Peru onshore-deepwater basins should have large potential

PERU'S COASTAL BASINS-2

Perupetro's recent announcement that 13 offshore exploration blocks of nearly 1 million acres each will be offered for bids in the fourth quarter of 1998 (OGJ, Aug. 3, 1998, p. 73) has reawakened interest in this extensive, largely unexplored area (Fig. 1 [110,471 bytes]).

The new government policy, combined with the results of modern, deep-probing seismic surveys, has already led to a stepped-up search for oil and gas that will probably escalate. This year Occidental and Perez Companc acquired offshore blocks (Fig. 2 [114,103 bytes], Table 1 [55,354 bytes]), joining PetroTech and Repsol as offshore operators.

Perupetro's predecessor, Petroperu, announced in 1973 that a call for bids on offshore blocks would be forthcoming. The notice circulated to industry worldwide was accompanied by a map showing 27 blocks. However, shortly thereafter the Peruvian government decided against accepting any bids, triggering the widespread belief that all of the open offshore areas would be reserved for Petroperu, wholly owned by the government.

This policy, leaving the offshore in limbo, changed only after Petroperu was partially privatized in 1993 and its role as negotiator and supervisor of contracts was assumed by Perupetro, a new government entity.

Most of Peru's ten coastal basins are entirely offshore, but at both ends of the 1,500-mile coastline the sedimentary basins stretch from onshore across the continental shelf and down the continental slope. Two of these basin areas, both in the north, have commercial production (Fig. 1). The third, straddling the country's southern border, has never been drilled either on land or offshore. The Peruvian sectors of these three basins total roughly 50,000 sq miles in area, 75% offshore.

All have major oil and gas potential. They are described individually in this article, an update in the ongoing studies last reported at the 1998 Offshore Technology Conference¹ and in the first article of this series.²

Tumbes-Progreso basin

This irregularly shaped basin of about 10,000 sq miles (barely 12% of it within Peru) is named for the Peruvian town of Tumbes and the Ecuadorian town of Progreso (Fig. 2).

It is defined by a thick sequence of Neogene sediments³ deposited at least partly on top of the older (pre-Oligocene) Talara basin (Fig. 3 [150,529 bytes]). From a range of hills about 20 km inland, the Tumbes-Progreso basin extends west across the Gulf of Guayaquil (a broad shelf) to the continental slope. It appears to be divided diagonally by the NE-striking Dolores megashear, a major zone of transcurrent faulting that includes the Guayaquil and Puji* geosutures.^{4 5}

On the northwesterly side of the shear zone, the Progreso sub-basin wraps around the productive area of the Santa Elena Peninsula, which is on a pre-Neogene high. This sub-basin of about 5,000 sq miles lies entirely within Ecuador and is not included in the present study.

Landward from the shear zone, the Tumbes sub-basin extends up the coast from northern Peru into Ecuador. It takes in the shallow nearshore waters, including Amistad gas field just north of the international border. Amistad reserves are in the Miocene Subibaja formation-up to 400 net ft of conglomeratic sandstone at around 10,000 ft. The productive area of the field is about 6,400 acres, and the estimated gas reserves exceed 1 tcf.

Natural gas in commercial quantities has also been discovered in Peru, but-like Amistad-the five discoveries have yet to be developed because there has been no market for the gas. The onshore Peruvian sector of the sub-basin includes the old Zorritos oil field, discovered in 1863 and produced continuously until 1965. The reservoir here is the Miocene Zorritos sandstone, which yielded final cumulative production of 4 million b/d of light crude. In both sub-basins Mio-Pliocene clastics are underlain by Paleogene and Cretaceous strata that in places rest on Paleozoic basement.

In part of the southern Tumbes sub-basin, Oligocene sediments lie directly on late Paleozoic strata.⁶

The sedimentary sequence in this sub-basin is entirely clastic, characterized by shallow marine and fluvial sandstones, deepwater marine shales, and heavy conglomerates that were probably deposited by turbidity currents (Fig. 4 [61,320 bytes]).

Besides the Zorritos sandstone, the Upper Miocene Tumbes sandstone is also prospective. The Neogene sequence reaches a maximum aggregate thickness in excess of 20,000 ft. Beneath it lies the Talara basin sequence, here represented by a maximum aggregate 28,000 ft of Paleogene sediments on top of some 20,000 ft of Mesozoic strata. These impressive thicknesses explain why sedimentary velocities can be seen down to seismic (two-way) reflection times of as much as 10 sec in some places.

