--> ABSTRACT: Provenance Controls on Microscopic Reservoir Heterogeneities, Eastern Venezuelan Basin, by J. I. Guzman; #91021 (2010)

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Provenance Controls on Microscopic Reservoir Heterogeneities, Eastern Venezuelan Basin

GUZMAN, JOSE I.

Variations in sediment source and dispersal have a primary control on porosity types and permeability in Cretaceous and Tertiary reservoirs of the North Monagas trend of Eastern Venezuela. Modal compositions of clastic detritus indicate a shift from cratonic to recycled-orogen provenance, as the basin evolved from an Atlantic-type passive margin to a foredeep. Sediments derived from the Craton in the Late Cretaceous show a mean composition of Q[92.6]F[4.8]L[2.6] and Qm[51.5]F[4.8]Lt[43.8]. The main source for this apparently contradictory composition are the various feldespathic, quartzose, and lithic sandstones of the Precambrian Roraima Group, located within the Guayana Shield to the south. Partial or complete leaching of potassium feldspars and rock fragments results in significant secondary porosity generation in this interval, but precipitation of authigenic illite reduces permeability due to obstruction of pore throats.

Late Oligocene-Early Miocene sediments are also derived from the Guayana Shield, but a longer transport path through a westerly drainage system results in an average composition of Q[99.2]F[0.3]L[0.5] and Qm[73.2]F[0.3]Lt[26.5]. Porosity is mostly primary, reduced by mechanical compaction and quartz cementation, and locally by authigenic kaolinite. Middle Miocene sandstones are poorly sorted and show a mean composition of Q[46.9]F[0]L[53.1] and Qm[32.4]F[0]Lt[67.6]. This is clearly indicative of lithic recycled orogenic provenances from the thrusted terrains of the Serrania del Interior to the north. Primary porosity is destroyed by early calcite cements which show no evidence of dissolution at their present depth of burial. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.