Attractive potential seen in 10 sub-basins off Peru

PERU'S COASTAL BASINS-1

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

With 10 basins spread over some 67 million acres, Peru's continental shelf and slope stretch 1,500 miles from northwest to southeast-a distance equal to the U.S. Gulf Coast's perimeter from Brownsville, Tex., to the Florida Everglades ( Fig. 1 [120,241 bytes], Fig. 2 [112,108 bytes]).

Modern 2D seismic profiles show that the basins have a substantial sedimentary section (in places down to more than 7 sec, or in excess of 40,000 ft). The seismic records also indicate a variety of trapping features, from rollovers and updip closures against faults to turbidite channel deposits and onlap onto old highs.

Of the 10 basins, six lie entirely offshore. The other four straddle the coastline, and three of those extend well beyond the present edge of the continental shelf into deeper water.

Only three of the 10 basins have seen any offshore drilling (Table 1 [123,614 bytes]); one of those, the Talara basin, has well-established commercial production offshore.¹ Another, the Tumbes-Progreso basin, has recorded offshore gas discoveries that have yet to be developed, as well as commercial oil production onshore. The third (the Trujillo basin) has been penetrated by only two wells-one shallow, the other of intermediate depth-that were drilled 27 years ago; neither was tested.

Our studies indicate that all the basins have the potential to produce oil or gas or both. Modern seismic control suggests that, with a few local exceptions, the tectonic framework and general structural style are fairly consistent in the coastal region. The main stratigraphic groups (Paleozoic, Mesozoic, Paleogene, and Neogene) are recognizable from basin to basin. On that basis, the long-established productivity of the Talara basin and the proven potential of the Tumbes-Progreso basin would point to similar potential in the other coastal basins.

Exploration

Petroleum has been known in Peru's coastal region since prehistoric times.

Tar-pits inland from the port of Talara were used by the indigenous people for waterproofing their woven baskets and porous earthenware, for paving the roads of the Inca empire, and for mummifying the dead. Later the Spaniards used the pitch for caulking their ships and waterproofing cordage.

About 250 km to the south, submarine oil seeps were noted by Spanish navigators as a sign that they had reached Peruvian waters.

Onshore drilling for oil commenced in 1863, not long after Col. Drake drilled his first wells in Pennsylvania. Offshore drilling began in the 1950s as production on land in the Talara basin (Fig. 2) was extended out under the continental shelf.

Cumulative production from the Talara basin overall has exceeded 1.6 billion bbl, including wells in the shallow offshore (out to 370 ft water depth) that have accumulated around 280 million bbl of light crude and about 700 bcf of natural gas. No platforms have been set, nor wells drilled, in deeper water in the Talara basin.

Owing to the fairly thick cover of calcareous shallow-marine deposits of Pleistocene age, which characterizes the onshore part of the Talara basin, early seismic survey results were usually very poor. Geoscientists therefore relied mainly on gravity and magnetic maps in conjunction with well and outcrop data. Modern marine seismic surveys, however, have generated fair to excellent information (Table 2 [97,302 bytes], Fig. 3 [119,461 bytes]), with some promising surprises:

• Far from being thin and discontinuous, the total sedimentary column can reach as much as 10 sec in thickness on seismic records, while velocity data confirm that the deep rocks are indeed sedimentary. In places these strata reach all the way to the Peru-Chile trench.
• Thick sequences of Paleozoic and Mesozoic strata are present under the entire continental shelf and slope, while Tertiary sediments show attractive structures and considerable thickness in many places.
• Lower Tertiary strata, like the underlying Cretaceous and older rocks, are generally continuous over wide areas. It is only the youngest Tertiary rocks (Miocene and Pliocene) that reflect the separation into the different sedimentary basins along the coast and offshore.
• The 10 basins differ considerably in their structural features. Northern basins are characterized by extensive normal faulting and some wrench faulting, while southern basins, close to the Nazca Ridge, show prominent compressive features on seismic sections.

Geology and history

The plate divergence that formed the Atlantic Ocean did not become appreciable until Senonian time.

Only from the late Cretaceous onwards was there convergence of tectonic plates along the west coast of South America. Prior to that, the area was a part of the perimeter of western Gondwanaland.

During the early Paleozoic the area evidently formed part of a great massif, the Patagonian Shield. Toward the interior of Gondwana, the Amazon depression separated the Brazilian and Guayana shield areas, with drainage westward into the sea north of the Patagonian Shield; that is to say, in pre-Tertiary time the ancestral Amazon River flowed in the opposite direction.

Consequently, as there was no volcanic arc in the area of interest, the basinal areas were not forearc basins, although they have been so characterized in the past. Rather, they were linear downwarps that filled intermittently with clastic debris from the adjacent continent and also received deposits of shallow and deep marine limestones.

The overall shape of the depositional area during the Paleozoic and Mesozoic has yet to be delineated in detail, but it seems evident that what is now the Peruvian coastal region was part of a single extensive basin covering the whole region of interest and extending farther to the east.

Probably the basin also continued well to the west, as seismic profiles that were carried far enough seaward show sedimentary strata extending all the way to the axis of the Peru-Chile Trench. Late Cretaceous intrusives, as well as late Cretaceous-early Tertiary volcanic rocks, are known along the western Cordillera of the Andes; otherwise, the Mesozoic rocks are largely carbonates and clastic sediments.

Submarine ridges

The position, shape, and size of the offshore Tertiary basins are substantially controlled by two main subsurface ridges ^{2 3} which reach the sea floor in some places.

