Abstract

This chapter presents an overview of the geochemical composition of 15 heavy oils from producing and exploration wells onshore Suriname aiming to determine the organic facies generating them. The inferred facies are integrated with the geologic framework of the Guyana Basin in a two-dimensional basin model to further assess their thermal history in the shelfal area of the basin.

Detailed biomarker and carbon isotope geochemistry indicates that two compositional groups occur onshore Suriname. Oils produced from Cenozoic reservoirs (Group A) possess compositional attributes characteristic of oils generated from a distal marine shale. Their composition suggests the Upper Cretaceous shales of the Canje Formation as their possible source. In contrast, oil shows in the Upper Cretaceous strata (Group B) have biomarker relationships diagnostic of oils derived from a proximal marine depositional system rich in terrestrial organic matter. Uncertainty exists as to the age and spatial distribution of this organic facies. A Late Jurassic–Early Cretaceous age is provisionally proposed. Oil-maturity estimates indicate generation from source rocks at the mid-oil window for all the sample set.

Thermal maturity modeling suggests that generation from the Upper Cretaceous (Canje Formation) and Upper Jurassic–Lower Cretaceous source rock facies started in the early Oligocene. The Upper Cretaceous clay-rich facies has only transformed 30% of its potential in the shelf with expulsion starting in the middle Pliocene. Accordingly, entrapment of Canje-generated oils in the onshore Tambaredjo trapping structure is suggested to be younger than the middle Pliocene. The Upper Jurassic–Lower Cretaceous facies in most of the shelf area has nearly reached peak generation with expulsion commencing in the late Miocene given the input parameters. In the shelf area, updip migration of hydrocarbon expelled from these two organic facies is dominant and terminates around the onshore Tambaredjo area.