KAOLINITIC BEDS AS CORRELATION HORIZONS IN THE SUBSURFACE OF THE COASTAL PLAIN OF SURINAME

As a result of intensive weathering of Paleocene-Eocene sediments during the so called bauxite hiatus in the Eocene and Oligocene , the well-known economic bauxite occurrences have been formed in the coastal plain of Suriname. These bauxites belong to the Onverdacht Formation and are developed as duricrusts on various hills which either crop out or are covered by younger sediments. The lower part of the Onverdacht Formation is characterized by kaolinitic clays and sands. The entire Onverdacht Formation can be regarded as a weathered sedimentary section which rest unconformably on weathered Precambrian rocks of the Guiana Shield.

Toward the North the sediments of the Onverdacht Formation change laterally in the sediments of the Saramacca Formation, consisting of a regular alternation of marine to fluvial sands and kaolinitic clays. Locally, various sands of the Saramacca Formation are oil bearing, giving rise to three oil fields which are currently being exploited by Staatsolie Maatschappij Suriname. In the course of time more than 1000 exploration and production well have been drilled in the coastal plain. The majoririy of these wells have been logged by gamma ray, density and resisitivity tools and the relevant logs have also been used for correlation purposes from well to well. Given the erratic nature of the depositional environment of the Saramacca Formation, it was not possible to establish a consistent lithological subdivision of this formation. Recently, the spectral gamma ray tool was introduced in the logging operations. This tool measures the natural gamma radiation of: 40K (Potassium), 232Th (Thorium) and 238U (Uranium).

A decreasing Th/K ratio indicates a base level lowering, initiating the formation of kaolinite. In this context a sea level lowering corresponds to this event. Several of these events, representing different sea-level movements, were recorded in numerous wells of the Saramacca Formation. This enables a consistent, new correlation framework throughout the entire coastal plain of Suriname. Recognition of these sedimentary sequences will highly contribute to the prediction of sand and clay occurrences which are essential in oil exploration.