PERU OFFSHORE-2: Peru offers deepwater blocks south of Talara area

The total area is 1.2 million ha. The blocks are geologically located partly over the Talara basin and partly over the South Lancones-Sechura basins (Fig. 1). The blocks offer indications of geological models of sedimentary clastics deposits of transgressive/regressive systems environment in the three basins, making it plausible to find reservoirs of appropriate size to support the economics (Fig. 1).

The blocks are located in the south and east extension of the Talara-Peru Oil Province, where 1.3 billion bbl of oil has been produced. The blocks have never been explored, and only a few 2D regional seismic lines are available.

Organic contents resemble good sources for unaltered oils, although a different facies with more marine conditions is required. Based on these conclusions, the hydrocarbons must have migrated farther than originally suspected. Thus, geochemical analyses give indications of farther westward extension, where more marine conditions should be present and where hydrocarbons would have been generated, mostly offshore in the westernmost portion of the blocks or even farther west in the ocean.

Onshore Lancones basin

The Lancones structural-sedimentary basin in northwestern Peru could be classified as a "forearc basin" type. Its geologic location is described as westward from the Andes cordillera and eastward from metamorphic outcrops of the Paleozoic Amotape formation that in this case act as basement.

The elongated basin is generally oriented NE-SW. Its north limit is in Ecuador. The south limit extends towards the sea in the so-called Sechura basin (Fig. 1). Sediments filling the basin are formed mainly by cyclic sequences of sandstones and shales. Towards the east, these sequences interfinger with extrusive andesite volcanics. Total thickness of the Cretaceous sequence, reconstructed by outcrops, is estimated at more than 4,500 m (Fig. 2).

The Lancones basin presents two areas at the north and south of Huaypira fault, with characteristic geologic and topographic features. At the north, sediments of Cretaceous age outcrop, both upper and middle, forming an undulating geomorphology: In some areas, geomorphology is abrupt with relief of up to 800 m above sea level. The southern part is mainly covered by Tertiary and Quaternary sediments, and the Chira River marks the rough southern boundary of the Lancones basin (Fig. 1).

The Lancones basin is composed of two structural sub-basins separated by a fault trending west-east and dipping southward. In the north sub-basin, a Cretaceous-age sedimentary-volcanic sequence is totally exposed at surface. In the southern sub-basin, older Chira-Verdun sediments are at the surface.

Structural conditions are attractive for the formation of oil-bearing traps in both sub-basins. However, structural closure and sealing conditions are not clearly favorable, possibly being washed by meteoric waters. Faulting in blocks, typical of Talara basin, has not been observed in Lancones. Geochemical conditions are favorable for hydrocarbon generation, expulsion, and migration. The presence of the Muerto formation should be confirmed in the subsurface.

Reservoir conditions are limited to sedimentary rocks of low porosity and permeability, and they are, on the other hand, limited to interfingering with andesitic volcanic facies. In short, based on currently available information, they are not too attractive to be considered as reservoir rocks. In Lancones it has not yet been possible to confirm the oil bearing sedimentary sequence of Eocene typical of the Talara basin, but more exploratory activities with modern technology could give more positive results. Hydrocarbon indications in general have been poor. Fluorescence and drilling mud cut by gas could have been confirmed only in two wells. Oil seeps are known in Quebrada Pocitos.

S. Lancones-Sechura desert

The Sechura desert or Sechura basin covers more than 20,000 sq km on the northwestern coast between 4 degrees 50' and 6 degrees 30' S. Lat. and between 79 degrees 45' and 81 degrees 14' W. Long. It is 150 km north-south and 100-160 km east-west (Fig. 1).

Tertiary formations cover no more than 2% of the desert. The Paleozoic and Mesozoic series affected by plutonic intrusions (Illescas Hills) are calculated to occupy 20%. Uplifted sea-floor terraces (Quaternary-Pleistocene age) and modern deposits represent 78%.

The following geological units can be recognized in the Northwestern Peru Region (Fig. 1):

1. The Andean geosyncline, mainly the eugeosyncline characterized by its magmatism and representing the eastern border of the Sechura desert.
2. The Paleozoic arc, represented by the "Lobos de Afuera" and "Lobos de Tierra" Islands offshore, and Illescas Hills onshore.

Between these structures we found a north-south longitudinal plain or para-andine depression, which includes in the north part of the Lancones Cretaceous rock outcrops and the Sechura Tertiary basin, where most series of the lower Tertiary typical of Talara basin productive formations are absent here.

In order to determine the oil bearing possibilities of the Sechura onshore area, it is necessary to carry out an analysis of the lithological and reservoirs conditions of the different rock units, evaluated and described by 51 wells already drilled.

Paleozoic

Of 28 wells drilled since 1930, 16 stopped in the Amotape. The Paleozoic thickness averaged 57 m with a minimum of 9.5 m in Viru 5X-2 and a maximum of 174 m in SX-37 (Fig. 4).

Lithology characterized by shale, schists, phyllites, and metaquartzites has not allowed the accumulation of hydrocarbons due to its lack of porosity and permeability. Nevertheless, since the South Talara-Portachuelo area proves economical production from Paleozoic accumulated in fractures caused by diastrophic events, we can only expect favorable prospectivity from the Sechura basin's northwest because of:

1. Possible migration conditions coming from hydrocarbon generating formations from Rio Chira's northwest;
2. A seal of subcrop Cretaceous units.
3. torage in Paleozoic fractured quartzites in a favorable structural position.

Cretaceous

According to available information such as well history, stratigraphic columns, electric logs, fossil lists, and other data, the Cretaceous has been registered in the following wells:

La Huaca 1, Viru 69-X-1, Viru 45-X-1, Viru 4-X-2, Viru 4-X-1, Viru 5-X-1, Inca 5-1, SX-38, Z2-75-55X (La Casita), Z2-47-75X (Chira West).

The greater thickness was found in Tamarindo 1, Peoco 7-1, La Huaca 1, Viru 69-X-1 and Viru 4-X-1.

Upper Cretaceous has been found in 5 of 15 wells. According to log shows, 8 of the 15 wells had signs of hydrocarbons, tested weak flows of dry gas, or traces of liquid and/or dead oil. These wells are: Tamarindo 3, P.G. 39-X-1, Peoco 3-1, Peoco 3-2, La Huaca 1, Viru 69-X-1, Viru 4-X-1, Viru 5-X-1.

Only well Peoco 7-1 presents somewhat better signs of petroleum in the Ancha formation; it was tested, but without enough success so as to start commercial production. The other 14 wells, which did not find Cretaceous rocks, may have been located outside the area where this sequence was deposited, or the lack of sediments of this period may be due to pre-Verdun erosion.

These results are discouraging since neither the Copa Sombrero, Redondo, nor Ancha formations gave evidence of advantageous petroleum values.

This would leave only the Pananga and Muerto Aptian-Albian limestones, which have not been found except in Peoco 3-1, in which the black limestone of the Muerto formation gave slight fluorescence and signs of tar.

Tertiary

In Northwest Peru (La Brea y Parinas, Lobitos, El Alto) are 7,620 m of Tertiary sediments of which 4,572 m correspond to the Paleocene and Eocene (Fig. 3).

Throughout the Sechura area, with exception of the erratic presence of the Talara, Salina and Mal Paso formations, all remaining Tertiary formations are practically nonexistent.

In the South Sechura area, the Verdun formation (or Verdun-Chira Group) is better represented than the oldest Tertiary units. Its presence has been confirmed in 14 wells, and in 6 other wells in a kind of undivided Verdun-Chira group. It has also been found in the old International Petroleum Corp. wells east of Illescas Hill. Wells Viru 5-X-1 and Viru 5-X-2 have shown the presence of a basal conglomerate in this formation, which in Viru 5-X-2 penetrated 113 m.

This formation is moderately productive in La Brea y Parinas, Lobitos, and Carpitas; nevertheless, in Sechura it gave dry gas in Viru 4-X-1 and Viru 69-X-1, with somewhat favorable commercial production. Referring to the Chira formation, widely distributed throughout the Sechura area, no productive horizon has been proven. Its possibilities as a hydrocarbon generator are moderate, as a reservoir very doubtful, and its importance is as a seal. Traces of dry gas were found in Viru 45-X-1 and Viru 4-X-1.

Oligocene

The Oligocene has been detected in the following wells: Minchales 1, Inca 5-1, Viru 4X1, Viru 4X2, Viru 5X1, Viru 5X2, Viru 45X1, Viru 69X1, Venturosa 9X1, Venturosa 10X1, Expectativa 1X1, P.G. 11 NX 1, and Z2-75-55X.

The Mancora formation and the Oligocene Group are generally saturated with highly saline brine. A weak flow of methane has only been registered in Viru 4X1, Minchales 1, and Venturosa 9X1.

The Heath formation has been recognized in Viru 5X1, Inca 5-1, Minchales 1, Venturosa 9X1, and Venturosa 10X1. The conglomerates and sandstones found towards the top of this formation are productive in Zorritos field; however, Sechura registered traces of a weak flow of methane only in Minchales 1, Inca 5-1, and Viru 5X1.

Miocene

The Miocene consists of two lithological units: the lower Miocene, sandy, marine, and thinner unit named Montera formation and another much thicker, overlying unit, the Zapallal formation, mainly clayey and marine. These sediments are the most widely distributed in all Tertiary deposits of the Sechura desert area, but they are found neither in the basin's northwest region nor in the Talara basin. The greatest thickness of 1,194 m is in Minchales 1.

In the old Bayovar wells, the Montera formation could not be distinguished from Zapallal. The Miocene series became sandier and semi-continental in this part of the basin, probably originating in brackish or fresh waters.

The Montera formation overlies parts of the Heath formation, which in the Zorritos area is considered as a source and reservoir. Nevertheless, uncertain data on petroleum evidence only exists in Bayovar 2. Slight signs of dead oil, tiny hardened tar grains, and some dry gas were found in Inca 5-1 and Minchales 1.

The Zapallal formation consists of clayey, quartziferous sandstone of fine to thick grain, interbedded with shales and limolites, bentonite, and calcareous ash.

In the lower part of the Miocene series, calcareous sandstones and impure lenticular limestones are found. Secondary lithic components: layers of plaster, beds of concretionary phosphate, discontinued shell belts usually loaded with the remains of fish, fossil wood, pyrites, and even fragments, thus revealing a nearby coast.

No sign nor oil shows have been found in Zapallal formation outcrops. Nevertheless, the presence of small grains of solid tar or fossil petroleum found at intervals in the drilled section of Inca 5-1 and similar traces found in Minchales 1 may have some significance.

In Minchales 1, as well as in the old well Bayovar 2, the Zapallal brought some signs of hydrocarbons being generated. Further, gas is found related to salt and/or sulphurous water with a local acid odor.

With this entire handicap, possibilities in the Zapallal formation are very poor due to the lithology and global shore or even semi-continental character. Therefore, it should be considered an inadequate hydrocarbons source rock, with the exception of its purely marine and diatomic characteristics, mainly developed in the middle of its sequence, which diatomites are equivalent to the tripolites of Monterrey in the California Miocene. On the other hand, the Zapallal formation has an acceptable ability for storing hydrocarbons, only lacking the sealing protecting rocks.

General conclusions

Invasion of drilling mud can be an indicator of formation permeability. Seven formation tests carried out in the Cretaceous recovered gas with different flows indicating capacity for trapping accumulations of hydrocarbons.

Two formation tests carried out in the Paleozoic Amotape formation recovered gas; one of these with a weak blow to surface gave possibilities of the existence of hydrocarbons accumulation.

However, the only formation that gave positive results in its evaluation was the Verdun, presenting itself as a gas reservoir. Fluorescence was only obtained in ditch samples treated with carbon tetrachloride. Through tests taken, all formations have shown to contain water with various saline degrees, the Tertiary formations being those most invaded with saline water.

Z6/Z7 offshore area

Blocks Z6 and Z7 must be considered as an extension into the sea of the Lancones basin, and the offshore extension may show better characteristics and better chances of containing hydrocarbons than have been found in the onshore formations.

Nevertheless, the structural panorama may be altered due to the basement's relatively shallow depth, which in some way contributes towards a greater faulting in the formation overlying it. On the other hand, the Cretaceous-Paleozoic discordance and the basement relief may have some influence on the entrapment of hydrocarbons.

It is logical to assume that some of the formations must be locked against the raised basement causing possible pinchout entrapments; however, the presence of hydrocarbons in these potential traps is very unlikely, at least statistically according to the negative results obtained from onshore.

