Potential untested under 50 million acres in Peru

March 15, 1999
Along the extensive continental margin of Peru 1 ( Fig. 1 [127,766 bytes] ) ( Fig. 2 [103,263 bytes] ) are at least 10 sedimentary basins-some partly onshore, 2 some entirely on the continental shelf, 3 others on the continental slope 4 -and only two have really been tested for oil and gas. They are the Talara basin, which has already accumulated close to 300 million bbl from its offshore sector, and the Tumbes sub-basin, with several shut-in gas fields discovered offshore.
Fernando Zúñiga-Rivero, Hugh Hay-Roe, Tom s Vargas
BPZ & Associates Inc.
Houston
Along the extensive continental margin of Peru1 (Fig. 1 [127,766 bytes]) (Fig. 2 [103,263 bytes]) are at least 10 sedimentary basins-some partly onshore,2 some entirely on the continental shelf,3 others on the continental slope4-and only two have really been tested for oil and gas. They are the Talara basin, which has already accumulated close to 300 million bbl from its offshore sector, and the Tumbes sub-basin, with several shut-in gas fields discovered offshore.

Only one other basin has seen the drilling of offshore tests (two wells, one very shallow, 27 years ago). Two further basins have had limited testing onshore. One of these has commercial gas onshore, and the other was tested over 40 years ago with a single 3,100-ft well. The remaining five basins have never been tested (Table 1 [218,722 bytes]). Still other basins may be present on the lower continental slope, in areas not yet covered by modern multichannel 2D seismic surveys. The total untested area is around 50 million acres.

Working with modern geophysical data supplemented in part by subsurface and outcrop geologic information, we have been able to estimate that the entire sedimentary area underlying the continental shelf and slope could contain up to 13 billion bbl of recoverable oil or, if natural gas rather than oil were found, possibly as much as 77 tcf of recoverable gas (Table 2 [77,397 bytes]).

Peru's 1,500-mile coastline is similar in length to the distance along the U.S. Gulf Coast from Brownsville, Texas, to the tip of Florida.

Possibilities overlooked

Probably the key finding of our ongoing studies is that the previous subsurface geologic picture of Peru's offshore was misleading in important ways.

The Talara basin, which has been an active oil area for over a century, has sometimes been considered an anomaly-even after successful drilling out to 350 ft water depth confirmed that the offshore part of the basin was essentially a westward extension of the Eocene/ Cretaceous basin known from thousands of onshore wells. Likewise both the Tumbes and Progreso sub-basins, initially known from onshore drilling, have been tested in the shallow offshore, and commercial hydrocarbons (both oil and natural gas) have been confirmed.

The extent, both lateral and vertical, of sedimentary strata beneath the continental shelf was greatly underestimated in the past. Much of the early seismic shooting over the continental slope was single-fold and of low penetration. Similarly, early multichannel shooting provided no deep data, leading geophysicists to conclude that the sedimentary section was relatively thin.

Now, estimates based on seismic data from the mid-1990s indicate a sedimentary fill anywhere from 2.7 to 4.4 times as thick as earlier estimates. At the same time, the longest lines perpendicular to the coast confirm that sedimentary strata extend westward all the way to the axis of the Peru-Chile Trench.

In contrast, interpretations of the earliest marine seismic records suggested that most or all of the basins held only a rather thin Tertiary section on top of a basement composed of igneous rocks or metamorphosed sediments. That picture is clearly erroneous. Interval velocities computed from all the modern seismic records confirm that sedimentary velocities are present in most places down to at least 5 sec (about 23,000 ft) and in many places to as much as 7 sec (about 33,000 ft). In fact, interval velocities on many of the records never reach more than 5,500 ft/sec.

Now, after examining modern seismic records from nine of the continental margin basins, we can confirm that those basins all have substantial Mesozoic and Tertiary stratigraphic sequences, in most cases overlying still more sediments of Paleozoic age. Thickness and lithology of course vary materially from basin to basin, along with certain structural characteristics.

Another important change involves the presence of potential trapping structures. It is obvious on modern seismic profiles off the coast of Peru that the least disturbed strata are the shallowest, of inferred Neogene age in most of the basins. In many places they are flat and featureless.

