Print this pagePorosity Distribution in Relation to Unconformities and Its Implications
GHOSH, SANTOSH K.
Subaerial unconformities, common worldwide, are created by eustatic sea level
fall and/or tectonic uplift. The increased porosities in sandstones at
unconformities result from dissolution of labile grains like feldspar, lithic
fragments, and other unstable mineral grains and cements by undersaturated
carbon dioxide-rich meteoric waters. Even quartz and chert may dissolve, as
observed in the Roraima table mountains in Venezuela. Dissolution is most
effective in the coarsest, best sorted and permeable sandstones. Other
porosity-
controlling
factors include mineralogy, grain size, nature and extent
of initial diagenesis, climate, duration of subaerial exposure, and timing of
hydrocarbon migration.
A common attribute of these unconformity-related sandstones is that they were
subaerially exposed in a humid, warm setting, characterized by abundant
rainfall. In the Maracaibo Basin, various northwest-southeast trending
fault-bounded reservoirs in the Eocene deltaic-paralic Misoa Formation exhibit a
systematic trend of diagenetic evolution related to the post-Eocene uplift. The
reservoirs are truncated by the erosional SB-39.5 unconformity. Both porosity
and permeability show maximum values (twenty-four percent and 2000md
respectively) near the truncated erosional edge. The minimum values (twelve
percent
and 5Omd) occur farthest from the unconformity subcrop in areas beyond
the influence of undersaturated waters. Additionally, saturated meteoric waters
from the subcrop edge percolate downdip, mix with deeper connate water and
precipitate 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.