In formation evaluation, resistivity logs are typically used to identify pay zones because of the resistivity contrast between the oil zone and water zone. But not always. When the pay zone exhibits low resistivity, these logs cannot identify oil zones. LA field is a mature oil field in the Malacca Strait PSC in Indonesia that has been on production since 1984. Oil is produced from the Upper Sihapas (US) and Lower Sihapas (LS) sandstones. The LS sands have provided 98% of the oil produced to data. The US sands are thin, low permeability, low pressure and limited in volume (sand lenses) compared to the LS sands.

High water cuts in LS sands have made wells uneconomic to operate. Rather than abandon the wells, US sands were identified and re-completed alone (US sands will not flow when commingled with LS sands). But how to identify oil sands in the US? Resistivity of oil sands and water sands is almost the same. Gas readings and oil show give clues of oil in low-resistivity sands, but depth control is not accurate enough to know exactly which thin sand has oil.

The LA field has seven US producers. Production varies from well to well due to sand heterogeneity. We created a flowchart to determine oil potential, likely reserves and deliverability. Our experience is that low resistivity US sands can produce commercially at low water cut, albeit at low rate due to low pressure and low deliverability.

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