
<PAGE>

                            Kelley Drye & Warren LLP
                                281 Tresser Blvd.
                           Stamford, Connecticut 06901



                                                    WRITER'S DIRECT LINE
                                                          (203) 351-8023

                                                         WRITER'S E-MAIL
                                              Jschifferli@kelleydrye.com



                                            October 30, 1998




3DX Technologies Inc.
12012 Wickchester, Suite 250
Houston, TX  77079

Dear Ladies and Gentlemen:

                  We have acted as Counsel to 3DX Technologies, Inc., a Delaware
corporation (the "Company"),  in connection with the proposed public offering of
shares (the "Shares") of the Company's common stock, $.01 per share (the "Common
Stock") as  described  in the  Registration  Statement  on Form S-3, as amended,
Registration No. 333-63119 (the  "Registration  Statement") filed by the Company
with the Securities and Exchange  Commission (the "Commission')  pursuant to the
Securities  Act  of  1933,  as  amended  (the  "Act"),  to  which  this  opinion
constitutes an exhibit.

                  We have examined the Company's  Certificate  of  Incorporation
and  By-laws,  each as  amended  to date;  minutes  of the  Company's  corporate
proceedings  through the date hereof, as made available to us by officers of the
Company;  the Stock Purchase  Agreement dated as of November 3, 1993 between the
Company and certain  investors;  the Series C Preferred Stock Purchase Agreement
dated as of July 26, 1995  between the Company and certain  investors;  a Common
Stock Subscription  Agreement dated June 3, 1998 between the Company and certain
investors;  the Common  Stock  Purchase  Agreement  dated as of August 21,  1998
between  the  Company  and  Santa Fe Energy  Resources,  Inc.,  or as  permitted
transferees  of persons who so acquired  such  Shares;  an executed  copy of the
Registration  Statement  and all  exhibits  thereto  in the form  filed with the
Commission,  and such  matters of law  deemed  necessary  by us to  deliver  the
opinions set forth herein. In the course of our examination, we have assumed the
genuineness  of all  signatures,  the  authority of all  signatories  to sign on
behalf of their principals,  if any, the authenticity of all documents submitted
to us as original  documents  and the  conformity  to original  documents of all
documents  submitted  to us as certified or  photostatic  copies.  As to certain
factual matters, we have relied upon information  furnished to us by officers of
the Company.

                  Based on the foregoing,  and solely in reliance thereon, it is
our  opinion  that the Shares have been duly  authorized  and fully paid and are
non-assessable.

                  We hereby  consent to the filing of this opinion as an exhibit
to the Registration Statement and to the reference to our firm in the Prospectus
included therein under the caption "Legal  Matters." In giving such consent,  we
do not admit that we are in the  category of persons  whose  consent is required
under Section 7 of the Act.

                                                 Very truly yours,

                                                 KELLEY DRYE & WARREN LLP

                                                 By:/s/ Jay R. Schifferli
                                                      A Member of the Firm


