Oscar R. Lopez-Gamundi1 and Héctor J. Villar2
1 ChevronTexaco, Bellaire, TX
2 FCEN-UBA, Buenos Aires, Argentina
The tripartite cycle identified in the Triassic asymmetric half-grabens of west Argentina is characterized by a conspicuous fining-upward to coarsening-upward signature: (1) an early stage where fault-related subsidence exceeds sediment supply expressed by an aggradational or backstepping stacking pattern, (2) a climax stage with the highest rate of fault-related subsidence and expansion and deepening of the lakes adjacent to the footwall, and, (3) a late stage with waning rate of fault-related subsidence and progradational stacking pattern.
Along the fault margin of the northernmost Las Peñas-Tamberías half-graben lacustrine source rocks are made up of calcareous shales with up to 13% of TOC and abundant amorphous, oil-prone, type I/(II) kerogen. Hydrogen index (HI) values range between 700 and over 1,000 mgHC/gTOC. Pristane/phytane (Pr/Ph) ratios are around 1 indicating persistent anoxia and low terrigenous input.
In the prolific Cacheuta half-graben, the lacustrine shales have average TOC values around 4% (locally reaching 20%), with some terrigenous component but dominant, amorphous, algal-like organic matter Pr/Ph ratios range between 2 and 3. Oils derived from these source rocks are predominantly waxy. All these geochemical characteristics suggest balanced to overfilled lake conditions.
Lacustrine shales in the Ischigualasto half-graben indicate TOC values of less than 4% and HI values up to 230 mgHC/gTOC and abundant type III, gas-prone organic matter. HI values are not greater than 350 mgHC/gTOC. The low to moderate TOC and HI values and abundant type III kerogen suggest an overfilled lake-basin type with significant clastic dilution due to strong fluvial input.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005