“Sub-prime” is the term by which became known the debt market segment that served low quality housing in the US. Essentially these were products supporting mortgages to low-middle class families, that in 2006/07, up against the simultaneous rise in interest rates and commodities prices, produced a wave of defaults that lead to the 2008 financial crisis.
The rise in petroleum prices was a key element to the 2008 crisis, but would eventually bring something positive to the US. Petroleum is usually extracted from large underground cavities known as reservoirs. However, it is formed at greater depth, within source rocks, where organic matter is slowly cooked by the internal heat of the planet until it degrades, first into petroleum and finally into gas. Prices persistently above 100 dollars per barrel meant that beyond traditional reservoirs it also became feasible to drill deeper for petroleum, down to source rocks and other rock formations of low permeability.