Shelf petroleum system of the Columbus basin,offshore Trinidad,West Indies.II.Field geochemistry and petroleum migration model

摘要

Petroleum migration patterns in the present-day shelf area of the Columbus basin reflect a complex interaction of structural evolution,stratigraphic architecture,and fault-seal behavior.Thermogenic charge access is ultimately controlled by the geographic distribution of sand-prone carrier beds and how these vertically stacked stratigraphic units relate spatially to one another along their basinal limits.Migration in this system consists of two components,cross-stratal migration through mud-dominated,deep-water (slope and basin floor) sediments followed by horizon-parallel flow along laterally extensive,sand-prone,shelf carrier beds.Thermogenic charge access to any individual sand occurs only in a linear geographic area where that sand is not shielded from vertical migration by stratigraphically older sands.The geochemical characteristics of the trapped petroleum in this system are primarily a reflection of migration distance,with the earliest-expelled (lowest maturity) products having progressed farthest along the migration pathway.Because of the stratigraphic architecture and structural geometry involved,a pattern of increasing thermogenic content and maturity with depth at any location is created,despite the fact that a significant portion of the migration is horizon-parallel.In parts of the basin,this simple pattern has been overprinted by complex mixing of maturity fractions in single accumulations as a result of late-stage differential source-rock uplift.The observations are inconsistent with previously proposed models for this basin that emphasize petroleum fractionation accompanying vertical migration through the shelf stratigraphic section.