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. • Purchases of minerals in place.  In 2018, total purchases of minerals in place of 6.8 MMBOE were primarily attributable to developed non-producing wells and undeveloped acreage acquired in the Midland Basin as part of an acreage trade, as further described in Note 3. Acquisitions and Divestitures  in the Notes to Consolidated Financial Statements . • Revision to previous estimates.  In 2018, the upward revisions of prior reserves of 6.1 MMBOE consisted of improved PUD reserves of 5.8 MMBOE with improved proved developed reserves of 0.3 MMBOE. PUD revisions are a result of our successful drilling efforts in the Midland Basin as well as improved commodity prices. Proved undeveloped reserves (“PUDs”) decreased from 75,201 MBOE to 62,815 MBOE or 16%, as of December 31, 2019 compared to December 31, 2018. PUDs represent 66% of our total proved reserves. Certain previously booked PUDs were reclassified as proved developed reserves due to successful drilling efforts. Revisions of prior estimates include certain PUDs that were reclassified to unproved categories due to development plan changes and increased well spacing. In accordance with our 2019 year-end independent engineering reserve report, we plan to drill all of our individual PUD drilling locations within the five years of original classification. Changes in our PUD reserves for the years ended December 31, 2019 and 2018 were as follows ( in MBOE ): Proved Undeveloped Reserves Proved undeveloped reserves at December 31, 2017(1) 60,015 Conversions to developed (4,419) Extensions and discoveries 13,734 Sales of minerals in place (4,702) Purchases of minerals in place 4,735 Revision to previous estimates 5,838 Proved undeveloped reserves at December 31, 2018 (2) 75,201 Conversions to developed (10,254 ) Extensions and discoveries 1,230 Revision to previous estimates (3,362 ) Proved undeveloped reserves at December 31, 2019 (3) 62,815 (1) Includes 34,029 MBOE attributable to noncontrolling interests. (2) Includes 41,560 MBOE attributable to noncontrolling interests. (3) Includes 34,243 MBOE attributable to noncontrolling interests. Conversions to developed . In our year-end 2018 plan to develop our PUDs within five years, we estimated that $103.8 million of capital would be expended in 2019 for the conversion of 30 gross / 12.3 net PUDs to add 9.9 MMBOE, which was consistent with the $111.5 million actually spent to convert 32 gross / 13.4 net PUDs adding 10.3 MMBOE to developed reserves. Extensions and discoveries . Additionally, 1.2 MMBOE were added as extensions and discoveries due to successful drilling results on our acreage positions because of the wells we drilled. The increase was also supported by successful drilling results by other operators directly offsetting and in close proximity to our acreage. Revision to previous estimates.  Revisions of 3.4 MMBOE were primarily due to reduced commodity prices. 2019 Changes in Proved Undeveloped Reserves 41