Sec Form 4 Filing - Yorktown Energy Partners XI, L.P. @ Carbon Natural Gas Co - 2018-02-01

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Yorktown Energy Partners XI, L.P. 2. Issuer Name and Ticker or Trading Symbol Carbon Natural Gas Co [ CRBO] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O YORKTOWN PARTNERS LLC, 410 PARK AVENUE, 19TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 02/01/2018 (Street) NEW YORK, NY10022 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common stock, par value $0.01 per share 02/01/2018 M 1,527,778 ( 1 ) A 1,527,778 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Warrant ( 2 ) 02/01/2018 M 1,527,778 ( 3 ) 02/15/2017 02/15/2024 Common Stock, par value $0.01 per share 1,527,778 ( 3 ) ( 2 ) 0 D Series B Convertible Preferred Stock, par value $0.01 ( 4 ) 04/06/2018 P 50,000 04/06/2018 ( 5 ) Common Stock, par value $0.01 per share 50,000 $ 100 50,000 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Yorktown Energy Partners XI, L.P. C/O YORKTOWN PARTNERS LLC 410 PARK AVENUE, 19TH FLOOR NEW YORK, NY10022 X Signatures Yorktown Energy Partners XI, L.P., By: Yorktown XI Company LP, its general partner, By: Yorktown XI Associates LLC, its general partner, /s/ Peter A. Leidel, Manager 04/09/2018 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Shares received upon exercise of Warrant No. 1, issued on February 15, 2017 (the "Warrant") to the reporting person. ( 2 )The consideration the reporting person paid for the full exercise of the Warrant was the transfer and assignment to Carbon Natural Gas Company (the "Issuer") of 11,000 Class A Units of Carbon California Company, LLC, a Delaware limited liability company. ( 3 )On March 15, 2017, the Issuer effected a one for twenty reverse stock split of the Issuer's issued and outstanding common stock, as reported on the Current Report on Form 8-K filed by the Issuer with the SEC on March 16, 2017. As a result, the number of shares of common stock underlying the Warrant was proportionately reduced from 30,555,556 to 1,527,778. ( 4 )The Series B Convertible Preferred Stock converts into shares of the Issuer's common stock at the election of the holder and will automatically convert into shares of the Issuer's common stock if and when the Issuer completes a qualifying equity financing. The number of shares of common stock issuable upon conversion is dependent upon the price per share of common stock issued in connection with any such qualifying equity financing, but has an initial floor conversion price equal to $8.00 per share. The conversion price will be proportionately increased or decreased to reflect changes to the outstanding shares of common stock, such as the result of a combination, reclassification, subdivision, stock split, stock dividend or other similar transaction involving the common stock. ( 5 )The Series B Convertible Preferred Stock has no expiration date. However, it converts into shares of the Issuer's common stock at the election of the holder and will automatically convert into shares of the Issuer's common stock if and when the Issuer completes a qualifying equity financing.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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