<PAGE>
 
                                                                   Exhibit 10.70

CONTRACT  No.:  GCI-003-99

SELLER:      ARGOSY ENERGY INTERNATIONAL AND
OBJECT:      SANTANA CRUDE  SALE/PURCHASE
PERIOD:      FEBRUARY 1, 1999 TO DECEMBER 31, 1999
AMOUNT:      UNDETERMINED QUANTITY


The contracting parties, on one hand, the EMPRESA COLOMBIANA DE PETROLEOS
"ECOPETROL", which hereinafter will be called "ECOPETROL",  an Industrial and
Commercial State owned Company,  authorized by Law 165 of 1948 and regulated by
its approved statutes by Decree 1209 of 1994, with main headquarters located in
the City of Santafe de Bogota, D.C., represented by LUIS AUGUSTO YEPES G.,  of
legal age, identified by Colombian Identity Card No. 19.125.070 issued in
Bogota, residing in Bogota, who declares:  a.  That he acts in his capacity of
Vice President and in exercise of legal and internal norms of Ecopetrol.-b.  As
party of the second part, ARGOSY ENERGY INTERNATIONAL,  an existent and
organized company in accordance with the laws of the State of Utah, of The
United States of America, domiciled in Salt Lake City, State of Utah, with a
subidiary established in Colombia, duly constituted by Public Deed number 5323
conceded in the Seventh Notary of Bogota on October 25, 1983, and registered in
the Chamber of Commerce of Bogota, under registry No. 200848, on November 23,
1983, represented by SANTIAGO GONZALEZ ANGULO, of legal age, identified by
Colombian Identity Card No. 5.584.373, issued in Barrancabermeja, company which
henceforth will be designated as THE SELLER, who enter into this purchase and
sale contract of Santana Crude, contained in the following clauses: CLAUSE ONE:
- OBJECT AND QUANTITIES.  THE SELLER is obliged  to sell and to deliver to
ECOPETROL, under the conditions here set forth, the crude oil produced under the
contract of  "Santana" Risk Participation, that corresponds to THE SELLER, in
the quantity and quality that is stipulated and in agreement with that provided
in Clause Two of this document and that in consequence, ECOPETROL is obliged to
receive and pay.  PARAGRAPH 1:  The "Santana"Risk Participation contract was
entered into on May 27, 1987, with effective date that of July 27, 1987, between
ECOPETROL AND ARGOSY ENERGY INTERNATIONAL AND NEO ENERGY, INC, which was
modified by means of Public Deed No. 00064 dated January 19, 1999, conceded by
Notary 50 in Bogota.  PARAGRAPH 2:  All purchases will be made by ECOPETROL in
accordance with the preliminary crude purchasing program agreed upon by the
parties for six (6) month periods, that ECOPETROL could modify, giving previous
notice to THE SELLER with at least 30 days anticipation.  CLAUSE TWO:  
<PAGE>
 
QUALITY. The quality of the crude, subject matter of this contract, will have
the following specifications: a) the gravity of the crude oil will be that with
which said crude oil is obtained, through the operations that are carried out
for its production. b) The content of water and sediment in the crude oil,
cannot be greater than 0.5% in volume and its determination will be made
respectively by means of the ASTM-D4377 methods, "water in crude oil by
distillation" (Karl Fisher), last revision, ASTM-D473, "sediments in crude and
fuel oil by extraction", last revision. The parties agree that the Karl Fisher
method will be implemented within a 90 day period as of the subscription of this
contract. Meanwhile, the parties will continue using method ASTM D-95. c)The
content of sulphur in the crude oil with which said crude oil is obtained,
through the operations carried out for its production; its determination will be
made by the ASTM - D2622 method, last revision, "sulphur analysis by X-ray". d)
The salt content cannot be greater than 20 pounds for each 1,000 barrels of
crude oil and its determination will be made by the ASTM - D3230 method, " crude
salts (electrometric method), last revision. When any of the previously
mentioned parameters are not fulfilled or the previously mentioned
specifications are not within the permitted margin, ECOPETROL reserves the right
to reject the crude oil. But should ECOPETROL choose to accept with salinity
over the specifications, the price of the crude oil will be penalized in
accordance with the following chart:
<TABLE>
<CAPTION>
 