Such unprospective-looking beds naturally appeared more prominent on the old shallow-penetration records, where deeper and more interesting features were seen indistinctly or not at all. With modern records it is possible to interpret not only anticlines and potential fault traps in the Neogene and pre-Neogene rocks, but also diapirs (Fig. 3 [127,115 bytes]), turbidite channel fills, and various pinchout geometries.

One further interesting characteristic, noted in the Talara basin, is likely present in other basins as well: The normal faulting that defines the present-day reservoirs appears to be less intense toward the west. This suggests that reservoir blocks should be bigger offshore, with fewer wells required for efficient drainage.

The hydrocarbon potential of this region, whose total area exceeds 75,000 sq miles (landward from the Peru-Chile Trench), is suggested not only by production from the northern basins but also by the presence of submarine seeps,5 and by both bottom-simulating reflectors (BSRs) and methane hydrates in bottom samples.6

The forearc basins of the Peruvian continental margin today present varied configurations of shelf, slope, terraces, and ridges, probably reflecting the original geological setting. The entire system can be classified as a Contracted Continental-Margin Arc-Trench System (classification of Dickinson and Seely); in Klemme's system, Intermediate Crustal Group of Type 6-7 or Intermontane Transverse-Strike; or, in Bally's classification: fore-arc basins associated with B-subduction zone (Type C1a).

In this class of basins, nearly 35 giant fields have been found, as in Indonesia, Los Angeles, Ventura, Cook Inlet, Gippsland, Talara, and others. In such tectonic settings, major strike-slip faulting has accelerated the formation of hydrocarbons in younger basins by serving as an avenue for the upward transfer of heat.

During the early Cretaceous, Peru's paleogeographic setting comprised the following major elements, from east to west (Fig. 4 [370,391 bytes]):

  • A foreland basin, the Eastern Peruvian Megabasin, adjacent to the Braziliãn Shield;
  • The Western Péruvian Megabasiñ (including the Cajamarca, Bagua, Oyón, and other basins) between the Marañon Uplift and the coastal massif-arc; and finally,
  • The Peruvian Forearc Megabasin, extending from the coastal massif-arc all the way to the original trench. Some seaward uplifts are presumably represented today by the subsurface ridges that mark Neogene basin boundaries;4 others may have suffered tectonic erosion during subduction since the mid-Mesozoic.
Tectonism created an accommodation space that was filled by sequences favorable for the development of reservoirs, source rocks, and seals like the Morro Solar-Pamplona-Atocongo sequence that now crops out along the coast near Lima; it was deposited to the west of the massif-arc.

The lithostratigraphic units of the fill consist of (1) a sequence of sandstones with few interbedded siltstones that attained a thickness around 6,500 ft, overlain by (2) a carbonate sequence (Pamplona and Atocongo formations). In Fig. 5 [240,934 bytes], Exxon's global eustatic cycles are included to indicate the regional trend of the global transgression during the Cretaceous.

Uplifted areas in the present offshore also supplied sediments to these basins, as revealed by the sedimentary structures with an ENE direction of sediment transport.7 A similar tectonic-sedimentary scenario occurred during the Late Paleocene in southern coastal Ecuador, where the Santa Elena basin was filled with coarse turbidites derived from two opposites sources: the Western Cordillera, lying to the east, and the Colonche Cordillera, to the southwest.8

The main petroleum production from fore-arc basins in Ecuador has come from the Azucar Group reservoirs (mainly the Atlanta sandstone), which are quartz rich, high-density turbidites. More than 120 million bbl has been produced from fields in the Santa Elena Peninsula.

The play-types to be sought in Lower Cretaceous strata will be the transgressive system tract (TST), highstand tract (HST), and lowstand tract (LST), as well as the carbonate platform complexes (Pamplona-Atocongo sequence). Additionally, there is a high probability of finding reservoirs in naturally fractured volcanic rocks. Structural traps were formed during the local(?) Albian coastal tectonic phase and during the Peruvian Phase9 (Fig. 5) that formed folds and block faulting.

Several petrographic analyses of Morro Solar sandstones show regional differences with the "Circum-Pacific Volcanic-Plutonic Graywacke Suite" of Dickinson.10 The triangular compositional diagram (Fig. 4) indicates that these clastics can be derived from granitic rocks, associated coarse-grained metamorphic rocks, or quartzose sedimentary rocks.7 This mineralogical stability leads to the formation of porous sandstone reservoirs.

