highest known oil elevation and the potential exists for an associated natural gas cap, proved oil reserves may be assigned in the structurally higher portions of the reservoir only if geoscience, engineering or performance data and reliable technology establish the higher contact with reasonable certainty. Reserves which can be produced economically through application of improved recovery techniques (including, but not limited to, fluid injection) are included in the proved classification when: (i) successful testing by a pilot project in an area of the reservoir with properties no more favorable than in the reservoir as a whole, the operation of an installed program in the reservoir or an analogous reservoir, or other evidence using reliable technology establishes the reasonable certainty of the engineering analysis on which the project or program was based; and (ii) the project has been approved for development by all necessary parties and entities, including governmental entities.

Developed oil, natural gas and NGL reserves are reserves of any category that can be expected to be recovered through existing wells with existing equipment and operating methods where production can be initiated or restored with relatively low expenditure compared to the cost of drilling a new well.

The information provided below on our oil, natural gas and NGL reserves is presented in accordance with regulations prescribed by the SEC. Our reserve estimates are generally based upon extrapolation of historical production trends, analogy to similar properties and volumetric calculations. Accordingly, these estimates will change as future information becomes available and as commodity prices change. These changes could be material and could occur in the near term.

Presented below is a summary of changes in estimated reserves for 2017, 2016 and 2015:

<table><tr><td rowspan="2"></td><td>Oil</td><td>Gas</td><td>NGL</td><td>Total</td></tr><tr><td>(mmbbl)</td><td>(bcf)</td><td>(mmbbl)</td><td>(mmboe)</td></tr><tr><td>December 31, 2017</td><td></td><td></td><td></td><td></td></tr><tr><td>Proved reserves, beginning of period</td><td>399.1</td><td>6,496</td><td>226.4</td><td>1,708</td></tr><tr><td>Extensions, discoveries and other additions</td><td>62.7</td><td>3,694</td><td>44.9</td><td>723</td></tr><tr><td>Revisions of previous estimates</td><td>(168.1)</td><td>(315)</td><td>(31.0)</td><td>(252)</td></tr><tr><td>Production</td><td>(32.7)</td><td>(878)</td><td>(20.9)</td><td>(200)</td></tr><tr><td>Sale of reserves­in­place</td><td>(0.9)</td><td>(418)</td><td>(0.8)</td><td>(71)</td></tr><tr><td>Purchase of reserves­in­place</td><td>0.1</td><td>21</td><td>—</td><td>4</td></tr><tr><td>Proved reserves, end of period(a)</td><td>260.2</td><td>8,600</td><td>218.6</td><td>1,912</td></tr><tr><td>Proved developed reserves:</td><td></td><td></td><td></td><td></td></tr><tr><td>Beginning of period</td><td>200.4</td><td>5,126</td><td>134.1</td><td>1,189</td></tr><tr><td>End of period</td><td>150.9</td><td>4,980</td><td>134.9</td><td>1,116</td></tr><tr><td>Proved undeveloped reserves:</td><td></td><td></td><td></td><td></td></tr><tr><td>Beginning of period</td><td>198.7</td><td>1,370</td><td>92.2</td><td>519</td></tr><tr><td>End of period(b)</td><td>109.3</td><td>3,620</td><td>83.6</td><td>796</td></tr></table>