Salinity Content           Penalty in U.S.   Charged to
Pounds per 1,000            Dollars/Barrel
Barrels                      (US$/Barrel)
<S>                 <C>    <C>               <C>
 
20.01        30.0            0.160           THE SELLER
                                          
30.1         40.0            0.180           THE SELLER
                                          
40.1         60.0            0.200           THE SELLER
                                          
60.1         80.0            0.220           THE SELLER
                                          
80.1        100.0            0.240           THE SELLER
</TABLE>

It is understood that THE SELLER  will make every effort possible to deliver the
contracted crude oil, with a salt content of less than twenty (20) pounds for
each one thousand (1,000) barrels of crude oil.  e)  Whatever variation
referring to the quality specifications mentioned previously, that be accepted
by both parties, will be consigned in a document subscribed 
<PAGE>
 
between ECOPETROL Representatives and THE SELLER. CLAUSE THREE: PLACE OF
DELIVERY AND OWNERSHIP. The crude oil, subject of this contract, will be turned
over by THE SELLER to ECOPETROL in the Santana terminal where the crude will be
later measured and analyzed. Then, from the referred point, it will be
transported by ECOPETROL to the terminal in Tumaco. The property and risk title
will pass from THE SELLER to ECOPETROL at the moment in which the crude oil
passes the exit check point of the measuring system located in the Santana
Terminal. PARAGRAPH 1: If the Santana Association constructs the Santana-Orito
pipeline, the measurement and transfer of the ownership of the crude oil will be
made effective in Orito. Also, as of that date, the sales-purchase value of the
crude will be modified, as now the tariff for transport from Santana-Orito will
not be taken into account, in accordance with Resolution 6031 of February 8,
1994, from the Ministry of Mines and Energy. PARAGRAPH 2: The capacity for
receipt of Santana crude will be limited to the capacity of the pipeline,
Santana-Orito that exists at that moment. CLAUSE FOUR: DURATION. This agreement
will have an initial duration of eleven (11) months counted from the first of
February of 1999 until December 31, 1999, and could be extended for additional
periods of one (1) year by written agreement between the parties, before the
expiration of this contract. CLAUSE FIVE: TRANSPORTATION. For invoicing
purposes, the tariff approved by the Ministry of Mines and Energy will be
applied based on the total number of barrels transported. It is understood that
the Pipeline Transportation Regulation will be incorporated to this contract
once the parties approve it and once it has been submitted to the required
official approvals. CLAUSE SIX. PRICE. The price that ECOPETROL will pay THE
SELLER for the crude is defined in the following manner: Price = Base price plus
or minus quality adjustment less transport less handling and marketing.
PARAGRAPH 1: The basic price will be the average weighted of the export
shipments invoiced by Ecopetrol during that month. PARAGRAPH 2: The quality
adjustment that will be applied in this contract, will be for API Gravity and
Sulphur content that will be calculated monthly in accordance with the procedure
described in Annex 1. PARAGRAPH 3: The transportation cost, Santana-Tumaco, as
well as the respective transport tax will be adjusted to that designated by the
statutory clauses in force and remain subject to whatever modification
introduced by said provisions during the duration of the contract. In accordance
with that established in Resolution number 6031 of May 8, 1998, from the
Ministry of Mines and Energy, the Santana-Tumaco transport tariff to January 31,
1999, amounts to 1.6809 US$/BL. The amount of the transport tax corresponds to
2% (two percent) of the rate in accordance with that established in Article 17
of Decree 2140 of 1955, adopted as the  
<PAGE>
 
