With European Central Bank President Mario Draghi hinting at further easing by the central bank next month, markets are busy trying to work out whether the next move will be to lower deposit rates even further into negative territory or ramp up asset purchases. Or perhaps both.

The terms of the ECB's quantitative easing, or QE, program mean that it theoretically has a fixed universe of assets to purchase - a limit that could be hit earlier than its intended September 2016 deadline if the bank substantially increases the size of its purchases.

Under its current rate of 60 billion euros ($65 billion) a month, the ECB should be more than capable of purchasing the 893 billion euros of agency and government bonds with yields above the deposit rate planned through next September.

However, if the rate of purchases is ramped up then it could come close to running out of available assets, according to Bloomberg Intelligence economist David Powell.

Instead, he said, the ECB could cut the deposit rate to increase the investible universe of assets, as well as significantly increasing QE purchases.

"BI Economics calculates that a decline in bond yields of 25 basis points across the curve would shrink the universe of bonds to 1.3 trillion euros," he said. "In that instance, a cut to the deposit rate would be required to implement asset purchases of 90 billion euros much beyond September 2016 - the total through September would be roughly 1.2 trillion euros of overall purchases of agency and government bonds."