Our estimate is that this event negatively impacted our EBITDA by approximately $5 million.
For the first quarter of 2021, revenues decreased to $182.6 million compared to $243.8 million in the first quarter of the prior year.
Operating loss for the first quarter was $10.5 million compared to an adjusted operating loss of $13.2 million in the first quarter of the prior year.
EBITDA for the first quarter was $7.8 million compared to adjusted EBITDA of $25.8 million in the same period of the prior year.
Loss per share was $0.05 in the first quarter of this year compared to adjusted loss per share of $0.04 in the first quarter of 2020.
Cost of revenues during the first quarter of 2021 was $146.2 million or 80.1% of revenues compared to $181.9 million or 74.6% of revenues during the first quarter of 2020.
Selling, general and administrative expenses decreased to $30.6 million in the first quarter of 2021 compared to $36.5 million in the first quarter of the prior year.
Depreciation and amortization decreased to $17.8 million in the first quarter of 2021 compared to $39.3 million in the first quarter of the prior year.
Our Technical Services segment revenues for the quarter decreased 24.2% compared to the same quarter in the prior year.
Significant operating -- segment operating loss in the first quarter of 2021 was $5.8 million compared to a $12.2 million operating loss in the first quarter of the prior year.
Our Support Services segment revenues for the quarter decreased 38% compared to the same quarter in the prior year.
Segment operating loss in the first quarter of 2021 was $2.9 million compared to an operating profit of $1.5 million in the first quarter of the prior year.
On a sequential basis, RPC's first quarter revenues increased 22.9% to $182.6 million from $148.6 million in the prior quarter.
Cost of revenues during the first quarter of '21 -- 2021 increased by $28.3 million or 24% to $146.2 million due to expenses, which increased with higher activity levels, such as materials and supplies and employment expenses.
As a percentage of revenues, cost of revenues increased slightly from 79.3% in the fourth quarter of 2020 to 80.1% in the first quarter of 2021.
Selling, general and administrative expenses during the first quarter of 2021 increased 17.6% to $30.6 million and $26 million in the prior quarter.
RPC incurred an operating loss of $10.5 million during the first quarter of 2021 compared to an adjusted operating loss of $11.3 million in the prior quarter.
RPC's EBITDA was $7.8 million in the first quarter of 2021, which was the same as adjusted EBITDA of $7.8 million in the fourth quarter of 2020.
Our Technical Services segment revenues increased by $33.7 million or 24.2% to $172.6 million in the first quarter due to increased activity levels in most of the segment service lines.
The Technical Services segment incurred a $5.8 million operating loss in the current quarter compared to an operating loss of $11.3 million in the prior quarter.
Our Support Services segment revenues increased by $310,000 or 3.2% to $10 million in the first quarter.
Operating loss was $2.9 million in the current quarter compared to an operating loss of $2.6 million in the prior quarter.
First quarter 2021 capital expenditures were $11.8 million, and we currently estimate full year 2021 capital expenditures to be approximately $55 million, comprised primarily of capitalized maintenance of our existing equipment and selected growth opportunities.
We are in the final stages of upgrading another of our fleets to dual-fuel capability, after which 2/3 of our deployed frac capacity will be ESG-friendly.
At the end of the first quarter, RPC's cash balance was $85.4 million, and we remain debt-free.

Summary:
For the first quarter of 2021, revenues decreased to $182.6 million compared to $243.8 million in the first quarter of the prior year.
Loss per share was $0.05 in the first quarter of this year compared to adjusted loss per share of $0.04 in the first quarter of 2020.