**Exhibit
      10.21**




**SECOND
      AMENDMENT TO**


**MORTGAGE,
      DEED OF TRUST, ASSIGNMENT OF PRODUCTION, SECURITY**


**AGREEMENT,
      FIXTURE FILING AND FINANCING STATEMENT**





**FROM**





SONTERRA
      OIL & GAS, INC. (f/k/a SONTERRA RESOURCES, INC.),
      a
      Delaware corporation





**TO**


WALTER
      H.
      WALNE, III,
      AS
      TRUSTEE





**FOR
      THE BENEFIT OF**





**VIKING
      ASSET MANAGEMENT, LLC**





**NOTICE
      OF CONFIDENTIALITY RIGHTS: IF YOU ARE A NATURAL PERSON, YOU MAY REMOVE OR STRIKE
      ANY OF THE FOLLOWING INFORMATION FROM THIS INSTRUMENT BEFORE IT IS FILED FOR
      RECORD IN THE PUBLIC RECORDS: YOUR SOCIAL SECURITY NUMBER OR YOUR DRIVER’S
      LICENSE NUMBER.**




**A
      CARBON, PHOTOGRAPHIC, FACSIMILE OR OTHER REPRODUCTION OF THIS INSTRUMENT IS
      SUFFICIENT AS A FINANCING STATEMENT.**




**PORTIONS
      OF THE MORTGAGED PROPERTY ARE GOODS WHICH ARE OR ARE TO BECOME AFFIXED TO OR
      FIXTURES ON THE LAND DESCRIBED IN OR REFERRED TO IN** **EXHIBIT
      A** **HERETO. THIS FINANCING STATEMENT IS TO BE FILED FOR RECORD OR RECORDED, AMONG
      OTHER PLACES, IN THE REAL ESTATE RECORDS OR SIMILAR RECORDS OF EACH COUNTY
      IN
      WHICH SAID LAND OR ANY PORTION THEREOF IS LOCATED. THE MORTGAGOR IS THE OWNER
      OF
      RECORD INTEREST IN THE REAL ESTATE CONCERNED. THIS INSTRUMENT IS ALSO TO BE
      INDEXED IN THE INDEX OF FINANCING STATEMENTS.**




1




**SECOND
      AMENDMENT TO**


**MORTGAGE,
      DEED OF TRUST, ASSIGNMENT OF PRODUCTION, SECURITY**


**AGREEMENT,
      FIXTURE FILING AND FINANCING STATEMENT**




**THIS
      SECOND AMENDMENT** **TO MORTGAGE, DEED OF TRUST, ASSIGNMENT OF PRODUCTION, SECURITY AGREEMENT,
      FIXTURE FILING**  **AND
      FINANCING STATEMENT** (this
      “ **Amendment** ”)
      is
      made as of the 14 th day of
      February 2008, by SONTERRA OIL & GAS, INC. (f/k/a SONTERRA RESOURCES, INC.),
      a Delaware corporation, whose address for notice is 300 East Sonterra Boulevard,
      Suite 1220, San Antonio, Texas 78258 to **WALTER
      H. WALNE, III** ,
      as
      Trustee, whose address for notice is 17 South Briar Hollow, Houston, Texas
      77027
      (“ Trustee ”),
      for
      the benefit of **VIKING
      ASSET MANAGEMENT, LLC** ,
      a
      California limited liability company, 600 Montgomery Street, 44th Floor, San
      Francisco, CA 94111, on its own behalf and in its capacity as collateral agent
      for the benefit of the holders of the Notes (as defined in the Mortgage
      described below) (together with its successors and assigns, the “ Mortgagee ”).
      Capitalized terms used but not defined herein shall have the respective meanings
      ascribed to such terms in the Mortgage describe below.




**R
      E C I T A L S**




A.  Mortgagor
      has previously executed and delivered in favor of Mortgagee a certain Mortgage,
      Deed of Trust, Assignment of Production, Security Agreement, Fixture Filing
      and
      Financing Statement made as of August 3, 2007, and recorded on August 10, 2007
      in the office of the County Clerk in Calhoun County, Texas as Instrument #107284
      and on August 14, 2007 in the office of the County Clerk in Matagorda County,
      Texas as Instrument #03500175898001, Year 2007, No. 076321, as amended by that
      certain First Amendment to Mortgage, Deed of Trust, Assignment of Production,
      Security Agreement, Fixture Filing and Financing Statement made as of August
      29,
      2007 and recorded on September 6, 2007 in the office of the County Clerk in
      Calhoun County, Texas as Instrument # 107792 and on September 18, 2007 in the
      office of the County Clerk in Matagorda County, Texas as Instrument
      03500176874001, Year 2007, No. 077293 (as the same may be further amended,
      restated, supplemented or otherwise modified from time to time, the
“ **Mortgage** ”),
      which
      Mortgage encumbers Mortgagor’s interest in the land legally described on Exhibit
      A attached
      hereto (the “ **Land** ”),
      in
      addition to various other real and personal property pledged to the Mortgagee
      as
      more fully described in the Mortgage.




B.  Mortgagor
      and Mortgagee have agreed to modify the Mortgage upon the terms and conditions
      contained herein.




**NOW
      THEREFORE** ,
      in
      consideration of the premises and other good and valuable consideration, the
      receipt and legal sufficiency whereof are hereby acknowledged, the parties
      hereby agree as follows:




Recitals **.**




The
      Recitals set forth above are incorporated herein by this reference thereto
      as if
      fully set forth herein.




2


Amendment
      of Mortgage **.**




Effective
      as of the date hereof, Section 1.03 of the Mortgage is hereby amended by adding
      a clause (e) thereto to read as follows:




“(e)  Payment
      of and performance of any and all indebtedness, obligations and liabilities
      of
      Mortgagor pursuant to that certain Guaranty, dated as of February 14, 2008,
      among Mortgagor, the other Persons from time to time thereto and
      Mortgagee.”




(b)  Effective
      as of the date hereof, the Mortgage is hereby amended by deleting each reference
      to the words “Sonterra Resources, Inc.” and in each case substituting therefor
      the words “Sonterra Oil & Gas, Inc.”




No
      Further Amendment **.**




This
      Amendment is given solely to amend and modify the Mortgage as set forth herein.
      No further amendment or modification of the Mortgage is made or intended, and
      the respective terms and provisions thereof shall, as expressly amended and
      modified hereby, continue in full force and effect after the date hereof. The
      warranties, representations, covenants and agreements contained in the Mortgage
      as herein expressly amended, are hereby ratified, approved and confirmed in
      every respect. Mortgagor also hereby **(i)** agrees
      that, from and after the date of this Amendment, each reference in the Mortgage
      to the term “Indebtedness” shall mean and be a
      reference to such term as defined in the Section 1.03 of the Mortgage, as such
      Section 1.03 is amended pursuant to **Section
      2** of this
      Amendment , **(ii)** expressly ratifies and confirms, as of the date of the Mortgage and as of the
      date hereof, the grant by Mortgagor of the lien on the Land and all of the
      other
      property and interests in property created or intended to be created by the
      Mortgage, in each case as amended and modified hereby and **(iii)** represents and warrants that Mortgagor has not created or suffered or permitted
      to exist any other lien upon or in any such property or interests in property
      subsequent to the execution and delivery of the Mortgage, other than as
      expressly permitted pursuant to the terms and provisions thereof. Mortgagor
      has
      no claims, claims of offset or causes of action against Mortgagee, and no
      defenses to its performance of all Indebtedness (as such term is defined in
      the
      Mortgage after giving effect to this Amendment).




No
      Release **.**




The
      indebtedness, liabilities and other obligations secured by the Mortgage are
      continuing obligations and nothing contained herein shall be deemed to release,
      terminate or subordinate any lien created or evidenced thereby and all such
      liens and the priority thereof shall relate back to the recordation date for
      the
      Mortgage as referenced herein. This Amendment is not intended and shall not
      be
      deemed or construed to in any way affect the enforceability or priority of
      the
      Mortgage or constitute a novation, termination or replacement of all or any
      part
      of the indebtedness, liabilities or other obligations secured
      thereby.




3


Governing
      Law; Severability **.**




This
      Amendment shall be governed by and construed in accordance with the laws of
      the
      State of New York. The invalidity, illegality or unenforceability of any
      provision of this Amendment shall not affect or impair the validity, legality
      or
      enforceability of the remainder of this Amendment or the Mortgage, and to this
      end, the provisions of this Amendment are declared to be severable.










[Remainder
      of page intentionally left blank;


Signature
      page follows]






4


**IN
      WITNESS WHEREOF** ,
      Mortgagor has executed this Amendment on the date set forth in the
      acknowledgments hereto, to be effective as of the date first above
      written.




MORTGAGOR:**SONTERRA
                OIL & GAS, INC.** (f/k/a SONTERRA RESOURCES, INC.), a Delaware
                corporationBy:
            /s/
                Michael J. PawelekName:
            Michael
                J. PawelekTitle:
            President












**[** **Signature
      Page to Second Amendment to Mortgage]**




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