Impact of Freshwater Diagenesis on Reservoir Quality: B-6 Sands, Mosoa Formation, Venezuela: ABSTRACT

Abstract

Various northwest-southeast-trending fault-bounded Eocene reservoirs in the Maracaibo basin exhibit a systematic trend of diagenetic evolution as a response to changing chemical and physical conditions related to two phases of subsidence (Eocene and Miocene) separated by a period of major Oligocene uplift. The initial subsidence, more important than that of the Miocene, caused compaction, cementation, and dissolution, reducing original porosity from 40% to about 20%. The Oligocene regional uplift created abundant secondary porosity, specifically in the vicinity of the truncated erosion surface at the northwest limit of the reservoirs, through intensive meteoric leaching of framework grains and cements by undersaturated CO/sub 2/-charged waters. Both the porosity and permeability systematically decrease southeastward as the vertical distance from the unconformity surface increases. The sands immediately below the erosional unconformity have the best reservoir quality. They show profuse leaching of monocrystalline and polycrystalline quartz and are virtually free of unstable rock fragments, feldspar, and authigenic clays. In contrast, labile grains largely persist in the areas away from the unconformity where cements of calcite and clays have moderately reduced the reservoir quality. Some of the dissolved silica, resulting from quartz and feldspar dissolution near the unconformity surface, would have precipitated in the areas to more » the southeast where a patchily welded texture commonly was observed. Similar processes of silica dissolution through meteoric water leaching occur in outcrops of Precambrian karstic quartzarenites in Venezuela. These modern analogs in areas of heavy rainfall and copious production of inorganic and organic acids corroborate the efficacy of a freshwater flushing mechanism and consequent enhancement of reservoir quality in the Eocene sequence. « less