Unravelling the Role of Microbes in Tuffaceous Sediment Geobodies From Pozo D-129 Formation: Clue to Understand its Renewed Potential

DANIELA HUNZIKER; MURIEL PACTON; CRISOGONO VASCONCELOS; PAMELA CAROLINA DROEVEN; LEANDRO CESAR GALLO

Conferencia; Unconventional Resources Technology Conference; 2016

The Golfo San Jorge Basin (Southern Patagonia, Argentina) is one of the biggest oil and gas accumulation basins in Argentina of which the Pozo D-129 Formation (Fm.) (Barremian- Lower Aptian) is the main source rock. The Pozo D-129 Fm. is formally divided into an Upper, Medium and Lower Section. A sub-unit of the Upper Section, represented by black shales and informally named the ?mudstone horizon?, has recently emerged for its world-class shale-gas/oil potential. Moreover, the Upper Section at the Northern Flank revealed outstanding gas production from tight reservoirs. Reservoir quality varies in this basin primarily reflecting heterogeneous sedimentary deposits in a fluvial/lacustrine system. Coarser grained deposits in ancient channels and fans with high primary porosity alternate with fine grained volcaniclastic sediments and ash, which modify flow rheology by increasing density while reducing selection and primary porosity, which largely reduces reservoir quality. Diagenetic processes later modify those primary features, when pore space is closed by precipitating cement or, more rarely, when mineral dissolution or dolomitization increases secondary porosity. Previous studies have already demonstrated the high impact of mineral dissolution on petrophysical properties in tuffaceous deposits, yet, the mechanisms behind those processes have not been elucidated. To identify possible factors triggering mineral dissolution, we combined sedimentological, petrological and logging data to characterize reservoirs in the Upper Section of the Pozo D-129 Fm. and explore the potential enhancement of secondary porosity through microbial processes. Porosity and permeability values from reservoirs of the Upper Section are within the range of tight reservoirs. Concomitant log analysis has shown that porosity positively correlates to gamma ray count rate and possibly to potassium concentration. Using this correlation, high potassium concentrations derived from spectral gamma ray (SGR) logs could help detecting this type of reservoir. We propose that microbial metabolism, which released organic acids in these rocks could have influenced physico-chemical parameters and thereby enhanced preferential mineral dissolution. Thus, these biological processes have the potential to impact physical rock properties and, as a consequence, improve reservoir quality.