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Physical Properties of Carbonate Reservoirs in South Sumatra
Widespread deposition of platform and reefal carbonates of the Baturaja Limestone occurred during the Miocene epoch in the South Sumatra Basin. Significant oil and gas accumulations have been discovered in the porous facies. However, since porosity within the Baturaja Limestone has been observed to vary widely between tight platform facies and highly porous reefal facies, predrill prediction of porosity is an important exploration objective.
It is well known that porosity and certain physical properties of carbonates are directly related. Consequently, prediction of these properties in seismic data could provide indirect evidence of porosity occurrence, and by corollary, its absence. In order to do this, there needs to be a good understanding of the effect of porosity change on those physical properties of limestones which define the nature of seismic reflections. This paper documents the measured values of three of the physical properties of the Baturaja Limestone from PT Stanvac Indonesia acreage in the South Sumatra Basin: P-wave velocity, density and electrical resistivity. These parameters were investigated for their relationship with porosity. Established modeling equations like the Time-Average Equation, Raymer's Equation, and Gassmann's Equation were tested against the data base of measured values, and it was found that the Time-Average Equation calculates velocities that are in best agreement with the data.
Prediction of velocity from electrical resistivity using a combination of Archie's Equation and the Time-Average Equation is shown to be possible; therefore modeling can be done in areas where electrical logs are the only well log data available.
A general trend of decreasing porosity with depth was noted for the Baturaja Limestone samples, indicative of consolidation changes that are a function of overburden pressure. Also, comparison of acoustic impedances of Telisa shales with those of gas-charged limestones indicates that in contrast with conventional belief, direct hydrocarbon indicators may be seen over limestone reservoirs.
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