Oil, natural gas and NGL revenues were $96.8 million and $52.5 million for the three months ended September 30, 2021 and 2020, respectively. Average net production volumes were approximately 25.1 MBoe/d and 27.7 MBoe/d for the three months ended September 30, 2021 and 2020, respectively. The change in production volumes was primarily due to natural decline. The average realized sales price was $41.89 per Boe and $20.63 per Boe for the three months ended September 30, 2021 and 2020, respectively. The increase in average realized sales price was primarily due to the increase in commodity prices. Commodity prices were depressed in the third quarter of 2020 due to the impact of the pandemic and the effects of OPEC production related to supply and demand decisions.
Lease operating expense was $34.5 million and $27.6 million for the three months ended September 30, 2021 and 2020, respectively. The change in lease operating expense is primarily due to platform structure inspections at our Beta properties which are performed approximately every 10 years and increase workover expenses. On a per Boe basis, lease operating expense was $14.92 and $10.86 for the three months ended September 30, 2021 and 2020, respectively. The increase in lease operating expense on a per Boe basis is primarily driven by higher costs and lower production.
Gathering, processing and transportation was $5.0 million and $5.3 million for the three months ended September 30, 2021 and 2020, respectively. On a per Boe basis, gathering, processing and transportation was $2.18 and $2.07 for the three months ended September 30, 2021 and 2020, respectively. The change in gathering, processing and transportation on a per Boe basis is due to higher costs and lower production.
Taxes other than income were $6.0 million and $3.8 million for the three months ended September 30, 2021 and 2020, respectively. The increase in taxes other than income is due to an increase in production taxes as a result of the increase in commodity prices. On a per Boe basis, taxes other than income were $2.61 and $1.48 for the three months ended September 30, 2021 and 2020, respectively. The change in taxes other than income on a per Boe basis was primarily due to the increase in commodity prices.
Depreciation, depletion & amortization (“DD&A expense”) was $7.0 million and $8.0 million for the three months ended September 30, 2021 and 2020, respectively. The change in DD&A expense was primarily due to a decrease in production from natural decline.
Impairment expense. No impairment expense recorded for the three months ended September 30, 2021 and 2020, respectively.
General and administrative expense was $6.4 million and $6.4 million for the three months ended September 30, 2021 and 2020, respectively. The change in general and administrative expense was primarily related to a decrease of $0.3 million in legal expenses and a decrease of $0.1 million in professional services partially offset by an increase of $0.3 million in stock compensation expense.
Net losses (gains) on commodity derivative instruments of $46.7 million were recognized for the three months ended September 30, 2021, consisting of $24.1 million decrease in the fair value of open positions and $22.6 million of cash settlements paid on expired positions. Net losses on commodity derivative instruments of $14.4 million were recognized for the three months ended September 30, 2020, consisting of a $28.4 million decrease in the fair value of open positions offset by $14.1 million of cash settlement received on expired positions.
Given the volatility of commodity prices, it is not possible to predict future reported mark-to-market net gains or losses and the actual net gains or losses that will ultimately be realized upon settlement of the hedge positions in future years. If commodity prices at settlement are lower than the prices of the hedge positions, the hedges are expected to partially mitigate the otherwise negative effect on earnings of lower oil, natural gas and NGL prices. However, if commodity prices at settlement are higher than the prices of the hedge positions, the hedges are expected to dampen the otherwise positive effect on earnings of higher oil, natural gas and NGL prices and will, in this context, be viewed as having resulted in an opportunity cost.
Interest expense, net was $3.1 million and $3.4 million for the three months ended September 30, 2021 and 2020, respectively. The change in interest expense is primarily related to a decrease of $0.4 million in interest expense primarily due to lower interest rates related to our Revolving Credit Facility.
Average outstanding borrowings under our Revolving Credit Facility were $234.9 million and $274.5 million for the three months ended September 30, 2021 and 2020, respectively.