Stratigraphic Controls on the Source Rock Distribution, Llanos Orientales Basin, Colombia

Juan C. Ramon, Andres Fajardo, Jorge Rubiano, and Andres Reyes

All available rock and oil geochemistry analyses were tied to a high-resolution stratigraphic framework for more than 50 wells in the Central Llanos Orientales Basin. New Tertiary generation input 15 proposed. The best source rock intervals are at the base and top of the Gacheta Formation (Upper Cretaceous) and in the middle of the Barco-Cuervos (Paleocene) and Mirador (Eocene) formations.

These organic-rich zones contain type II and III kerogen. TOC contents range from about 1% up to 15%. The four source rock intervals occur within marine shales near condensed sections, at the position maximum accommodation/ sediment-supply (A/S) ratios. The development of conditions that allow accumulation and preservation of anomalously high fractions of organic matter might be explained by two mechanisms. Increased A/S ratio results in retention of more sediment in the coastal plain, thus reducing the tendency for siliciclastic sediment to dilute the organic matter accumulating on the shelf. Also, deeper water might restrict circulation, enhancing bottom anoxic conditions. In the more transitional and continental sequences, increased A/S ratio is associated with higher phreatic wate level. A high ground water table enhances preservation of coaly intervals. The sea-level rise brings marine water into valleys and low-gradient coastal plains. The resulting embayments, marsh and swampy areas are organic-prone, contributing to the source rock potential of strata associated with high S/A conditions and base-level rise-to-fall turnaround positions.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California