permanent norm by Law 10 of 1961.  Said tax will be paid according to the
Law 141 of 1994, which until January 31, 1999, is of 0.0336  US$/Bl.  These
rates will be readjusted annually according to that established in the
previously mentioned Resolution No. 6031.  PARAGRAPH 4:  The handling and
marketing cost will be of 0.165 US$/BL.  PARAGRAPH 5:  If during one month or
consecutive months, there is no exportation of crude, the price would be that of
the preceding month (M-1).  This will be adjusted in the following month (M + 1)
according to the new price calculated for the exports of said month M + 1.  The
volume of Santana crude received in the corresponding month will be taken and
the adjustment for quality will be calculated with the average of  price
quotations corresponding to the three months preceding the adjusted month (Month
M). PARAGRAPH 6.   In the event that there are cabotages from the crude Tumaco
blend (South Blend), to the Cartagena refinery, the conditions for sale and
pricing that Ecopetrol will recognize to the Seller, will be the same obtained
by the International Trade Management for the crude blend (FOB Tumaco) received
by the Cartagena refinery plus or minus the quality adjustment, less the cost
for Santana-Tumaco transport and the respective tax, less marketing (USD 0.165
per barrel).  In the event that the Cartagena Refinery purchases the South
Blend, without this resulting from a bidding process, the parties will agree on
a method to calculate this crude's price. PARAGRAPH 7. Not withstanding that
expressed in Paragraph 2, regarding the quality adjustment procedure, the
parties can agree to a revision of the crude supply and of the current method
used, and they could resort to a mutual agreement on the crude which should be
eliminated or added to the current supply. If an agreement is reached, it will
be applied to the entire contract. Additionally, the parties could also hire an
external consultant who could determine whether to continue with the current
method, or whether a new method should be implemented. In the event that the
parties do not reach an agreement, the current method will prevail. CLAUSE
SEVEN.  BILLING AND FORM OF PAYMENT.  THE SELLER will bill ECOPETROL in its
Bogota offices, within the ten (10) first days of each month, for the crude oil
of its ownership, delivered to ECOPETROL during the preceding month, after
having deducted the volume corresponding to royalties, contributions and shares.
Within seven (7) days of the before-mentioned deadline, ECOPETROL will supply
THE  SELLER with the information that he requires to draw up the corresponding
invoice.  The payment will be made on a monthly basis at thirty (30) days from
the date of receipt of the invoice by ECOPETROL, prior to that withheld
according to law, if there were occasion for it.  Twenty-five percent ( 25% ) of
the amount will be paid in Colombian Pesos and seventy-five percent (75%) in
United States Dollars.  The invoicing will be made on the basis of net volume,
free of water and 
<PAGE>
 
sediment, adjusted to 60 degrees F. For the portion in Colombian Pesos, the
representative exchange rate for the market will be used according to the
Banking Superintendency, calculated as an arithmetical average, corresponding to
crude deliveries. PARAGRAPH 1: In the event of delay in payment of that
stipulated in the contract, for the payment in Dollars over invoices not
objected to opportunely by ECOPETROL, ECOPETROL will acknowledge to the SELLER
as interest paid in Dollars, the equivalent rate to the Prime Rate indicated by
the Chase Manhattan Bank of New York, during the days corresponding to the
effectual delay elapsed, plus 2.0%.: In the event of delay in payment of that
stipulated in the contract, liquidated in Pesos, over the respective quantity in
delay in Pesos, Ecopetrol will acknowledge the maximum monthly interest in
accordance to the certificate issued by the Banking Superindentency. The
invoices for interest charge in Pesos or in Dollars, will be cancelled within
ten (10) days following the receipt of the invoice by ECOPETROl. PARAGRAPH 2: If
ECOPETROL does not have in its possession and at its disposal, Dollars from the
United States of America, or could not obtain from the Colombian Government or
from its authorized agencies, Dollars to cover the purchases corresponding to
the contracted crude oil, Ecopetrol will quickly notify THE SELLER in writing
without detriment to the conditions stipulated in the preceding Paragraph 1, and
the parties will prepare for a maximum period of thirty (30) calendar days,
counting from the notification from ECOPETROL, as by mutual agreement, arrive at
an adequate solution. PARAGRAPH 3: ECOPETROL will dispose of fifteen (15)
working days to revise, correct or object to the invoices presented by THE
SELLER. Whatever invoice that has not been objected to within this period, will
be considered as final and correct. Whatever adjustment or correction that must
be made in the invoice, will occasion that the valid date of its presentation be
considered from the date in which said adjustment had been effectively made
before ECOPETROL. ECOPETROL will inform THE SELLER within the permitted term
over whatever invoice that is objected to so that it can be adjusted and
corrected, specifying clearly the items that must be adjusted or corrected and
the corresponding reason. CLAUSE EIGHT: INSPECTION AND MEASUREMENT. For the
purposes of Clause Two, the determination of quality will be made according to
the operative procedures that are established by mutual agreement between the
parties in accordance with the written record of proceedings. The cost of these
procedures will be shared by the parties in proportion to the crude ownership by
ECOPETROL and THE SELLER. In Addition to these operative procedures, whichever
of the parties could designate when they wish, an independent inspector to
certify quality and quantity, carry out examinations on the tanks or the
calibration of the volume measuring
<PAGE>
 
