Andrew P Bowman1, Howard D Johnson2, Neville Jones3
(1) Imperial College, University of London, London, United Kingdom
(2) Imperial College, University of London
(3) BP, United Kingdom
ABSTRACT: Analogues for Shallow Marine Sandstone Reservoirs in the Columbus Basin
Recent discoveries (e.g. Red Mango) have raised the profile of the Columbus Basin, offshore East Trinidad. The basin is late Miocene to recent in age, extensional and filled with up to 6km of sediment. It lies at the eastern end of the Caribbean/South American plate boundary zone. Pliocene to Pleistocene age stacked shallow marine reservoirs occur at the top of hundreds of metre thick upward coarsening sediment packages. Reservoir units (up to 80% sand) have excellent properties: porosities 25-30% and permeabilities of 1000md.
New analysis of equivalent age rocks exposed in southeast Trinidad, adjacent to the Columbus Basin, shows that they are also arranged into upward coarsening sediment packages. These contain basal slumped slope deposits, passing upwards into shelf mudstones with upper sand-rich units.
The upper sand-rich units comprise 5 to 15m thick parasequences with inner shelf mudstones and HCS sandstones/siltstones (30-50% net sand) passing up into middle/upper shoreface amalgamated SCS sandstones (95-100% net sand). Storm processes controlled depositional architecture with alongshore currents supplying sand from sources on the palaeo-Orinoco delta. Growth faults locally alter facies architecture and net: gross distribution.
Thicker transgressive and regressive parasequence sets record high rates of subsidence, sediment supply and relative sea level changes. Amalgamated SCS sandstones 10 to 50m thick occur above candidate sequence boundaries.
The outcrops provide good architectural analogues for the nearby subsurface reservoirs. Inner shelf/lower shoreface deposits provide evidence for vertical and lateral flow barriers. Parasequence flooding surfaces will compartmentalize reservoirs. The highest quality reservoir sands occur above candidate sequence boundaries.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado