Caprock Characterization: from Integrated Approach in a Gas Field, Southeastern Brazil
Seal capacity of a caprock is determined by its petrophysical properties (pore throat size distribution, threshold pressure, porosity). The present work demonstrates that these properties are mostly controlled by the sedimentogical, petrological and geochemical conditions of the rock history, ultimately commanded by its stratigraphic framework, depositional environment, and diagenetic history.
It is very well stabilized that sedimentology and compaction control the sealing properties. However it was not the main aspect directly involved in sealing capacity presented here.
Ten core samples of a gas field caprock in a Brazilian continental margin basin were analysed.
The microfossil assemblage of the mudstone indicates that these sediments were deposited during a relative sea level rise, typical of transgressive system tract.
Palynofacies studies and the very low total organic carbon (TOC < 1%) content suggest that the mudstones were deposited in a proximal oxic shelf with high biogenic activity.
All samples are silt-rich mudstones (50 % silt, 35% clay and 15 % very fine grained sand), commonly strongly bioturbated, with no fissility, occasionally laminated. Compositionally the very fine sand and silt fractions are made of quartz, K-feldspars, plagioclase, micas, and pyrite, whereas clay fraction is chiefly composed by chlorite.
The predominance of chlorite in the clay fraction is due to severe diagenetic transformation during burial diagenesis of a smectite-rich precursor similary to contemporaneous sandstones in the same basin.
The presence of high content of silt and very fine sand and the pervasive bioturbation would indicate rocks with low seal capacity. However, this was not observed in this study case probably because the bioturbation promoted the mixture of different grain sizes generating a framework where the clay fraction is the controlling factor of the pore throats. In addition, the intense succeeding burial diagenesis completely obstructed the porous system.
High pressure (up to 60000 psi) mercury injection measurements showed very high values of threshold pressures (from 12000 to 19800 psi) characterizing a very effective seal rock. The same analysis revealed a distribution of pore throats size with major frequency values with less than 5 nm.
The discrepancy between sedimentological controls and the entry pressure may be consequence of post deposicional process such as the compaction and/or diagenesis of the chlorite that fill the pores.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009