Reservoir Architecture and Recognition Criteria for Confined Sheet Reservoirs: Deepwater Gulf of Mexico
Deepwater turbidite sandstone reservoirs are commonly deposited as channels or sheets. Sheet deposits are classified as confined or unconfined, depending on how the turbidite transport system interacts with local basin margins. In the supra-salt setting or outboard of the salt canopy, confined sheet reservoirs can be recognized by a lobe shaped seismic amplitude response. In the sub-salt setting, seismic frequency content is too low (~9-12 Hz.) to delineate depositional features, so additional recognition criteria are needed to reliably characterize confined sheet reservoirs.
Confined sheet deposits are typically composed of thick-bedded sandstones. Depositional net to gross (N:G) sandstone commonly exceeds 90% within the bulk of confined sheet reservoirs. High N:G and highly amalgamated sandstones result from turbidite flows that are large relative to local basin accommodation. Log character alone is not a unique diagnostic indicator, but multiple well penetrations in individual confined sheets generally reveal a highly correlative log motif. Small scale features may be correlative over long distances (miles), except where complicated by post-depositional faulting or deformation.
In core, the dominant lithofacies in confined sheets is thick-bedded structureless amalgamated fine-grained sandstone, commonly referred to as Bouma Ta turbidite beds. Sandstones typically appear to be poorly sorted, and may have other features that suggest rapid deceleration. Cross-beds, planar laminations, ripple laminations and mudclast beds may occur but are typically rare. Siltstone and shale is very rare to absent. Recognition criteria derived from appraisal and development assets with multiple cored intervals in confined sheets help to reduce uncertainty in exploration projects with limited wells. Examples from Paleocene and Miocene reservoirs are used to illustrate recognition criteria for confined sheets in the deepwater Gulf of Mexico.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California