instruments. In the latter case, the cost will be covered by the party
requesting the measurement. CLAUSE NINE: TERMINATION. The Seller or ECOPETROL
can terminate the sale/purchase contract by written notice with an advance
notice of sixty (60) days. If THE SELLER decides to export his Santana crude
directly, he will advice ECOPETROL in writing with an advance notice of sixty
(60) days, a term within which Ecopetrol and the Seller can revoke this
contract. CLAUSE TEN: DESTINATION. ECOPETROL can give its acquired crude oil,
the destination that is considered appropriate for its interests, provided that
said destination be permitted by legal applicable arrangements that are in force
at that time. CLAUSE ELEVEN: TRANSFERS. Neither of the parties could cede, sell
or transfer the totality or any part of their rights and obligations here
contracted to a third party, without previous and written consent from the other
party. CLAUSE TWELVE: FORCE MAJEURE. Neither ECOPETROL nor THE SELLER will be
responsible for lack of compliance of all or any of their obligations according
to this agreement, if said failure is caused by events constituted as "force
majeure"or act of nature duly proven. Force Majeure is an unforseen event which
cannot be controlled, which cannot be blamed on the obliged party and that
places said party in a position where it cannot fulfil its obligations, such as:
natural phenomena (earthquakes, floods, landslides) or public order events
(strikes, Terrorism, mutiny, sabotage, pipeline ruptures). The "force majeure"
will not liberate ECOPETROL from its payment obligation to THE SELLER for those
invoices for the sale of crude oil that has been delivered by THE SELLER, in
conformity with the stipulated terms in Clause Eight of this contract. CLAUSE
THIRTEEN: APPLICATION OF COLOMBIAN LAWS. For all the purposes of this contract,
the parties confirm their domicile in the City of Santafe de Bogota, D.C.,
Republic of Colombia. This contract is in force in all of its parts by Colombian
law and the parties submit to the jurisdiction of Colombian courts and renounce
the intention of diplomatic claims in all that touches upon their rights and
obligations proceeding from this contract, except in the case of denial of
justice. For all effects of this contract, it is understood that the
dispositions incorporated in it from Article 25 of Law 40 of 1993, and that of
the Second Chapter of the Title Three of Law 104 of 1993, or from whatever other
norm that modifies or adds to them. CLAUSE FOURTEEN: NOTIFICATIONS. All
notifications made in accordance with this contract, should make reference to
this Clause and to the pertinent Clause. Said notifications should be sent by
certified mail, fax, or be delivered at the addresses that follow, are indicated
and will be considered as received in the respective address, on the date that
appears in the receipt of the letter or on the date in which the fax is sent.
<PAGE>
 
EMPRESA COLOMBIANA DE PETROLEOS - ECOPETROL, Calle 37 No. 7-43, Fax No. 3382585,
Attn.. International Trade and Gas Vice Presidency.  THE SELLER, ARGOSY ENERGY
INTERNATIONAL, Avenue 13 (Autopista Norte), No. 122-56, floor 4, Fax No. 6-
195460 , Santafe de Bogota, D.C., Attn.: Santiago Gonzalez A., President. All
changes of address should be informed in writing, with anticipation, to the
other party.  CLAUSE FIFTEEN:  TAXES AND EXPENSES.   All taxes and expenses that
are caused for entering into and performing this contract and its extentions or
modifications will be considered the exclusive duty of THE SELLER.  CLAUSE
SIXTEEN:  DISAGREEMENTS.  A)  In the event of disagreements between the parties
concerning legal issues related to the interpretation and  performance of the
contract, that cannot be arranged in an amicable manner, shall be subjected to
the knowledge and decision of the jurisdiction branch of  Colombian Sovereign
Power.  B)  Any difference in deed or of a technical character that arises
between the parties, with motive for the interruption or enforcement of the
contract, that cannot be settled in an amicable manner, will be subjected to the
decision of experts, named as follows:  one for each party and a third
designated by mutual agreement by the principal experts named.  If the two
experts did not agree on the nominated third party, this will be named by
petition of any of the parties,  by the Board of Directors of the Colombian
Society of Engineers that has its headquarters in Santafe de Bogota, D.C.  Any
difference of an accounting nature that arises between the parties caused by
interpretation and performance of the contract that cannot be arranged in an
amicable manner, will be subjected to the opinion of experts,  who should be
certified accountants, designated as follows:  one for each party and a third
named by the two principal experts.  Lacking an agreement between these and on
the petition by either of the parties, said third expert will be named by the
Central Board of Accountants of Bogota, and lacking this, by the Colombian
Society of Engineers. D)  Both parties declare that the decision by the experts
will have all the effects of a transaction between them and consequently, such a
decision will be definite.  E)  In the event of a disagreement between the
parties over technical quality, countable and/or legal, of the controversy, this
is considered as legal and the literal will be applied A)  from this Clause,
that agreed to in the Clause is understood without damaging the special
procedures provided in this contract.