Offshore both oil and gas have been confirmed by a series of exploratory wells (Table 2 [98,339 bytes]).

The entire Peruvian sector of the Neogene basin is now under license, along with major parts of the Paleogene [Talara] basin (Table 1). Seismic coverage in the Peruvian sector totals 3,700 line miles, excluding the most recent shooting by Oxy in Block Z-3.

Talara basin

The best known of all Peru's coastal basins, the oil-rich Talara basin (Fig. 5 [84,492 bytes]), reflects Paleogene sedimentation over some 9,000 sq miles on top of the regional Paleozoic-Mesozoic basin.2 Productive for over 130 years, this basin has yielded some 1.6 billion bbl of light crude (average 34°) and an estimated 3.5 tcf of associated gas from about 12,000 wells, mainly in the central and northern parts of the onshore sector.

It appears that in many if not all of the coastal basins, tectonic movements during the Paleozoic established the structural and depositional framework for subsequent periods. Some major faults originally became active during the Paleozoic; since then they have been reactivated several times, occasionally with movement opposite to the original direction of offset. Normal faulting is an important aspect of the structural style of this basin, as are low-angle gravitational slide faults and large, essentially vertical transcurrent faults.

Being older than the adjacent Tumbes-Progreso and Sechura-Salaverry basins, the Talara basin in effect underlies parts of those Neogene basins. The stratigraphic sequence is primarily Eocene (up to 28,000 ft aggregate thickness), overlying in excess of 5,000 ft of Paleocene (maximum aggregate thickness), which in turn lies on up to 6,700 ft of Cretaceous (Fig. 4).

The uppermost Paleozoic units, known both from outcrop and drilling, are also sedimentary; in fact, commercial oil production occurs from the Pennsylvanian Amotape quartzite in two onshore fields. The Paleocene-Eocene sedimentary sequence is entirely clastic, characterized by shallow marine, deltaic, and fluvial sandstones, marine shales, and turbidites. The underlying Cretaceous is about 75% clastic rocks, but it does include some thick and widespread oolitic, reefy, and micritic limestones that are considered to be the most important hydrocarbon source rocks. The Eocene sequence includes littoral and beach sands, fluviodeltaic sands, and (in places) coarse conglomerates, as well as turbidite channel sands.

The sandy units are separated and sealed by marine shales, both shallow- and deepwater deposits. Altogether the Talara oilfields have about a dozen different reservoir units, most of them Eocene, with up to a half dozen producing formations in various localities. Some units are themselves composed of multiple sands separated by shales.

In the Talara basin much or all of the hydrocarbon migration took place following a mild compressive phase, for distribution of oil and gas suggests original entrapment in anticlinal or domal closures. Subsequent normal faulting modified the structure and also redistributed the hydrocarbons to some extent.

Offshore in the south-central sector, both oil and gas have been tested by a series of exploratory wells (Table 3 [86,852 bytes]). As of this writing it appears that the southern third of the basin is more likely to yield gas or gas/condensate. Two long east-west seismic lines confirm that sedimentary strata, while thinning westward, do extend all the way to the Peru-Chile trench (Fig. 6 [149,986 bytes]). We consider the best future prospects in this basin are still within the thick Paleocene-Eocene sequence.

The Peru Bank is a large (400,000-acre) uplift in the northernmost part of the Talara basin. In places on the Bank, water is less than 200 m deep. It was formerly thought to be a basement uplift bordering the Tumbes-Progreso basin, capped only by Pleistocene sediments, but modern data confirm that it contains a thick sequence interpreted as Paleogene-Cretaceous-Upper Paleozoic (Fig. 3).

This extensive positive area with moderate water depth can be considered one of the most prospective offshore features in the basin. In July 1998 Oxy did additional seismic shooting over the Peru Bank, but so far it remains untested by the drill.

Mollendo-Moquegua basin

The coastal-offshore area of southern Peru is occupied by an extensive composite basin. Its components are the offshore (Mollendo) and onshore (Moquegua) sub-basins, which are partially separated by a range of coastal hills in which rocks of Precambrian to Cretaceous age are exposed (Fig. 7 [84,341 bytes]). In the past some researchers have called the offshore part the Arequipa basin.

