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Petrographic Characterization of Matrix Vs Fracture Porosity in Tight Sandstones: The Huamampampa Formation from an 8 km-Deep Well in the Sub-Andean Fold-and-Thrust Belt of Bolivia

LACR Andes Basins Research Symposium 2020
AAPG Distinguished Lecture

Authors: Laura I. Net (presenter), Maria Ochoa, Daniel Peña, Alia Ponz Garcia, Massimo Bonora, Camilo Restrepo, Tomas Zapata, Repsol

Drilling of well BUY-X2 in the Bolivian fold-and-thrust belt launched a petrographic study on Devonian-aged Huamampampa Fm tight sandstones using cutting samples from a thrust sheet at ~8,000 m of burial depth. Optical and scanning electron microscopy, cathodoluminiscence, microprobe elemental mapping, fluid inclusion microthermometry, and X-ray diffraction were combined with image log and petrophysical data to better understand the rock matrix and its porosity network. Sandstone matrix is made up by tightly-packed, fine to medium-sized detrital grains of dominant quartz with subordinated feldspars and rock fragments. Intense diagenesis resulted in cementation mostly by quartz overgrowths, minor authigenic and/or recrystallized clays, and traces of titanium oxide and pyrite. Matrix porosity is very scarce, and limited to: a) primary intercrystalline microporosity associated to interstitial, intergranular pore filling clays, b) slot porosity at quartz overgrowth cement-cement contacts, and c) secondary intragranular microporosity due to partial dissolution of non-quartz grains. Petrographic image analysis (PIA) on CL-BSE photographs shows matrix porosity values ~3% in agreement with avg. PHI in petrophysical interpretation and regional data. This 3% matrix porosity is proposed as the minimum threshold that Huamampampa may achieve during deep burial. A tectonically-overimposed, secondary fracture porosity network is observed from macroscale in image log down to microscale in thin sections. Presence of oversized, euhedral quartz crystals trapping fluid inclusions with Ths from ~130°C to 190°C -close to bottom hole temperature- confirms the existence of an open, natural fracture system, into which authigenic quartz may still be precipitating at present reservoir conditions.


Laura I. Net, Repsol

Laura earned a BSc from the Universidad de Buenos Aires (1995), a PhD also from the Universidad de Buenos Aires (2000), and a MSc from the University of Texas at Austin (2003), all three in Geological Sciences.

With 16 years of experience in the industry, Laura is Geologist Advisor, Clastic Reservoir Quality, Geosciences Specialists Team (since 2010) at Repsol in Madrid, Spain, where she is in-house consultant specialized in topics related to clastic petrology, sandstone diagenetic modeling and reservoir quality assessment of sandstone and shale reservoirs. Laura previously worked for E&P projects targeting both conventional and unconventional reservoirs with Pioneer National Resources Argentina (2004-2006), Apache Corporation Argentina (2006-2009), and YPF SA (2009-2010) in Buenos Aires, Argentina.

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