As written evidence, duly signed in Santafe de Bogota, D.C., on the Sixteenth
day of the month of February of Nineteen Ninety Nine (1999), on safety paper for
the original and the copy
<PAGE>
 
EMPRESA COLOMBIANA DE PETROLEOS    ARGOSY ENERGY INTERNATIONAL
          ECOPETROL



LUIS AUGUSTO YEPES G.               SANTIAGO GONZALEZ ANGULO
VP International Trade and Gas        Legal Representative
<PAGE>
 
                                   ANNEX   1


             CALCULATION OF THE ADJUSTMENT FOR QUALITY COMPENSATION


The procedure for quality adjustment will be applied for the deliveries of the
preceding month in accordance with the following procedure:

Information required:

+   Regression equation for the calculation of the referenced prices, according
    to the information supplied at the end of this annex.
+   Volume and quality (API & %wtS) of the shipment (s) of crude from the Tumaco
    terminal for export.
+   Volume and quality (API & %wtS) of Santana crude delivered to the Santana
    terminal, as well as the deliveries in the Mary, Miraflor, Toroyaco and
    Linda fields.
+   Price base of the South Blend crude delivered in Tumaco, that is exported.

Procedure:

1.  Calculate the reference price for the South Blend withdrawn in Tumaco and
    for the Santana crude delivered at the Santana terminal, by using a
    regression equation according to its respective quality.
2.  With the volume of the South Blend oil withdrawn in Tumaco and of the
    Santana crude delivered at the Santana terminal and the reference prices
    previously calculated, calculate the amount in US$ of each one.
3.  Calculate the fraction of the amount of Santana crude in the South Blend,
    dividing the US$ for the Santana crude by the US$ of the crude-Blend.
4.  Calculate the volume of Santana crude compensated, multiplying the volume of
    crude Blend withdrawn in Tumaco by the fraction of the cost of the Santana
    crude.
5.  Calculate the compensation factor of the Santana crude, dividing the
    compensated volume of Santana crude by the Santana crude delivered in the
    Santana terminal.
6.  Calculate the amount of the compensation as follows:

Amount of the compensation = Base Price * (compensation factor minus 1)
The price is the amount of the export of  the South Blend oil FOB Tumaco.

Example:  All of the information presented as follows, is presented as an
example.
<PAGE>
 
Required information:

1.   Regression Equation: Price=Bo+B1*SG+B2*%wtS.

     B2 = - 0.7995          B1 =-3.8822      Bo = 20.1712
     Where:
     Bo = Independent Term
     B1 = Regression coefficient associated with the specific gravity.
     SGR = Specific gravity of the crude.
     B2 = Regression coefficient associated with the percentage in weight of
          sulphur.
     %wtS = Percentage in  crude sulphur weight.

2.   South Blend crude withdrawn in the Tumaco terminal.

Volume:  400 Net Bls.
Quantity:  28.90 deg. API (0.882 SGR), 0.790 %S
Base Price of crude-Blend delivered in Tumaco:  16.0000 US$/Bl.

3.  Santana crude delivered in Santana terminal.

Volume:  150,000 Bls.
Quality:   26.50 Deg. API (0.896 SGR).

