Exploration for Deep Gas in the Devonian Chaco Basin of Southern Bolivia: Sequence Stratigraphy, Predictions, and Well Results

Kenneth E. Williams, Barbara J. Radovich, James W. Brett

In mid 1991, the basin modeling and sequence stratigraphy teams were assembled at Texaco's Frontier Exploration Department (FED) to define a well position for assessing the hydrocarbon potential of the Chaco Basin of Southern Bolivia. The Texaco Miraflores #1 was drilled in fall 1992 and confirmed the predicted stratigraphic trap in the Mid-Devonian with gas discovered in two highstand and transgressive sands. These are classic 'low resistivity ' sands in a deep basin 'tight gas' setting. Assessment of this play is complicated by drilling fluid interactions at the well bore which reduced porosity of these already low porosity/permeability sands.

This basin has no structures of any size, but offered several high risk, large stratigraphic traps. Extensive outcrop work and previous seismic studies conducted by Texaco's research groups had documented over 300 meters of fine-grained, reservoir quality Devonian-age sands in the Andean thrust belt pinching out to the east somewhere on the Texaco block (partner at the time was Santa Fe Energy Resources). Wells in Paraguay show only distal shales and silts.

The main target of the well was the sand-rich, mid-Devonian Huamampampa formation. This unit is deposited as a series of third-order highstand sequences prograding to the east. Regional seismic data and log signatures show a major second-order sequence boundary within the formation. Sands deposited on top of this sequence boundary in a transgressive systems tract are the main target, and show a regional seismic amplitude response which abruptly terminates basinward. Tertiary thrusting reversed the basin dip. This presents a favorable stratigraphic trap as the updip transgressive sandy lithofacies change into, and are encased in basinal, source quality, gas-generating shales. Distal highstand paralic sands were a back-up target. The Texaco Miraflores #1 found the two gas-charged sands s predicted.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995