IDENTIFYING NEW POTENTIAL IN A MATURE FIELD: A MIXED SILICICLASTIC-CARBONATE K-LIMESTONE RESERVOIR CHARACTERIZATION IN SUPAT FIELD, SOUTH SUMATRA BASIN

摘要

The Supat Field in South Sumatra is a mature oil field which has produced from the Talang Akar Formation for 28 years. The declining of the oil production has resulted in a re-analysis of the entire stratigraphic sequence to determine if any bypassed potential remains in the field. One such interval investigated was the Miocene K-Limestone. This interval overlies the Talang Akar Formation and while drilling the development wells, exhibited oil shows. The wireline data over the K-Limestone interval suggested a low porosity rock with high water saturation (Sw), which was at odds with the drilling data. In order to determine whether the K-Limestone was oil bearing and moveable, well tests were conducted on two wells. Both tests confirmed an oil bearing low permeability rock which was capable of flowing oil with a minor water cut which contradicts the high calculated Sw (80% and 100%). In order to determine the potential of this reservoir unit, a full reservoir characterization was conducted. This work included interpretation of the 2D seismic data, post-stack seismic inversion for structural framework and reservoir identification, stratigraphic well correlation for reservoir facies discrimination and distribution and petrophysical analysis to determine the reservoir presence and saturation. From this information, a geological model was built, therefore the reservoir complexity and connectivity could be simulated. The primary uncertainty in evaluating the potential of the K-Limestone is the calculation of the reservoir properties. The petrophysical calculations for the reservoir quality and saturation rely heavily on the core data. Unfortunately, there are no conventional core data in the K-Limestone so petrophysical models which does not rely on the core data were created. In the first model, the K-Limestone was assumed to be a pure limestone which resulted in a high calculated Sw and contradicted with the test data. In the second model, the K-Limestone was treated as a mixed carbonate and clastic reservoir where the volume of shale was determined. This resulted in a lower Sw, which could then explain the test results. This observation also supported by petrography data from Sidewall Core (SWC) which shows that the K-Limestone is a clastic sedimentary rock made up of a mixture of siliciclastic and carbonate material. Additionally, K-Limestone geometry interpreted from the seismic data suggests that it is neither a carbonate build-up nor a platform facies. Defining the bypassed potential in mature oil and gas fields is critical to maximize the recoverable hydrocarbons. In Supat Field, assumptions of rock lithology led to an interpretation that the K-Limestone did not contain pay. Careful examination of the cuttings and mud log data resulted in a re-interpretation of the lithology of the K-Limestone and re-calculation of pay using a new petrophysical model. This approach has made it possible to realize the full potential of the K-Limestone in the Supat Field.