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Supra-Salt Stacked Condensed Sections (SCS): Potential Indicators of Subsalt Stratigraphy*

Dwight "Clint" Moore¹, Frank C. Snyder², and Stefan "Steve" Rutkowski³

Search and Discovery Article #60020 (2009)

Posted March 23, 2009

*Adapted from presentation and accompanying extended abstract at GCS SEPM 16^th Annual Research Conference, December, 1995,  p. 195-196.

¹Anadarko Petroleum Company, Houston, Texas; current address: ION Geophysical Corporation ([email protected])

²Phillips Petroleum Company, Bellaire, Texas; current address: Anchorage, Alaska ([email protected])

³Anadarko Petroleum Company, Houston, Texas  

Abstract 

The development and understanding of allochthonous salt sheets has been the focus of intense scientific (McGuinness and Hossack, 1993, and others) and economic (Moore and Brooks, 1995) interest in recent years. Concurrently enhanced techniques in biostratigraphic analysis have advanced the comprehension of depositional and structural interaction to a level of greater precision. Depositional sequences of clastic sediments can be identified by the occurrence of bioevents and abundance patterns (Armentrout and Clement[1990] and others) in microfossil assemblages. Recognition and correlation of these patterns can be translated into useful techniques of subsalt stratigraphic prediction. 

Reservoir prediction is a critically important component of subsalt petroleum exploration. It is dependent upon paleoenvironments of deposition, structural development (both pre- and post-salt sheet emplacement), and the impact of salt movement upon sediment transport fairways. This interaction of salt tectonism and sedimentation creates complex systems of potential subsalt hydrocarbon traps. 

Over thirty wells have now penetrated allochthonous salt sheets on the offshore Louisiana and Texas continental shelf and slope and have encountered significant thicknesses of subsalt sedimentary section (Moore and Brooks, 1995). In addition, several hundred wells have been drilled into the tops of these sheets, yielding not only useful salt sheet well control, but also delineating useful stratigraphic information which can be used to interpret salt mechanics and subsalt stratigraphy. Most of these wells contain non-condensed normal sedimentary sections deposited during the evacuation of the salt after sheet emplacement. These sections are often expanded intervals associated with large regional listric faults and have been laterally displaced over significant distances on a decollement surface along the top of the sheet. However, in upwards of forty of these salt top wellbores, multiple condensed sections, stacked conformably in thin sequences on top of each other, are encountered in various thicknesses above the salt. Each of these condensed sections represents a time period of slow sediment accumulation marking an extended period of hemipelagic sedimentation. It is important to realize the significant time period represented by these thin stratigraphic intervals. Often four or five of these condensed sections, representing approximately 2-3 million years (Paleo Data, 1992), are encountered stacked on top of each other above the top of salt, and they represent a period of minimal clastic deposition. These intervals can be termed as stacked or mega-condensed sections, and their occurrence offers an opportunity to better understand both the suprasalt and subsalt stratigraphy. They are unique and offer an invaluable point of stratigrapahic control when encountered. 

Within the wells that penetrated the top of salt and subsalt wells, at least four types of supra-salt sequence occurrences are evident. They are: 1) stacked condensed sections conformably deposited upon and coupled to the salt (coupled SCS), 2) stacked condensed sections that have decoupled from, and extended along, the top of the salt (decoupled SCS) as a unit, 3) coupled or decoupled normal thicknesses of individual sequences resting conformably upon the salt, and 4) coupled or decoupled normal sequences that are significantly younger than the surrounding salt flank intervals, suggesting subaqueous erosion, non-deposition, or overthrusting. 

Thus, three scenarios exist for the age relationship between supra-and subsalt sediments. First, a large sedimentary time gap may be present. This most often occurs along the updip edge of salt sheets where a large listric fault system formed above the salt. Second, the section can be conformable with little apparent missing section. Third, a prominent repeated section can occur when a stacked condensed section is present above the salt sheet. These repeated sections are of primary interest in understanding subsalt stratigraphy.  

The most notable repeated sections occur in the publicly released South Marsh Island 200 #1 and Vermilion 356 #1 wells, but several other unreleased wells have also encountered repeated section (McGuinness and Hossack, 1993). All of these wells encountered stacked, condensed sections above salt and then penetrated subsalt time equivalent sediments that were from five to ten times as thick as the supra-salt sediments. In each of these two released wellbores, at least one of the repeated subsalt sedimentary sequences contained thick reservoir quality sandstone. 

McGuinness and Hossack (1993) concluded that repeat sections demonstrated the extrusive glacier model of sheet development. They stated that these sediments can only be translated laterally with the sheet as it lengthens and they could not be preserved during sill-styled injection. These initial supra-salt sediments are deposited on sheet inflated highs, which minimizes the deposition of significant sediment thicknesses above the salt. 

This is evidenced in models of supra-salt,bypassed-sand deposition from Simmons and Bryant (1992) on the modern day salt-supported continental slope. High resolution sonar-derived physiographic images demonstrate intrabasinal paleo structures underlain by inflating salt structures. These structures have only thin layers of subparallel sediments on top and are surrounded by deep interlobal and supralobal mini-slope basins filled with thick sand-rich sediments (i.e., Keathley Canyon 255 and Green Canyon 908 wells). Subsequent periods of salt canopy remobilization can extend the sheet fronts and edges over an ancestral mini-basin, carrying the equivalently aged, thin-stacked condensed secions (SCS) with it. It is significant that these thick sands encountered beneath salt are deposited in a lower bathyal (or deeper) paleoenvironment, which appears similar to some recently detailed accounts for significant deepwater GOM discoveries (Mahaffie, 1994; McGee, 1994). Sand-rich subsalt sediments can thus be penetrated after identifying the positions of the updip depocenters and upper slope fairways from where sediments can be transported downdip to lower slope-abyssal depocenters. 

McGuinness and Hossack (1993) demonstrated a reconstruction-dependent method of subsalt age-prediction in which sediment extension was equivalent to sheet lengthening and the translational fault extension could be used to determine the footwall cutoff “by measuring back from the hangwall cutoff on top of the sheet a distance equal to the amount of extension.” This is not always easily determined in the contorted, deeply buried salt environment of the present-day shelf and upper slope, but is a refined valid reconstructive technique, and reconstructions are useful in fully understanding these sediment/salt relationships.