ABSTRACT: Meteroic Diagenesis and Porosity Enhancement in Fluvio-Deltaic Reservoir Facies: B-6 Sands, Maracaibo Basin, Venezuela

Santosh Ghosh, Bernabe Aguado, Jesus Maguregui, Juan Di Croce, Andreina Isea

The Eocene northwest-southeast-trending fault-bounded reservoirs reveal a systematic diagenetic evolution due to changing physics1 and chemical conditions related to an Oligocene uplift, separating an Eocene and a subordinate Miocene phase of subsidence. Compaction, cementation, and dissolution during Eocene subsidence reduced original porosity to about 20%. The Oligocene uplift created abundant secondary porosity, raising the total porosity to around 30%, especially near the truncated erosion surface on the northwest. Here, abundant lamellar porosity is characteristic. Intense leaching action by undersaturated meteoric CO₂-charged waters may be responsible for the regenerated porosity. The sandstones immediately below the unconformity surface show the best res rvoir quality. Leaching of monocrystalline and polycrystalline quartz, feldspar, rock fragments, and clays has produced a highly porous and permeable diagenetic quartzarenite.

Porosity and permeability decrease systematically southeastward as the vertical distance from the unconformity surface increases. Labile grains largely persist, and compaction and cementation (calcite, silica, and clays) reduce the reservoir quality. A patchily welded texture observed in the southeast may be due to silica cement originated from grain dissolution near the unconformity surface on the northwest.

Presence of highly degraded crudes lends additional support to the influence of meteoric waters on these reservoirs.

Similar processes of silica dissolution through the action of acidic meteoric water presently occur in extensive outcrops of Precambrian karstic quartzarenites in Venezuela. These tropical analogs support the efficacy of freshwater flushing mechanism and consequent enhancement of reservoir quality in the Eocene sequence.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990