OTC 20936
Revisiting the Subsalt Trap Archetype Classification Scheme
After Eight More Years of Gulf of Mexico Subsalt Drilling
William H. Hart and Martin L. Albertin, BP
Copyright 2010, Offshore Technology Conference
This paper was prepared for presentation at the 2010 Offshore Technology Conference held in Houston, Texas, USA, 3–6 May 2010
.
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Subsalt exploration in the Gulf of Mexico (GOM) remains an area of intense focus by the oil and
gas industry. Successfully accessing economic hydrocarbon accumulations beneath the
extensive allochthonous salt sheets in the northern GOM basin requires solving a number of
geophysical, geological, and drilling challenges. In 2001, Hart and Albertin presented a geological
tool which, given imaging challenges associated with complex allochthonous salt, could be used
for qualitatively ranking subsalt exploration prospects according to their structural attributes.
The ranking scheme proposed by Hart and Albertin is comprised of a collection of subsalt
trap archetypes, each archetype representing an important structural variation with specific trap
merits and risks. These archetypes are grouped into four genetic play families, calibrated for
overall prospectivity by a statistical analysis of subsalt well success rate. The statistical analysis
in 2001 used a database of 69 subsalt wells; since then, at least 122 new subsalt traps have
been drilled by the industry. We used this new subsalt well data to revisit the classification
scheme and answer a few specific questions: does the trap family priority hold up given the
subsalt successes and failures of the past eight years, do the original archetypes still represent
the full range of observed salt styles, and are any revisions to the classification scheme
warranted?
At a high level, the subsalt trap family prioritization still holds. All but two of the 45 new
subsalt discoveries can be positively attributed to the top ranked autochthon-rooted trap family.
With only two possible exceptions, where root style could not be determined, none of the other
subsalt trap families yielded a single discovery in the past eight years. Further analysis of the 107
new autochthon-rooted subsalt tests suggests the following descriptive and statistical updates to
the autochthon-rooted trap family:
1) the overall success rate of autochthon-rooted traps, which currently stands at 42%, has
been favorably impacted by the emergence of the Paleogene (Wilcox) play trend,
2) the trap family has been broadened to include cases where traps form above deep
Cretaceous salt allochthons, and above Louann salt that has crept onto younger oceanic
basement,
3) northward and westward expansion of the deepwater subsalt play fairway necessitates
the addition of new end-member archetypes within the autochthon-rooted family,
4) the industry will be challenged to maintain the high success rate for the autochthon-
rooted trap family as subsalt exploration focus within the GOM shifts from the lower slope
to more complex subsalt traps of the middle to upper slope.