Note 8 - Supplemental Information Relating to Oil and Gas Producing Activities (Unaudited) - Standardized Measure of Discounted Future Net Cash Flows (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Future cash inflows from sales of oil and gas	$ 67,050	$ 37,232
Future production and development costs	(36,575)	(22,716)
Future asset retirement obligations, net of salvage	(15,871)	(16,360)
Future income tax expense	(321)	(132)
Future net cash flows	14,283	(1,976)
Effect of discounting future net cash flows at 10% per annum	(9,839)	(3,078)
Standardized measure of discounted future net cash flows	$ 4,444	$ (5,054)	$ (9,924)