Oil, gas potential in shallow water: Peru's continental shelf basins

PERU'S COASTAL BASINS-3

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

This third article of a series highlights the three sedimentary basins that underlie the 16 million acres of continental shelf adjacent to a 650-mile stretch of Peruvian coastline. This area lies roughly between the ports of Chiclayo and Pisco (Fig. 1 [108,044 bytes] and Fig. 2 [121,677 bytes]).

These basins offer a variety of reservoirs, traps, and source-rock potential in water depths of less than 1,000 ft. They are characterized by a thick sequence of Neogene strata, underlain by Paleogene, Mesozoic, and Upper Paleozoic sediments down to as much as 7 sec two-way time on modern seismic records.¹ In some places the sedimentary section may reach an aggregate thickness in excess of 50,000 ft.

From north to south these contiguous shelf basins are the Sechura-Salaverry, Huacho, and Pisco basins. This basin trend is divided only at the Neogene level by transverse highs. The pre-Neogene strata reflect laterally continuous deposition for the entire distance from the north end of the Sechura sub-basin to the south end of the Pisco basin. Vertical continuity is broken by major unconformities (Fig. 3 [111,230 bytes]).

Sechura-Salaverry basin

This basin, the largest of the shelf basins, underlies the broadest part of Peru's continental shelf (the maximum width is 80 miles). It contains a significant volume of geophysical data, including some 12,700 line km of seismic records, plus gravity and magnetics. In addition, there are many geological reports and well reports from the onshore part of the basin.

Earlier studies^{2 3 4} treated the Salaverry shelf basin as separate from the onshore Sechura basin;⁵ but our study of the latest available seismic data revealed no tectonic or structural boundary between the two. Accordingly, we are treating them as essentially continuous parts (sub-basins) of one basin covering some 23,000 sq km (14.8 million acres), with over two-thirds of it lying offshore (Fig. 1). The aggregate sedimentary sequence (the total of individual formation thicknesses) may reach 58,000 ft.

On the seaward side this basin is separated from the adjacent Trujillo basin by a narrow, linear subsurface high that we have called the Inner Ridge.⁶ The ridge is evidently a late Paleogene fold that for most of its length runs along or close to the edge of the continental shelf, as defined by the 200 m isobath. The ridge has been penetrated by a single shallow well, drilled 27 years ago and abandoned without testing (this well will be discussed in the next article of this series).

Seismic data suggest that at the north end of the Salaverry sub-basin the Inner and Outer ridges converge into a single linear uplift that comes onshore and continues as part of the Illescas Hills, the Paita Saddle, and the Amotape Mountains.⁸ Likewise a part of this uplift are the Lobos de Afuera and Lobos de Tierra Islands, the site of submarine oil seeps that have been known since the time of the Spanish Conquest.

No wells have been drilled in the Salaverry sub-basin itself, but its northward extension onshore, underlying the so-called Sechura Desert, has around 50 wells that range from very shallow to medium depth, and include Olympic's recent development drilling for natural gas. The tectonic style is different from that of the adjacent Talara basin, showing large gentle folds and diapiric structures, as well as high-angle normal faulting that is less intense than the faulting that characterizes the eastern Talara basin (Fig. 4 [257,910 bytes]). Some structures mapped with marine seismic data have an areal extent of more than 30,000 acres, and vertical closure can exceed 2,500 ft.

The Sechura-Salaverry basin contains late Paleozoic and Mesozoic sediments with a fairly thick cover of Tertiary strata, consisting of about 2,300 ft of Neogene rocks above a Paleogene sequence (Fig. 3) that includes most of the stratigraphic units recognized in the Talara basin.

Onshore well control indicates that the Paleozoic and Mesozoic sequences are also similar to the units in the Talara basin, except that rocks of Aptian age are the oldest Mesozoic in the Talara basin, whereas the Sechura sub-basin contains both Triassic and Jurassic strata.

The Jurassic sequence contains reef carbonates that may include intervals with source potential. However, the principal source rocks are probably the thick Cretaceous carbonates, which in onshore outcrops appear to have excellent source potential. Nevertheless, the natural gas encountered to date is in Upper Eocene sandstones. Micritic limestone and numerous shales throughout the sedimentary sequence (mainly in Cretaceous and Tertiary strata) will provide effective seals.

Huacho basin

This basin of slightly under 21/2 million acres, located entirely in shallow water, was not previously recognized as a separate Mio-Pliocene basin, in part because seismic control was sparse. The seismic reflection character on profiles of the 1993-94 spec survey, plus outcrop geology in the adjacent onshore area, would indicate that the basin fill of up to 28,000 ft is primarily Mesozoic overlain by Paleogene strata, and underlain by upper Paleozoic sedimentary rocks. The Mesozoic-Paleogene sequences are overlain by a comparatively thin Neogene section.

The Huacho basin is separated from the adjacent Salaverry and Pisco basins (Fig. 2) by minor transverse subsurface ridges on which the Neogene thickness reaches a minimum. Shoreward the entire Tertiary sequence wedges out against the Cretaceous (Fig. 5 [248,087 bytes]); in places the wedge-edge of Tertiary sediments is concealed beneath Quaternary deposits. In a seaward direction the Inner Ridge separates the Huacho basin from the Lima basin, which underlies the upper continental slope.