Varied types of prospects

Given the length of the coast of Peru, it is not surprising to find a variety of tectonic configurations and stratigraphic settings from basin to basin (Table 1). The two northern basins, Tumbes and Talara, can be assessed using both subsurface and marine geophysical data.1 2 But the remaining basins are known largely from geophysical data, and in part from the geology of the nearest onshore outcrops.3 4

The estimates of hydrocarbon potential given in Table 2 are based on the assumptions shown in Table 3 [102,282 bytes], as applied to individual basins. The arbitrary input values vary greatly, in accordance with the quality and quantity of available data. Gas and oil estimates are mutually exclusive, i.e., the simplifying assumption was made that discoveries would be either all gas or all liquid hydrocarbons. In areas of sparse control, all parameters were severely trimmed ("handicapped"), which accounts for the variation in the resulting estimates.

Peru's continental margin is also attractive from a logistical standpoint. The entire coast is virtually storm-free year-round. The desert climate along the entire coast, due to the cold Humboldt Current that flows north from Antarctica, accounts for the exceptionally high proportion of sunny days. Only a few areas experience winter overcast and occasional fog.

Logistics of marine operations are also simplified by the availability of port facilities. North of Lima, sizable seaports are found every 50-100 miles; south of Lima, every 100-200 miles, with a number of smaller ports.

As Peru is currently a net importer of petroleum, the local market is attractive. Export markets would include Chile, U.S. West Coast, and the NW Pacific rim. Gas is also needed for thermal power generation. Greater Lima, with a population exceeding 8 million, is the key market for electricity, but not the only one. Power generated anywhere along the coast can enter the national grid for distribution.

Opportunities now

Possibilities for exploring Peru's offshore have recently improved a great deal.

While negotiating with Repsol on Block Z-4 (and also with Repsol on the enlargement of Block Z-29), Perupetro is getting ready to announce two bid rounds for 1999. The 13 blocks that are expected to be included in the first bid round cover parts of five basins, from the Talara basin in the north to the Mollendo sub-basin in the extreme south (Figs. 1, 2).

In addition, Perupetro may be prepared to enter direct negotiation on other blocks. An important option for companies wishing to negotiate is the possibility of beginning with an evaluation phase prior to the regular exploration phase (see Table 4).

Acknowledgments

The cooperation of Perupetro, in providing information for this article and the four preceding ones, is greatly appreciated. M.P. Zuñiga-Pflucker prepared the estimates of hydrocarbon potential shown in Table 2. J. Bettis drafted the maps.

References

  1. Zúñiga-Rivero, F., Keeling, J.A., and Hay-Roe, H., Attractive potential seen in 10 sub-basins off Peru, OGJ, Sept. 7, 1998, pp. 117-122.
  2. Zúñiga-Rivero, F., Keeling, J.A., and Hay-Roe, Peru onshore-deepwater basins should have large potential, OGJ, Oct. 19, 1998, pp. 88-95.
  3. Zúñiga-Rivero, F., Keeling, J.A., and Hay-Roe, H., Oil, gas potential in shallow water: Peru's continental shelf basins, OGJ, Nov. 16, 1998, pp. 92-96.
  4. Zúñiga-Rivero, F., Keeling, J.A., and Hay-Roe, H., New hunting grounds studied on Peru's continental slope, OGJ, Dec. 7, 1998, pp. 87-92.
  5. Perupetro, Peruvian Petroleum: A renewed exploration opportunity (revised edition), Perupetro SA, Lima, Peru, 1995, pp. 29, 41.
  6. Miller, John J., Lee, M.W., and von Huene, R., An analysis of a seismic reflection from the base of a gas hydrate zone, offshore Peru, AAPG Bull., May 1991, p. 910.
  7. Wilson, J.J., Cretaceous stratigraphy of central Andes of Peru, AAPG Bull., Vol. 47, No. 1, January 1963.
  8. Jaillard, E., et al., Basin development in an accretionary, oceanic-floored fore-arc setting: Southern coastal Ecuador during Late Cretaceous-Late Eocene time, AAPG Memoir 62, Petroleum Basins of South America, 1995.
  9. Megard, F., Estudio Geológico de los Andes del Peru Central, Ingemmet-Orstom, Lima, Peru, November 1979.
  10. Dickinson, W.R., Composition of sandstones in circum-Pacific subduction complexes and fore-arc basins, AAPG Bull., Vol. 66, No. 2, February 1982.

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