New hunting grounds studied on Peru's continental slope

PERU'S COASTAL BASINS-4

Fernando Zúñiga-Rivero, J.A. Keeling, Hugh Hay-Roe
BPZ & Associates Inc.
Houston

Among the 10 sedimentary basins along the coast of Peru,¹ at least four have been recognized on the upper and middle continental slope (Fig. 1 [145,459 bytes], Fig. 2 [142,398bytes]Fig. 3 [143,109 bytes]). All are strongly linear and essentially parallel to the coastline and to the axis of the Peru-Chile Trench. They cover a total area of 15 million acres or more-without including the Talara and Mollendo-Moquegua basins,² which extend from onshore across the shelf and slope.

The total slope area off Peru that is underlain by sedimentary strata, taken from the 650-ft isobath to the 10,000-ft isobath (without reference to individual Neogene basins) is over 32 million acres.

All the available information including modern seismic data suggests that the pre-Neogene reservoir strata off the coast of Peru were deposited in a fluviatile to shallow marine environment, between periods of deep marine deposition-all of course unrelated to present-day water depths. It was probably not until Senonian time that tectonic plates along the west coast of South America started to converge, beginning the evolution of the present-day configuration offshore.³

The basinal areas that now underlie both the shelf and continental slope began as linear downwarps that filled intermittently with clastic debris from the adjacent continent; they also received deposits of shallow and deep marine limestones and shales. Deposition probably continued well to the west, as the few seismic profiles carried all the way across the Peru-Chile Trench show sedimentary strata extending to the trench axis.

It appears, then, that the prospectiveness of the four known basins confined to the continental slope should be comparable to that of the other basins of Peru's continental margin. At present only one of the four-the Trujillo basin-has been licensed for exploration.

Trujillo basin

This basin is the best known of the slope basins,⁴ with two exploratory wells (drilled in 1971)⁵ and extensive geophysical coverage, including 7,524 line-miles of seismic data (plus recent surveys conducted by Repsol in 1996-98). We consider that it is also the most misunderstood of these basins, because of misinterpretation of data from the two wells. It is a long, narrow basin on the upper continental slope, extending about 230 miles southeastward off the city of Chiclayo (Fig. 1). Width averages about 30 miles. Water depth between the eastern and western limits of the basin ranges from 550 ft to about 8,000 ft.

The north end of this basin is adjacent to the Lobos de Afuera Islands, which are the surface expression of the subsurface high we have named the Outer Ridge. The islands are the site of submarine oil seeps known since 1540, when they were reported by Fr. José de Acosta in his History of the Conquest of the Indies. Hydrocarbon shows were recorded from a core-sample (ODP Site 684),⁵ and prominent amplitude anomalies have been observed on seismic records.

In November 1994 the Spanish petroleum company Repsol obtained an exploration license on Block Z-29, a newly defined offshore lease block covering practically the entire basin. Together with partners Brasoil, Ranger, and YPF of Argentina, Repsol has conducted additional geophysical surveys during 1998 (the next and final article in this series will carry details on current license blocks offshore).

Structurally this basin appears to be similar to the Talara basin, with prominent high-angle normal faults forming horsts and grabens (Fig. 4 [259,553 bytes]). The seismic records show a number of such large, promising structures. Gentle folds, rollovers associated with high-angle faults, updip onlaps, and pinchouts are also visible on seismic profiles, as well as turbidite channel deposits and prominent amplitude anomalies.

The stratigraphic sequence also appears to be similar to that of the Talara basin, with a thick sequence of Mesozoic (Triassic, Jurassic and Cretaceous) sediments and Tertiary rocks deposited over an Upper Paleozoic section of indurated marine sediments (Fig. 5 [233,568 bytes]). The best potential source rocks are Mesozoic sediments, most probably those equivalent to the mid-Cretaceous Muerto limestone. The deepest part of the basin may have sufficiently mature Tertiary sediments for additional hydrocarbon sources. The maximum sedimentary thickness, inferred from seismic profiles, is estimated at 45,000 ft.

The best reservoir prospects are probably Tertiary sandstones, the equivalents to those of the Talara basin. There are also excellent potential reservoirs in the Cretaceous Farrat sandstones, which crop out onshore. Inferred turbidite sequences offer another possibility. The drilled sequence confirmed the presence of cap rocks; the section exposed onshore likewise shows good indications of sealing strata.