The content of sulphur is obtained using the average of the sulphur content in
the crude from the Mary, Miraflor, Linda and Toroyaco fields, weighed by the
volume delivered per field, as follows:
<TABLE>
<CAPTION>
 
       CRUDE               % Weight of  DELIVERED
     (FIELDS)                    S        FIELDS
                                           BLS.
<S>                      <C>          <C>
     MARY                      0.579      81.661
     MIRAFLOR                  0.644      13.454
     LINDA                     0.481       3.766
     TOROYACO                  0.527      60.164
 
     TOTAL DELIVERIES                    159.045

     SULPHUR CONTENT                      0.550

     WEIGHED

Quality of sulphur:  0.550 %S
</TABLE>
<PAGE>
 
Note:  The sulphur content from the different fields is that supplied by
Ecopetrol's Services Management of the Refining and Marketing Vicepresidency,
which for this end will be in charge of contracting an independent inspection
firm, whose expenses will be cancelled in accordance with Clause Eight of this
contract.

Procedure for Calculation:

1.  Replacing the quality of each of the crudes in the regression equation .

CrudeBlend Price=20.1712+(-3.8822)*0.882+(0.7995)*0.790=16.1155 USD/BL.
Santana crude Price=20.1712+(-3.8822)*0.896+(-0.7995)*0.550=16.2530 USD/BL

2.  Amount in US$ of each crude:

Crude Blend=Volume*Price=400.000 Bls*16.1155 USD/BL.=USD 6'446.200.00
Santana crude=Volume*Price=150.000 Bls*16.2530 USD/BL=USD 2'437.950

3.  Fraction of Santana crude volume in crude Blend.
 
    2'437.950.00/6'446.200.00=0.3782

4.  Volume of the Santana compensated crude

    400.000 Bls.*0.3782=151.280 Bls.

5.  Compensation factor for Santana crude.

    151.280.00 Bls./150.000.00 Bls.=1.0085

6.  Amount of compensation = 16.00 USD/BL.*(1.0085-1)=0.1360 USD/BL.


INFORMATION REGARDING THE REGRESSION EQUATION

For the calculation of the regression equation, the following chart has been
drawn up of 14 international crudes.  The prices of these crudes correspond to
an arithmetical average of their quotations in the three months previous to the
quality compensation evaluation.  Said information, as much the price as the
quality, will be obtained from the monthly report published by Platt's and
titled as follows:  " PLATT's OILGRAM PRICE REPORT".

The chart of the 14 international crudes is as follows:
<TABLE>
<CAPTION>
 
                                     Price (USD/Bl.)
CRUDE            API    SGR    %wtS  Month X Year X
<S>             <C>    <C>     <C>   <C>
FATAH           30.70  0.8724  1.90       21.50
ARAB LIGHT      33.40  0.8581  1.77       22.53
ARAB MEDIUM     28.50  0.8844  2.85       21.55
</TABLE>
<PAGE>
 
<TABLE>
<CAPTION>
 
                                     Price (USD/Bl.)
CRUDE            API    SGR    %wtS  Month X Year X
<S>             <C>    <C>     <C>   <C>                                        
ARAB HEAVY      27.40  0.8905  2.80       20.79
BRENT BLEND     38.00  0.8348  0.30       24.04
BONNY LIGHT     35.70  0.8463  0.14       24.01
CANO LIMON      29.50  0.8789  0.45       24.45
FORCADOS        31.00  0.8708  0.20       24.06
FLOTTA BLEND    36.00  0.8448  1.20       23.49
ISTHMUS         33.00  0.8602  1.30       23.16
KUWAIT          31.40  0.8686  2.52       21.26
MANDJI          30.50  0.8735  1.10       22.08
MAYA            22.00  0.9218  3.40       19.83
ORIENT          29.50  0.8789  0.90       22.06
</TABLE> 

The preceding international supply of crudes may be modified by mutual agreement
between the parties.

NOTE 1:

When the quality of the crude supplies varies during a month, an average will be
taken of the three quality prices corresponding to those same months in which
the quotations were taken.

NOTE 2:

When no quotation appears for one of the crude supplies in one month, an average
will be taken of the two previous months instead of the three quotations
established for the price calculation of that month and will be agreed upon by
the parties in writing if the supply is definitely reduced or it will be
replaced by whatever other crude in the price calculation for the following
month.




This is a fair and accurate English translation of the original document which
is in the Colombian language.

                                     /s/ James L. Busby
                                     ------------------
                                     James L. Busby
                                     Secretary and Treasurer
                                     of Aviva Petroleum Inc.