Oil Trapping in the Onshore Oil Fields of Suriname and Implications for Prospectivity in a Sub‐Regional Context
Staatsolie has been producing oil from Paleocene, Eocene and Miocene sands in the central onshore area of Suriname, for almost 27 years. The many exploration wells that have been drilled for additional fields have not resulted in any further commercial discoveries. Staatsolie has now embarked on a Play-based Exploration study to assess the remaining exploration potential of the Nearshore and Shallow Offshore. As part of that, this study was carried out with the objective to come to an understanding of the trapping mechanism(s) in the onshore fields and its relation to the existing and potential oil accumulations in surrounding Onshore and Nearshore Blocks, in order to understand the risks and volume potential of the identified prospects and leads in the Nearshore as well as the Shallow Offshore Blocks. A regionally consistent stratigraphic and structural framework has been established, gross depositional environment (GDE) maps, Post Well Analyses (PWA), Reservoir-Seal Pair established, and hydrocarbon charge models were constructed and / or evaluated. The focus of the study was on the Paleocene-Eocene section since most of the oil onshore production derives from this section. Five (5) Reservoir-Seal pairs (RSP’s) were distinguished in the Paleocene-Eocene section (2A, 2B, 3, 4, 5/6), and evaluated. The deposits in the Nearshore and Shallow Offshore represent a largely unstructured northward thickening sediment wedge. Nevertheless, several structural trends with subtle faults can be seen. A trend with NE-SW oriented faults is thought to be linked to reactivation of Precambrian basement weaknesses; ENE-WSW & EW to WNW-ESE oriented faults probably originated during the opening of the Central Atlantic. Re-activation of the faults during the Cenozoic is linked to the West to East movement of the Caribbean plate and far-field Andean compression. The Bakhuys structural nose, which was previously thought to have a major impact on the migration and trapping of oil in the onshore oil fields, is outlined based on an integration of well and seismic data. The high is extremely subtle in the onshore and disappears rapidly towards the North (nearshore). The structural feature is therefore thought to be more likely to have effected trapping rather than migration. The overall depositional setting in the onshore oil fields is mainly continental to transitional marine. Due to relatively high sediment input in the Calcutta/Coppename area, sand/shale ratios are higher there and the marine influence is forced more northwards (i.e. basinwards). Two marked flooding phases linked to relative high sea-level stands led to the deposition of the two regional seals during deposition of RSP2/3 and RSP 5/6. The remaining sections comprises a heterogenous system of mostly discontinuous sand and shales. In RSP’s 2A/2B/3 oil is trapped due to favorable sand/shale alternations (updip sand pinch-outs), and sands encased in shaly/calcareous sections where trapping can be locally fault induced. RSP4/5 consist of local oil accumulations (attics), mostly with OWC’s, where vertical migration occurs from the underlying sections due absence/incisions of Seal 3, and further upward migration along local seals also occurs. Oil accumulations in the RSP 5/6 shale occurs in sands encased in the shaly section. The charge model for the Paleocene-Eocene accumulations is still to some extent enigmatic, comprising a dominant contribution from the proven deep offshore Upper Cretaceous (ACT) kitchen with marine type II source rocks. Migration from the deep ACT kitchen is difficult to map due to the complex (aerially and stratigraphically) distribution of seals in combination with an overall distal to proximal trend from the offshore to the onshore. It is therefore thought likely that the migration took place in a diffuse manner, with broad focus areas. Where the seals are more regional and intact such as Seal 2/3, they have a guiding and focusing role for charge. Implications for the Paleocene-Eocene Prospects Nearshore: It is thought that any offshore accumulations are likely to be less compartmentalized than the onshore fields. With a greater proportion of (shallow) marine sediments, reservoirs will be more continuous, with (sub) regional sealing units linked to flooding events, such as Seal 2/3 and Seal 4, improving the chances for larger trap geometries in RSP 3 and 4.
AAPG Datapages/Search and Discovery Article #90361 © 2019 AAPG Latin America & Caribbean Region Geosciences Technology Workshop, Recent Discoveries and Exploration and Development Opportunities in the Guiana Basin, Paramaribo, Suriname, November 6-7, 2019