Shale compaction profiles from about 100 wells from western Indonesia Tertiary basins have been constructed from sonic logs, using Raiga-Clemenceau equation for calculating shale porosities. This equation is much better suited than the Wyllie’s equation, as it accounts for high porosity values between 37% to 50%. The resulting porosity depth plots do not resembles compaction curves as suggested by previous workers, but they can be best described as being to consist of linear segments. Each segment appears to correspond to a stratigraphic unit and segment boundaries coincides with sequences boundaries. Stratigraphic units or sequences appear to have their own shale compaction curves, consisting initially of a exponential curve near the surface until a certain depth where the porosity value is 38%, (called the inflection point), followed downward by a linear curve until it is constant further downward. In all the sections studied, the near surface exponential curve appears to have been eroded before the next sedimentary sequence is deposited. By assuming the inflection point- to be constant initially at about 500 m depth, the present depth of the corresponding shale porosity value of 38% (D10: depth of inflection observed) indicates the presence of an unconformity or a noncompacting interval. If DID is located at depth less than the theoretical depth of inflection point (DIT) of 500 m, then uplift and erosion have taken place before the sedimentary sequence was deposited. When 1310 is deeper than DIT (the theoretical depth of inflection point), then non compaction have taken place along a depth interval between DIT and 1310. Using this sort of procedure, it is possible to determine the thickness of missing sections due to erosion. However, in reality, it becomes more complicated because of the presence of overpressured shale and noncompacting intervals.
Authors : R.P. Koesoemadinata, Srikanti I. Qivayanti & Asep H.P. Kesumayana