--> Integrated Analysis of the Upper Jurassic Bossier Deltaic Complex, East Texas: A Revisit of Deposition Seaward of the Delta-Front, J. B. Wagner and K. M. Stevens, #90093 (2009)

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Integrated Analysis of the Upper Jurassic Bossier Deltaic Complex, East Texas:  A Revisit of Deposition Seaward of the Delta-Front

 

 

J. B. Wagner and K. M. Stevens

 

Nexen Petroleum USA, Dallas, Texas

   

 

EXTENDED ABSTRACT

 

The sandstones encased within the Bossier Shale Member of the Cotton Valley Sandstone in East Texas are subdivided into three genetically related stratigraphic cycles.  The lower deltaic cycle is a seaward stepping unit that becomes reworked as a result of delta switching.  The upper two cycles are aggradational to progradational units whose facies range from prodelta to delta-front to distributary channel deposits.

 

Previous interpretations have ranged from submarine-fan to braided river with individual cycles interpreted to be bounded by regionally extensive marine flooding surfaces.  Detailed sedimentologic, petrologic, and biostratigraphic analysis of cores, well cuttings, and well logs, however, indicate that the stacking pattern of the Bossier deltaic complex is controlled by autocyclic lobe-switching as a result of varying sediment supply (overall increase) associated with the large Cotton Valley fluvial system.  In particular, detailed biostratigraphic analysis (palynology and kerogen) suggests that bounding shale intervals and “flooding surfaces” exhibit a high terrigenous/marginal marine signature.  True marine flooding events are associated only with the source-rock shales in the underlying Lower Bossier Shale interval.  Additionally, the abundance of distributary channels associated with all cycles suggests the entire Bossier Sandstone section is a river-dominated system subordinately influenced by marine processes.

 

Depositional history of the Bossier shelf margin began with an episode of salt displacement that set up an early ramp margin profile.  Early Bossier time was characterized by the deposition of transgressive marine shale source rocks (Bossier shale) associated with high subsidence rates.  Deposition of the Bossier sands followed this marine transgression, as indicated by an increase in sediment supply associated with the advancing Cotton Valley fluvial system.  Rapid abandonment of sediment supply due to delta-switching and associated relative transgressions encased the Bossier sands within these organic-rich marine shales.  The nature of the receiving basin or basin configuration can be characterized as an embayed, low slope ramp-style basin morphology.  We agree with the climatic change interpretation (from tropical humid to cooler more temperate) by Klein and Chaivre (2002) as a mechanism for sea-level lowering and associated rapid sedimentation.

 

The entire Bossier system is a river-dominated (fluvial) system in which marine processes (predominantly waves and storms) play a subordinate role.  Several analyses of the palynology and kerogen type data indicate that the studied sections of the Bossier Sand interval were deposited in a nearshore marine, strongly delta-influenced environment.  Autocyclic processes of channel avulsion and lobe switching controlled the stacking pattern, internal architecture and partitioning of the cycles typical of river dominated systems (Coleman and Gagliano, 1964).  Changes in reservoir architecture are observed related to a fluvial-dominated system (lower delta cycle) undergoing destructive processes by increased wave and tidal energy related to delta-lobe switching (upper delta cycles).  Due to abandonment and reworking related to decreasing sediment supply and increasing accommodation, a stronger shallow marine signature is recognizable between the individual deltaic cycles.  The sandstones of the Bossier Shale Member represent the distal equivalent of the Cotton Valley Sandstone and are subdivided in this study into three genetically related deltaic cycles (Bonner, Moore, and Taylor) with facies ranging from prodelta to delta front to distributary channels.  Palynology, sedimentology, ichnofacies, and seismic reflection characteristics all support a shallow marine fluvial/deltaic environment modified by wave and tidal processes.  Seismic expression of these deltaic systems are observed as low-relief lens shaped progradational geometries (~150 ft), that internally exhibit a series of divergent reflections toward the basin.  Opportunity for development of organized submarine fan deposition would be associated with the overlying large Cotton Valley fluvial system that truncates the tops of the underlying Bossier deltaic units.  These shelf margin deltaic systems essentially prograded the upper slope and provided a sediment source for mass-wasting processes to displace sediments downslope as gravity flow deposits which could fully encase these sandstones within marine shales.  These delta-fed gravity flow (turbidite) deposits would have good lateral continuity and higher pore-pressures potentially providing excellent flow rates as compared to their updip more shelfal equivalents.  Recent drilling seaward of the Bossier delta-fronts have proved this model correct by results observed today within EnCana’s “Deep Bossier Amoruso” Field. 

 

 

REFERENCES CITED

 

Coleman, J. M., and S. M. Gagliano, 1964, Cyclic sedimentation in the Mississippi River Delta plain:  Gulf Coast Association of Geological Societies Transactions, v. 14, p. 67-80.

 

Klein, G. D., 2002, Sequence and seismic stratigraphy of the Bossier Formation (Tithonian), western East Texas Basin:  Gulf Coast Association of Geological Societies Transactions, v. 52, p. 551-561. 

 

 

 

Wagner, J. B., and K. M. Stevens, 2009, Integrated analysis of the Upper Jurassic Bossier Deltaic Complex, East Texas:   A revisit of deposition seaward of the delta-front:  Gulf Coast Association of Geological Societies Transactions, v. 59,       p. 777-778.

 

AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana