unweighted average of the first of the month prices of the WTI spot prices which equates to $65.56 per barrel and $51.34 per barrel, respectively. The natural gas prices as of December 31, 2018 and 2017 are based on the respective 12-month unweighted average of the first of month prices of the Henry Hub spot price which equates to $3.10 per MMBtu and $2.98 per MMBtu, respectively. The natural gas liquids prices used to value reserves as of December 31, 2018 and 2017 averaged $28.81 per barrel and $22.59 per barrel, respectively. All prices are adjusted by lease or field for energy content, transportation fees, and market differentials. All prices are held constant in accordance with SEC guidelines. A summary of our changes in quantities of proved oil, natural gas and NGLs reserves for the years ended December 31, 2018 and 2017 are as follows: Oil (MBbl) Natural Gas (MMcf) NGLs (MBbl) Total (MBOE) Balance - December 31, 2016 7,111 20,401 1,540 12,051 Extensions and discoveries 19,558 29,644 6,264 30,763 Sales of minerals in place (1,833 ) (6,853 ) (1 ) (2,976 ) Purchases of minerals in place 28,176 46,709 9,950 45,911 Production (1,828 ) (3,260 ) (500 ) (2,872 ) Revision to previous estimates (3,857) 4,447 215 (2,901) Balance - December 31, 2017 47,327 91,088 17,468 79,976 Extensions and discoveries 10,148 17,673 3,116 16,209 Sales of minerals in place (2,651 ) (14,300 ) (1,562 ) (6,596 ) Purchases of minerals in place 3,532 9,890 1,629 6,810 Production (2,370 ) (3,610 ) (655 ) (3,627 ) Revision to previous estimates 3,048 12,476 947 6,075 Balance - December 31, 2018 59,034 113,217 20,943 98,847 Proved developed reserves: December 31, 2016 6,052 13,545 1,051 9,361 December 31, 2017 11,949 23,336 4,123 19,961 December 31, 2018 14,325 26,110 4,969 23,646 Proved undeveloped reserves: December 31, 2016 1,059 6,856 489 2,690 December 31, 2017 35,378 67,752 13,345 60,015 December 31, 2018 44,709 87,107 15,974 75,201 As of December 31, 2018 Oil (MBbl) Natural Gas (MMcf) NGLs (MBbl) Total (MBOE) Proved developed 7,917 14,430 2,746 13,068 Proved undeveloped 24,709 48,140 8,828 41,560 Total proved 32,626 62,570 11,574 54,628 As of December 31, 2017 Oil (MBbl) Natural Gas (MMcf) NGLs (MBbl) Total (MBOE) Proved developed 6,775 13,232 2,338 11,318 Proved undeveloped 20,059 38,415 7,566 34,028 Total proved 26,834 51,647 9,904 45,346 Notable changes in proved reserves for the year ended December 31, 2018 included the following: • Extensions and discoveries.  In 2018, total extensions and discoveries of 16.2 MMBOE was a result of successful drilling results and well performance primarily related to the Midland Basin. • Sales of minerals in place.  Sales of minerals in place totaled 6.6 MMBOE during 2018, which consisted of 4.7 MMBOE resulting from the disposition of non-operated properties in the Midland Basin as part of an acreage trade and 1.9 MMBOE related to the disposition of non-operated Eagle Ford properties, both further described in  Note 3. Acquisitions and Divestitures  in the Notes to Consolidated Financial Statements. 39 The table below presents the quantities of proved oil, natural gas and NGLs reserves attributable to noncontrolling interests as of December 31, 2018 and 2017: