Diagenetic Processes in Sabkha Deposits and Exploration Potential of the Intractonic Parecis Basin, Western Brazil

AAPG Annual Convention and Exhibition 2009

Paper presented at the 2009 AAPG Annual Convention & Exhibition. Denver, CO.

The Parecis Basin is a large intracratonic basin in western Brazil, with more than 6 km os sediments. The petrologic study of the Paleozoic Pimenta Bueno Formation in the PB-01-RO drill core revealed the occurrence of sabkha deposits, in which playa lake evaporites are interbedded with fluvial sandstones. The diagenetic processes observed in these rocks are largely facies-controlled. The evaporites show early silicification of originally microbial, micritic mud, followed by extensive replacement by dolomite and coarse anhydrite. Gypsum and silica replaced dolomite and anhydrite under eodiagenetic and telodiagenetic conditions. In the fluvial sandstones, diagenetic products and processes are numerous. During eodiagenesis, hematite and clay coatings covered the grains, followed by multiple phases of quartz (and locally K-feldspar) overgrowths. Mechanical compaction of mud intraclasts led to the formation of pseudomatrix, which was commonly replaced by dolomite during mesodiagenesis. The main mesodiagenetic process was the precipitation of sulfates (anhydrite and barite) and carbonates (calcite, dolomite and Mn-Fe-rich dolomite). Telodiagenetic processes include the formation of pore-filling hematite, gypsum nodules and veins, and kaolin, and during a second mesodiagenetic stage illite and anhydrite were formed. Fluvial sandstones in the studied core show relatively good porosity and occurrences of residual bitumen. Such evidence suggests a good potential for hydrocarbon exploration in the Parecis Basin.