Field service expense increased $0.6 million or 19.3% from $3.1 million for the third quarter 2017 to $3.7 million for the third quarter 2018 and increased $1.0 million, or 19.3% from $9.2 million for the nine months ended September 30, 2017 to $10.2 million for the nine months ended September 30, 2018. Field service expenses primarily consist of salaries and vehicle operating expenses which have increased during the nine months ended September 30, 2018 over the same period of 2017 as a direct result of increased services and utilization of the equipment.
Depreciation, depletion, amortization and accretion on discounted liabilities remained flat from $7.8 million for the third quarter 2017 to $7.9 million for the third quarter 2018 and $23.8 million for the nine months ended September 30, 2017 to $23.7 million for the nine months ended September 30, 2018, reflecting the increased reserve base and production related to new wells placed on production late in 2017 and mid 2018.
General and administrative expense increased $3.9 million, or 56.5% from $6.9 million for the nine months ended September 30, 2017 to $10.8 million for the nine months ended September 30, 2018, and decreased $0.3 million, or 10.8% from $2.5 million for the three months ended September 30, 2017 to $2.25 million for the three months ended September 30, 2018. This increase in 2018 reflects the combination of a reduction in reimbursements related to the decrease in gains on sales of properties from 2017 to 2018 and increases in personnel costs.
Gain on sale and exchange of assets of $3.2 million and $42.1 million for the nine months ended September 30, 2018 and September 30, 2017, respectively consists of sales ofnon-essential oil and gas interests and field service equipment.
Interest expense increased from $0.6 million for the third quarter 2017 to $0.8 million for the third quarter 2018 and from $1.7 million for the nine months ended September 30, 2017 to $2.6 million for the nine months ended September 30, 2018. This increase reflects the increase in current rates and borrowings under our revolving credit agreement.
Income tax expense decreased $10.1 million due to a lower effective tax rate and lowerpre-tax income. The effective tax rates for the nine months of 2018 and 2017 were 25% and 32.1%, respectively. The decrease in the effective tax rate is primarily due to the impact of the Tax Cuts and Jobs Act changes that are effective January 1, 2018.
LIQUIDITY AND CAPITAL RESOURCES
Our primary sources of liquidity are cash flows generated from operations, through our producing oil and gas properties, field services business and from sales ofnon-core acreage.
Net cash provided by our operating activities for the nine months ended September 30, 2018 was $25.5 million compared to $21.4 million provided by operating activities for the nine months ended September 30, 2017. Excluding the effects of significant unforeseen expenses or other income, our cash flow from operations fluctuates primarily because of variations in oil and gas production and prices or changes in working capital accounts. Our oil and gas production will vary based on actual well performance but may be curtailed due to factors beyond our control.
Our realized oil and gas prices vary due to world political events, supply and demand of products, product storage levels, and weather patterns. We sell the vast majority of our production at spot market prices. Accordingly, product price volatility will affect our cash flow from operations. To mitigate price volatility we sometimes lock in prices for some portion of our production through the use of derivatives.
If our exploratory drilling results in significant new discoveries, we will have to expend additional capital in order to finance the completion, development, and potential additional opportunities generated by our success. We believe that, because of the additional reserves resulting from the successful wells, we will be able to access sufficient additional capital through bank financing.
We currently maintain a credit facility totaling $300 million, with a borrowing base of $90 million. The Company currently has $55 million in outstanding borrowings and $35million in availability under this facility. The bank reviews the borrowing base semi-annually and, at their discretion, may decrease or propose an increase to the borrowing base relative to a redetermined estimate of proved oil and gas reserves. The next borrowing base review is scheduled for December 2018. Our oil and gas properties are pledged as collateral for the line of credit and we are subject to certain financial and operational covenants defined in the agreement. We are currently in compliance with these covenants and expect to be in compliance over the next twelve months. If we do not comply with these covenants on a continuing basis, the lenders have the right to refuse to advance additional funds under the facility and declare all principal and interest immediately due and payable. Our borrowing base may decrease as a result of lower natural gas or oil prices, operating difficulties, declines in reserves, lending requirements or regulations, the issuance of new indebtedness or for other reasons set forth in our revolving credit agreement. In the event of a decrease in our borrowing base our ability to borrow under our revolving credit facility may be limited and we could be required to repay any indebtedness in excess of the redetermined borrowing base.
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