<table><tr><td rowspan="2"></td><td>Oil</td><td>Gas</td><td>NGL</td><td>Total</td></tr><tr><td>(mmbbl)</td><td>(bcf)</td><td>(mmbbl)</td><td>(mmboe)</td></tr><tr><td>December 31, 2016</td><td></td><td></td><td></td><td></td></tr><tr><td>Proved reserves, beginning of period</td><td>313.7</td><td>6,041</td><td>183.5</td><td>1,504</td></tr><tr><td>Extensions, discoveries and other additions</td><td>191.2</td><td>1,798</td><td>89.0</td><td>580</td></tr><tr><td>Revisions of previous estimates</td><td>(58.9)</td><td>598</td><td>2.8</td><td>43</td></tr><tr><td>Production</td><td>(33.2)</td><td>(1,050)</td><td>(24.4)</td><td>(233)</td></tr><tr><td>Sale of reserves­in­place</td><td>(14.7)</td><td>(1,190)</td><td>(28.1)</td><td>(241)</td></tr><tr><td>Purchase of reserves­in­place</td><td>1.0</td><td>299</td><td>3.6</td><td>55</td></tr><tr><td>Proved reserves, end of period(c)</td><td>399.1</td><td>6,496</td><td>226.4</td><td>1,708</td></tr><tr><td>Proved developed reserves:</td><td></td><td></td><td></td><td></td></tr><tr><td>Beginning of period</td><td>215.6</td><td>5,329</td><td>158.0</td><td>1,262</td></tr><tr><td>End of period</td><td>200.4</td><td>5,126</td><td>134.1</td><td>1,189</td></tr><tr><td>Proved undeveloped reserves:</td><td></td><td></td><td></td><td></td></tr><tr><td>Beginning of period</td><td>98.1</td><td>712</td><td>25.5</td><td>242</td></tr><tr><td>End of period(b)</td><td>198.7</td><td>1,370</td><td>92.2</td><td>519</td></tr><tr><td>December 31, 2015</td><td></td><td></td><td></td><td></td></tr><tr><td>Proved reserves, beginning of period</td><td>420.8</td><td>10,692</td><td>266.3</td><td>2,469</td></tr><tr><td>Extensions, discoveries and other additions</td><td>61.1</td><td>805</td><td>35.3</td><td>231</td></tr><tr><td>Revisions of previous estimates</td><td>(110.0)</td><td>(4,191)</td><td>(75.8)</td><td>(885)</td></tr><tr><td>Production</td><td>(41.6)</td><td>(1,070)</td><td>(28.0)</td><td>(248)</td></tr><tr><td>Sale of reserves­in­place</td><td>(16.6)</td><td>(195)</td><td>(14.3)</td><td>(63)</td></tr><tr><td>Purchase of reserves­in­place</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Proved reserves, end of period(d)</td><td>313.7</td><td>6,041</td><td>183.5</td><td>1,504</td></tr><tr><td>Proved developed reserves:</td><td></td><td></td><td></td><td></td></tr><tr><td>Beginning of period</td><td>229.3</td><td>8,615</td><td>198.5</td><td>1,864</td></tr><tr><td>End of period</td><td>215.6</td><td>5,329</td><td>158.0</td><td>1,262</td></tr><tr><td>Proved undeveloped reserves:</td><td></td><td></td><td></td><td></td></tr><tr><td>Beginning of period</td><td>191.5</td><td>2,077</td><td>67.8</td><td>605</td></tr><tr><td>End of period(b)</td><td>98.1</td><td>712</td><td>25.5</td><td>242</td></tr></table>

(a) Includes 1 mmbbl of oil, 20 bcf of natural gas and 2 mmbbls of NGL reserves owned by the Chesapeake Granite Wash Trust, of which 1 mmbbl of oil, 10 bcf of natural gas and 1 mmbbl of NGL are attributable to noncontrolling interest holders

(b) As of December 31, 2017, 2016 and 2015, there were no PUDs that had remained undeveloped for five years or more.

(c) Includes 1 mmbbl of oil, 23 bcf of natural gas and 2 mmbbls of NGL reserves owned by the Chesapeake Granite Wash Trust, 1 mmbbl of oil, 12 bcf of natural gas and 1 mmbbl of NGL of which are attributable to the noncontrolling interest holders.

(d) Includes 1 mmbbl of oil, 32 bcf of natural gas and 3 mmbbls of NGL reserves owned by the Chesapeake Granite Wash Trust, 1 mmbbl of oil, 16 bcf of natural gas and 2 mmbbls of NGL of which are attributable to the noncontrolling interest holders.