TECHNICAL PROGRESS

SEQUENCE STRATIGRAPHIC FRAMEWORK

The project team has developed a firm sequence-stratigraphic framework that integrates the genetic stratigraphic framework and systems tracts within and between Starfak and Tiger Shoal fields. As deduced in part from biostratigraphic data, the chronology of the genetic cycles coincides closely with that of published syntheses. This achievement represents a fundamental step in the reservoir characterization of these two fields and lays the critical groundwork for all subsequent, more focused analyses of reservoir-specific attributes and the identification of previously undetected gas and oil prospects one of the principal objectives of this project. The model is useful in myriad ways:

1. It establishes a robust genetic framework from which finer scale (that is, reservoir-specific) attributes can be predicted and derived.
   
   
2. It allows precise delineation of key chronostratigraphic surfaces (sequence boundaries and flooding surfaces), identification of sand-bearing depositional systems tracts within the reservoir-bearing succession, prediction of the areal geometry of sand-body reservoirs (and after full petrophysical analysis, identification of reservoir flow units), and formation of a robust reference system within which all engineering, production, and petrophysical data can be placed.
   
   
3. It allows interpolation of boundaries within the field areas and prediction of field-scale depositional attributes, observations that can extend our knowledge of hydrocarbon prediction to other areas of the region.

The sequence-stratigraphic framework of the Miocene Series can be readily extrapolated to surrounding offshore fields using regional biostratigraphical markers. The overall regressive, progradational Miocene succession comprises (in ascending order) slope and slope-fan deposits, distal shelf and lowstand-delta (lowstand deltaic wedge) facies, and medial and proximal shelf systems. Inferred coastal-plain deposits occur within 200 to 300 ft above the top of the study interval. Delineation of the third-order sequences within these general facies tracts allows further division of the slope and shelf systems into finer scale lowstand, transgressive, and highstand systems tracts that honor precise time-stratigraphic boundaries.

Lowstand Systems Tract

All four elements of the lowstand systems tract are collectively represented in Starfak and Tiger Shoal fields. They include basin-floor fans, slope fans, lowstand deltaic wedges, and incised-valley fills. The first three components compose lowstand prograding wedges.

In contrast to Starfak field, lowstand deltaic wedges are absent in Tiger Shoal because Tiger Shoal is in a generally more proximal depositional position relative to Starfak. Isolated wells located south (basinward) of Tiger Shoal field exhibit inferred deltaic wedge sandstones in some of the same intervals (sequences) that contain wedge deposits in the southern part of Starfak field.

Lowstand prograding wedges and other time-equivalent distal-shelf to upper-slope successions typically overlie, or are cut by, numerous penecontemporaneous (growth) faults in Starfak field and in the untested area between the two fields. This relation suggests that movement on these faults influenced the position of the shelf edges and, therefore, the updip limits of lowstand prograding wedges. In Starfak field, these faults represent at least two generations of syndepositional-faulting episodes that probably (1) controlled the positions of the shelf/slope break and, consequently, the location of lowstand-deltaic sedimentation and (2) enhanced accommodation rates for downdip-thickening lowstand-deltaic deposits.

The recognition of lowstand prograding wedges within the Miocene succession is one of the more significant findings of this study thus far. Starfak field contains four third-order prograding wedges that can be clearly imaged using seismic data in areas outside of well control.

Sandstone zones that display prominent coastal-onlap geometry within the third-order lowstand systems tracts form potential stratigraphic traps for hydrocarbon accumulation in the untested region between Starfak and Tiger Shoal fields. Because these third-order lowstand prograding wedges are proven producers in Starfak field and seismic data indicate that the sandstone-rich wedges, with the similar internal sandstone geometries, extend to the east, we believe there is high potential for new discoveries in the untested interfield area. Moreover, the block-faulted lowstand systems tracts within deeper third-order sequences, which also extend into the untested interfield area and are prominent producers in Starfak field, are also very likely candidates for new hydrocarbon discoveries.

Incised Valley Fill
In Starfak and Tiger Shoal fields, facies of lowstand systems tracts in on-shelf successions consist of the basal, fluvial portions of incised-valley fills. On-shelf facies compose 50 to 60 percent of the study interval. Erosional surfaces occur at the bases of thick (50 to 250 ft) blocky to spiky log facies that represent incised-valley fills composed of lowstand fluvial facies overlain by inferred estuarine and bayhead-delta deposits of transgressive systems tracts. Juxtaposition of these fluvial facies over neritic on-shelf shales represents significant basinward shifts in coastal onlap.

Changes in log motif (indicating changes in net-sandstone or sandstone/shale distribution) that characterize incised-valley-fill facies are associated with the spatial location of the lowstand valley relative to the proximal, medial, or distal paleoshelf. The magnitude of valley incision increases and valley fills generally thicken to the south, downdip on the paleoshelf. Log signatures of the valley fill in those reaches of the valleys incised into proximal and medial paleoshelf settings are blocky, blocky serrated, or upward fining. This character records well-developed (low-clay-content), high-porosity stacked fluvial-channel architectural elements. These are dominantly lowstand fills inferred to have aggraded during late lowstand time when relative sea level was stable or slowly rising. Lowstand fluvial fills are overlain by valley-confined transgressive deposits. In contrast, thicker valley fills (as much as ~250 ft) are located in more distal shelf locations. These valleys form areally restricted topographic "notches" in the paleoshelf margin. Valleys are located immediately upslope from lowstand prograding wedges that were fed with sediments cannabalized from erosion of the underlying highstand shelf deposits as "knickpoints" eroded landward, forming the final lowstand river profiles.

Transgressive Systems Tract

The thin (10- to 40-ft) backstepping, progradational successions (retrogradational parasequence sets) that compose transgressive systems tracts disconformably overlie the transgressive surfaces within lowstand valley-fill deposits and above lowstand wedges and merge with the sequence boundaries in interfluve areas. These parasequence sets form stacked, upward-fining log-facies successions that culminate in a high gamma-ray response signifying the maximum marine flooding event, the marine condensed section In most of the study area, however, the condensed sections are composed of regionally correlatable organic-rich shales deposited under tranquil marine conditions during inundation of the shelf.

Amplitude stratal slice from 3-D seismic data of the J sand interval depicting the areal distribution of a lobate to digitate highstand delta within the greater Starfak and Tiger Shoal area. Note apparent control by the major growth fault bounding the north side of Starfak field on distributary-channel development and transport directions.

Highstand Systems Tract

Sediments of highstand systems tracts form progradational parasequence sets that generally display upward trends of parasequence thinning and increase in percentage of sand within parasequences. These upward-coarsening successions are typically 150 to 200 ft thick in most of the study interval, but they range from as thin as 60 ft in the most proximal shelf locations to as thick as 600 ft in the distal-shelf areas.

Sands within late-highstand parasequences, the uppermost parasequences of the progradational parasequence sets, represent delta-front deposits, locally developed distributary channel fills, delta-mouth bars, and interdeltaic shoreface facies. Early-highstand parasequences originated as more distal variations of these same deltaic and shoreface environments of deposition. Seismic stratal slices of highstand successions illustrate generally lobate and digitate aerial geometries.

Amplitude stratal slice from 3-D seismic data of the J sand interval depicting the areal distribution of a lobate to digitate highstand delta within the greater Starfak and Tiger Shoal area. Note apparent control by the major growth fault bounding the north side of Starfak field on distributary-channel development and transport directions.