Abstract In many compressive zones, there is a risk of undercharged hydrocarbon prospects as a result of timing, that is, the growth of the structure is younger than the main fluid migration phase. The North Ucayali Basin represents a setting of this type, where locating the earliest structures is crucial for well placement. In the North Ucayali Basin, the variable amount of erosion at the top of the structures shows that they are not uniformly recent. Although the growth of early structures may be explained by the reactivation of inherited features during shortening, evaporitic pillows may represent an alternative factor controlling the localization of deformation in the studied area. Indeed, subsurface data reveal the presence of flat salt domes that have an influence on thrust localization. The existence and tips of such efficient gliding surfaces concentrate the strain and, as a result, localize the early zones of deformation. Analogue models designed to study these phenomena highlight the crucial role of salt pillows as potential weak zones localizing deformation ahead of the propagating wedge. These models also emphasize the need to constrain certain parameters that have been previously disregarded, including the thickness of the brittle layers between the main décollement level (if present) and the pillows, as well as their burial depth.

ABSTRACT The provenance of middle Permian to Maastrichtian sandstones from the subsurface of the Marañon and Ucayali Basins was determined through U-Pb dating of 113 detrital zircon samples from 21 hydrocarbon exploration wells. An additional 52 samples representing many of the subsurface lithostratigraphic units drilled in the Marañon and Ucayali Basins were collected from 42 outcrop localities in the Huallaga Basin and one outcrop locality in the Pachitea sub-Basin for U-Pb dating. The exposed units were analyzed to determine whether the outcrop sandstones had the same provenance as their subsurface counterparts. Analytical results show that profound temporal changes in long-term detrital zircon provenance were observed in all the basins; spatial changes in detrital zircon populations between and within the Marañon and Ucayali Basins appear to be less significant. Western pre-Andean “Peruvian” source areas were major contributors of detrital zircons to sandstones in all the basins, especially during the middle Permian to Late Jurassic. Zircons with ages that are contemporaneous with the deposition of these sandstones were contributed by active continental arcs or locally via erosion and recycling of Permo-Triassic plutons. Abundant Neoproterozoic–Cambrian zircons were likely derived locally from the Pampean arc and Puncoviscana Formation (now buried in Peru) that formed in western Amazonia prior to emplacement of the Marañon Complex. Conversely, the primary sources for Archean to middle Paleoproterozoic detrital zircons were through local erosion and recycling of Paleozoic and Mesozoic sandstones and from recycling of Solimões Basin sandstones. Direct provenance from more distant central and eastern Amazonian cratonic source areas is possible but is considered unlikely. Regional Late Middle Jurassic to Early Cretaceous uplift of the ancestral Solimões Basin, the eastern Marañon and Ucayali Basins, and western Amazonia caused a major shift in regional detrital zircon provenance, from local and western “Peruvian” sources to proximal western Amazonian cratonic sources. The development of new fluvial drainage areas in western Amazonia rapidly replaced less important long-distance detrital zircon sources from central and eastern Amazonia. Sources of local and west-derived “Peruvian” detrital zircons diminished by the Late Cretaceous (Cenomanian) as the Pampean arc and Puncoviscana Formation were buried from north to south; contemporaneous zircons were largely trapped within a deep back-arc basin west of the Marañon Complex. Detrital zircons from local “Peruvian” source areas continued to be important for Upper Cretaceous sandstones in the Huallaga and southwestern Ucayali Basins but were replaced by proximal western Amazonian cratonic-sourced zircons by the end of the Cretaceous. Similar detrital zircon-age populations observed in middle Permian to Lower Cretaceous sandstones suggest that subsurface correlations may be imprecise in certain areas. Local recycling and redeposition of zircons from older sandstones is regarded as a more important mechanism for the formation of key hydrocarbon reservoir sandstones than was previously known.