The most prominent submarine ridge is the aseismic Nazca Ridge (Fig. 1), a broad transverse feature whose crest lies at a depth of some 2,000 m, putting it more than 2,000 m above the abyssal plain (and 4,000 m above the axis of the trench). The northeastward movement of this ridge within the Nazca oceanic plate was presumably a factor in the compression that led to folding and thrusting of the pre-Neogene sediments in nearby basins.

The uplift of the Andean chain was the result of extensive tectonic activity in the area of the present coast and the adjacent belt to the east. Two narrow but very long anticlinal ridges formed in late Cretaceous-early Tertiary time, generally parallel to the Andes and the coast (Fig. 2). These two and various shorter, transverse uplifts strongly affected the area by dividing it into a series of more restricted areas of late Tertiary sedimentation, superimposed on the major Paleozoic-Mesozoic basin already described.

Today both submarine ridges lie mainly in the subsurface. However, the easterly (shoreward) ridge, which has been called the Inner Ridge, does reach the seafloor as a topographic feature in some places and even emerges locally as islands. For much of its length the Inner Ridge roughly follows the edge of the continental shelf (water depth around 200 m). The westerly anticline, called the Outer Ridge, roughly divides the upper continental slope from the middle slope, at a water depth of about 3,000 m.

On the basis of early seismic data both ridges were thought to be basement features, but it is now clear that they are late Cretaceous-early Tertiary folds involving both Paleozoic and Mesozoic strata, buried in many places beneath younger Tertiary.

Structure and trapping features

Potential hydrocarbon traps, present in all the basins, vary in type from basin to basin in accordance with the local tectonic regime. In the northernmost basins, moderate folding probably controlled accumulation of oil and gas from Cretaceous source rocks; but the mid- to late Tertiary extensional regime was associated with the characteristic high-angle normal faulting that redistributed the hydrocarbons, leaving them primarily in fault-block reservoirs.

Seismic and subsurface data suggest that the faulting is most intense in the eastern (onshore) parts of the northern basins, lessening somewhat in a seaward direction.

In the southernmost basins, close to the Nazca Ridge (Fig. 2), a compressional regime was important during the late Paleogene: the pre-Neogene strata are folded and thrust-faulted. The basins between the northern and southern extremes-the Sechura-Salaverry, Trujillo, Pimentel, and Huacho basins-are characterized by more gentle folding along with some normal faulting.

Source, reservoir rocks

Regional studies indicate that most of the potential source units are found in the late Triassic, Jurassic, and Cretaceous, including both micritic limestones and marine shales ( Table 3 [59,270 bytes]). Upper Paleozoic and Lower Tertiary units are also important in some areas. In addition to offshore oil and gas seeps, methane hydrates have been detected beneath the sea floor off the Peruvian coast. ⁴

Reservoir-quality rocks are common in the Cretaceous and Tertiary of the Peruvian coast, particularly in the Paleogene. In the Talara basin, Lower Eocene formations such as the Pariñas sandstone can have porosities exceeding 25% with permeabilities of more than 5 darcies. In the southern Tumbes-Progreso basin, the productive Zorritos sandstone (Miocene) has calculated porosities as high as 35%. A more southerly example is the onshore sector of the Pisco basin, where the Lower Eocene Paracas formation contains well-sorted porous sandstone.

Conclusions

The 10 basins mentioned in this article can be characterized as sub-basins separated by two linear ridges roughly parallel to the coast, and several transverse highs. The separation affected only the younger Tertiary strata; the older rocks were deposited in a single large basin that covered the entire region in late Paleozoic, Mesozoic, and early to mid-Tertiary time.

Excluding the basin areas that already have commercial oil production or demonstrated gas potential, the remaining coastal and offshore basins contain attractive exploration areas characterized by:

• Thick sedimentary sequences with favorable (normal) temperature gradients.
• A variety of trapping geometries-folds, faults, angular unconformities, turbidite channels.
• Many of the structures mapped with modern seismic exceed 100 sq km (24,000 acres) in area, with 200-1,000 m of vertical closure (or more, in some instances). Depending on the size of the basin, the number of mapped closures may range from 10 to more than 30.
• Tertiary clastic rocks including porous sandstones, in places overlain by sealing shales.
• Mesozoic and Upper Paleozoic carbonates and clastic rocks, including both potential source rocks and possible reservoirs.
• Submarine seeps (both gas and oil) and methane hydrates in some places, as well as seismic amplitude anomalies that may be indicative of hydrocarbons.

As exploration and development are ongoing, the present article and those to follow can be considered as progress reports. Future articles in this short series will describe in greater detail the three categories of coastal-offshore basins (classified by tectonic position), with individual assessments of their oil and gas potential.

Acknowledgments

We appreciate the cooperation of Perupetro's management and technical staff in providing technical data used in this study. Tomas Vargas and Joe Bettis prepared the key illustrations.

References

1. Travis, R.B., Gonzales, G., and Pardo, A., Hydrocarbon potential of coastal basins of Peru, in Halbouty, M., Maher, J., and Lian, H.M., eds., Circum-Pacific Energy and Mineral Resources, 1976, p. 331.
2. Masias, Juan A., Morphology, shallow structure, and evolution of the Peruvian continental margin, 6° S. to 18° S., Oregon State University Masters thesis, 1975, p. 52.
3. Azalgara, Carlos, Structural evolution of the offshore forearc basins of Peru, including the Salaverry, Trujillo, Lima, West Pisco and East Pisco Basins, Rice University Masters thesis, 1993, p. 102.
4. 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.

The Authors