LONDON, Nov 19 (Reuters) - Northwest European diesel barge profit margins fell to about $8.50 a barrel on Friday, their lowest since early August, amid threats to demand from spreading COVID-19 cases in Europe. Oil prices fell on Friday below $80 a barrel as a fresh surge in COVID-19 cases in Europe threatened to slow the economic recovery while investors also weighed a potential release of crude reserves by major economies to cool prices. Austria became the first country in western Europe to reimpose a full coronavirus lockdown this autumn to tackle a new wave of COVID-19 infections across the region. [USN:L1N2SA0KQ] European refiners produced almost 9.6 million barrels per day (bpd) of oil products in October, more than 2% less than in September, but 8.5% higher than a year earlier, Euroilstock data showed on Friday. [USN:S8N2GW07Q] Gasoil stocks held in independent storage in the Amsterdam-Rotterdam-Antwerp (ARA) refining and storage area rose by about 1.7% to 1.94 million tonnes in the week to Thursday, data from Dutch consultancy Insights Global showed.