http://www.afghanbooks.com/beartrap/bt_infi.htm
I also believed that the Soviets were sensitive in the north. Not only was their base area for the entire war effort just north of the Amu, but northern Afghanistan had had great commercial value to the Soviet Union for many years. In 1960 Soviet exploration had discovered several substantial natural gas fields near Shibarghan (see Map 6) in the northern province of Jozjan. It had an estimated reserve in excess of 500 billion cubic metres. In 1968 a 15-kilometre pipeline was opened, carrying the gas into the Soviet Union. Later, oil was discovered at Sar-i-Pul and Ali Gul 200 kilometres further west. Copper, iron, gold and precious stones are among the other profitable minerals that have been located in the northern and eastern parts of Afghanistan centred on or near the cities of Kabul, Kunduz and Mazar-i-Sharif. Precisely the areas that coincided with the Soviets military dispositions.