The Geology of selected Peruvian basins

The Talara Basin is located in the northwestern coastal region of Peru. The basin occupies the onshore coastal plain, the offshore continental shelf and slope. The basin is separated from the Sechura Basin to the south by the Nuto High.

Hydrocarbon entrapment in the Talara Basin is mainly controlled by a complex mosaic of fault blocks. The major structural plays comprise lenticular reservoirs of Eocene deltaic or submarine fan sandstones sealed generally by intraformational shales which are conformable or unconformable on the reservoirs. The marine and near shore clastic sequences of the Paleocene and Eocene are very well developed and their formations have produced about 90% of the oil in this region. In many fields, highly organic shales interbedded in the Eocene sandstones reservoirs are believed to have provided good sources for locally generated oils. Good source rocks occur also in the Cretaceous section (e.g. Redondo Formation).

In terms of wildcat and seismic density, the basin (particularly the productive region) is highly mature. The plays which formed the basis for previous discoveries are thought to be mostly exhausted. However, significant reserve additions from exploration are still thought to be very possible based on new concepts or a focusing on relatively unexplored parts of the basin. Deeper drilling in many areas may also have the potential to discover new reserves. The offshore potential of the basin, even in shallow water, has barely been tapped and is thought to be considerably higher than in the mature onshore portion. The Talara Basin represents the oldest and longest producing basin in South America with what is now the La Brea Parinas Field beginning production in 1871 and still producing today. Before that indigenous people in the area used the oil seeps to caulk boats. Historical exploratory success in the Talara Basin is calculated by Perupetro as the highest in the country at 47.4%.

The Trujillo Basin is situated on the upper slope of the continental margin of Peru. The Trujillo Basin formed as an extensional system due to gravitational collapse of the thrust wedge above the subducting Nazca Plate. In absence of any discoveries, there are no proven plays or reservoir rocks in the Trujillo Basin. Seismic interpretation indicates that major potential reservoirs are probably Lower Tertiary deep sea sands. In the Talara Basin to the north, Eocene marine turbidite sandstones are important producing reservoirs. Tertiary shales and mudstones in the Trujillo Basin can be regarded as good oil-prone source rocks. There are no proven seals, though Eocene shales may make potentially good regional seals.

The Trujillo Basin is underexplored. It contains potential Tertiary reservoirs (Eocene deep sea sands) and source rocks, reasonable seals and potential structural traps related to extensional block faulting. Unrisked potential reserves are estimated at 1,700 MMBO (with Salaverry Basin).

A structural high separates the Salaverry Basin from Trujillo Basin. Potential petroleum systems within the basin are multiple with source rocks ranging from Jurassic to Miocene. Oil and gas seeps were observed in the top of Lower Cretaceous section on wildcat well. Only one exploration wildcat has ever been drilled in the basin along with a few stratigraphic test wells. It was not successful. A total of 1,487 line km of seismic has been acquired in the basin (mostly regional).

The Maranon Basin is a Subandean foreland basin east of the Andes in jungle terrain in the northeast part of Peru. It is entirely onshore. The basin is elongate and parallel to the Andean mountain front. Three proven play groups exist in the basin: Cretaceous Structural, Tertiary Structural and Cretaceous Stratigraphic/Structural. The first is the largest play with about 75% of the basin's reserves and includes both subtle foreland anticlines (55%) as well as fold/thrust anticlines (20%) in the Subandean Zone. Underexplored plays exist within the Pucara Structural Play, where one gas discovery has been made. The Contaya/Cabanillas structural play has only been lightly explored.

Shallow-marine shelf to deltaic and continental sandstones and carbonates of the Upper Cretaceous Raya, Agua Caliente, Chonta and Vivian formations are the reservoirs in the present fields. Multi-reservoir fields are the rule with stacked pays. Sandstones on top of the Lower Cretaceous Cushabatay Formation (fluvial to deltaic) and at the base of the Tertiary Tena, Cachiyacu and Pozo/Orteguaza formations (fluvial to lacustrine) are minor reservoir targets. Sandstone porosity and permeability in producing reservoirs is high (about 25% and up to several darcies). The Devonian Cabanillas Group, the Ordovician Contaya Formation and the Permian Ene Formation are potential source rocks in the southeast part of the basin, which have not yet been typed to oils in fields. The Cabanillas Group is in the late mature oil-generating stage in the southern and southwestern parts of the basin. Cretaceous and Triassic-Jurassic proven source beds are thermally immature in the area of the oil fields, which are located well to the east of the depocenter. Marginally mature source rocks are present in the deeper central and western portion of the basin. Cretaceous and Tertiary reservoirs are sealed mainly by intraformational shales and tight carbonates. Vivian Formation reservoirs are sealed also by Paleogene red beds. Tight Pucara carbonate and calcareous shales are possible internal seals to Pucara carbonate reservoirs. Shaly interbeds in the Cabanillas and Contaya speculative reservoir sections are well documented in the Ucayali Basin to the south.

Exploration in the Maranon Basin has been focused primarily on one type of trap for the last 20 years (Cretaceous Structural, Tertiary Structural and Cretaceous Structural/Stratigraphic plays) with modest economic success in the early phase of exploration. Several unproven reservoirs of older age are underexplored in the basin.

The Huallaga Basin is located in the upper jungle foothills of the Eastern Cordillera in the north-central Peru region. It is bounded to the west by the Maranon Arch and to the east by the Cushabatay and Pauya uplifts produced by the Sub-Andean frontal thrust system.

