Edward J. Kovas¹, Andres Ansa¹

(1) Repsol-YPF, 1035 Buenos Aires, Argentina

ABSTRACT: An Example of the Impact of Petroleum Systems Analysis on Exploration Prospectivity: A Case Study of the Montero Block, Chaco Basin, Bolivia

The Repsol-YPF Montero Block lies within the Chaco Basin of Central Bolivia, 40km northeast of the city of Santa Cruz. Within the block, the Montecristo Field has minor production from Carboniferous stratigraphic traps and shallow Devonian structurally trapped sands. In turn, deeper Devonian fractured sandstones offer potential for significant gas. The Montecristo Deep play, a high-risk/high-reward "deeper potential" target, has four-way closure, a regional reservoir and associated shallow production. However, timing/volume (charge) issues directly impact play prospectivity.

The petroleum systems picture is more complex than it appears. Two systems are hypothesized: 1) Silurian/Early Devonian shales charging overlying and intercalated structurally trapped sands, sealed by Devonian shales, and 2) Middle Devonian shales that charge stratigraphically trapped Carboniferous reservoirs. This study focuses on the older source sequence.

Source rocks are mature and generative in situ, but a dilemma exists. Modeling suggests that most generation/migration took place during the Devonian, converting most kerogen during that time. Because independent structural closure did not occur until mid Tertiary time, prior expulsion has been lost and remaining generative potential is minimal. While minor expulsion has been fairly continuous, Andean foreland basin compaction in the Tertiary, increased hydrocarbon expulsion. This mechanism is hypothesized to have charged intercalated reservoir sands, possessing some fracture enhanced, preserved primary porosity. Heightened awareness of timing/volume issues helps assess risk. In this case, it is estimated that since trap development (30 Ma to present), the volume of hydrocarbons available to charge the deep structure exceeds its pore volume by nearly two times.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado