Abstract: Porosity Enhancements at Unconformities and Their Implications

Santosh K. Ghosh

Subaerial unconformities, common worldwide, are created by eustatic sea level fall or tectonic uplift. The increased porosities in sandstones at unconformities result from the dissolution of unstable grains like feldspar, lithic fragments and carbonate/sulphate cements by undersaturated carbon dioxide-rich meteoric waters. Even quartz and chert may dissolve, as observed in the Romima tepuys in Venezuela. Dissolution is pronounced in the coarsest, best sorted and permeable sandstones. Other porosity-controlling factors are mineralogy, grain size, nature and extent of initial diagenesis, duration of subaerial exposure, and timing of hydrocarbon migration.

A common attribute of these unconformity-related reservoirs is that they were subaerially exposed in a humid, warm setting, characterized by copious rainfall. In the Maracaibo Basin, various northwest-southeast-trending fault-bounded reservoirs in the Eocene fluvio-deltaic Misoa Formation exhibit a systematic trend of diagenetic evolution related to the post-Eocene uplift (SB-39.5). The fault-bounded reservoirs are truncated by the erosional SB-39.5 unconformity. Both porosity and permeability show maximum values (24% and 2000 md) near the truncated erosional edge. The minimum values (12% and 50 md) occur furthest from the unconformity subcrop in areas beyond the influence of undersaturated waters. Additionally, from the subcrop edge saturated meteoric waters percolate downdip, and mi with deeper connate water and precipitated cements in the southeast, further reducing reservoir quality.

In-depth exploration of unconformity related plays will yield significant dividends but it would require integrated analysis of depositional facies, petrology, diagenesis, and burial history, in relation to timing of hydrocarbon migration and structural development.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela