ABSTRACT The tectonic evolution of the Marañón Basin and its related basins, the Huallaga and Santiago Basins, in northern Peru, spans more than 250 m.y. of Mesozoic–Cenozoic subsidence. Basin evolution began with an initial rifting in the Late Permian–Early Triassic. This period of extension was accommodated by inherited structural inhomogeneities and a southwest-oriented extension, which dissected the Paleozoic sequences into a series of roughly northwest-southeast-trending grabens and half grabens filled with volcanic and continental-derived sediments. Fault-controlled subsidence was followed by regional postrift subsidence, and a thick section of Triassic to Jurassic marine to transitional sediments was deposited over the preexisting extensional features. These included one of the potential source rocks for the western part of the basin (the Aramachay Formation). The Late Jurassic to Early Cretaceous Jurua orogeny and later peneplanation produced a major regional unconformity. Subsequent Cretaceous sedimentation, mostly controlled by eustatic processes and related regressive–transgressive cycles, is composed of a thick section of continental to proximal and shallow marine deposits, comprising the main reservoirs and the main source rock for the basin, responsible for most of the oil discoveries in the northeastern region. Several compressional and structural inversion episodes, related with the subduction of the Nazca plate in Late Cretaceous and that culminated in the Miocene, have modified the basin and isolated the Huallaga and Santiago subbasins to the west, both structured by complex thrust systems. Cenozoic deposits constitute the foreland basin system infill and contain more than 4000 m (13,120 ft) of mostly fluvial and deltaic deposits with minor marine incursions. In the sub-Andean zone they constitute the “molasses” from the rising Andean cordillera to the west. This history of tectonic evolution is reflected in a complex structural framework in the western part and a well-developed foreland system to the east with a broad topographic high, known as the Iquitos Arch, which corresponds to the present forebulge.

ABSTRACT The Huallaga–Marañon retroforeland basin system of northern Peru is deformed by both thick- and thin-skinned tectonics. The thrust system is complex and resulted from the reactivation of a west-verging Permian fold and thrust belt capped by an important salt detachment. This chapter presents 2-D petroleum modeling from an updated balanced cross section and sequential restoration through the Huallaga–Marañon wedge-top basin. The sequential restoration has been calibrated by thermochronological dating and thickness variations in Cenozoic synorogenic sediments. It shows two important stages of the deformation (Middle Eocene and Late Early Miocene). Late Triassic/Early Jurassic Pucara Group and Late Cretaceous (Raya and Chonta formations) classic source rocks are present in the Huallaga–Marañon foreland basin, but the revision of the stratigraphy replaced in its updated structural context allowed us to highlight a new Late Permian source rock (Shinai Formation). 2-D modeling of kerogens maturity evolution and hydrocarbon (HC) accumulations in the sequential restoration shows that first Andean structures (Middle Eocene and Late Early Miocene) could preserve HC accumulations in the Chazuta thrust sheet footwall. In the eastern Marañon basin, more recent structures (Late Miocene–Pliocene) such as Santa Lucia could also have been charged. Deep subthrust structures stay unexplored in the Peruvian fold and thrust belts. The Huallaga–Marañon foreland system is probably the best example of subtrap attractiveness in Peru.

ABSTRACT The quartz-rich sandstones of the Upper Cretaceous Vivian Formation constitute the most important reservoirs in the prolific Marañón foreland basin of northern Peru. The Vivian sandstones are largely fluvial in the northeast and transition to a marine shoreface and open shelf in the west. The formation consists of two sand-rich units (Lower and Upper Vivian) with better reservoir characteristics in the Lower Vivian. Porosity versus depth analysis for the dominantly fluvial Vivian sandstones shows a simple trend of decreasing porosity with depth, with facies-dependent variations for wave-reworked facies. This simple linear trend (correlation coefficient R2 > 0.77) indicates that overburden stress is the dominant factor that determines the porosity reduction. Further west, in the wedge-top Santiago Basin, the Vivian sandstones exhibit anomalously low porosities at relatively shallow depths, a distinct diversion from the regional trend. The depth difference between these low porosities and the porosities of the regional trend was used as a proxy for the uplift that the basin has suffered during the Andean deformation in the latest Miocene. The resulting values (up to approximately 2600 m [8530 ft]) are consistent with uplift estimates derived from other methods and suggest that parts of the foredeep have been partially uplifted.

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 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.

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.