ABSTRACT Stratigraphic, geochemical, and biomarker data from the Huallaga Basin suggest that organic carbon-rich shales and limestones of the Upper Triassic to Lower Jurassic Aramachay Formation of the Pucará Group, previously identified as potential hydrocarbon source rocks in Peruvian sub-Andean basins, were deposited under low oxygen or anoxic conditions within a semirestricted basin. Rock-Eval and total organic carbon (TOC) data from surface and subsurface locations show that although most Aramachay Formation shale and limestone outcrop samples have relatively high organic carbon content, the unit has little remaining genetic potential; T max data indicate that the thermal maturity of nearly all outcrop samples ranges from wet to dry gas. Visual kerogen analyses show that type II amorphous kerogen is the dominant type in the Aramachay Formation. Cretaceous rocks within the Huallaga Basin are dominated by type II/III and type III kerogen and generally lack sufficient TOC to be effective source rocks for oil. Geochemical and biomarker data indicate that rock extracts and seep oils were derived from mixed shale and carbonate source facies dominated by marine algal and bacterial organic matter and are similar to “Jurassic” oils described from the Marañon and northwestern Ucayali Basins. Hydrocarbon generation and expulsion models suggest that the generation and expulsion of oil from the Aramachay Formation (likely the middle Aramachay Formation) began from west to east in the Huallaga Basin, starting in the now-exhumed western part of the basin during the Early Cretaceous, extending through the middle Oligocene in the central part of the basin and into the Present in the eastern part of the basin. Estimates of vitrinite reflectance (R o) based on biomarker data indicate that Marañon Basin oils derived from the Aramachay Formation were likely generated during the peak oil phase of generation; oils in the northwestern Ucayali Basin were generated during the late oil phase of generation. Petroleum extracts from outcrop samples in the northern part of the basin and oils from seeps along the southeastern frontal thrust of the basin indicate a late oil level of thermal exposure. Migration of oils into the Marañon and northwestern Ucayali Basins likely occurred prior to the early Pliocene, when formation of the Andean frontal thrust cut off migration routes from the Huallaga Basin.

Abstract Sub-Andean Peru comprises the Maranon, Ucayali, and Madre de Dios basins which, together with three subsidiary basins, cover an area of 370,000 km2. These basins extend considerable distances northward into Ecuador and Colombia and southeastward into Bolivia. More than 5 billion bbl of recoverable oil have been discovered in these basins, of which over 1 billion bbl of oil and almost 7 tcf of gas are in Peru. The Tertiary foreland basins in front of the Eastern Cordillera are filled with up to 4 km of Tertiary molasse sedimentary rocks. The basins are mainly of Miocene age and overlie older Paleozoic and Mesozoic depocenters. Three major compressional episodes are recognized: a Middle Triassic event, an Early Cretaceous event associated with major unconformities in some areas, and a regionally pervasive late Miocene-Pliocene (Quechua III) event expressed in thrusting and compressional folding over most of sub-Andean Peru. Two families of oils are differentiated in the Maranon basin, related to Permian and Cretaceous source rocks. Three groups of oils in the Ucayali basin derive from Devonian, Carboniferous, Permian, and Triassic sources. Oil samples in the Madre de Dios basin correlate to Devonian and Carboniferous shales. A variety of trap types have been identified. The foreland can be divided into areas where preexisting faults have been reversed by late Tertiary compression and flexural uplift, and areas unaffected by this deformation where older, more subtle traps are important. To the west, the sub-Andean belt comprises regions where basement- involved thrusts predominate and other areas characterized by thin-skinned thrusting. The Oriente-Maranon- Ucayali basin complex has at least one large hydrocarbon accumulation in each trap type. The level of exploration is low, and many areas are virtually unexplored.

Abstract The Oriente, Ucayali, and Madre de Dios basins, located in the Amazon drainage of Colombia, Ecuador, and Peru, are part of a series of large asymmetric depressions between the Andes Cordillera and the Guyana and Brazilian shields. They are separated by basement arches and have areas of 458,000, 200,000, and 95,000 km 2 , respectively. The whole area is topographically low, covered by heavy rain forest, traversed by numerous huge tributaries of the Amazon, and very sparsely populated. From early Paleozoic time until the Maestrichtian, seas repeatedly invaded the area, depositing a variety of sediments, but mostly calcareous and silicate clastic material, over wide areas. At the beginning of the Tertiary, predominantly marine deposition gave way to nonmarine deposition, reflecting the Andean orogeny and topographic development of the Andes mountains. The depositional cycle of major importance for hydrocarbons took place in the Cretaceous. A complete marine cycle of miogeosynclinal sedimentation is represented; the maximum thickness is 2,500 m, but the sequence thins consistently and becomes sandier toward the east. Although the sequence consists mainly of sandstones and shales, limestones and sandy limestones are important potential reservoirs. Most prospective structures in the basin are anticlines, generally fault-bounded and somewhat steeper on the east. Salt domes and other diapiric structures are also present. Amplitude of structures and intensity of deformation decrease eastward. The formation of structures and the migration and entrapment of hydrocarbons in them appear to have occurred at various times in the Tertiary. The Colombian part of the Oriente basin is in an advanced state of exploration and has little potential for future significant discoveries. In Ecuador, although the peak of exploration activity has been passed, the future potential is thought to be substantial. In Peru, exploratory drilling in the Oriente basin has already discovered reserves on the order of 800 million bbl of oil. Based on these facts and on information from Colombia and Ecuador, the total potential of the Oriente basin is estimated to be 25 – 35 billion bbl. About 20 wildcats have been drilled in the Ucayali basin; two small oil fields and an undeveloped, but potentially large, gas field have been discovered. On the basis of these results and estimates for the Oriente basin, the Ucayali basin has an estimated potential of 5 – 10 billion bbl. In addition. Paleozoic sedimentary rocks have some potential. The Madre de Dios is the least explored of the three basins. Favorable conditions are known to exist and oil seeps are present. A potential of 6 – 12 billion bbl is estimated. The total estimated potential for the three basins is about 45 billion bbl.

