplitudes, in many instances extending across the data set.
The orientation of these bands is quite consistent, ranging from 20 to 30 degrees east of north. In some instances the bands narrow and even appear to pinch out. Commonly the bands appear well defined on one side and poorly defined on the other.
Map of summed amplitudes corresponding to 20 percent of the total amplitude of the highest amplitude trace segment within a 24 msec interval bracketing the shelf ridge. The data gap corresponds to a channelized feature cutting across the area.
B) Reflection amplitude extraction on a horizon slice 30 msec below reference datum, illustrating two small shelf ridges; note that they overlap slightly.
Figure 2 illustrates two shelf ridges in another area offshore Java.
Note their strikingly linear appearance. A clearly defined leading edge and a vaguely defined trailing edge characterize both of these examples.
The two shelf ridges represent extremes in the sense of shelf ridge dimensions observed. The shelf ridge illustrated in Figure 2A reaches a thickness of 17 m, predominantly sandstone, whereas the shelf ridge is only 2 m thick at its maximum, again consisting predominantly of sandstone.
In both instances, borehole data confirms the absence of sandstone outboard of the leading edge of the ridge, suggesting the potential for stratigraphic trapping of petroleum along this pinchout.
An example of the profile expression of a shelf ridge is shown in Figure 3. Note the abrupt limit of the ridge towards the west-northwest, and the more tapering limit to the east-southeast.
Figure 4 illustrates various seismic attributes that correspond to the interval shown in Figure 3A. The well-defined edge of the feature corresponds to its leading edge, or, in other words, the direction of ridge migration.
Of particular interest are the sharply different seismic facies patterns that contrast the areas outboard of the leading edge compared with outboard of the trailing edge.
This contrast suggests that the lithology is markedly different on either side of the ridge, which, interpreted within the context of a shelf ridge model, suggests a possible stratigraphic sandstone pinchout at the leading edge but not at the trailing edge.
Note also the arcuate forms superimposed atop the ridge as shown in Figures 4C and D. These arcuate forms are believed to be megaripples or dunes traveling piggy-back along the ridge.
Conclusions:
Sand-prone transgressive shelf ridges have been identified and mapped in the Miocene section of offshore Java using a variety of seismic attribute analyses.
They have a distinctively linear form and are observed to occur either as isolated features or in clusters. They are characterized in most instances by a sharply defined leading edge that corresponds to the direction of migration of these large-scale bed forms.
In some instances the leading edge corresponds to a sandstone pinchout and has potential for stratigraphic trapping of petroleum. Shelf ridges observed here range from 2-15 m thick, 0.3-2 km wide, and 3-12 km long.
The application of seismic attribute analysis was critical in identifying and subsequently determining the stratigraphic architecture of these sandstone bodies.
The shelf ridges observed and described in this study represent one of the first unequivocal examples of a stratigraphic feature common on modern sea floors but notably absent from the geologic literature with regard to ancient deposits.
This suggests that under certain paleo-environmental conditions, shelf ridges do have the potential for preservation within the rock record, thus opening the door for the possible reinterpretation of similar deposits elsewhere.
Source:WWW.CGG .COM
By Henry W. Posamentier, Atlantic Richfield Indonesia Inc.
