UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of report (Date of earliest event reported): September 29, 2016

SEMGROUP CORPORATION

(Exact Name of Registrant as Specified in Its Charter)

Delaware

(State or Other Jurisdiction of Incorporation)

Two Warren Place

6120 S. Yale Avenue, Suite 700

Tulsa, OK 74136-4216

(Address of Principal Executive Offices) (Zip Code)

(918) 524-8100

(Registrant’s Telephone Number, Including Area Code)

Not Applicable

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Introductory Note

On September 30, 2016, SemGroup Corporation, a Delaware corporation (“SemGroup”), completed its previously announced merger with Rose Rock Midstream, L.P., a Delaware limited partnership (“RRMS”), pursuant to that certain Agreement and Plan of Merger (the “Merger Agreement”), dated May 30, 2016, by and among SemGroup, RRMS, PBMS, LLC, a Delaware limited liability company and an indirect wholly owned subsidiary of SemGroup (“Merger Sub”), and Rose Rock Midstream GP, LLC, a Delaware limited liability company and the general partner of RRMS (the “RRMS General Partner”), whereby Merger Sub merged with and into RRMS with RRMS being the surviving entity in the merger (the “Merger”). Upon consummation of the Merger, SemGroup indirectly acquired all of the outstanding common units representing limited partner interests in RRMS (the “Common Units”) that SemGroup and its subsidiaries did not already own. On September 30, 2016, following the consummation of the Merger, SemGroup and certain of its subsidiaries commenced a series of transactions, including (i) the merger of the RRMS General Partner with and into SemGroup and (ii) the merger of RRMS with and into SemGroup, (each a “Clean-up Merger” and, together, the “Clean-up Mergers”), with SemGroup, in each instance, continuing as the surviving entity.

New Credit Agreement

On September 30, 2016, following the consummation of the Merger, SemGroup entered into an Amended and Restated Credit Agreement (the “Restated Credit Agreement”), together with various lenders and Wells Fargo Bank, National Association (“Wells Fargo”), as administrative agent. The Restated Credit Agreement amends and restates that certain Credit Agreement, dated as of June 17, 2011 (as amended prior to September 30, 2016, the “Existing Credit Agreement”) among SemGroup, Wells Fargo and various lenders.

The Restated Credit Agreement increases the aggregate commitments available to SemGroup to $1.0 billion, an increase of $500 million (the “Incremental Commitments”), and extends the maturity date to March 15, 2021 (the “Maturity Date”). The Incremental Commitments are increased commitments from lenders under the Existing Credit Agreement as well as commitments from new lenders. In addition, SemGroup may request an increase of up to an additional $300 million in commitments from either new lenders or increased commitments from existing lenders under the Restated Credit Agreement.

At SemGroup’s option, amounts borrowed under the Restated Credit Agreement will bear interest at either the Eurodollar rate or an alternate base rate (“ABR”), plus, in each case, an applicable margin. Until the date the financial statements relating to the quarter ending September 30, 2016 have been delivered, the applicable margin relating to any Eurodollar rate loan is 2.0% and the applicable margin relating to any ABR rate loan is 1.0%. After the date the financial statements for the quarter ending September 30, 2016 have been delivered, the applicable margin will range from 2.0% to 3.0% in the case of a Eurodollar rate loan and 1.0% to 2.0% in the case of an ABR rate loan, in each case, based on reference to a leverage-based pricing grid.

The Restated Credit Agreement includes customary representations and warranties and affirmative and negative covenants, which were made only for the purposes of the Restated Credit Agreement and as of the specific date (or dates) set forth therein, and may be subject to certain limitations as agreed upon by the contracting parties. Such limitations include the creation of new indebtedness, liens, sale and lease-back transactions, new investments, disposition, making fundamental changes including mergers and consolidations, making of dividends and other distributions, entering into certain transactions with affiliations, making material changes in business, modifying certain documents or entering into certain documents that limit the ability of subsidiaries to make distributions, entering into certain swap agreement and making significant accounting changes.

In addition, the Restated Credit Agreement contains financial performance covenants as follows:

The Restated Credit Agreement includes customary events of default, including events of default relating to non-payment of principal and other amounts owing under the Restated Credit Agreement from time to time, including in respect of letter of credit disbursement obligations, inaccuracy of representations and warranties in any material respect when made or when deemed made, violation of covenants, cross payment-defaults of SemGroup and its restricted subsidiaries to any material indebtedness, cross acceleration to any material indebtedness, bankruptcy and insolvency events, the occurrence of a change of control, certain unsatisfied judgments, certain ERISA events, certain environmental matters and certain assertions of or actual invalidity of certain loan documents. A default under the Restated Credit Agreement would permit the participating banks to terminate commitments, require immediate repayment of any outstanding loans with interest and any unpaid accrued fees, and require the cash collateralization of outstanding letter of credit obligations.

The Restated Credit Agreement is guaranteed by all of SemGroup’s material domestic subsidiaries and secured by a lien on substantially all of the property and assets of SemGroup and the other loan parties, subject to customary exceptions.

The foregoing description of the Restated Credit Amendment does not purport to be complete and is qualified in its entirety by reference to the Restated Credit Amendment, a copy of which is attached as Exhibit 10.1 to this Current Report on Form 8-K and incorporated into this Item 1.01 by reference.

Merger Agreement

The description of the Merger Agreement and the Merger in the Introductory Note is incorporated into this Item 2.01 by reference.

