taxes as a percentage of natural gas revenues remained consistent at 6% in each of the nine months ended September 30, 2020 and 2021.
Impairment of oil and gas properties. Impairment of oil and gas properties decreased from $156 million for the nine months ended September 30, 2020 to $70 million for the nine months ended September 30, 2021, a decrease of $86 million, or 55%, primarily related to lower impairments of expiring leases between periods. During both periods, we recognized impairments primarily related to expiring leases as well as design and initial costs related to pads we no longer plan to place into service.
Depletion, depreciation, and amortization expense. DD&A expense decreased from $652 million for the nine months ended September 30, 2020 to $564 million for the nine months ended September 30, 2021, a decrease of $88 million, or 13%, primarily as a result of increased proved reserve volumes between periods due to higher commodity prices as well as lower production volumes between periods. DD&A per Mcfe remained relatively consistent at $0.67 per Mcfe and $0.63 per Mcfe during the nine months ended September 30, 2020 and 2021, respectively.
General and administrative expense. General and administrative expense (excluding equity-based compensation expense) increased from $84 million for the nine months ended September 30, 2020 to $94 million for the nine months ended September 30, 2021, an increase of $10 million, or 11%. The increase was primarily due to higher salary and wage expense between periods, which includes our annual incentive program that was significantly reduced during 2020. We had 520 and 506 employees as of September 30, 2020 and 2021, respectively. On a per-unit basis, general and administrative expense excluding equity-based compensation increased from $0.09 per Mcfe during the nine months ended September 30, 2020 to $0.10 per Mcfe during the nine months ended September 30, 2021 as a result of lower production volumes and higher overall costs between periods.
Equity-based compensation expense. Noncash equity-based compensation expense decreased from $17 million for the nine months ended September 30, 2020 to $15 million for the nine months ended September 30, 2021, primarily due to equity award forfeitures, partially offset by new awards granted to employees. When an equity award is forfeited, expense previously recognized for the award is reversed. See Note 9—Equity Based Compensation and Cash Awards to the unaudited condensed consolidated financial statements for more information on equity-based compensation awards.
Marketing Segment
Marketing. Where feasible, we purchase and sell third-party natural gas and NGLs and market our excess firm transportation capacity, or engage third parties to conduct these activities on our behalf, to optimize the revenues and mitigate costs from these transportation agreements. We have entered into long-term firm transportation agreements for a significant portion of our current and expected future production to secure guaranteed capacity to favorable markets.
Net marketing expenses decreased from $133 million, or $0.14 per Mcfe, for the nine months ended September 30, 2020 to $65 million, or $0.07 per Mcfe, for the nine months ended September 30, 2021. The decrease was driven by higher marketing volumes and margins that mitigated some of our excess firm transportation expense.
Marketing revenues increased from $202 million for the nine months ended September 30, 2020 to $563 million for the nine months ended September 30, 2021, an increase of $361 million due to increased marketing volumes.
Marketing expenses increased from $335 million for the nine months ended September 30, 2020 to $628 million for the nine months ended September 30, 2021, an increase of $293 million, or 87%. Marketing expenses include firm transportation costs related to current excess firm capacity as well as the cost of third-party purchased gas and NGLs. Firm transportation costs included in the expenses above were $122 million and $81 million for the nine months ended September 30, 2020 and 2021, respectively.
Equity Method Investment in Antero Midstream Corporation
Antero Midstream Corporation. Revenue from the Antero Midstream Corporation segment decreased from $697 million for the nine months ended September 30, 2020 to $682 million for the nine months ended September 30, 2021, a decrease of $15 million, or 2%, primarily due to lower fresh water delivery revenue as a result of decreased well completions period-over-period and lower gathering volumes, partially offset by higher compression revenues as a result of increased throughput between periods. Total operating expenses related to the segment decreased from $929 million for the nine months ended September 30, 2020 to $255 million for the nine months ended September 30, 2021, primarily due to impairments by Antero Midstream Corporation during the nine months ended September 30, 2020 of $89 million on its freshwater pipelines and equipment and impairment of goodwill of $575 million. Antero Midstream Corporation’s impairment expense was $2 million for the nine months ended September 30, 2021 due to a lower of cost or market adjustment for pipe inventory.