Royalty income during 2020 decreased approximately $10,394,000, or 22.2 percent, compared to 2019 royalty income, which had decreased approximately $5,659,000, or 10.8 percent, from 2018 royalty income.
Revenues generated by sales of oil and gas decreased in 2020 from 2019 as a result of decreased oil and gas prices ($11.1 million) and higher operating expenses ($1.0 million); offset somewhat by higher natural gas and oil production volumes ($1.0 million) and lower taxes ($0.8 million).
Gas volumes increased from 7,516,325 thousand cubic feet ("Mcf") in 2019 to 8,015,576 Mcf in 2020 after increasing from 7,138,837 Mcf in 2018. The average price per Mcf of gas received by the Trust decreased from $2.27 in 2019 to $1.70 in 2020 after decreasing from $2.67 in 2018. A warmer-than-normal summer in 2018 contributed to higher gas prices during the first half of the year, but an active hurricane season on the east coast and in the Gulf of Mexico led to decreased demand and, consequently, lower prices for the second half of 2018. Continued robust production in the U.S. led to sustained high inventories which kept the price of natural gas low throughout 2019. The COVID-19 pandemic that hit in March 2020 and the subsequent sharp decrease in demand due to lockdowns worldwide caused the price of natural gas to decrease significantly. The restrictions in activity through the rest of 2020 and the economic uncertainty that went with it continued to put pressure on natural gas prices.
Oil volumes sold increased to 700,177 barrels ("Bbls") in 2020 from 697,089 Bbls in 2019 after increasing from 689,799 Bbls in 2018. The average sales price of oil decreased to $41.63 per Bbl in 2020, from $51.44 per Bbl in 2019 after decreasing from $58.91 per Bbl in 2018. The price of oil in 2018 was affected by continued economic stability and industrial increase in demand. Beginning with the fourth quarter of 2018, however, oil prices decreased significantly. The downward pressure on oil prices continued in 2019 due mainly to the shale oil production boom that continues. Geopolitical tensions in the Middle East during the latter part of 2019 caused an increase in oil prices, however short-lived that increase was. For the first two months of 2020, oil prices stayed at higher levels until COVID-19 was discovered in the U.S. in March. The subsequent lockdowns led to a steep decrease in demand, causing the spot price of oil briefly to go negative. The price of oil recovered somewhat in the third and fourth quarters of 2020 as restrictions began to ease.
Interest income was approximately $31,000 in 2020, which had decreased from approximately $166,000 in 2019, which had increased from approximately $142,000 in 2018. Changes in interest interest income are the result of changes in interest rates and funds available for investment.
General and administrative expenses increased to approximately $3,057,000 in 2020 from approximately $2,857,000 in 2019. This is due mainly to increases of approximately $145,800 for digitizing certain permanent files of the Trust and an increase in revenue processing of approximately $88,600. These increases were offset somewhat by decreases in Escrow Agent/Trustee fees of approximately $17,800 and professional fees of approximately $12,100.
General and administrative expenses increased to approximately $2,857,000 in 2019 from approximately $2,599,000 in 2018 due mainly to increases in Escrow Agent/Trustee fees of approximately $162,000; legal and professional expenses of approximately $71,500; and revenue processing services of approximately $47,300. These increases were offset somewhat by a decrease in Unit holder and printing expenses of approximately $22,200.
The cash received by the Trust is primarily from purchasers of the Trust's oil and gas production and consists of gross sales of production less applicable severance taxes. In August 2020, the Trust received a refund of $122,556 (tax year 2018) from the State of New Mexico. In January 2019, the Trust received refunds of $309,879 (tax year 2015) from the State of Oklahoma and a refund of $140,474 (tax year 2015) from the State of New Mexico. In March 2019, the Trust received a refund of $334,624 (tax year
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