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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Classification of Syndepositional Structural Systems,
Northern Gulf of Mexico
By
1Shell Oil Co.
2Samson Offshore
Syndepositional structures form in response to sedimentary
loading. In the northern Gulf of Mexico, deltaic and turbidite
complexes comprising the sediment load cover tens of miles, and
the resultant structures are of similar size. We have come to recognize
that hydrocarbon-trapping structures are actually quite
small components of larger scale structural systems. We define
syndepositional structural systems as complexes comprising a
number of different, but genetically related, structures that occur
in generally repetitive and predictable patterns, linked causally
with reservoir distribution and the nature of the unstable substrate
being loaded. Three primary types of syndepositional
growth-fault
systems are observed in the northern Gulf of
Mexico: shale-based detachment
fault
systems, salt-withdrawal
mini-basin systems, and salt-based detachment (roho) systems.
Shale-based detachment fault
systems are strike-linear gravity
slides. Characteristically they exhibit a highly listric master
expanding
fault
, a rollover anticline, a planar synthetic
fault
, a
syncline, a planar antithetic
fault
, and a horst block.
Compressional toe folds may be visible, but they are usually
obscured by younger systems. The listric faults share a common
sole plane, most often a locally thick transgressive shale. Deltaic
progradation stalls at the head of the system. Sands tend to be
stacked in the rollover or transported, as if on a conveyor belt,
down dip along the
fault
ramp to form rotated wedges, while the
synclinal region is typically filled with prodelta slope facies.
Salt-withdrawal amini-basin systems consist of large (12-20 miles), elliptical withdrawal basins. They consist of a deep turtle structure, a syncline, and a series of subsidiary faults and diapirs that outline the basin margin. In the slope region, tabular salt may obscure all or part of the underlying mini-basin.
Overall geometry of a mini-basin and the nature of the sedimentary fill within the basin vary considerably, depending on whether the mini-basin was loaded primarily in a shelf or in a slope environment. In mini-basins loaded by shelf deposits, the shelf margin characteristically coincides with the hinge faults rimming the basin, resulting in a section of stacked deltaic sands and shales that overlie massive prodelta shales. In mini-basins loaded with slope deposits, ponding of turbidites is generally continuous as long as there is salt withdrawing from the basin.
Salt-based detachment faults, or roho, are combination gravity
slide and salt evacuation structures. Roho systems undergo two
phases of evolution. Evacuation of salt from a starved minibasin
first forms an extensive salt wing, which is subsequently
remobilized by deposition of upper slope and outer shelf sediments
on the wing. The deposition initiates formation of a series
of listric, down-to-basin faults that sole into remnant salt resulting
primarily from salt evacuation into the edges of the wing.
Growth faults of roho systems are generally a series of nested
horseshoe-shaped faults along which deltaic sands and shales
tend to be stacked. The listric normal faults grade laterally into
strike-slip fault
zones and dip-oriented salt diapirs, and the system
has a complex toe thrust zone of salt and compressional
structures at the down dip edge of the salt wing. Because of the two-phase evolution, roho separates two regions of discordant
structure, a geopressured slope section below and a hydropressured
shelf section above.
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