Application of New Generation Logging Tools in Recognizing and Quantifying Dolomitization in Carbonate Reservoirs

Rajiv R. Tiwari
B&S Asset, ONGC, Mumbai, India

Amongst the post-depositional alterations which occur in carbonates, the process of dolomitization is a long going process and in its initial stage of occurrence it may go unnoticed both on conventional logs as well as on cores. As we know, the process of dolomitization increases the porosity because of partial replacement of calcium ions with magnesium ions. However, this increase in porosity will not be accounted for unless the user invokes dolomite as a mineral in his model and has got tools to solve for it. In other words we need to know the volume of dolomite in order to quantify the enhancement in porosity. In the absence of any lithological tool this job is being done with the help of Photoelectric log which, however, cannot always be relied upon as it is affected by a number of environmental factors.

The new generation Elemental Capture Spectroscopy is capable of directly providing the elemental concentration of magnesium at each sample level, which in turn has a direct bearing on the volume of dolomite at that level. The dry weight percentage of magnesium can be directly introduced into the solver (ELAN model) to solve for dolomite volume. An ECS driven ELAN model run in a carbonate environment of a field in Mumbai offshore shows an overall 5% increase in porosity compared to the conventional ELAN without ECS. Other factors remaining same, this is bound to increase the in-place reserves.

Another new generation tool, Combinable Magnetic Resonance, is capable of providing porosity which is free of any lithological effect. This porosity can, in turn, be used in porosity-density transform to compute the actual matrix density at each level, which otherwise is taken as a constant (normally 2.71 gm/cc in carbonate reservoirs). The solver uses this information to compute the volume of dolomite once the CMR porosity is invoked in the model.

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