The following details the allocation of stock-based compensation to functional expense categories (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2022		2021		2022		2021
Direct operating expense	$	372	$	319	$	1,083	$	986
Brokered natural gas and marketing expense		663		446		1,868		1,339
Exploration expense		393		368		1,163		1,116
General and administrative expense		10,402		9,845		32,245		28,632
Total stock-based compensation expense	$	11,830	$	10,978	$	36,359	$	32,073

The following assumptions were used to estimate the fair value of TSR-PSUs granted during first nine months 2022 and 2021:


	Nine Months Ended September 30,
	2022			2021
Risk-free interest rate		1.4	%		0.2	%
Expected annual volatility		68	%		75	%
Grant date fair value per unit	$	27.90		$	12.58

The following is a summary of our non-vested internal performance awards outstanding at September 30, 2022:


	Number of Units			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2021		1,095,355		$	7.80
Units granted (a)		153,089			20.38
Vested and issued (b)		(243,482	)		9.64
Forfeited		—			—
Outstanding at September 30, 2022		1,004,962		$	9.27


(a)	Amounts granted reflect the number of performance units granted; however, the actual payout of shares will be between zero and 200% depending on achievement of specifically identified performance targets. Units granted in first quarter 2022 were to our CEO, CFO and COO only.
(b)	For the awards issued during 2019, the aggregate payout was approximately 116% of target with a positive performance adjustment of 158,793 shares.

The following is a summary of our non-vested TSR-PSUs award activities:


	Number of Units			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2021		1,147,994		$	7.60
Units granted (a)		111,828			27.90
Vested and issued (b)		(314,152	)		11.34
Forfeited		—			—
Outstanding at September 30, 2022		945,670		$	8.76