FINNERAN, JOSEPH M, and Y. KEITH BALLY Amoco Energy,Trinidad

Abstract: Petrophysical Characteristics of an Emerging Giant Gas Province: The Columbus Basin Southeast Coast of Trinidad

Beginning in 1993, exploration drilling in the Columbus Basin by Amoco resulted in 16 of 21 successful wells and over 13 TCF of newly discovered gas reserves. Two factors have combined to produce these exceptional results: gas market development and excellent petrophysical characteristics of the gas reservoirs. This paper will use examples to illustrate the petrophysical characteristics of these new reservoirs, focused on volumetric estimations (porosity and water saturation) and flow rate (permeability and pore throat radius). The Columbus Basin on the southeast coast of Trinidad consists of up to 20,000 feet of Pliocene and Pleistocene fine grained sandstones and shales. Sandstones are mature, averaging 90% quartz, 2% to 5% feldspar and minor detrital clay. Diagenesis is limited to mechanical compaction and minor amounts of pyrite, siderite and quartz overgrowth cement in the deeper reservoir sands. Due to these factors, porosity is controlled by depth of burial and depositional energy or clay content. Permeability is controlled by grain size, and indirectly by porosity and clay content. Capillary pressure data show consistent relationships of increased entry pressure, dominant pore throat radius and irreducible water saturation with decreased porosity and permeability. Based on porosity-permeability and capillary pressure data, petrophysical rock types have been defined where water saturation can be calculated in a wellbore independent of resistivity measurement. Archie constants, n and m, increase with depth from a low of 1.65 shallow to almost 2.0 in the deeper compacted reservoirs. Archie constants less that 2 favorably impact water saturation calculations and gas-in place estimates.