ABSTRACT The last stratigraphic and structural assessments of the Ene Basin (Peru, Block 108) defined a prospective petroleum system. Over the past 50 years, this basin has been studied by a number of oil companies and scientists. Surface geology and ~750 km (~460 mi) of 2-D seismic sections have provided most of the information, since no exploratory well has been drilled to date. The presence of well-known source and reservoir rocks, several hydrocarbon manifestations at the surface, and large anticlines define two main plays, currently making the Ene Basin the frontier basin with greatest exploration potential in Peru. The Ene Basin is part of the Peruvian sub-Andean system developed ~300 km (185 mi) east from the Pacific trench, cratonward of the Eastern Andes. As shown by their common stratigraphy, the evolution of this intermontane basin was related to the southern Ucayali and Camisea Basins until they were separated by the uplift of the Otishi and Shira basement blocks during the Andean orogeny. The Ene Basin is divided into two main structural domains based on their different mechanical stratigraphy that imprinted contrasting structural styles: (1) the northwestern domain displays marked stratigraphic similarities with the northerly Pachitea Sub-Basin, being characterized by a thick Mesozoic succession, salt domes, and a deformation style related to the inversion of the southeastern rim of the Triassic–Jurassic Pucará extensional basin; (2) the southeastern domain is affected by thin-skinned structural deformation and exhibits a similar stratigraphy than the easterly Ucayali Basin, characterized by thin to absent pre-Cretaceous Mesozoic units and variable Cretaceous–Paleozoic unconformable relationships. Contrasting structural styles and the uneven distribution of shortening are related to differing degrees of interaction between the two main structural domains and the surrounding basement blocks.

ABSTRACT The Camisea multi-trillion cubic feet (tcf) gas and condensate fields are located at the southern edge of the Ucayali Basin of southeastern Peru. The Ordovician to Neogene sedimentary succession was deformed by late Miocene to Present Day contraction related to the Peruvian flat-slab subduction regime. This produced thin-skinned, north-northeast-vergent thrust-fault-related folds that form the traps of the Camisea fields. The architecture of the frontal thin-skinned thrust system is characterized by a faulted detachment fold system at Cashiriari and a gently dipping north-northeast-vergent thrust ramp system and associated kink-band hanging-wall anticlines and back-thrusts at San Martin. At San Martin, these form brittle thrust wedge systems that terminate in triangle zones in the Paleogene–Neogene strata of the foreland basin at the leading edge of the fold-and-thrust belt. The basal detachment of the thin-skinned system is located at the top of the Ordovician–Silurian synrift sequence and at the base of the Devono–Mississippian postrift units. Steep Ordovician–Silurian extensional faults offset the basement and form half-graben structures that influence the topography of the postrift strata and the basal detachment geometry. The Cashiriari Anticline is modeled as gentle inversion fault-propagation fold at the early stages of the Andean deformation and then was amplified forming a detachment fold during the late Miocene to Present Day phase of strong contraction. Small displacement limb-break thrusts displace the Cashiriari fold limbs. In contrast, the San Martin fault-fold system is modeled as a simple shear fault-bend fold that forms a wedge thrust and a triangle zone. The San Martin folds are hanging-wall kink-band-style fault-bend systems where the positions of the underlying thrust ramps were controlled by the basement fault systems and the topography of the postrift units. The hinterland of the Camisea frontal thin-skinned fold-and-thrust belt is interpreted to be a system of large inverted basement fault blocks that were uplifted and exhumed as the Andean deformation moved outboard from the hinterland to the foreland and transferred displacement onto the thin-skinned sedimentary wedge at the edge of the basin. This study shows how the underlying basement fault architectures and rift basin geometries can control the styles of the thin-skinned Andean deformation in the sub-Andean system.

