ABSTRACT

Interbedded Sand Analysis (XISA) is a bimodal model that computes a resistivity profile which makes use of a new, extended range, high resolution induction tool (XHR*) along with existing high resolution shale indicators such as a shallow dielectric log.

The XHR is used to provide an accurate resistivity profile in thinly bedded turbidites where the conventional devices show low contrast in these hydrocarbon-bearing intervals. This unique device uses a pulse source and two simultaneously sampled receiver arrays that are offset. Each array provides a digitally sampled log representing the average resistivity over a 12-inch interval. Since the two assemblies are offset, they do not measure the same 12-inch interval and therefore overlap to provide greater coverage and information. The logs from the two assemblies combine to form a composite curve that can be correlated with other high resolution devices.

In this turbidite example, the XISA analysis uses the XHR and EPT** logs to generate a resistivity profile for thin beds. XISA uses the EPT as a shale indicator and reconstructs the resistivity based on the assumption of a constant shale resistivity, R_shale, and computes R_sand. From this information, XISA computes high resolution resistivity with the combination of the maximum value of the two XHR curves. This new deep resistivity (XISA=Rt) can then be used in a straightforward water saturation evaluation based on as assumed constant porosity in the clean layers as defined by the shale indicator.

* Mark of MPI

** Mark of Schlumberger

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