BALDIS, BRUNO A. J., and RICARDO D. MARTINEZ, Institute of Geology, National University of San Juan, San Juan, Argentina

ABSTRACT: On the Existence of Potential Source Rocks in the Lower Paleozoic Carbonate Sequence of the Precordillera, San Juan, Argentina

The Precordillera of western Argentina is a terrane that comprises a thick Upper Cambrian-Lower Ordovician carbonate sequence corresponding to a stable platform developed over a "Pampean" basement. Several authors have considered this terrane as allochthonous, on the internal border of the Andean Mobile Belt; but it is more possible to assign it to a parautochthonous origin with minor relative displacements.

The presence of kerogen has been detected at an interval comprising the Zonda (Cambrian) and San Roque (Cambrian-Tremadocian) formations, which correspond to calcareous intertidal to lagoonal sequences deposited in restricted carbonate platforms and grading into supratidal facies to the east (shoreline). The Zonda Formation includes dark wackestones, with oolites, intraclasts and peloids; gray to brownish carbonates with high fenestral porosity, showing fine lamination and stromatolites; and light-colored dolomitic mudstones with voids filled with sparite. As the result of sea-level changes, this part of the sequence shows typical dissecation structures and increased fracture porosity with kerogen fillings. The San Roque Formation is mainly formed by dolomitic mudstones showing bird's eyes filled with kerogen and strong evidence of dissecation and dehydration. Hydrocarbons always appear as partial fillings of fractures; also included in granular sparite veins or between microsparite bands in the laminated carbonates. These kerogen shows are the first described in Lower Paleozoic rocks in Argentina, and are tentatively assigned to changes in sea-level and the last stages of the Cambrian Great Cycles in South America.

These new potential source rocks open new frontiers for petroleum exploration in the Cuyo Basin and prove potential plays in Lower Paleozoic rocks, as have been described in several basins of North America, China, and Australia.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.