The only two wells in the Trujillo basin were drilled in 1971 by Occidental Petroleum. One was located at the eastern edge of the basin, on the subsurface high we have called the Inner Ridge.¹ The other was located 4 miles west, down the flank of that ridge. The crestal well, Ballena X-1, penetrated 2,365 ft of Miocene, followed by 354 ft of Middle to Lower Eocene; it was stopped at 3,168 ft in presumed pre-Tertiary (possibly Cretaceous) rocks. The flank well, Delfin X-1, found a thicker Paleocene section (totaling 4,492 ft) below the Miocene and was stopped above the basal Tertiary at a final depth of 8,725 ft. Both wells penetrated potential reservoirs but were abandoned without testing, because at the time it was considered that they had reached economic basement without any shows.

Repsol and partners reportedly plan to drill their first two exploratory tests in 1999.

Lima basin

The largest of the known slope basins, the Lima basin⁶ extends southward along the upper continental slope from west of the port of Chimbote (230 miles north of Lima), almost to the port of Pisco in the south-a distance of some 350 miles (Fig. 2). Separated from the Trujillo basin to the north and the Paracas basin to the south by poorly defined transverse highs, it is bounded on the landward side by the Inner Ridge and on the seaward side by the Outer Ridge (another basin analogous to the Pimentel basin may lie down- slope from the Lima basin, with sediments extending all the way to the Trench).

Between the continental shelf edge and the 10,000 ft isobath (approximately) this basin encompasses an area of nearly 8 million acres. Part of the basin is in relatively shallow water, as shown by the most recent seismic data (Fig. 6 [284,944 bytes]). Currently available seismic coverage is sparse-about 750 line-miles backed by gravity and magnetic data. No wells have been drilled, and no exploration licenses are in force in this basin.

Broad folds (some bounded by normal faults) involving Cretaceous and Paleogene strata show up on seismic records. Up-to-basin faults may provide additional trapping. In the southern part of the basin seismic profiles show strong folding, similar to that of the Paracas basin.

The sedimentary section probably exceeds 25,000 ft. It is inferred to consist of a Paleozoic and Mesozoic sequence, similar to that exposed along the coast of the Lima area, covered by a Tertiary section resembling that of the Pisco basin (Fig. 5). Potential reservoir rocks are most probable within the Cretaceous section, particularly the Morro Solar sandstones, but there are also prospective reservoirs in Paleogene and Neogene clastic sediments.

The most likely source rocks are the dark Cretaceous shales and micritic limestones belonging to the Pucusana-Atocongo-Chillón group, which is exposed in outcrops along the coast in the Lima area. An important indication of hydrocarbons is the presence of black sediments, with total organic carbon in the range of 2% to 12%, within which methane hydrate zones were found in cores taken from the deep seaward edge of the basin.⁷

Paracas basin

In the past some researchers have called this the "West Pisco basin" to distinguish it from the continental shelf ("East Pisco") basin, which we call simply the Pisco basin.⁸ The Paracas basin is named for the Paracas Peninsula, which forms the northern end of the coastal range of hills that is the onshore expression of the Inner Ridge.

Seismic profiles (Fig. 7 [306,284 bytes]) suggest that the tectonic style of this basin is complex. During one phase of development the style was compressional, evidently due to effects related to the presence of the Nazca Ridge.⁹ Promising features on seismic sections are structurally complex, with large, prominent folds (some overturned), thrust faults with possible decollement surfaces, and apparent diapiric structures-features that are seldom if ever noted in other basins. The Paracas basin is evidently separated from the Mollendo basin to the south by the Outer Ridge.

The sedimentary section, as interpreted from seismic reflection character and outcrops onshore in the Pisco basin area, consists of Paleozoic sediments, overlain by Jurassic-Cretaceous and Paleogene sequences-the latter highly contorted-followed by flat-lying Upper Tertiary (most likely Plio-Pleistocene) sediments. Cores from the Peru-Chile Trench and the Nazca Ridge recovered Miocene and Pliocene planktonic foraminifera. The most likely source rocks are the Jurassic and Cretaceous limestones and shales, which crop out in the nearby onshore part of the Pisco basin, and show high organic content. The aggregate thickness of sediments is probably in excess of 40,000 ft.

Potential reservoir rocks are most probably the sandstones in the Jurassic-Cretaceous sequence, as well as those of Tertiary age, which are very likely similar to those of the Pisco and Lima basins. The Jurassic-Cretaceous sequence and the Tertiary contain adequate sealing shales and fine-grained limestones.

No wells have been drilled in this basin, and none of the acreage has been licensed. Geophysical coverage includes about 1,140 line-miles of seismic, together with gravity and magnetic surveys. There is ample geological information about the adjacent Nazca Ridge.