The approximately N-S–trending Andean retroarc fold-and-thrust belt is the locus of up to 300 km of Cenozoic shortening at the convergent plate boundary where the Nazca plate subducts beneath South America. Inherited pre-Cenozoic differences in the overriding plate are largely responsible for the highly segmented distribution of hydrocarbon resources in the fold-and-thrust belt. We use an ~7500-km-long, orogen-parallel (“strike”) structural cross section drawn near the eastern terminus of the fold belt between the Colombia-Venezuela border and the south end of the Neuquén Basin, Argentina, to illustrate the control these inherited crustal elements have on structural styles and the distribution of petroleum resources. Three pre-Andean tectonic events are chiefly responsible for segmentation of sub-basins along the trend. First, the Late Ordovician “Ocloyic” tectonic event, recording terrane accretion from the southwest onto the margin of South America (present-day northern Argentina and Chile), resulted in the formation of a NNW-trending crustal welt oriented obliquely to the modern-day Andes. This paleohigh influenced the distribution of multiple petroleum system elements in post-Ordovician time. Second, the mid-Carboniferous “Chañic” event was a less profound event that created modest structural relief. Basin segmentation and localized structural collapse during this period set the stage for deposition of important Carboniferous and Permian source rocks in the Madre de Dios and Ucayali Basins in Peru. Third, protracted rifting that lasted throughout the Mesozoic provided the framework for deposition of many of the source rocks in the Subandean belt, but most are not as widely distributed as the Paleozoic sources in Bolivia and Peru. We attribute variations in the style of Andean deformation and distribution of oil versus gas in the Subandes largely to differences in pre-Cenozoic structure along the fold belt. The petroleum occurrences and remaining potential can be understood in the context of three major geographic subdivisions of the Subandes. Between Colombia and central Peru, rich, late postrift Cretaceous source rocks occur beneath Upper Cretaceous–Cenozoic strata that vary significantly in thickness, yielding large accumulations around the Cusiana field in Colombia and within the Oriente Basin in Ecuador, but weak or nonviable petroleum systems elsewhere, where the cover is too thin or too thick. The central Subandes of southern Peru and Bolivia evaded significant Mesozoic rifting, which kept Paleozoic sources shallow enough to delay primary or secondary hydrocarbon generation. After post-Oligocene propagation of the fold belt, however, numerous accumulations were formed around Camisea, Peru, and in the Santa Cruz–Tarija fold belt of central Bolivia. In the southern Subandes of Argentina, Paleozoic crustal thickening caused by terrane accretion and associated arc magmatism created a tectonic highland that precluded deposition and/or destroyed the effectiveness of any Paleozoic source rocks. Here, all accumulations in the foreland and in the fold belt rely on Triassic and younger source rocks deposited in both lacustrine and marine environments within narrow rifts. The trap styles and structurally restricted source rocks in the southern segment have yielded a much smaller discovered conventional resource volume than in the northern and central Subandean segments. In comparing the Subandean system to other global fold belt petroleum systems, it is undoubtedly the rule rather than the exception that the robustness of hydrocarbon systems varies on a scale of a few hundred kilometers in the strike direction. Our work in the Andes of South America suggests that this is because most continents possess heterogeneous basement, superposed deformation, and subbasin stratigraphy that vary on roughly this length scale.

ABSTRACT The Andean foreland basin formed throughout the Cenozoic in a retro-arc setting in front of the advancing orogen. A 2500 km (1553 mi) long segment of this basin system passes through eastern Peru and Bolivia and comprises, from north to south, the Marañón, Ucayali, Madre de Dios, Beni, and Chaco Basins. The Andean foreland basin contains substantially thick units of Cenozoic sediments, which overlie Mesozoic and Paleozoic successions and Precambrian crystalline basement. In the deeper parts of the foreland basin, no wells have penetrated the full, pre-Andean sedimentary section and the sheer thickness of the sediments makes it difficult to seismically image crystalline basement in some areas. Thus, the thickness of the pre-Andean sediments and the existence of basins that pre-date the Andean orogeny are partly obscured. Areally extensive gravity and magnetic data sets have been used to build a structural and tectonic framework for the area. Gravity and magnetic 2-D forward modeling and 3-D inverse gravity modeling, constrained by seismic interpretation and well data, enabled base Cretaceous and top crystalline basement horizons to be derived. This approach allowed lateral extrapolation of the detailed but localized seismic interpretation into areas without seismic coverage, and it also extended this interpretation by including the depth to top crystalline basement. The results of this analysis indicate the presence of large pre-Cretaceous depocenters underlying the Andean foreland basin. These include a major depocenter extending from the central Marañón Basin north-northeastward across the Iquitos Arch, two depocenters underlying the Madre de Dios Basin and four depocenters beneath the Beni/Chaco Basins.