The smaller onshore part occupies some 10,000 sq miles, while the offshore portion extends from the strandline to the outer continental slope; it also continues farther south into Chilean waters. The total area of the Peruvian offshore sector is about 25,000 sq miles, making it the largest of all the offshore basins. In this area the continental shelf varies in width from 6 to 20 miles; the continental slope shows a pronounced flattening between 1,500 ft and 6,500 ft water depth, attaining widths of 25 to 45 miles.

Lying west of the old Arequipa massif, the area was probably part of the regional Paleozoic-Mesozoic basin. Onlap onto older rocks, pinchouts of porous facies, possible reefs, and turbidite channels may have provided early traps; but structural closures of large vertical and areal extent evidently date from the Tertiary. Seismic coverage to date amounts to 500 line miles, backed by gravity and magnetic data.

Outcrops in the coastal range reveal a thick sequence of Mesozoic sediments lying on Paleozoic strata (Fig. 8 [66,220 bytes]) and in places on Paleozoic basement; the maximum aggregate thickness totals some 40,000 ft.

The most prospective source rocks are considered to be limestones within the Jurassic Socosani formation and Yura Group; the latter also contains sandstones that could be prospective reservoirs. Sandstones of the Oligocene Camana formation likewise offer reservoir potential.

Because no wells have been drilled in either part of this basin, the stratigraphic sequence is known only from outcrops, and indirectly from marine seismic profiles. On 1971 seismic records the inferred Mesozoic sequence thickens in a seaward direction, as does the inferred Lower Tertiary (Fig. 9 [133,392 bytes]-adapted from Perupetro's interpretation of seismic profile 71-97), creating potential stratigraphic traps in a shoreward direction. Conversely, the inferred Middle Tertiary thins seaward, with pronounced onlap onto a Lower Tertiary high.

At present, no license blocks have been awarded in this basin.

Marine exploration outlook

Inasmuch as all of the Tumbes sub-basin and a substantial part of the Talara basin are now under license, we can expect some interesting results in the near-term.

Of particular interest are three shallow-water plays:

1. Natural gas prospects in the Tumbes sub-basin;
2. Oil and gas possibilities just west of active producing areas in the Talara basin, in both Block Z-2B and Z-4; and
3. The untested Peru Bank in Block Z-3.

As a rank wildcat area, the Mollendo sub-basin in southern Peru is also of considerable interest. With its 16 million acres are large structures underlying both the continental shelf and slope, largely in less than 6,000 ft of water. Further geophysical surveying will be needed to delineate the most promising areas for wildcat tests.

The next article in this series will review the four basins that occupy the portion of the continental slope between the Talara basin on the north and the Mollendo sub-basin on the south.

Acknowledgments

We gratefully acknowledge the collaboration of Perupetro's management and professional staff in making available some of the technical data used in this study. Tomas Vargas provided essential data on the northern coastal basins and designed many of the figures. Joe Bettis handled the drafting.

References

1. Zuniga-Rivero, F., Keeling, J.A., and Hay-Roe, H., Uncovering Peru's offshore petroleum potential, presented at Offshore Technology Conference, Houston, May 6, 1998.
2. Zuniga-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.
3. Delfaud, J., Marocco, R., Megard, F., Cordova, E., and Leon, I., Peculiarities of the sedimentary filling of Talara-Tumbes basins on the North Peruvian active margin (in French), Sixth European Regional Meeting of Sedimentology, 1985.
4. Jaillard, E., Ordonez, M., Benitez, S., Berrones, G., Jimenez, N., Montenegro, G., and Zambrano, I., Basin development in an accretionary, oceanic-floored fore-arc setting: southern coastal Ecuador during Late Cretaceous-Late Eocene time, AAPG Memoir 62, 1995, p. 615.
5. Shepherd, G.L., and Moberly, R., Coastal structure of the continental margin, northwest Peru and southwest Ecuador, in Kulm, et al., GSA Memoir 154, 1981, p. 351.
6. Sanz, V.R., Geology of the Department of Tumbes (in Spanish), Seventh Peruvian Congress of Geology, Lima, 1991, p. 367.

The series

Part 1-Zuniga-Rivero, F., Keeling, J.A., and Hay-Roe, H., Attractive potential seen in 10 sub-basins off Peru, OGJ, Sept. 7, 1998, p. 117.

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