Pursuant to the Merger Agreement, at the effective time of the Merger (the “Effective Time”), each outstanding Common Unit (other than Common Units that, immediately prior to the Effective Time, were (i) subject to outstanding RRMS restricted unit awards (“RRMS Restricted Unit Awards”), (ii) held by SemGroup or any entities partially or wholly owned, directly or indirectly, by SemGroup or (iii) held by RRMS (each such Common Unit other than the foregoing excluded Common Units, an “RRMS Public Common Unit”)) was converted into the right to receive 0.8136 shares of validly issued, fully paid and non-assessable SemGroup Class A common stock, par value $0.01 per share (“SemGroup Common Stock”). No fractional shares of SemGroup Common Stock were issued in the Merger and holders of RRMS Public Common Units are, instead, entitled to receive cash in lieu of fractional shares of SemGroup Common Stock from the exchange agent.

Pursuant to the Merger Agreement, at the Effective Time, each RRMS Restricted Unit Award previously granted pursuant to the Rose Rock Midstream Equity Incentive Plan that was not vested and did not vest in accordance with its terms as a result of the transactions contemplated by the Merger Agreement and that was outstanding as of immediately prior to the Effective Time, including RRMS Restricted Unit Awards held by named executive officers of the RRMS General Partner, ceased to represent an award with respect to Common Units and was converted into an award with respect to shares of SemGroup Common Stock (a “SemGroup Award”), subject to the same vesting and forfeiture provisions as were applicable to such RRMS Restricted Unit Award immediately prior to the Effective Time, with the number of shares of SemGroup Common Stock subject to each such SemGroup Award being equal to the number of Common Units subject to each such RRMS Restricted Unit Award immediately prior to the Effective Time multiplied by 0.8136 (rounded down to the nearest whole share), with any corresponding accrued but unpaid Unit Distribution Rights (as defined in the Rose Rock Midstream Equity Incentive Plan) with respect to any RRMS Restricted Unit Awards being assumed by SemGroup, remaining outstanding and continuing to represent an obligation with respect to the applicable SemGroup Award.

Pursuant to the Merger Agreement, SemGroup issued approximately 13.3 million shares of SemGroup Common Stock (i) to the holders of RRMS Public Common Units and (ii) pursuant to the conversion of RRMS Restricted Unit Awards, each as described above.

The summary of the Merger Agreement in this Current Report on Form 8-K does not purport to be complete and is qualified by reference to the full text of the Merger Agreement, which is filed as Exhibit 2.1 to SemGroup’s Current Report on Form 8-K filed with the Securities and Exchange Commission on May 31, 2016 and incorporated into this Item 2.01 by reference.

On September 30, 2016, SemGroup entered into the Restated Credit Agreement as described under Item 1.01 above. The description of the Restated Credit Agreement under Item 1.01 is hereby incorporated into this Item 2.03 by reference.

The information included under Item 3.03 is hereby incorporated into this Item 2.03 by reference.

The information included under Item 2.01 is hereby incorporated into this Item 3.03 by reference.

On September 30, 2016, following the consummation of the Clean-Up Mergers, SemGroup entered into:

The Restated Credit Agreement restricts the Company’s and its subsidiaries’ ability to make certain types of payments relating to its and their capital stock, including the declaration or payment of dividends.

Copies of each of the 2021 Notes Supplemental Indenture, 2022 Notes Supplemental Indenture and 2023 Notes Supplemental Indenture (collectively, the “Supplemental Indentures”) are attached as Exhibits 4.1, 4.2 and 4.3, respectively, to this Current Report on Form 8-K and incorporated into this Item 3.03 by reference). The foregoing description of the Supplemental Indentures does not purport to be complete and is qualified in its entirety by reference to the applicable Supplemental Indenture.

In connection with the Merger and in anticipation of its granting of the SemGroup Awards, the Compensation Committee of the Board of Directors of SemGroup (the “Comp Committee”) adopted a form of Restricted Stock Award Agreement that has terms substantially similar to each of (i) the RRMS Restricted Unit Awards and (ii) existing SemGroup restricted stock award agreements previously granted by the Comp Committee, except that the terms of the SemGroup Awards were adjusted to reflect the consummation of the Merger, including the carrying over of any accrued but unpaid Unit Distribution Rights, as described in Item 2.01 above. The form of Restricted Stock Award Agreement with respect to SemGroup Awards is attached hereto as Exhibit 10.2.

The information included under Item 2.01 is hereby incorporated into this Item 5.02 by reference.

SemGroup held a Special Meeting of Stockholders (the “Special Meeting”) on September 29, 2016. At the Special Meeting, the SemGroup stockholders were requested to consider and vote upon: (1) a proposal to approve the issuance of shares of SemGroup Common Stock pursuant to the terms of the Merger Agreement in connection with the Merger (the “Stock Issuance Proposal”); and (2) a proposal to approve the adjournment of the Special Meeting, from time to time, if necessary or appropriate to solicit additional proxies if there were insufficient votes at the time of the Special Meeting to approve the Stock Issuance Proposal (the “Adjournment Proposal”). A total of 52,814,950 shares of SemGroup Common Stock were entitled to vote as of August 22, 2016, the record date for the Special Meeting. There were 46,428,776 shares present, in person or by proxy, at the Special Meeting (or 87.91% of the outstanding shares).

The following are the final voting results on the proposals considered and voted upon at the Special Meeting, each of which is more fully described in SemGroup’s definitive proxy statement filed on August 26, 2016:

On September 29, 2016, SemGroup issued a press release announcing the voting results of the Special Meeting. A copy of such press release is attached hereto as Exhibit 99.1 and is hereby incorporated into this Item 8.01 by reference.

On September 30, 2016, SemGroup issued a press release announcing the completion of the Merger, a copy of which is attached hereto as Exhibit 99.2 and is hereby incorporated into this Item 8.01 by reference.

The following exhibits are filed herewith.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

EXHIBIT INDEX