ABSTRACT The Madre de Dios Basin, located in the northern sub-Andean zone of Bolivia is an underexplored sub-Andean basin. A complete stratigraphic revision, including biostratigraphy, core description, and sismo-stratigraphy, has been carried out; it suggests some changes in the historical sedimentary models and allows the identification of several reservoir and seal pairs. This revision not only integrates the results of previous studies but also provides new and original interpretations of the existing data set. The geochemical study indicates the existence of an Upper Devonian world-class source rock, in which, the Frasnian interval is characterized with a type I-II kerogen and a source potential index (SPI) higher than 6 ton/m 2 ; the Famennian interval has a type II kerogen and its SPI reaches 3 t/m 2 . The Carboniferous and Permian formations have levels with notable content of organic matter but do not classify as source rock in this area because of their low SPI. To evaluate the hydrocarbon potential of the basin, a 3-D dynamic model has been built. The thermal calibration of the temperature and maturity data is only possible taking into account an increase of the heat flow during Triassic–Jurassic time. As a consequence, 90% of the hydrocarbons are expelled before Cretaceous times by the identified kitchen in the center of the basin. The remaining 10% were expelled between the Oligocene and present time. Considering a petroleum system yield of 1%, the yet to find (mean) of the studied area is evaluated at 7 Gbbl of oil equivalent. The main challenge of the basin remains in finding traps.

Abstract New information from two core holes in the Madre de Dios basin of northern Bolivia suggests that the Paleozoic foreland basin (Altiplano and Cordillera Oriental outcrop areas) had correlative units there. The Pando X-l and Manuripi X-l wells were cored to 1968 m and 1542 m, respectively, with the former reaching crystalline basement and the latter terminating in the Devonian. Both cores penetrated the Bala (Tertiary) and Beu (Mesozoic) formations, with the latter occurring disconformably on upper Paleozoic units. Madre de Dios basin stratigraphy is applied. Devonian lithologies include fine- to medium-grained quartz arenite, clay mudstone, and rare conglomerate. Upper Carboniferous lithologies include carbonate wackestone and fossiliferous packstone, anhydrite, clay mudstone, and occasional fine- to medium-grained quartz arenite, in descending rank of abundance. Biostratigraphic results, including palynomorphs, brachiopods, and foraminifera, suggest that terrigenous clastic deposition began in the latest Emsian or earlier and continued through at least Bashkirian time. Devonian clastic depositional units represent muddy shelf, shoreface, prodelta, tidal flat, dropstone, and submarine slump paleoenvironments. It appears that some Lower Carboniferous units were removed by the intra-Carboniferous erosion event. Late Carboniferous units represent delta front and delta plain, evaporative lagoon, lagoon, tidal flat, and carbonate inner platform paleoenvironments. Although basal units reflect influence of the local craton (as a source and basin-limiting feature), middle and upper units display geologic development that parallels coeval units in the Lake Titicaca and Altiplano sequences to the south. These include (1) Upper Devonian dropstone, (2) Carboniferous eolian sandstone, and (3) Upper Carboniferous carbonate. We suggest that these wells document the intracratonic connections between the Upper Amazon, Peruvian, and Bolivian basins. Permian beds may have been removed by pre-Cretaceous erosion (Pando X-l well)

ABSTRACT Different depositional environments (fluvial, deltaic, lacustrine, tidal-marine) have been proposed for the Upper Miocene to Pliocene Madre de Dios Formation exposed in the upper reaches of the Amazon River catchment in the Andean retroforeland region. This study constrains the stratigraphy, depositional environment, and drainage evolution in southwestern Amazonia through petrographic and provenance analysis of the sand and mud fractions of the three recognized members (A–C) of the Madre de Dios Formation at three stratigraphic sections measured previously along riverbank outcrops: Cerro Colorado, Piedras River, and Candelaria. Petrographic analyses of thin sections of sand separates from 32 sandy samples showed them to be litho-quartzose to quartzo-lithic in composition, with variable feldspar content and a recycled-orogen provenance. Sand components were predominantly monomineralic to polymineralic quartz, and sedimentary and metamorphic lithic fragments. Muscovite, potassium feldspar, plagioclase, and volcanic lithics were less abundant. These sand components are consistent with derivation from the Andean range to the west. Quartz and feldspar content generally increased up section from member A to member B. Sand composition in member C is similar to the modern river sand composition, consistent with recycling of Madre de Dios Formation sand into the modern river system. Petrographic analyses of 144 smear slides of the mud fractions showed no significant changes in silt composition, i.e., mainly quartz, feldspar, and mica, among the three members. X-ray diffraction of eight mud samples showed their clay mineralogy to be dominated by kaolinite and illite, with some smectite and chlorite in member B. None of the 144 smear slides or the 32 thin sections contained marine or marginal-marine biogenic debris. Mud-rich samples from the Madre de Dios Formation exhibit six main colors that characterize distinct intervals (designated I to VI) that occur in the same stratigraphic order in each measured section, from I at the base to VI at the top. The boundaries of these intervals do not correspond directly to member (A, B, C) boundaries; therefore, the colors are at least partly secondary. If primary, the red to orange to brown mudstone (which is dominant in Madre de Dios Formation members A and C) would suggest development in oxidizing environments, consistent with fluvial systems. Based on its light olive-gray color and smectite content, interval IV in member B may have been deposited in, or subjected to, a more reducing environment, such as a lacustrine-deltaic setting, with low-lying topography and poor drainage. In sum, the Madre de Dios Formation exhibits up-section compositional and thickness trends that are consistent with changes in depositional environment from fluvial (member A) to lacustrine/deltaic (member B) to fluvial (member C), as proposed by previous workers.

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 Madre de Dios Basin of Bolivia represents two distinct phases of tectonic development that illustrate the linked stratigraphic responses to a changing basin style. The first phase associated with the Paleozoic is characterized as an intracratonic setting. The second, which began during the late Mesozoic and persists today, is the development of the Sub-Andean Foreland Basin. Hydrocarbons occur primarily within stratigraphic traps, potential reservoirs and seals are Paleozoic to Late Mesozoic in age. Paleozoic depositional environments identified from core indicate major changes in climatic conditions have occurred and include fluvial/deltaic, eolian dune, coastal sabkha, and shallow marine carbonate facies. A Late Devonian marine source rock with total organic carbon (TOC) content of up to 18% also occurs within the basin. Cretaceous age sediments contain an incised valley system of 10 to 15 kilometers in width and 300 meters in depth. Valley fill facies represent low sinuosity, braided fluvial systems grading upwards into estuarine muds. Terracing of the valley margin formed in response to multiple cut and fill episodes (baselevel fluctuations) of valley formation. Recurrent movements of basement involved fault blocks related to migration of the advancing forebulge, controlled the location and magnitude of valley incision and drainage incisement patterns. Large-scale variations in depositional environments, duration of geologic time (450 to 60 MY) represented by the stratigraphic section within a changing tectonic style, provides the Madre de Dios Basin as an example of the process to response interplay between tectonics, eustasy, climate and sediment supply.

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 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 In the Peruvian sub-Andean foreland basin system, the construction of serial balanced cross-sections from a good set of structural data and an extensive knowledge of the stratigraphy and geodynamic evolution allow a more refined definition of the unexplored plays, as subthrusts, duplexes, or pre-Andean structures. Sequential restorations are proposed by coupling thermochronologic analyses with growth strata studies. The results show significant north–south variations in geometry, timing and rates of deformation, and foreland sedimentation. These latitudinal variations are not only related to the pre-Andean basins’ inheritance but also to the interactions between thrusts propagation, erosion, and sedimentation. Thermochronologic ages correspond to the most recent thrust-related uplifts and are supplemented by the study of stratigraphic foreland basin records that can bring to light oldest tectonic events. North of the Peruvian sub-Andean zone, thrusts propagation is controlled by thick-skinned and thin-skinned salt tectonics. Northern thick-skinned tectonics has westward vergence and is inherited from a Middle Permian fold-and-thrust belt. To the south, thrusts deformation is largely controlled by the geometry of the preserved Paleozoic sedimentary wedge and becomes progressively thin skinned. Total sub-Andean shortening varies between 70 km (43 mi) in the north and 47 km (29 mi) in the south. Sub-Andean deformation started in the Late Cretaceous. After a period of quiescence during the middle Eocene, it reactivated and is still active. Three stages of sub-Andean deformation are clearly identified and help to define the preservation time in the suggested petroleum plays.

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.