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Figure Captions
 Figure 1. Location of the
study area, Rodador field in the state of Tabasco, southeastern Mexico.
 Figure
2. Rodador field layout, Tabasco,
southeastern Mexico.
 Figure
3. Interpretation of seismic line
L-64/39, with wells Rodador 173, 175, 177, 179, and 278. Location of line in
Figure 2.
 Figure
4. Cross section A-B
through wells Rodador
62, 83, and 197, showing the Encanto Formation top and base. Line of
section shown in Figure 2.
 Figure
5. Cross section X-9
through wells Rodador
173, 175, 177, 179, and 278. Line of section shown in Figure 2.
 Figure
6. Cross section Z-7
through wells Ají 1,
Rodador 62, 81, 193, 175, and 157. Line of section shown in Figure 2.
 Figure
7. (a) Sand 17-A top structural
configuration, Rodador field, Encanto Formation. (b) Rodador-85  well ,
showing detail of sand 17-A.
 Figure
8. (a) Sand 18 top structural
configuration, Rodador field, Encanto Formation. (b) Rodador-278 well
showing detail of sand 18.
Click here for sequence of
Figure 7a and Figure 8a.
 Figure
9. Well in Rodador field, showing sand
12 (A-12).
 Figure
10. Rodador field wells, showing
intervals with exploitation possibilities.
 Figure
11. Schematic sedimentary model of
Rodador field, southeastern Mexico. (a) Schematic sedimentary model for
the Encanto Formation. (b) Schematic block diagram of the Encanto
Formation.
 Figure
12. Cross section Z-5
through wells Rodador 63, 85, 197, 179, and 258, illustrating seal rocks and traps.
Line of section shown in Figure 2.
 Table 1.
Time-stratigraphic
and biostratigraphic subdivision of the Ají-1 well. Modified after
Rodríguez (1999), using data from Berggren et al. (1995).
The Rodador field is in
the state of Tabasco, southeastern Mexico. This field is in the onshore
portion of the Isthmian Saline Basin, part of the province known as the
Southeastern Tertiary Basins (Figure 1). The purpose of this study is to
estimate the original hydrocarbon volume in place, which will be used to
rank the potential of the sands for new exploitation plans.
The sequence-stratigraphic
concepts and terminology used in this study are based on Van Wagoner et
al. (1988, 1990) and Mitchum et al. (1993). Seismic line L-64/39 is
crucial to the study, because it crosses through wells Rodador 173, 175,
177, 179, and 278 (Figures 2 and 3). The study includes 25 well logs
(Figure 2), as well as the stratigraphic data from the Ají-1 well,
located to the southwest of the field (1.4 km S29°W of the Rodador-1
well). The Ají-1 well was chosen because cutting samples from 1900 m to
3835 m contain fossils that were studied by Rodríguez (1999), who
identified the species outlined in Table 1.
Based on well logs, the
top of the Encanto Formation is established at the contact of a shalier
section with a sandier one. According to correlation of well logs and
supported by paleontological data furnished by the Rodador-195D well
drilled in 1997, the top of the Encanto Formation was established with
the following microfossils: Uvigerina hispida, Reophax encantoensis,
and Melonis pompilioides.
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By 1953, several
unpublished gravity, refraction, and reflection seismology surveys had
resulted in identification of several geologic blocks considered to be
attractive for oil exploration. Jiménez (1976) interpreted the northern
portion of the Rodador field to evaluate available commercial
hydrocarbon reservoirs. His study defined the structure as a faulted
anticline with a southeast-northwest-trending axis. The long axis of the
structure measures 3 km, and the width measures about 2 km. It has a
small closure dipping to the northwest.
This
study encompasses the southern area of the field and concurs with
Jiménez (1976) in that the cumulative sand horizons in the southeastern
part of the field appear to be better developed. The maps of sand tops
provided in this study indicate that these sand bodies are potential
reservoirs. Guerrero (1993) estimated the original hydrocarbon volume
and reserves for the Rodador field. He proposed the existence of eight
reservoirs divided into 18 blocks in the Encanto Formation. These
reservoirs are at depths that range from 2400 m to 3450 m.
The Rodador field was
discovered by wildcat Rodador-1 in 1971. This well was drilled to a
depth of 3647 m and became a dual producer of oil and gas from intervals
3220-3228 m and 3458-3470 m. These intervals correspond to middle
Miocene-lower Pliocene sand bodies of the Encanto Formation. The Rodador
field was developed mainly to the south and southeast of the Rodador-1
well. Forty-six wells had been drilled by 1980, of which 36 were
producers and 10 were dry (Figure 2). The spacing between wells was 400
m.
Only seismic line L-64/39, which crosses
the field in a southwest-northeast direction (Figure
2), was analyzed.
This line ties to wells Rodador 173, 175, 177, 179, and 278 (Figure
3).
The velocity survey used was that of the Magallanes-850 well, about 10
km southeast of Rodador field. This velocity survey was used to convert
the seismic reflections from time to depth and to try to calibrate the
seismic information with the wells, because the Rodador field does not
have its own velocity survey.
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Jurassic anhydrite and salt are common
in the Southeastern Tertiary Basins of southern Mexico. In Rodador
field, 10 wells (Rodador 1, 2, 6, 20, 31, 32, 43, 52, 54, and 173)
reached the anhydrite-salt top. These wells penetrated from 1 m below
the top (Rodador-54) to 35 m (Rodador-6) and from 3453 m minimum depth
(Rodador-173) to 3791 m maximum depth (Rodador-6). The Ají-1 well in the
southwestern area reached the anhydrite top at 3853 m, penetrating 17 m.
On seismic line L-64/39, at the position
of wells 175 and 173 at approximately 2.1 to 2.3 s (equivalent to 2690
to 3010 m in depth), the following can be observed (Figure
3):
1) The pattern of imbricated turbiditic
flows deposited on the slope forms stratigraphic traps in the Encanto
Formation.
2) The line drawn across the lower part
of wells Rodador 173, 175, 177, 179, and 278 corresponds to the base of
the Encanto Formation at 2.48 s (3360 m) for Rodador-173 and at 2.55 s
(3430 m) for Rodador-278. These lower sands are better developed than in
the northern producing portion of the field. Below the base of the
Encanto Formation, the section becomes more shaly.
3) Southwest of seismic line L-64/39 (Figure
3), from approximately 1.8 to 2.4 s, the seismic reflections are
seen to rise upward, produced by the Cinco Presidentes salt dome to the
southeast (Figure 1).
4) The main producing area with
exploitation possibilities is located in the 2.0- to 2.5-s interval,
from 2500 m to 3350 m deep. These producing intervals belong to the
middle and lower part of the Encanto Formation, as illustrated on
seismic line L-64/39 (Figure 3). Note that the 850-m thickness of the
producing zone is a gross thickness, representing interbedded layers of
sand and shale.
The Encanto Formation consists of
interbedded sand and shale. Both lithologies have variable thickness
through the formation. The thickness of the sand bodies ranges from 4 m
to 65 m, and the thickness of shale horizons ranges from 3 m to 67 m.
The sand is light to dark gray in color, fine- to coarse-grained, and
the shale is soft to hard, green and dark gray.
The Concepción Inferior Formation
overlies the Encanto Formation and is characterized by interbedded
thick-shale and thin-sand bodies. This formation in the study area has
an average thickness of 260 m. Based on a correlation of electric logs,
the top of the Encanto Formation is characterized by the presence of
interbedded, well-defined sand and shale bodies located at an average
depth of 2240 m (Figure 4).
The Encanto Formation in the study area
has a minimum thickness of 1205 m (Rodador-67 well) and a maximum
thickness of 1376 m (Rodador-177 well). The Ají-1 well, southwest of the
field and 1.4 km southwest of the Rodador-1 well (Figure
2), penetrated
1468 m of Encanto Formation before encountering the top of the
anhydrite. On the basis of well logs, the base of the Encanto Formation
is established at the contact of a sandier section with a shalier one.
Most of the oil produced from the
Encanto Formation in southeastern Mexico is from shingled turbidites
(PEP-BP Exploration Report, 1994). The producing and potential turbidite
sands are found mainly in the middle and lower portions of the Encanto
Formation.
In the southern area of Rodador field,
two prograding complexes, referred to as A and B, have been recognized
as part of the Encanto Formation (Figures 5 and
6). Prograding complex A
in the lower and middle portions of the Encanto Formation includes the
31 to 13 sands at depths ranging from 2700 m to 3450 m and whose
thickness varies from 5 m to 69 m. Sands 22 to 17 (except 19) (Figure 5
and 6) show a good resistivity curve response because of the presence of
hydrocarbons, and thus they have exploitation possibilities. Sand 17-A
has been tested in wells 81, 82, and 175, and it also has good
possibilities in nine more wells (83, 85, 87, 179, 193, 195D, 197, 199,
and 278) (Figure 7).
The 18 sand in wells 195D and 278 is the
main producer in the field. The sand has a good response on the log
resistivity curve (well 278), and has an average thickness of 33 m and
an average porosity of 22%. It is considered a potential future producer
in wells 83, 85, 87, 179, 197, and 199 (Figure
8).
Prograding complex B comprises the sand
bodies at depths of 2200 m to 2650 m. This is the shallowest complex,
and it encompasses sands 12 to 8, whose thickness varies from 8 m to 65
m. Sand 12 in the Rodador-258 well has yielded good oil production and
has exploitation possibilities in well 179 (Figure
9), as indicated by a
high resistivity curve response. The 12 sand in well 179 reaches a gross
thickness of 65 m. It is important to mention that in the Rodador field,
there are intervals with exploitation possibilities, and they are shown
with well logs in Figure 10. Figure 11 depicts a conceptual sedimentary
model of the Rodador field, showing the approximate location of the
field in the prograding complex.
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Hydrocarbon reservoirs are typically
fine- to medium-grained gray sands, some of which are shaly.
Coarse-grained sand rarely occurs in the formation. In the study area,
sand reservoirs vary in thickness from 3 m to 33 m and have an average
porosity of 20%.
Seal rocks in the Rodador field are
shales of variable thickness interbedded with the reservoir sands of the
Encanto Formation. In some cases, lateral seals are faults or changes in
sedimentary facies (Figure 12; wells Rodador 63, 85, 197, 179, and 258).
An example of a lateral fault seal is
clearly seen between wells 179 and 258, where the impregnation of sand
A-11 in well 179 no longer appears in well 258. An example of facies-change,
lateral seal is shown where a sand body immediately below sand A-26 in
well 197 becomes shalier toward well 179. Finally, a pinch-out lateral
seal is illustrated with sands A-25 and A-26, where the pinching out of
the sands acts as a seal.
Traps are mainly stratigraphic as a
result of pinch-outs of sand bodies, as illustrated in Figure
12, where
sands A-25 and A-26 pinch out to form traps. Structural and combination
traps also occur between wells 179 and 278 for sand A-11.
1) Optimal hydrocarbon production in the
Rodador field is from a prograding complex in the middle Miocene-lower
Pliocene Encanto Formation.
2) The study area is not strongly
affected by structural deformation; traps are mainly stratigraphic.
3) A conceptual sedimentary model for
the Rodador field is proposed here on the basis of a multidisciplinary
study.
4) The detailed analysis of sand-top
maps was used to estimate the original hydrocarbon volume in place, and
this helps to rank the areas or sand bodies for exploitation purposes.
The calculated hydrocarbon reserves will remain confidential.
I thank Dr. Abelardo Cantú-Chapa (Instituto
Politécnico Nacional) for his suggestions for improving the manuscript.
I also thank Petróleos Mexicanos, Activo de Producción Cinco Presidentes,
Area Diseño de Explotación, Agua Dulce, for permitting me to publish
this paper.
Berggren, W. A., V. K. Dennis, C. Swisher, and M. P.
Aubry, 1995, A revised Cenozoic geochronology and chronostratigraphy,
in Geochronology time scales and global stratigraphic correlation:
Society for Sedimentary Geology (SEPM) Special Publication 54, p.
129-212.
Guerrero, C. G., 1993, Cálculo del Volumen Original y
Reservas de Hidrocarburos del Campo Rodador: PEMEX confidential report,
p. 2.
Jiménez, T. A., 1976, Interpretación Geológica Campo
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Mitchum, R. M., J. B. Sangree, and P. R. Vail, 1993,
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and biostratigraphy: Examples from the late Cenozoic of the Gulf of
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J. C., H. W. Posamentier, R. M. Mitchum, P. R. Vail, J. F. Sarg, J. S.
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