This document describes a risk factor model for pairs trading between futures contracts. It constructs a stochastic spread model to identify mispricing between paired futures. Backtesting of the strategy between 2003-2013 on WTI crude oil and natural gas futures achieved an annual return of 22.75% with a Sharpe ratio of 5.39, outperforming benchmarks while maintaining low correlation and negative beta. Risk controls including leverage limits, position limits, and stop loss limits help manage drawdowns.