Impact of Tectonic Setting and Diagenetic History from the Reservoir Potential of Eocene to Miocene from- and off-Shore, Deepwater Sandstones, Northern South America
The deep structure and stratigraphy of 3 to12-km-thick Eocene to Miocene clastic sedimentary basins have been studied using seismic reflection data and compilation of existing well data from on- and offshore basins of the Lesser Antilles arc and forearc, the Barbados accretionary prism, and the Maracaibo and Falcon basins of western Venezuela. Offshore basins of northern South America remain largely undrilled and consequently have yielded only a major giant gas field and small discoveries of oil and gas. In this study, we summarized results of a petrographic study of 80 sandstone samples from Eocene to Miocene wells and outcrops from the region of northern South America to show the importance of the basin’s tectonic setting and diagenetic history on the reservoir potential of Eocene-Miocene sands; we also compare producing onland reservoirs with potential offshore reservoirs of roughly equivalent age. Point counting of sandstone grain types show that samples from Eocene foreland basin settings in the Maracaibo basin (Q85F5L10), Falcon Basin (Q75F13L12), Trinidad (Q92F5L3), and Barbados (Q95F3L2) are compositionally mature with higher quartz percentages (>75%), characteristic of the recycled orogenic and cratonic provinces in Gazzi-Dickinson’s sandstone model. In contrast, outcrop samples from the Eocene forearc basin near Margarita Island show higher percentages of volcanic rock fragments and clay (Q42F33L25 consistent with their location adjacent to a now inactive part of the Lesser Antilles arc. Quartz cementation, especially quartz overgrowth is a major cause of primary porosity-loss and secondary porosity in all samples along with dissolution of feldspars, calcite cementation and leaching of other unstable rock fragments. Outcrop samples from Barbados exhibit unusual diagenetic effects not seen in other areas that include leaching and fracturing of quartz grains which cause much higher secondary porosity. Interpreted diagenetic stages for both outcrop and well samples include: clay rims, mechanical compaction, quartz overgrowths, generation of secondary porosity by dissolution, precipitation of kaolinite in the secondary pore, and iron-oxide cementation. Potential Eocene to Miocene reservoir rocks from the Falcon, Trinidad and Barbados show similar reservoir characteristics and quality as those known reservoirs in the Maracaibo Basin. However, basins near arcs like Margarita have less potential.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009