Parts of Blocks Z6/Z7 in more than 400 ft of water would be explored differently than those in less than 400 ft of water because of the vastly different economics related to drilling and production infrastructure. Another factor is the presence of Paleozoic and intrusive rocks near the surface. There are two sectors related to this factor that offer different prospecting attractiveness:

1. Possible South Sechura arch offshore extension. This represents an extension into the sea of the "South Sechura Arch" (detected on land); its surface appearing again out at sea on "Lobos de Tierra" island, which is made up at shallow subsea depth of Paleozoic and intrusive rocks. Well Minchales 1 found the granodioritic basement at 5735 ft. The NE-SW continuation of this arch is marked by the presence of Paleozoic in wells Venturosa 9X-1 at 5,090 ft and Venturosa 10X-1 at 5,644 ft in a high structural position. The Paleozoic outcrop intruded by granodiorites forms "Lobos de Tierra" island (Fig. 3).
2. Sector Illescas Hill offshore extension. This covers an area between the coastline and the 400 ft water depth contour. The presence of Paleozoic rocks in this sector with outcropping or near-surface granodiorite intrusions makes the geological model different from that of the other areas. It is evident that little sedimentary cover overlies the crystalline-Paleozoic basement (Fig. 3).

It is concluded that 10,000-12,000 ft of clastic sediments cover the possible prospecting areas. Nevertheless, differences that may permit a more attractive configuration are the possible tectonic style of the area in which a close relationship exist between the faults and fracture systems of the basement and overlying rocks.

In general, it can be concluded that the metamorphism varies with the distance of the granite intrusion. The degree of metamorphism is interesting as it depends on the crystalline character of the rocks; the more crystalline the rock, the greater its fracture generating potential. An important detail could be this fracture porosity or empty space between the walls of the cracks or fissures in Paleozoic formations.

Possible prospective areas

After analyzing and discarding the three areas described, one can still find prospective parts of blocks Z6 and Z7. The following information may apply to the eastern part of Block Z7.

The prospective areas are necessarily related to the possible thickness of sediments (not more than 12,000 ft) and the eventual traps to be detected. These areas are directly based on the morphostructural characteristics described on land and which continued in offshore areas (Fig. 3).

It is possible that the Verdun formation may be present as a reservoir. It is evident that throughout this area Verdun pinches out against the Paleozoic arch represented by the offshore Illescas Hill extension.

On land and on the same axis the Verdun section in well PG11X-1 (TD 7,128 ft) was a sequence of medium to thick moderately compacted grained sandstone interstratified by shales. Nevertheless, a basal conglomerate has been detected in this formation made up of pebbles probably cemented into a sandy matrix (Olympic wells). These conglomerates have been described in the Verdun outcrops as chaotic conglomerates made up of quartzite slates, granite, and andesites. These fragments go from subangular to rounded and were apparently originated in submarine slidings.

Facies deposition in Verdun is sandy, of shallow water, in the north part of Brea and Parinas; of predominantly clayey facies out of sea in Lagunitos and Mirador; turning sandy in Sechura. The deposition conditions, therefore, vary from sublitoral to epineritic and even bathyal.

In well PG11X-1, Verdun was tested (DST) in three stages producing 2,810 ft of salt water having 27,600 ppm. Out at sea, the deposition possibilities must apparently continue a similar trend; a thinning through pinchout is expected throughout the sedimentary sequence.

The Oligocene Mancora formation is prospective here. The sequence was found when well Venturosa 10X1 (TD 5,836 ft) was drilled on land.

It is important to mention that in the Sechura subsurface the Mancora formation lies over the Chira-Verdun. The characteristics of this contact have not been observed, but it forms an abrupt change in the rocks and facies; that is to say, marine mudstones from the Chira formation pass without transition to the Mancora basal conglomerate deposited in brackish water.

Nevertheless, in the Sechura Desert the Mancora formation and the Oligocene are generally found saturated by highly saline water. A weak flow of methane gas has only been registered in Viru 4X1, and signs of methane have been found in Minchales 1 and Venturosa 9X1.

Consequently, the sequence is discarded as an hydrocarbon source but it does contain the reservoir characteristics.

The reservoir and deposition characteristics must continue out to sea as the distance between Venturosa 10X1 and the 400 ft water depth contour is only 15 km (Fig. 2).

This offshore area represents an out-at-sea continuation of the sedimentary sequence detected in well Inca 5-1 (TD 5,994 ft) which is the southernmost well drilled in the Sechura Desert and located about 20 km from the coastal line (Figs. 1, 3).