2018 to $22 million for the three months ended June 30, 2019 due to the Series B Exchange and the adoption of the AMC LTIP as result of the Transactions.
Depreciation expense. Depreciation expense increased from none for the three months ended June 30, 2018 to $36 million for the three months ended June 30, 2019 as a result of our acquisition of Antero Midstream Partners on March 12, 2019.
Accretion and change in fair value of contingent acquisition consideration. Accretion expenses increased from none for the three months ended June 30, 2018 to $2 million for the three months ended June 30, 2019 as a result of our acquisition of Antero Midstream Partners on March 12, 2019.
Interest expense. Interest expense increased from $18 thousand for the three months ended June 30, 2018 to $32 million for the three months ended June 30, 2019 as a result of the acquisition of Antero Midstream Partners, which included the assumption of approximately $2.4 billion of debt.
Operating income. Total operating income increased from a loss of $12 million for the three months ended June 30, 2018 to operating income of $118 million for the three months ended June 30, 2019. The increase was due to operating revenues and expenses as a result of the acquisition of Antero Midstream Partners on March 12, 2019.
Equity in earnings of unconsolidated affiliates. Equity in earnings of unconsolidated affiliates for the three months ended June 30, 2018 represents Antero Midstream GP LP’s equity investment in Antero Midstream Partners. Equity in earnings of unconsolidated affiliates for the three months ended June 30, 2019 represents the portion of the net income from Antero Midstream Partners’ investments in Stonewall and the Joint Venture, which is allocated to us based on our equity interests for the three months ended June 30, 2019.
Income tax expense. Income tax expense increased from $7 million for the three months ended June 30, 2018 to $30 million for the three months ended June 30, 2019 due to increased income before taxes.
Six Months Ended June 30, 2018 Compared to Six Months Ended June 30, 2019
Revenue and Direct Operating Expenses. Revenues from Antero Resources and direct operating expenses reflect 110 days of revenue and operating expenses generated by Antero Midstream Partners after the completion of the Transactions on March 12, 2019.
General and administrative expenses. General and administrative expenses (excluding equity-based compensation expense) increased from $3 million for the six months ended June 30, 2018 to $21 million for the six months ended June 30, 2019. The increase was primarily due to the inclusion of general and administrative expenses of Antero Midstream Partners after the completion of the Transactions on March 12, 2019. Equity-based compensation increased from $18 million for the six months ended June 30, 2018 to $33 million for the six months ended June 30, 2019 due to the Series B Exchange and the adoption of the AMC LTIP as result of the Transactions.
Depreciation expense. Depreciation expense increased from none for the six months ended June 30, 2018 to $44 million for the six months ended June 30, 2019 as a result of our acquisition of Antero Midstream Partners on March 12, 2019.
Accretion and change in fair value of contingent acquisition consideration. Accretion expenses increased from none for the six months ended June 30, 2018 to $3 million for the six months ended June 30, 2019 as a result of our acquisition of Antero Midstream Partners on March 12, 2019.
Interest expense. Interest expense increased from $14 thousand for the six months ended June 30, 2018 to $38 million for the six months ended June 30, 2019 as a result of the acquisition of Antero Midstream Partners, which included the assumption of approximately $2.4 billion of debt.
Operating income. Total operating income increased from a loss of $21 million for the six months ended June 30, 2018 to operating income of $128 million for the six months ended June 30, 2019. The increase was due to operating revenues and expenses as a result of the acquisition of Antero Midstream Partners on March 12, 2019.
Equity in earnings of unconsolidated affiliates. Equity in earnings of unconsolidated affiliates for the six months ended June 30, 2018 represents Antero Midstream GP LP’s equity investment in Antero Midstream Partners. Equity in earnings of