Abstract: The Petroleum Systems of the Boomerang Area, Bolivia

Laffitte, Guillermo , Silvia Del Vo - YPF; Enrique Aguilera, Victor Goitia, Gustavo Rebay, Daniel Lanussol - Andina

The Boomerang Hills region, with a surface of roughly 16,000 sq. Km, is one of the most prolific Bolivian oil districts and is located in the northern Chaco Basin (Fig. 1). A basement "high," with a general E-W orientation within a setting structured by Silurian extensional faulting, controlled the E-W sedimentation trend of the basin, resulting in a sharp thinning of the Silurian and Devonian sequences. All these features contributed to the accurate geometry of the Boomerang Hills and their oblique bearing to the general tectonic transport direction. The Andean compressive stresses and their tectonic interference with the basement during the Late Tertiary, generated a series of anticlines which are lined up following the Boomerang Hills trend. In this geological setting, many oil, gas and condensate accumulations have been found. These hydrocarbons are stored in Silurian (El Carmen Fm.), Devonian (Roboré Fm.), Cretaceous (Ichoa, Yantata and Cajones Fms.) and Tertiary (Petaca Fm.) reservoirs.

The source rock interval includes marine shales at the top of El Carmen Fm. (Late Silurian) and at the base of Roboré Fm. (Early Devonian). The TOC values of this interval are between 2 to 3% and the S₂ reaches values up to 14 mg/g. The visual kerogen analysis shows a predominance of amorphous and liptinitic organic matter. The hydrogen and oxygen indices of low-maturity samples indicate a type II kerogen on a Van Krevelen like diagram (HI vs. OI). These data show that the source rocks have a very good oil potential. The available information from deep wells demonstrates that such intervals may be more than 200 meters thick. The oil-source rock correlations show that most of the Boomerang oils correlated fairly well with El Carmen (Silurian) and Roboré (Devonian) rock extracts.

The TAI and Ro% data and the thermal modeling show that the hydrocarbon kitchen covers a zone that surrounds the Boomerang from its western and southern flanks. Such modeling suggests that the distal parts of the kitchen generated and expelled significant amounts of oil from the Late Devonian until the Middle Miocene, when the gas generation and expulsion began. During the secondary migration, the hydrocarbons could have initially traveled from the kitchen through the Siluro-Devonian sandstones updip. Close to the Boomerang, the hydrocarbons could have reached the Cretaceous and Tertiary traps. This vertical migration could have taken place along the reactivated distensive faults and/or could have been controlled by the efficiency loss of the seals. Several examples documented the erosion of the Limoncito Fm. (Middle Devonian) shales in the central Boomerang, due to a Cretaceous unconformity. Consequently, the sandy Cretaceous Ichoa Fm. overlies the Lower Devonian beds allowing the hydrocarbons vertical migration towards the younger reservoirs (Fig. 2).

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil