Bernard C. Duval, Carlos Cramez
Total Exploration Production
Paris
Gustavo E. Valdes
Petroleos de Venezuela SA
Caracas
This article results from 12 years of close scientific and technical collaboration between Total and Pdvsa.
When the Venezuelan petroleum industry was nationalized in 1976, the remaining hydrocarbon potential of South America was thought to be very limited, mainly because most visible traps had already been drilled. The discovery of four giant oil fields in the northern part of South America in the last 10 years has proved how wrong that judgment was (Fig. 1 (25034 bytes)).
Occidental and Corpoven discovered Cano Limon-Guafita field.
Maraven discovered Ceuta-Tomoporo field, which is now aggregated with the old Centro field.
Lagoven discovered El Furrial field, and Corpoven extended it westward with the discovery of Carito and Tejero fields.
A consortium of BP, Total, and Triton discovered Cusiana field.
The reserves of these fields are either confidential or not fully determined. However, existing public information indicates that they are all well over the 1 billion bbl of oil equivalent mark, and that El Furrial-Carito is the largest, followed in descending order of size by Cusiana, Cano Limon-Guafita, and Ceuta-Tomoporo.
The aim of this article is to illustrate our view of how it can help to look at things in a very regional way:
- To understand the petroleum systems,
- To establish similarities and differences between the systems, and
- To explain why the giant fields are there.
EASTERN VENEZUELA
Cross-section A-A' through El Furrial-Carito field in eastern Venezuela shows that the post-Pangean first order stratigraphic cycle is made up by the stacking of several types of sedimentary basins that control the petroleum system (Figs. 2 (32294 bytes)and 3 (29870 bytes)).
From bottom to top we identify:
- A rift type basin during Triassic and Jurassic times,
- A craton and an "Atlantic type" divergent continental margin from the Lower Cretaceous to the Neogene,
- A foredeep basin northward of the Oficina area, active since the Neogene,
- A folded belt northward of the Morichito area, since the Upper Miocene/Pliocene.
It is important to note that normal faults created during the stretching of the continental crust were only active during rift basin time.
However, the Pirital and Turimiquire thrusts are considered younger than the Furrial thrust. They are interpreted as a Plio-Pleistocene reactivation of old Jurassic faults in response to the release of the geostatic pressure related to the erosion of the Cordillera.
El Furrial-Carito field is located below the Pirital thrust fault in the buried foothills of the interior ranges of eastern Venezuela.
The structural configuration of the main reservoir of El Furrial is shown in the simplified depth map (Fig. 4 (33018 bytes)). The different faulted blocks are individualized by several thrusts and their related lateral ramps, which implies a relatively shallow plane of decollement limiting shortened sediments above from undeformed or slightly deformed sediments below. Note that the longitudinal extension of this structural complex is greater than 50 km.
The dip seismic line shows both the low angle Miocene faults flattening onto a decollement surface as well as the high angle Pirital fault. The lateral ramps of the thrust faults are clearly identified on the strike lines (Figs. 5 (33018 bytes) and 6 (33424 bytes)).
The geological interpretation shows the following features:
- The high angle deep Urica fault, which corresponds to an old normal fault complex affecting the Paleozoic and the basement.
- The decollement surface, which separates the shortened sediments above from the undeformed sediments below.
- The upthrown faulted blocks of the thrusts: they constitute the structural traps of the El Furrial trend.
- The lateral ramps.
- The "triangular"' zones between the successive thrust faults. Incidentally, their existence explains the absence of reservoirs in certain locations.
With regard to the migration-entrapment subsystem, it can be said that in eastern
Venezuela the shortening of the sediments is mainly "in sequence," and most of the structural high points have always been highs. There is no tectonic inversion, so that the possibility of several hydrocarbon expulsion from the potential source rocks is in no way an handicap and, on the contrary, increases the possibility of large accumulations.
If we look at the development of petroleum parameters chronologically, mentally putting aside the shortening of sediments, we see the following (Table 1 (15080 bytes)):
- A prolific source rock ("La Luna" equivalent), as well as the sand reservoirs and the surrounding undercompacted Carapita shales related to the divergent margin mentioned before.
- A phase of hydrocarbon expulsion from the deepest part of the basin possibly took place before the shortening. Due to the low impedance of the system at that time, no trapping occurred in the Furrial area, but migration, up the ample regional dip to the north towards the Orinoco heavy oil belt, did take place.
- Shortening by low angle faulting mechanism without the involvement of basement took place during the Neogene, creating the traps described before.
- No inversion is observed, and lateral migration was prevalent as the foredeep depositional overburden initiated renewed hydrocarbon expulsion in the Lower Neogene.
In summary therefore we have:
- a divergent margin and overpressured seal,
- "in sequence" low angle shortening and lateral migration, and
- no inversion, so the early highs have remained high.
WESTERN VENEZUELA
Regional section B-B' (Figs. 2 (32294 bytes)and 7 (30246 bytes)) through Ceuta-Tomoporo field, shows that in the Maracaibo area (the lake and adjacent areas) two major sedimentary basins are actually superimposed:
- A Mesozoic back-arc basin overlying pre-Pangean Paleozoic sediments, and
- A Neogene foredeep basin.
These basins are easily recognized on a regional seismic line (Fig. 8 (35948 bytes)). The back-arc basin sequence extends from a Paleozoic substructure to the major Eocene unconformity (SB 39.5 million years), whereas the foredeep basin corresponds to the upper inverted strata.
The seismic line also indicates that the sedimentary shortening was a multiphase geological event caused mainly by the reactivation of the older Jurassic normal faults inducing a spectacular inversion. As an example, the Eocene structural high axis between the Urdaneta and Pueblo Viejo faults grew progressively during the regressive phase of the foredeep basin, a structurally low axis.
Ceuta-Tomoporo field is representative of the type of structural trap encountered in the back-arc basins of South America.
A Paleocene depth map derived from 3D seismic shot south of Lagunillas in east-central Lake Maracaibo illustrates the reactivation of the north-south Jurassic normal faults as well as the coeval synchronous transfer faults (Fig. 9 (47821 bytes)).
The north-south pre-existing faults were reactivated during the compressional tectonic regimes because they are perpendicular to the direction of maximum effective compressional stress. This reverse reactivation is evident in all east-west oriented seismic lines that cross the structural "pop-up" of Ceuta field.
In Tomoporo field the combination of an extensional structural trap, east-west oriented transfer faults, and the presence of thick sands led to the entrapment of a large hydrocarbon column. Relatively heavy oil (around 200 gravity) accumulated in the upper stratigraphic levels is probably post-compressional and related to the "tilting" of the basin.
To summarize some of the petroleum parameters of importance (Table 2 (16500 bytes)):
- In a back-arc context such as this, fault reactivation and high impedance conditions are present early on, before compression took place on the Maracaibo high.
- Multiphase shortening occurred later, causing strong structural inversions, so that we are dealing here with extensional and compressional traps, with a prevalence of high angle faulting.
- Expulsion and lateral as well as vertical migrations were possible before and after compression.
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