three months ended September 30, 2020. The impairment recorded during the three months ended September 30, 2020 was due to a significant decline in the trailing twelve month average of oil and natural gas prices, related to reduced demand for oil and natural gas as a result of COVID-19, the announcement of price reductions and production increases in March 2020 by members of OPEC and other foreign, oil-exporting countries, and other supply factors.
Marketing and Other Deductions
Our marketing and other deductions include product marketing expense, which is a post-production expense. Marketing and other deductions for the three months ended September 30, 2021 were $3.0 million, an increase of $0.5 million from $2.5 million for the three months ended September 30, 2020.
General and Administrative Expenses
General and administrative expenses for the three months ended September 30, 2021 were $6.8 million, an increase of $0.7 million from $6.1 million for the three months ended September 30, 2020. Included within general and administrative expenses are non-cash expenses for unit-based compensation as a result of the amortization of restricted units that have been issued by us over various periods. The increase in general and administrative expenses was primarily attributable to a $0.3 million increase in unit-based compensation expense cash general and administrative expenses resulting from increases in salaries and wages and our costs associated with company growth.
Interest Expense
Interest expense for the three months ended September 30, 2021 was $2.5 million compared to $1.6 million for the three months ended September 30, 2020. The increase in interest expense was primarily due to debt incurred in 2021 to fund the redemption of the Series A preferred units.
Comparison of the Nine Months Ended September 30, 2021 to the Nine Months Ended September 30, 2020
Oil, Natural Gas and NGL Revenues
For the nine months ended September 30, 2021, our oil, natural gas and NGL revenues were $122.8 million, an increase of $56.1 million from $66.7 million for the nine months ended September 30, 2020. The increase in oil, natural gas and NGL revenues was directly related to the increase in the average prices we received for oil, natural gas and NGL production for the nine months ended September 30, 2021 as discussed below.
Our revenues are a function of oil, natural gas, and NGL production volumes sold and average prices received for those volumes. The production volumes were 3,907,134 Boe or 14,312 Boe/d, for the nine months ended September 30, 2021, an increase of 128,243 Boe or 520 Boe/d, from 3,778,891 Boe or 13,792 Boe/d, for the nine months ended September 30, 2020. The increase in production for the nine months ended September 30, 2021 was primarily attributable to production associated with the Springbok Acquisition, which accounted for 192,024 Boe. The increase was partially offset by a reduction in production on our other assets as a result of the COVID-19 outbreak and international supply and demand imbalances and, to a lesser extent, the winter storms experienced in parts of the United States in February 2021, which caused the temporary shut-in of certain properties in which we have an interest. See Business Environment — COVID-19 Pandemic and Impact on Global Demand for Oil and Natural Gas for further discussion.
Our operators received an average of $61.99 per Bbl of oil, $3.28 per Mcf of natural gas and $26.27 per Bbl of NGL for the volumes sold during the nine months ended September 30, 2021 compared to $36.08 per Bbl of oil, $1.70 per Mcf of natural gas and $11.47 per Bbl of NGL for the volumes sold during the nine months ended September 30, 2020. These average prices received during the nine months ended September 30, 2021 increased 71.8% or $25.91 per Bbl of oil and 92.9% or $1.58 per Mcf of natural gas as compared to the nine months ended September 30, 2020. This change is consistent with prices experienced in the market, specifically when compared to the EIA average price increases of 71.0% or $27.01 per Bbl of oil and 93.0% or $1.74 per Mcf of natural gas for the comparable periods.