Normal (block) faulting is not as prevalent in the Huacho basin as in some other offshore Peruvian basins, but broad anticlinal features are evidently present. The main traps will likely be the large folds within the Cretaceous sequence. Source rocks are probably Cretaceous shales correlative with onshore outcrops of the Pucusana-Pamplona-Atocongo-Chill?n sequence (Fig. 3). The best potential reservoir rocks are likely those of the Morro Solar sandstone series, along with the sandstones within the Pucusana-Pamplona-Atocongo-Chill?n series. It is also possible that Tertiary sandstones unknown in the adjacent onshore area will be good reservoirs. The best sealing rocks are expected to be the shales and limestones of the Pucusana to Chill?n series, as well as some of the Tertiary shales.

To date no wells have been drilled in this basin. Geophysical control is sparse, consisting of only about 470 line-miles of 1993-94 seismic profiling, accompanied by gravity and magnetic data. Earlier geophysical information is supplemented by geological reports on the onshore geology. Attractions for the explorationist include shallow water, proximity to seaports (especially Callao, the principal port of Peru), moderate seas with essentially storm-free weather conditions, and broad seismic structures with a substantial sedimentary section.

Pisco basin

This elongated basin, whose offshore sector covers around 3.7 million acres, resembles the Sechura-Salaverry basin in having a larger offshore sector and a smaller onshore one. In the past it has sometimes been called the "East Pisco basin" to distinguish it from the upper continental slope basin ("West Pisco") that we are calling the Paracas basin, after the Paracas Peninsula (Fig. 2).

The coastal region underlain by the Pisco basin extends from the shallow waters just south of Huacho to the onshore area east of Puerto Caballas, behind the range of coastal hills.⁹ The westerly limit of the basin is the Inner Ridge. The basin comes onshore close to the town of Pisco, behind the Paracas Peninsula. This northern tip of the coast range is characterized by outcrops of the Upper Paleozoic Ambo and Copacabana sequences, overlain by the Triassic Zaña Group and younger Mesozoic, including intrusive rocks. Altogether this basin is about 300 miles long, with nearly two-thirds of it offshore. The broader offshore part averages about 35 miles in width, while the onshore sector has an average width of around 20 miles.

Seismic profiles (Fig. 6 [243,388 bytes]) suggest that the tectonic style of this basin is complex. During periods of extension, high-angle normal faults formed horsts and grabens; but compression also occurred, producing gentle folds and some thrust-faulting. The Cretaceous section is tensionally faulted while the overlying Tertiary has been draped over older beds with accompanying folding of sediments. Ten large structures have been mapped on a deep reflector, possibly of Late Cretaceous age.

The Cretaceous and Tertiary sandstones are probably the best reservoirs present in this basin, although Cretaceous limestones could also form good reservoirs. Based on measured outcrop sections onshore and marine seismic data, the Pisco basin has an estimated total sedimentary thickness of about 30,000 ft. The stratigraphic sequence is largely marine clastics of Paleozoic, Jurassic, Cretaceous, and Tertiary age. The predominantly clastic Eocene sequence is similar to that of the Talara basin, and the Lower Eocene (the most prolific interval in the Talara basin) may be present in thicker sequences offshore. The Oligo-Miocene section resembles that of the Sechura sub-basin and may also contain potential reservoir strata.

No wells have been drilled offshore, and a single onshore well, which was drilled in the early 1950s to a depth of 3,127 ft, was not tested.¹⁰ Geochemical studies to date have been limited to reconnaissance. Mississippian shales have shown 0.98% to 2.75% total organic carbon and 64 to 565 ppm hydrocarbon content. The Cretaceous contains black shales and bituminous limestones. The Cretaceous shales and micritic limestones, and some Eocene and Oligo-Miocene shales, can be expected to provide effective seals. As no offshore wells and no submarine seeps have been reported, there is no evidence regarding the type of hydrocarbons that may be present.

Shallow-water possibilities

The three continental shelf basins highlighted in this article offer several attractions in addition to shallow water and moderate target depths:

Available modern seismic data show a variety of potential trapping structures, including large subsurface closures (some of more than 20,000 acres).
Basin fill with an aggregate maximum thickness of 28,000 to 58,000 ft offers a variety of potential source and reservoir units including turbidite-channel systems, with temperatures suitable for hydrocarbon maturation.
Hydrocarbon indications range from actual production (Sechura sub-basin) to submarine seeps, methane hydrates, and prominent amplitude anomalies on seismic profiles, in Cretaceous through Neogene strata.

To date no licenses have been awarded in any of the three shelf basins covered in this article, but Perupetro plans to offer offshore blocks in the Salaverry and Pisco basins, as well as a block each in the onshore parts of the Pisco (OGJ, Aug. 3, 1998, p. 73) and Sechura-Salaverry basins.

The next article in this series will review the potential of the four basins so far known on the continental slope.

Acknowledgments

The authors thank Perupetro's management and technical staff for making available some of the basic data used in this study. Tom s Vargas and Joe Bettis prepared all the illustrations.

References

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Dunbar, R.B., and Baker, P.A. eds., Cenozoic geology of the Pisco basin, IGCP #156 Guidebook to field workshop, May 1988, 253 pp.
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