ABSTRACT

Miocene sandstones of Iberia and St. Mary Parishes, Louisiana, cored at depth of 12,000 to 16,000 ft (3600 to 4880 m), were deposited in fluvio-deltaic and shallow marine environments. The reservoir quality of these sandstones is not only dependent upon the environment of deposition, but also upon the diagenetic history of these rocks.

Pore volume reduction due to mechanical compaction (V_mc) was determined petrographically for the three sandstones by assuming V_mc = 40-(C+P), where 40 is the original porosity, C the amount of cement, and P the amount of pore space, all in percents. Of the three sandstones studied, the "S" Sand has experienced the least mechanical compaction and the Planulina 6 Sand the most. The difference in mechanical compaction between these sandstones is due to the depth at which calcite cementation effectively stopped compaction.

During early (shallow) stages of diagenesis, chlorite rims and quartz overgrowths precipitated in the pore spaces of the sands. As silica cementation proceeded, calcite cementation began. Mechanical compaction occurred contemporaneously with these cementation events but was hindered by the calcite cement when it developed in abundance. Mechanical compaction and calcite cementation completed at a burial depth of 6,300 ft (1920 m) for the "S" Sand. Partial to complete dissolution of this calcite cement occurred when a greater than 10,000 ft (3050 m) burial depth was reached, by fluids from nearby shales that had undergone smectite-to-illite conversion and organic maturation; dissolution created the present secondary porosity. Kaolinite precipitated in the sands during cement dissolution. As the pH of the pore fluids in the sand increased, late mixed-layer illite/smectite and chlorite precipitated.