September 30, 2023, an increase of $7 million, or 23%, primarily due to higher salary and wage expense and software license costs between periods. We had 554 and 605 employees as of September 30, 2022 and 2023, respectively. General and administrative expense on a per unit basis (excluding equity-based compensation) increased from $0.11 per Mcfe for the three months ended September 30, 2022 to $0.13 per Mcfe for the three months ended September 30, 2023 as a result of our higher overall general and administrative costs, partially offset by increased production volumes between periods.
Equity-based compensation expense. Noncash equity-based compensation expense increased from $10 million for the three months ended September 30, 2022 to $18 million for the three months ended September 30, 2023, an increase of $8 million, or 77%, primarily due to an increase in the annual equity awards granted during the fourth quarter of 2022 and the first half of 2023 as compared to prior years, which were temporarily and significantly reduced during 2020 and supplemented by our cash awards program. Our equity awards vest over three or four year service periods, and our equity incentive program began returning to normal levels during 2021. See Note 9—Equity Based Compensation and Cash Awards to the unaudited condensed consolidated financial statements for more information.
Depletion, depreciation, and amortization expense (“DD&A expense”). DD&A expense increased from $170 million, or $0.58 per Mcfe, to $176 million, or $0.55 per Mcfe, for the three months ended September 30, 2022 and 2023, respectively. This increase in DD&A expense was primarily due to higher production volumes between periods, partially offset by higher reserve volumes during the three months ended September 30, 2023.
Impairment of property and equipment. Impairment of oil and gas properties decreased from $34 million for the three months ended September 30, 2022 to $13 million for the three months ended September 30, 2023, a decrease of $21 million, or 60%, 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.
Contract termination, loss contingency and other operating expenses. Contract termination, loss contingency and other operating expenses of $18 million for the three months ended September 30, 2022 were primarily due to a payment for the cancellation of the Smithburg 2 gas processing plant. Contract termination, loss contingency and other operating expenses of $14 million for the three months ended September 30, 2023 were primarily due to a loss contingency.
Marketing Segment
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, in order to optimize the revenues from these transportation agreements. We have entered into long-term firm transportation agreements for a significant portion of our current and expected future production in order to secure guaranteed capacity to favorable markets.
Net marketing expense decreased from $25 million, or $0.09 per Mcfe, for the three months ended September 30, 2022 to $16 million, or $0.05 per Mcfe, for the three months ended September 30, 2023, primarily due to lower firm transportation commitments.
Marketing revenue. Marketing revenue decreased from $160 million for the three months ended September 30, 2022 to $53 million for the three months ended September 30, 2023, a decrease of $107 million, or 67%. This fluctuation primarily resulted from the following:
| ● | Natural gas marketing revenue decreased by $102 million between periods primarily due to lower natural gas prices and marketing volumes. Lower natural gas prices accounted for a $100 million decrease in year-over-year marketing revenues (calculated as the change in the year-to-year average price times current year marketing volumes), and lower natural gas marketing volumes accounted for an approximate $2 million decrease in year-over-year marketing revenues (calculated as the change in year-to-year volumes times the prior year average price). |
| ● | Ethane marketing revenues were $5 million for the three months ended September 30, 2022. There were no ethane marketing revenues for the three months ended September 30, 2023. |
Marketing expense. Marketing expense decreased from $185 million for the three months ended September 30, 2022 to $70 million for the three months ended September 30, 2023, a decrease of $115 million, or 62%. Marketing expense includes the cost of third-party purchased natural gas, NGLs and oil as well as firm transportation costs, including costs related to current excess firm capacity. The cost of third-party natural gas, ethane and oil purchases decreased $100 million, $5 million and $1 million, respectively, between periods. The total cost of third-party commodity purchases decreased primarily