* Sept WCS trades at $22.60/bbl below WTI
* Sept synthetic last at $3.00/bbl above WTI
CALGARY, Alberta, Aug 19 (Reuters) - Canadian heavy crude prices weakened slightly on Monday, with traders citing position adjustment on the last day of the market's August trading window.
Western Canada Select heavy blend for September delivery last traded at $22.60 per barrel below the West Texas Intermediate benchmark, according to Shorcan Energy brokers.
That compares with a settlement price of $22.15 per barrel below the benchmark on Friday.
Earlier in the session WCS traded as low as $25.00 per barrel below WTI. One Calgary-based trader said some players may have believed the market was short heavy crude and tried to sell into perceived demand.
"I guess they thought it was short and I guess they were wrong," he said.
Monday is the last day of the nearly three-week-long period beginning on the first of each month and lasting until pipeline nominations are due, in which the majority of Canadian crude trading takes place.
Market sources said pipeline nominations, the first since Enbridge Inc changed how it calculates how much shippers can nominate, are due 0700 MT on Tuesday.
Pipeline apportionment for September will be announced later in the week. Some traders have warned the new rules could lead to more rationing on the Enbridge export network, which may push Canadian crude prices lower if oil gets bottlenecked in Alberta.
Light synthetic crude from the oil sands for September delivery traded at $3.00 per barrel above WTI, compared with a settlement price of $3.50 per barrel above the benchmark on Friday.
Synthetic crude prices have been supported in recent months by tighter supply as a result of coker maintenance at Syncrude's oil sands project in northern Alberta.