STRATIGRAPHY AND EVOLUTION OF THE TERTIARY ARUBA BASIN

Abstract

The Aruba Basin (offshore Venezuela) is asymmetric, with the basin axis aligned parallel to, and just to the south of the Aruba uplift. The sedimentary section consists mostly of clays and shales, deposited since the Oligence, or possibly late Eocene, on a Crataceous basement of oceanic affinity. This monotonous lithology makes it difficult to compile a useful stratigraphic subdivision, but three seismic‐stratigraphic units separated by unconformities have been distinguished, and are known as the Lower, Middle and Upper Sequences.Sedimentation began in abyssal‐to‐bathyal depths, after the rapid subsidence of a weathered, volcanic terrain; the rate of sedimentation exceeded that of subsidence, causing a gradual shallowing of the sea‐floor. The sedimentation rate during deposition of the Middle Sequence was sufficiently high to prevent normal compaction, thus causing a thick zone of abnormally high pressures. Deformation by faulting as well as folding was contemporaneous with sedimentaion and subsidence.The main source area for the sediments lies to the south; the Aruba High has acted prinicpally as a sediment trap. On the north flank of the basin, some (bio‐)clastic contribution from the island may be expected.The type of organic matter in sediments penetrated by the three wells off shore Aruba, as determined by pyrolysis, is mostly gas‐prone Type III kerogen. The TOC is fairly low—1% on average; but microscopic inspection revealed that a high percentage of the organic matter consists of amarphous kerogen. Due to the low geothermal gradient (1d̀ F/100 ft), this organic matter is immature. Another drawback for the development of petroleum accumulations if the limited areal distribution of carrier and reservoir rocks. These two factors seriously restrict the petroleum potential of the Aruba BasinHowever, there are indications from which the generation of petroleum can be deduced. In the deeper parts of the basin to the south of the island, sufficiently high temperatures may have been reached for Oligocene and older, sediments to be within the “oil window”; and the platform area in the SW may be underlain by older, more mature sediments. Further exploration on the north flank of the Aruba Basin and on the SW is considered to be justified, based on the inferred presence there of both source and reservoir rocks.