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Revitalizing the Poza Rica Field: New Insights into the Tamabra Reservoir and a Tale of a Paradigm Lost
By
Andrew Horbury1, Hector Hernandez2, Alfredo Marhx3, Angel Ojeda4, Gabriella Ita3, and Jaime Estrada3
Search and Discovery Article #20027 (2005)
Posted June 12, 2005
*Adapted from extended abstract, prepared by the authors for presentation at AAPG International Conference & Exhibition, Cancun, Mexico, October 24-27, 2004.
1Cambridge Carbonates Ltd., Solihull, U.K ([email protected])
2Consultant, Poza Rica, Veracruz
3Pep, Poza Rica, Ver.
4Imp, Poza Rica, Ver.
Introduction
Models for the Tamabra Limestone of the Poza Rica field (Figure 1) initially indicated a ‘reef’ system downfaulted from its original depositional position alongside the Tuxpan Platform. This was based on the facies encountered in the most highly productive core of the field (Salas, 1949; Barnetche and Illing; 1956, Gúzman, 1967, Coogan et al., 1972). However, work carried out during the late 1960’s and 1970’s which integrated new data from the flanks of the field, plus information from 2D seismic lines that crossed both the field and the Tuxpan Platform towards the northeast, resulted in a generally-accepted re-interpretation of the model as an unfaulted slope carbonate system (Viniegra and Castillo, 1970; Enos, 1977, 1985, 1988). In turn, this resulted in the Poza Rica field being recognized as one of the largest slope carbonate fields in the world (Enos, 1985), with consequent widespread implications for this play type and play risk. The concept of the ‘Tamabra’ play that has emerged is one of slope carbonates that pinch-out updip into tight, laterally-sealing basinal facies.
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AimWe aim
to show that detailed
SeismicThe new 3D survery reveals that the Poza Rica field and Tuxpan Platform are separated by a basin (Figure 2a), contrary to existing models which show a continuously dipping slope system between the Tuxpan Platform and the eastern limit of the Tamabra reservoir. Seismically-defined units are present in this basin which are not developed over the Poza Rica field itself. Additional stratigraphy is seen within the Tamaulipas Inferior and Tamaulipas Superior formations, parts of the Tamabra Formation, most of the Agua Nueva Formation, and parts of the San Felipe Formation (Figure 3). The Mendez Formation appears to be universally present but thins over the crest of the Poza Rica field. The Poza Rica field is, therefore, developed on a paleohigh that had topographical elevation throughout the Cretaceous; it appears that this high plunges to the southeast and that there is also some expression of another basin at the limit of 3D acquisition to the southwest side of the field (between the Poza Rica and Tajin fields). Within the Poza Rica field, the Tamabra Formation shows a variety of seismic facies. There are strong, parallel to mounded seismic facies over the main productive area (Figure 3), passing into relatively opaque and topographically elevated seismic facies (Figure 4), prior to descending into adjacent basins via stronger amplitude but often chaotic/lensoid/mounded seismic facies. The belt of opaque/elevated geometries have been mapped where they occur in the southeast part of the field; these can be seen to be trending SW-NE on the southwest margin of the 3D cube (Figure 2a). Seismic coherency maps of the top-Tamabra (Figure 2b) have also revealed 100-200-m diameter ring-shaped structures which are best developed on the slopes of the Poza Rica field. Such features are analogous to karstic doline features reported from the Italian field of Rospo Mare (e.g., Heritier et al., 1991).
Core and LithofaciesA
thorough revision of core and core descriptions in internal reports and
in published literature reveals that the central part of the Poza Rica
field is dominated by thick packages of rudist rudstone facies, whereas
breccia deposits are mostly encountered on the southeast and northeast
flanks of the field. There is no evidence that there is any significant
increase in interbedding of basinal facies towards the southwest edge of
the field; indeed, many wells with almost completely cored sections on
the extreme (present) southwest limit have very coarse-grained breccia
or rudist rudstone facies with less than 10% of fine-grained basinal
sediment. In addition, the wells drilled outside the field limit, in the
Coatzintla area to the southwest, are uncored. These wells show broadly
similar
InterpretationOur
interpretation of the above data suggest that early models for the field
were largely correct in that they interpreted the central, most
productive part of the field as an in situ platform, although these
models are still flawed in that they assume chronostratigraphic
equivalence of the Poza Rica ‘reefs’ with the Tuxpan Platform.
Subsequent models (Viniegra and Castillo, 1970; Enos, 1977, 1985)
dismiss the ‘reefal’ model probably because these workers focused on the
abundant new data that had just been acquired from the flanks of the
field and thus re-interpreted the whole system as a carbonate slope. Our
model would suggest that the most productive Tamabra facies both in the
Poza Rica field and the Pital y Mozutla field on the flanks of the
Tuxpan Platform largely represent second-order shallow-water lowstand
platforms passing downdip into slope deposits that were deposited during
the Cenomanian-?Turonian when the crest of the Tuxpan Platform (and also
most other Mexican ‘middle’ Cretaceous platforms [Smith, 1986; Basáñez
et al., 1993]) were subject to subaerial exposure and massive
karstification. The cause of this event has recently been suggested by
Horbury et al. (2004) as being due to early Laramide uplift of the Gulf
continental margin, which in turn was a consequence of Pacific margin
subduction which re-commenced in the Aptian (Pindell and Kennen, 2001).
We think that the carbonates within the field are dominated by three 3rd-order
highstand systems tracts, while the full
Implications and ApplicationsThe
correct interpretation of the Poza Rica field is important for ongoing
field re-activation because it opens up the possibility of extending the
field to the southwest. There are concerns about increased Sw in this
direction, but given the fact that the Poza Rica paleohigh (and
therefore closure) extends to the southwest, the important issue is
finding the ‘sweet spots’ in which rudist rudstone facies are present
with their high macroporosity and therefore high permeability and low Sw,
compared to high Sw platform interior microporous facies or
low-porosity/low-permeability ‘true’ slope breccias. The extent of the
rudist rudstone facies belt is at present not
Future WorkThe study was undertaken on a sector of the field in which there are significant lateral changes in both seismic and core facies and in which there were perceived to be opportunities for field extension towards the southwest. Further study is required of seismic, wells, and cores towards the northwest. In the longer term, drilling of step-out wells would be best constrained by extending the 3D coverage towards the southwest.
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