There are no proven plays/proven reservoirs in the Huallaga Basin but potential reservoir rocks occur in the sandstones of the Cushabatay, Agua Caliente and Vivian formations. Active oil seeps take place along the eastern margin of the basin. Source rocks occur in the Triassic Pucara and Upper Cretaceous Chonta formations. Triassic source rocks are currently mature to over-mature. Cretaceous source rocks are at their peak generation. Shales in the Middle Cretaceous Esperanza Formation, the Upper Cretaceous Chonta Formation and the Maastrichtian Cachiyacu Formation form excellent regional sealing horizons.

The basin is very immature in terms of oil exploration. Only one well has been drilled and seismic data is sparse. There are good source rocks and potentially good reservoirs, seals and traps. Unrisked potential reserves of 4,240 MMBO have been estimated.

The Ucayali Basin is one of the sub-Andean basins of Peru formed by Andean compression that followed Paleozoic extension and rifting. The basin has a maximum of 5,000 m of sedimentary fill. At least three episodes of extension and rifting affected the Ucayali Basin during the Mesozoic, followed by flexural subsidence due to lithospheric loading accompanied by thrusting from the Andes to the west.

Six play types exist within the Ucayali Basin, the first three of which are proven by discoveries or producing fields: 1) Upper Cretaceous-Paleocene Structural Play; 2)Lower Cretaceous Structural Play; 3) Lower Permian Structural Play; 4) Upper Carboniferous Structural Play; 5) Mesozoic-Paleozoic Pucara-Copacabana Stratigraphic Play; and 6) Cretaceous Channel Stratigraphic Play.

Multiple reservoirs are proven within the Ucayali Basin. Fluvial/deltaic to shallow marine Cretaceous clastic reservoirs, the primary reservoirs of the sub-Andean basins to the north, are present. The Cretaceous reservoirs are the best quality in the basin and are proven by several large discoveries and small fields. Lower Permian Ene Formation sandstones and Lower Permian Red Bed Group reservoirs have been proven in the southern part of the basin. The depositional environments of the Ene Formation include lacustrine, restricted shallow marine and eolian environments. The Red Bed Group sandstones are completely eolian. The Green Sandstone Member of the Tarma Formation, shallow marine, glauconitic sandstone, has fair porosities. Unproven reservoirs, which could be prospective, include shelf carbonates of the Copacabana Formation, which have fair porosities and local evidence of karstification. Thick turbidite, shallow marine and fluvial/deltaic sandstones of the Devonian Cabanillas Formation remain largely untested. Marine Cretaceous source rocks of roughly Cenomanian-Late Campanian age, which source the major petroliferous basins from the Maranon Basin of Peru northward to Trinidad, are present, but they thin within the Ucayali Basin. Quality of the Cretaceous source rocks is mostly marginal, but sometimes good in the southernmost Ucayali Basin in the Agua Caliente Formation. Triassic marine source rocks of the Ene Formation are the richest in the basin, with excellent quality.

The Lower Permian and Mississippian section possess thick sequences of marginal to good quality marine source rocks. The Devonian contains potentially organic-rich marine flooding surfaces in numerous sequences. However, only marginal source rock quality has been analyzed to date. In the northwestern part of the Ucayali Basin, the Triassic-Jurassic Pucara possesses organic-rich facies that extend to the south. Both regional and local seals are encountered in the Ucayali Basin. Sealing capacity of the basal Tertiary Yahuarango Formation, which overlies the Upper Vivian Sandstone, is not clearly documented in these red beds. Proven regional seals are marine transgressive shales and mudstones of the Upper and Lower Cretaceous section. These include the Raya, Chonta, Cachiyacu and Huchpayacu formations. Numerous intraformational shales, which are possible seals, can be found in the Mississipian Ambo Group and the Devonian Cabanillas Formation.

The Ucayali Basin remains relatively immature and underexplored, despite the discovery of the giant Camisea area gas/condensate fields. Cretaceous subtle structural traps in updip positions from the fold belts, subthrust anticlines in Cretaceous and Paleozoic objectives and channelized stratigraphic traps are all much underexplored in this promising productive basin. Estimated potential unrisked volumetric reserves of 17 BBOE are reported in addition to the giant field already discovered.

The Madre de Dios Basin lies on the Amazonian side of the Andes and shares similarities with the Ucayali Basin. Proven plays are all of the structural type. Known reservoirs are sandstones of Late Devonian-Mississippian, Late Permian, and Mesozoic ages. Potential sandstone reservoirs are numerous and occur throughout the succession. The major source rocks are Devonian black shales, Mississippian organic-rich marine to transitional facies and Pennsylvanian-Permian carbonates and shales. The most significant seals are Tertiary and Cretaceous mudstone units. Other seals occur in the near-complete succession.

There are so far four fields or discoveries in the basin and oil and/or gas seeps are known in the Madre de Dios Sub-Andean Zone. Because of some of its source rocks, the basin seems to have a considerable potential. However it is still very poorly understood. Estimated unrisked exploratory reserves have been pegged at 9,814 MMBO and 73.0 TCFG.

Most of the Titicaca Basin is situated in Bolivia, where it is called Altiplano Basin. This is an intermontane basin, located between the Western and Eastern Cordillera of the Andes. There is only one proven play in the basin, giving rise to the Pirin Field in southern Peru. The Sipin-Ayavacas Structural Play comprises fractured limestones and sandstones of the Lower-Middle Cretaceous Sipin and Ayavaca formations, sealed by basal Tertiary molasse. Potential reservoirs include Upper Silurian-Permian sandstones, Upper Cretaceous-Paleocene limestones and sandstones and numerous Tertiary sandstones. Potential oil and gas source rocks occur from the Silurian to the Cretaceous sequence. Potential seals include several Upper Cretaceous-Oligocene mudstone and shale units. There are adequate seals in the Paleozoic and Mesozoic sequences.