Pimentel basin

The only basin so far identified entirely seaward of the Outer Ridge is the partially surveyed Pimentel basin, which lies downslope from the Trujillo basin and south of the deepwater part of the Talara basin (Fig. 1). Originally it was named Yaquina basin10 after the oceanographic research vessel being used when the basin was discovered; but a nearby geographic feature, the seaport of Pimentel, provides a more appropriate name.

The main part of this little-known basin underlies the middle continental slope; it may extend all the way to the Trench. Because even its landward side is the farthest offshore of any of the basins, it has sparse geophysical coverage. No blocks have been awarded yet, and no wells have been drilled. Geophysical data consist of around 550 line-miles of recent seismic, together with gravimetry and magnetometry.

The surface area is probably at least 3.5 million acres, but it can only be estimated because the western and southern limits of the basin have not been defined. The basin lies in water depths ranging from 600 ft to more than 10,000 ft.

The tectonic style of this basin is quite complex, showing a combination of high-angle normal faulting with low-angle gravity slide faults and large folds. The trapping mechanisms are likewise probably the same as those in the Talara basin, i.e., primarily block faulting, but from the latest seismic data it has been possible to map some large folds and updip closures against faults, as well as turbidite channel deposits.

Seismic reflection character indicates a sedimentary section similar to that of the Talara basin (Fig. 8 [369,242 bytes]). It is evidently thick, perhaps on the order of 60,000 ft (including Paleozoic strata). The inferred source rocks are probably the equivalents of the mid-Cretaceous Muerto limestone and the Upper Cretaceous and Tertiary shales known from the Talara basin. Potential reservoir rocks are also possibly of the same age and type as those of the Talara basin, i.e., primarily Paleocene and Eocene sandstones, conglomerates, and turbidite sequences.

The next and final article in this series will summarize the findings of the previous articles and offer estimates of the oil or gas potential of each basin.

Acknowledgments

The authors appreciate the cooperation of Ribiana Inc., owners of the latest available geophysical surveys, and the assistance of Perupetro's management and technical staff in making technical data available for our use in this study. Tom s Vargas and Joe Bettis prepared the illustrations.

References

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.
Zúñiga-Rivero, F., Keeling, J.A., and Hay-Roe, H., Peru onshore-deepwater basins should have large potential, OGJ, Oct. 19, 1998, p. 88.
Williams, K.E., Tectonic subsidence analysis and Paleozoic paleogeography of Gondwana, in Tankard, A.J., et al., Petroleum basins of South America, AAPG Memoir 62, 1995, p. 79
Bolaños, R., Geologic evaluation and petroleum possibilities of the Tertiary Salaverry-Trujillo basin, Petroperu internal report (in Spanish), 1986, 51 pp.
Schrader, H., and Cruzado-Castañeda, J.C., The Ballena and Delfin wells off central Peru: Revised ages, in Suess, E., von Huene, R., et al., Proc. Ocean Drilling Program, Sci. Results, Vol. 112, Texas A&M University, 1990, p. 209.
Azalgara, Carlos, Structural evolution of the offshore forearc basins of Peru, including the Salaverry, Trujillo, Lima, West Pisco, and East Pisco basins, Rice University Master's thesis, 1993, 178 p.
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.
Zúñiga-Rivero, F., Keeling, J.A., and Hay-Roe, H., Oil and gas potential in shallow water: Peru's continental shelf basins, OGJ, Nov. 16, 1998, p. 92.
Cande, S.C., Nazca-South American plate interactions since 50 mybp to present, in Hussong, D.M., et al., eds., Atlas of the ocean margin program-Peru continental margin, Region VI, Woods Hole (Marine Science International), 1985, p. 14.
Ballesteros, M.W., Moore, G.F., Taylor, B., and Ruppert, S., Seismic stratigraphic framework of the Lime and Yaquina forearc basins, Peru, in Suess, E., et al., Proc. ODP Initial Reports, Leg 112, 1988, pp. 77-90.

The series

Part 1-Zúñiga-Rivero, F., Keeling, J.A., and Hay-Roe, H., Attractive potential seen in 10 sub-basins off Peru, OGJ, Sept. 7, 1998, p. 117. Part 2-Zúñiga,-Rivero F., Keeling, J.A., and Hay-Roe, H., Peru onshore-deepwater basins should have large potential, OGJ, Oct. 19, 1998, p. 88. Part 3-Zúñiga-Rivero, F., Keeling, J.A., and Hay-Roe, H., Oil and gas potential in shallow water: Peru's continental shelf basins, OGJ, Nov. 16, 1998, p. 92.