The fair value hierarchy prioritizes the inputs used in valuation techniques into three levels, as follows:

The Company recorded its warrants issued from 2011 through 2013 and the conversion features associated with its convertible notes at fair value and designated them as Level 3 on their issuance date.

Warrants – Except for the warrants issued September 26, 2012, the Company’s warrants were valued using a Monte Carlo Binomial Lattice-Based valuation methodology, adjusted for marketability restrictions. The warrants issued September 26, 2012, due to no anti-dilution protection features, were valued using the Black-Scholes Valuation Model.

Significant assumptions used and related fair values for the warrants as of December 31, 2014 were as follows:

Significant assumptions used and related fair values for the warrants as of December 31, 2013 were as follows:

The estimated fair value of the warrants is affected by the above underlying inputs. Observable inputs include the values of exercise price, stock price, term and risk-free interest rate. As separate inputs, an increase (decrease) in either the term or risk free interest rate will result in an increase (decrease) in the estimated fair value of the warrant.

Unobservable inputs include volatility and market discount. An increase (decrease) in volatility will result in an increase (decrease) in the estimated warrant value and an increase (decrease) in the market discount will result in a decrease (increase) in the estimated warrant fair value.

The volatility utilized was a blended average of the Company’s historical volatility and implied volatilities derived from a selected peer group. The implied volatility component has remained relatively constant over time given that implied volatility is a forward-looking assumption based on observable trades in public option markets. Should the Company’s historical volatility increase (decrease) on a go-forward basis, the resulting value of the warrants would increase (decrease).

The market discount, or a discount for lack of marketability, is quantified using a Black-Scholes option pricing model, with a primary model input of assumed holding period restriction. As the assumed holding period increases (decreases), the market discount increases (decreases), conversely impacting the value of the warrant fair value.

Convertible Notes – The conversion feature imbedded in the convertible notes was valued using a Monte Carlo Binomial Lattice-Based valuation methodology, adjusted for marketability restrictions. The Company estimated the fair value of the conversion feature until the retirement of the convertible notes in December 2013.

Other Derivative Instruments – The Company’s other derivative instruments consist of commodity positions. The fair values of the commodity positions are based on quoted prices on the commodity exchanges and are designated as Level 1 inputs.

The following table summarizes fair value measurements by level at December 31, 2014 (in thousands):

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The following table summarizes fair value measurements by level at December 31, 2013 (in thousands):

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For fair value measurements using significant unobservable inputs (Level 3), a description of the inputs and the information used to develop the inputs is required along with a reconciliation of Level 3 values from the prior reporting period. The changes in the Company’s fair value of its Level 3 inputs with respect to its warrants were as follows (in thousands):