L. L. Brooks1, J. B. Wagner2, T. D. Sheffield1, P. A. Zippi3, M. A. Dablain1, K. M. Stevens1, B. Brown1, R. Offenberger1
(1) Pioneer Natural Resources, Irving, TX
(2) Pioneer Natural Resources, Dallas, TX
(3) Biostratigraphy.com, Garland, TX
ABSTRACT: Integrated Analysis Of The Upper Jurassic Bossier Deltaic Complex, East Texas
The sandstones encased within the Bossier Shale Member of the Cotton Valley Sandstone in East Texas are subdivided into 3 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 characterized primarily as aggradational to progradational units and facies range from delta-fed gravity-flow 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 analyses 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.
Rock physics and seismic modeling of the Bossier sands have demonstrated a seismic response strongly dominated by large acoustic impedance contrasts associated with porous sandstones, low porosity siltstones and over-pressured shales. Depositional and sedimentological characteristics of the Bossier sands strongly resemble characteristics of modern-day fluvial dominated deltaic system (e.g. Mississippi River) undergoing processes of delta-switching and abandonment.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado