Eocene Unconformity Research Articles

Sweet spot areas for shale oil and shale gas plays in the Upper Cretaceous rocks of the Middle Magdalena Valley, Colombia: insights from basin modeling

The Middle Magdalena Valley Basin (MMVB) in Colombia has a long history of conventional hydrocarbon exploration and production, with a cumulative production of 2.75 billion barrels of oil as of December 2021. Recent interest has been directed towards unconventional hydrocarbon plays within the basin due to the fine-grained nature of its Cretaceous source rocks and their mineralogy and mechanical properties. This study presents a three-dimensional basin model for three Upper Cretaceous source rocks, known as the “La Luna Formation,” within the MMVB. The model was developed using new data from five outcrops, 7,640 km of 2D seismic lines, and forty-one boreholes, as well as additional data sets such as X-ray diffraction analyses, pyrolysis analyses, well-log correlations, facies analysis, fracture pattern prediction, pore pressure analysis, heat flow estimations, and petrophysical data. The model estimates total retained oil and gas volumes to be 7.95 billion barrels and 4.21 trillion cubic feet in most probable scenarios, after a 15% recovery factor. Seismic interpretation reveals pre-Eocene structures beneath Paleogene and Neogene sediments, and the thickness variation of the source rocks from south to north of the basin. Petrophysical modeling shows effective porosities ranging from 2%–12% and organic porosities lower than 0.1%, with parts of the succession that might correspond to a carrier bed play. From a geomechanical perspective, we identified several brittle strata based on the higher concentrations of carbonates and quartz, and the areas with a higher probability of occurrence of natural fractures. Pore pressure analysis of multiple wells shows that wells in which the Middle Eocene unconformity has beveled the source rocks have no sealing capacity, becoming a risk for the play. The results suggest potential for unconventional hydrocarbon plays in the MMVB, with sweet spot areas being primarily controlled by porosity, thermal maturity, gas-oil ratios, and retained oil and gas volumes, as well as to a lesser degree, the probability of natural fractures and pore pressure conditions. However, further exploration is needed to constrain uncertainties regarding facies and source rock quality, particularly within the depocenters of the basin, in order to prove the economic feasibility of these unconventional plays.

Origin and kinematics of a basin‐scale, non‐polygonal, layer‐bound normal fault system in the Levant Basin, eastern Mediterranean

AbstractPolygonal, layer‐bound normal faults can extend over very large areas (>2,000,000 km2) of sedimentary basins. Best developed in very fine‐grained rocks, these faults are thought to form during early burial in response to a range of diagenetic processes, including compaction and water expulsion. Local deviations from this idealised polygonal pattern are common; however, basin‐scale, layer‐bound faults with non‐polygonal map view are not well‐documented and accordingly, their genesis is not well understood. In this study, we use 3D seismic reflection data, biostratigraphy and well logs from the Southern Levant Basin, offshore Israel, to develop an age‐constrained seismic‐stratigraphic framework and determine the geometry and kinematics of such basin‐scale fault system. The faults tip out downwards along an Eocene Unconformity, but unlike layer‐bound faults in the Northern Levant Basin, they do not reach the base of the Messinian evaporites, instead tipping out upwards at the top Langhian. On average, the faults in the Southern Levant Basin are 6.3 km long, have an average throw of 120 m, and consistently strike NW‐SE. Throw‐depth plots, accompanied by thickness changes, indicate that the faults accumulated growth strata during the Late Burdigalian and are spatially and kinematically associated with a WSW‐ESE‐striking strike‐slip fault. Unlike true polygonal faults, these faults propagated through ca. 2 km‐thick sandstone‐prone Oligocene‐Miocene strata. Whereas previous studies from the Northern Levant Basin associate fault nucleation and growth with burial‐related diagenesis, the sandstone‐prone character of the Oligocene‐Miocene suggests that this process cannot be readily applied to the Southern Levant Basin. Instead, we highlight potential tectonic events that occurred during and may have triggered thin‐skinned extension at times of fault growth.

The low-angle breakaway system for the Northern Snake Range décollement in the Schell Creek and Duck Creek Ranges, eastern Nevada, USA: Implications for displacement magnitude

AbstractDocumenting the kinematics of detachment faults can provide fundamental insights into the ways in which the lithosphere evolves during high-magnitude extension. Although it has been investigated for 70 yr, the displacement magnitude on the Northern Snake Range décollement in eastern Nevada remains vigorously debated, with published estimates ranging between <10 and 60 km. To provide constraints on displacement on the Northern Snake Range décollement, we present retrodeformed cross sections across the west-adjacent Schell Creek and Duck Creek Ranges, which expose a system of low-angle faults that have previously been mapped as thrust faults. We reinterpret this fault system as the extensional Schell Creek Range detachment system, which is a stacked series of top-down-to-the-ESE brittle normal faults with 5°–10° stratigraphic cutoff angles that carry 0.1–0.5-km-thick sheets that are up to 8–13 km long. The western portion of the Schell Creek Range detachment system accomplished ~5 km of structural attenuation and is folded across an antiformal culmination that progressively grew during extension. Restoration using an Eocene unconformity as a paleohorizontal marker indicates that faults of the Schell Creek Range detachment system were active at ~5°–10°E dips. The Schell Creek Range detachment system accommodated 36 km of displacement via repeated excision, which is bracketed between ca. 36.5 and 26.1 Ma by published geochronology. Based on their spatial proximity, compatible displacement sense, overlapping deformation timing, and the similar stratigraphic levels to which these faults root, we propose that the Schell Creek Range detachment system represents the western breakaway system for the Northern Snake Range décollement. Debates over the pre-extensional geometry of the Northern Snake Range décollement hinder an accurate cumulative extension estimate, but our reconstruction shows that the Schell Creek Range detachment system fed at least 36 km of displacement eastward into the Northern Snake Range décollement.

The Sedimentary Record of Paleogene Sequences in Sarawak, Malaysia

In this paper, we compare the oldest Tertiary sedimentary sequences in the south ern margin of the South China Sea based on outcrop and well data along the Sarawak margin, northern Borneo. Paleogene rocks in Sarawak are present in three tectono stratigraphic zones and represent three depositional settings. Outcrops and deep exploration wells in the Miri Zone indicate shelfal clastics, carbonates, and clay dominated neritic sediments. In the Sibu Zone (Rajang Fold Thrust Belt), Late Cretaceous to Late Eocene deep marine clastic sediments indicate a shallowing upward of the depositional basin, which was later buried to great depths (?) and metamorphosed. In the Kuching Zone, the Kayan and Plat eau sandstones represent a fluvial dominated non marine depositional setting. There are two major unconformities within the Paleogene of Sarawak: the Rajang Unconformity, dated as approximatively 37 Ma, and the younger near Top Eocene (a.k.a. Base Oligocen e) unconformity of 33.7 Ma. The likely presence of Eocene strata in the margins of Sundaland is associated with an early phase of regional extensional tectonism, which was also observed in the Penyu and Malay basins, offshore Peninsular Malaysia, and is probably related to the onset of rifting of the South China Sea continental crust.

Palynostratigraphy and lithostratigraphy of Upper Cretaceous and Paleogene outcrop sections, Mérida Andes (Maracaibo Basin), Western Venezuela

Detailed palynologic and lithostratigraphic studies of outcrop sections on the western flank of the Mérida Andes in Western Venezuela were undertaken to establish a framework for interpreting and correlating regional and local unconformities from the outcrop to subsurface data. The Upper Cretaceous to lower Paleogene section of the Maracaibo Basin consists of laterally continuous rock units attributed to the Mito Juan and Barco/Catatumbo formations, which are characterized by marine sediments deposited relatively close to shore. The continuity of plant and dinoflagellate species across the Cretaceous-Paleogene boundary in this area may have potential significance to K/Pg studies elsewhere. The lower to upper Paleogene Los Cuervos, Mirador, and Carbonera formations were predominantly deposited in non-marine environments, though palynologic and stratigraphic data from each unit show the variable influence of marine and lacustrine conditions. A new chronostratigraphic framework for the study area (the “Venezuelan Composite Standard”) was developed based on first and last appearances of spores, pollen, and dinoflagellates, and three pollen acme events and calibrated to the latest time scale. Graphic correlation plots of the most complete outcrop sections at Río Lobaterita, Río Lora, and Río Chama indicate the presence of significant hiatuses in time, due to erosion and non-deposition (or a combination of the two) reflecting the complex structural and depositional history of the southeastern Maracaibo Basin. A regionally significant upper Paleocene to lower Eocene unconformity, as well as several local and regional intra middle Eocene unconformities appear to have been caused by a drop in base level (sea level plus structural uplift) forming incised valleys over much of the study area. Unconformities are also present in the upper middle Eocene to Oligocene and represent ravinement surfaces caused by transgressive marine reworking of pre-existing units. These surfaces are often marked by the presence of dinoflagellates within the upper Eocene Carbonera Formation. Depositional environments during much of the Upper Cretaceous to Paleogene succession vary from fluvial-floodplain to marginal marine, with newly noted periods of widespread lacustrine deposition in all geologic units. Temporal differences between similar Paleogene lithostratigraphic units in northeastern Colombia and southwestern Venezuela are likely due to the influence and timing of structural events on regional and local depositional environments.

Palaeogene stratigraphy and chronology of the western Sivas Basin, central Anatolia (Turkey): Tectono‐sedimentary evolution of a well‐preserved basin along the northern Neotethys suture zone

AbstractThe Sivas Basin overlaps the northern Neotethys suture zone in central Anatolia (Turkey) and contains a ca. 12‐km thick succession of Cenozoic strata that provides an exceptional record of major tectonic transitions from subduction to continental assembly and tectonic escape. Consensus regarding the tectonic and palaeogeographic evolution of this segment of the Arabia‐Eurasia collision zone has been hindered by uncertainty in the tectonic setting of the early (Palaeogene) Sivas Basin, which has been variably interpreted as a remnant ocean, forearc, foreland, wedge‐top or intracontinental extensional basin. Here we integrate new geologic mapping (>5,000 km2), stratigraphy, facies analysis and U‐Pb geochronology from the western Sivas Basin to produce a detailed time‐stratigraphic framework and a refined Palaeogene tectono‐sedimentary model for the Sivas Basin. Earliest marine sedimentation occurred during the Maastrichtian‐Palaeocene in a remnant ocean setting due to incomplete closure of the northern Neotethys Ocean. Rapid accumulation of >2 km of basin floor turbidites during the middle to late Eocene reflect a transition to a foreland setting. A major late Eocene unconformity (ca. 38–34 Ma) coincides with the initiation of the north‐vergent southern Sivas fold‐thrust belt, which inverted and structurally partitioned the basin into a southern wedge‐top and a northern evaporitic foreland basin; the entire basin was dominantly nonmarine after this unconformity. Published palaeomagnetic data integrated with a newly discovered tuff dated at 31.9 Ma reveal that the 2.8‐km thick Altinyayla Formation was deposited from ca. 34 to 26 Ma in an unconfined braided fluvial system. Close stratigraphic correlations between the western, central and northern Sivas Basin suggest that the entire basin evolved from a remnant ocean to peripheral retro‐foreland basin during the middle to late Eocene, and finally a fragmented foreland basin since the Oligocene. This general evolution may be a characteristic of such sedimentary basins formed on suture zones across Anatolia and in other collisional orogens globally.

Cased-Hole Solution Assesses Tight Reservoirs

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 197423, “Cased-Hole Solution With Fast Neutrons and Sonic Reflective Waves in Tight Reservoirs,” by Ulises Bustos, Schlumberger, Carelis Moya, Equion Energia, and David Rose, SPE, Schlumberger, et al., prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed. The complete paper presents a solution that assesses tight matrices and natural fractures at a level not previously achieved. At the tight-matrix level, advanced nuclear spectroscopy is carried out with a new pulsed-neutron device that achieves simultaneous time- and energy-domain measurements. A new resistivity- and salinity-independent methodology is presented for obtaining gas saturation by a measurement known as fast neutron cross section (FNXS), oil saturation from the total-organic-carbon log, mineral volumes solved from formation elemental concentrations from the energy domain, and porosity from the hydrogen index obtained from the spectroscopy time domain. Reservoir Challenges The reservoirs under analysis belong to the Colombian Pauto Complex field (Fig. 1). Wells drilled in the area have confirmed a staked structure architecture, in which three main thrust sheets can be differentiated: Miche, Guamalera, and Pauto Main. The present study was focused on the Mirador formation of the Pauto Complex, which can be described as a fluvial and shallow marine environment deposit system. The Mirador formation is divided into two zones by Middle Eocene unconformity and is almost totally composed of quartz grains and mainly cemented by silica and significant kaolinite content. An additional characteristic of this reservoir is the presence of natural fractures that contribute strongly to well productivity. The main production is from the Mirador formation and is characterized by highly complex heterogeneity and anisotropy because of a multidiagenetic process, the accumulated hydrocarbon of which is a gas rich in condensate with variations in composition. The reservoirs are located at average depths of approximately 17,000 ft in the eastern foothills of the Colombian Andes, a region characterized by high-tectonic-stress regimes and reservoirs intercalated with reactive shales. Those factors lead to challenging drilling conditions where the combination of oil-based-mud systems and small wellbore diameters (approximately 6 in.) at the target zones are the only way to minimize wellbore-stability problems. Data acquisition is also challenged because logging-while-drilling technologies are sometimes limited in small wellbores and openhole wireline logging is sometimes jeopardized by hostile hole conditions. Under these scenarios, the authors explored formation evaluation in cased-hole conditions with wireline-conveyed technologies for both matrix and natural-fracture systems. The described spectroscopy device is wireline-conveyed and consists of a nonradioactive/high-energy pulsed-neutron generator and a highly sensitive set of multidetectors equipped with advanced electronics for carrying out complex neutron-burst schemes for accurate gamma-ray spectral analysis. This achieves a broad mineral-spectrum analysis, highly significant for two key formation evaluation parameters, porosity and saturation. The porosity typically is derived from neutron measurements incorporated in the same tool, where the matrix components over the interest zones provided by this spectroscopy device ensure an accurate porosity computation. The technology provides gas saturation from a hydrogen-index-independent gas-detection device and the liquids/hydrocarbon fraction from the carbon measurement; other rock elements containing carbon also are considered for a precise saturation computation.

Episodic out-of-sequence deformation promoted by Cenozoic fault reactivation in NW Argentina

The Central Andes are the product of contractional deformation related to subduction of the Nazca plate beneath the South American continent. In this geodynamic setting, thrust belt propagation in the Central Andean retroarc in NW Argentina and southern Bolivia has been linked with the critical wedge model implying progressive eastward migration of the deformation front during the Cenozoic. However, Eocene unconformities and growth strata within the orogenic hinterland of the Eastern Cordillera and Puna Plateau suggest selective reactivation of pre-Andean and Andean faults revealing a pattern of diachronous and spatially disparate range uplifts. The Calchaquí Valley in NW Argentina is characterized by basin-and-range morphology, which is intimately linked with the heterogeneous Neoproterozoic basement, Cretaceous rift structures, and the Cenozoic foreland. In this study, we analyze this complex deformation by reconstructing Cenozoic fault reactivation and sedimentation patterns in the Eastern Cordillera. Detailed structural mapping along the eastern Calchaquí border (Tonco area) reveals five Cenozoic unconformities in the former foreland deposits related to multiple episodes of fault reactivation during the Eocene-to-Pleistocene. A refined chronostratigraphy (U-Pb zircon), new AHe thermochronological data (apatite U-Th-Sm/He), and existing evidence for deformation indicate episodic out-of-sequence deformation and increased Eo-Oligocene (Puna Plateau) to Miocene (Eastern Cordillera) fault displacement apparently controlled by pre-Cenozoic heterogeneities. The coeval eastward propagation of deformation and continuing deformation in the west generally challenge the applicability of the critical wedge model.

Tectono-Sedimentary Evolution of the Offshore Hydrocarbon Exploration Block 5, East Africa: Implication for Hydrocarbon Generation and Migration

Sedimentary deposits in Block 5, offshore Tanzania basin have been imaged using two-dimensional (2D) seismic data. The seismic data and well data reveal four tectonic units representing different tectonic events in relation to structural styles, sedimentation and hydrocarbon potential evolved in Block 5. Results show that during Early to Late Jurassic, Block 5 was affected by the break-up of Gondwana and the drifting of Madagascar as evidenced by patterns of sediments and structural features. The chaotic and discontinuous reflectors are characteristics features on the sediments pattern indicating a possible transitional setting following the breakup of Gondwana. From the Late Cretaceous, Block 5 sits in more stable subsiding sag as the consequence of the high thermal subsidence. The period displayed continuous parallel reflectors with few markable faults. This was followed by the late post rift sedimentation that occurred after Middle Eocene Unconformity characterized by high wavy and sub parallel reflectors. The evolution of Block 5 through major tectonic events reveals a more complete petroleum system towards the south. Thus, Block 5 responded in both space and time to a complex interplay between tectonics and sedimentation. This indicates that structural styles and associated features are potential control for hydrocarbon generation and migration.

A rift-to-drift record of vertical crustal motions in the Faroe–Shetland Basin, NW European margin: establishing constraints on NE Atlantic evolution

The Upper Paleocene–Eocene rock record in the Faroe–Shetland Basin is punctuated by a series of unconformities that reflect a persistent tectonic instability throughout the syn- to early post-breakup period, a duration of about 20 myr. A particular focus is on a Late Paleocene subaerial unconformity, herein termed the Flett unconformity, which has been argued to have formed in response to a transient pulse of mantle convective uplift associated with a proto-Iceland plume. However, ambiguity over its presumed correlation with the Faroe Islands Basalt Group combined with stratigraphic and palaeogeographical analysis of the Upper Paleocene–Eocene succession indicates that it is just one of a series of subaerial unconformities that are spatially linked to the same part of the Faroe–Shetland Basin. Most of these Late Paleocene–Eocene unconformities post-date volcanism, and their formation coincides with vertical motions associated with phases of uplift, inversion and compressional deformation linked to the growth of structures, such as the Wyville Thomson and Munkagrunnur ridges, and the Judd Anticline. These deformation phases are broadly coeval with intraplate and plate-boundary events in the wider NE Atlantic region. The possibility that the Flett unconformity had a similar tectonic origin should not be discounted.

Seismic characteristics of the onshore Abrolhos magmatism, East-Brazilian continental margin

A 3D seismic volume was interpreted in order to study the extension of the Abrolhos magmatism in the onshore portion of the Mucuri Basin, East-Brazilian continental margin. The effusive rocks were divided into two volcanic-sedimentary cycles that encompasses at least three igneous pulses, which were recognized in the well analyses. The seismic boundary between the volcanic-sedimentary cycles is marked by a strong negative amplitude reflector associated to the Lower Eocene Unconformity surface. Diffuse features similar to hydrothermal vents were also described, which would have been responsible for paleo-seepages in the marine Eocene sediments. The paleo-surface coincident with the top of these structures is directly related to the top of the first cycle of the onshore Abrolhos magmatism and, consequently, to the Lower Eocene Unconformity. The second volcanic-sedimentary cycle onlaps onto the mounds of the previews cycle and is limited at the top by the Pre-Upper Eocene Unconformity. The seismic analyses presented herein suggest that the relationship between the Cenozoic magmatism, associated to the Abrolhos Formation, and the Paleogene-Neogene sequences would not be restricted only to the offshore portion of the basin, but would also have related deposits in the onshore portion of the Mucuri Basin.

Evolution of the outer basin high, Ceduna Sub-basin, southern Australia

Recent exploration in the Bight Basin has identified an uplifted area on the outer margin of the Ceduna Shelf, known as the outer basin high. The Tiger Supersequence, a potential source rock, thins onto the uplifted area, so the timing and extent of uplift has implications for petroleum system maturity. The outer basin high is 15–40 km wide and extends along the south-western flank of the Ceduna Sub-basin, beneath the outer flank of the Ceduna Terrace. Relative uplift of greater than 2000 m occurs within the area. The outer basin underlies a transitional zone of deformation between the extensional faulting and outer fold and thrust regions of the overlying White Pointer and Hammerhead delta systems. Detailed mapping using the recently released Ceduna 3D seismic dataset reveals two main episodes of relative uplift, in the Santonian and again in the Maastrichtian to Eocene. The first phase of relative uplift is associated with thinning of the Tiger Supersequence onto the high. The second phase is identified as a decrease in thickness of the Hammerhead Supersequence overlying the high. The exact timing of this uplift is difficult to constrain due to truncation of the Hammerhead Supersequence by the basin-wide Eocene unconformity, however, estimates of the amount of uplift are based on seismic mapping.

Origin of the Mt Ashmore structural dome, west Bonaparte Basin, Timor Sea

New insights into the 3D structure, composition and origin of the Mt Ashmore dome, west Bonaparte Basin, Timor Sea, are enabled by reprocessed seismic-reflection data and by optical microscopic, X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive spectrometry (EDS) and transmission electron microscopy (TEM) analyses of drill cuttings. The structural dome, located below a major pre-Oligocene post-Late Eocene unconformity and above a ∼6 km-deep-seated basement high indicated by marked gravity and magnetic anomalies, displays chaotic deformation at its core and a centripetal kinematic deformation pattern. A study of drill cuttings of Lower Oligocene to Lower Jurassic sedimentary rocks intersected by the Mt Ashmore 1B petroleum-exploration well reveals microbrecciation and extreme comminution and flow-textured fluidisation of altered sedimentary material. The microbreccia is dominated by aggregates of poorly diffracting micrometre to tens of micrometres-scale to sub-millimetre particles, including relic subplanar fractured quartz grains, carbonate, barite, apatite and K-feldspar. A similar assemblage occurs in fragments in basal Oligocene sediments, probably derived from the eroded top section of the dome, which protrudes above the unconformity. SEM coupled with EDS show the micrometre to tens of micrometres-scale particles are characterised by very low totals and non-stoichiometric compositions, including particles dominated by Si, Al–Si, Si–Ca–Al, Si–Al–Ca, Si–Mg, Fe–Mg–Ca, Fe–Mg and carbonate. XRD analysis identifies a high proportion of amorphous poorly diffracting material. TEM indicates internally heterogeneous, fragmented and recrystallised structure of the amorphous grains, which accounts for the low totals in terms of the high-volatile and porous nature of the particles. Another factor for the low totals is the uneven thin-section surfaces which affect the totals. No volcanic material or evaporites were encountered in the drillcore, militating against interpretations of the structure in terms of magmatic intrusion or salt diapirism. Such models are also inconsistent with the strong gravity and magnetic anomalies, which signify a basement high below the dome. An interpretation of the dome in terms of a central rebound uplift of an impact structure can not be proven due to the lack of shock metamorphic effects such as planar deformation features, impact melt or coesite. However, an impact model is consistent with the chaotic structure of the domal core, centripetal sense of deformation, microbrecciation, comminution and fluidisation of the Triassic to Eocene rocks. In this respect, an analogy can be drawn between the Mt Ashmore structural dome and likely but unproven impact structures formed in volatile (H2O, CO2)-rich sediments where shock is attenuated by high volatile pressure, such as Upheaval Dome, Utah. In terms of an impact hypothesis the Mt Ashmore dome is contemporaneous with a Late Eocene impact cluster (Popigai: D = 100 km, 35.7 ± 0.2 Ma; Chesapeake Bay: D = 85 km, 35.3 ± 0.1 Ma).

Large submarine slides on a steep continental margin (Camamu Basin, NE Brazil)

Abstract: We describe a set of thin-skinned structures from the offshore Camamu Basin. The margin is about 40 km wide and the sea floor has a slope of up to 10%. The sedimentary cover is up to 7 km thick. Neocomian strata accumulated during continental rifting. Aptian strata consist mainly of coarse clastic sediment and evaporite. Late Cretaceous strata are thin or absent and the Tertiary succession is about 1 km thick, above a prominent Eocene unconformity. The Neocomian and Aptian sequences contain abundant thin-skinned structures, which are extensional near the continental shelf and compressional toward the toe of the slope. The structures have detached on Aptian evaporite and on the base of Neocomian shale. One giant slide is about 5 km thick and 60 km wide. It displays mirror symmetry about a vertical plane perpendicular to the margin, as occurs in physical models. The main phase of sliding occurred between the Campanian and the middle Eocene. The trigger may have been a regional phase of uplift and exhumation, for which there is independent evidence on the shelf and inland.

Eocene climate record of a high southern latitude continental shelf: Seymour Island, Antarctica

A high-resolution record of Eocene paleotemperature variation is preserved within the high southern latitude, marine shelf succession of the La Meseta Formation on Seymour Island, off the NE side of the Antarctic Peninsula. 87 Sr/ 86 Sr ratios of bivalve shell carbonate indicate that the La Meseta Formation spans virtually the entire Eocene, and suggest the presence of an early middle Eocene unconformity. Paleoclimatic and paleoceanographic inferences are based on the stable oxygen and carbon isotope values of two genera of bivalves collected with a high degree of stratigraphic resolution within the formation, and with multiple replicate samples from each horizon. δ 18 O data indicate roughly 10 °C of cooling from the early Eocene climatic optimum (~15 °C) through the end of the Eocene (minimum ~5 °C), much of which took place in two comparatively short intervals at ca. 52 and ca. 41 Ma. Many features of the isotope curves generated from this Eocene shelf section are apparent in δ 18 O and δ 13 C data from the Southern and global oceans, including warm intervals that likely correspond to the early and middle Eocene climatic optima (EECO and MECO). A rapid middle Eocene shift to much more positive values is the most signifi cant in the section and refl ects a drop to universally cooler temperatures in the late middle and late Eocene that might also be associated with a short-lived glacial advance. However, even using a somewhat depleted value for δ 18 O of seawater in the Antarctic peninsular region, average Seymour Island shelf-water paleotemperatures did not reach freezing before the end of the Eocene. δ 13 C data similarly refl ect the documented middle Eocene surface ocean enrichment followed by more negative values, but depletion is much more pronounced on Seymour Island and persists for the remainder of the Eocene, suggesting a combination of upwelling, metabolic effects, and/or atypical carbon cycling on the shelf in this region. Isotope data capture information about changes in the paleoenvironment that also had consequences for the biota, as published paleontological records document marked change in the nature of terrestrial and marine biota at this time. The fact that middle Eocene cooling and biotic turnover in the Peninsular region correspond well in time to the proposed initial opening of Drake Passage suggests that the formation of gateways, in addition to changes in pCO 2 , had signifi cant consequences for the Earth’s climate system during the Paleogene.

Multiphase structural evolution of the western margin of the Girardot subbasin, Upper Magdalena Valley, Colombia

More than 1400 km of two-dimensional seismic data were used to understand the geometries and structural evolution along the western margin of the Girardot Basin in the Upper Magdalena Valley. Horizons are calibrated against 50 wells and surface geological data (450 km of traverses). At the surface, low-angle dipping Miocene strata cover the central and eastern margins. The western margin is dominated by a series of en echelon synclines that expose Cretaceous–Oligocene strata. Most synclines are NNE–NE trending, whereas bounding thrusts are mainly NS oriented. Syncline margins are associated mostly with west-verging fold belts. These thrusts started deformation as early as the Eocene but were moderately to strongly reactivated during the Andean phase. The Girardot Basin fill records at least four stratigraphic sequences limited by unconformities. Several periods of structural deformation and uplifting and subsidence have affected the area. An early Tertiary deformation event is truncated by an Eocene unconformity along the western margin of the Girardot Basin. An Early Oligocene–Early Miocene folding and faulting event underlies the Miocene unconformity along the northern and eastern margin of the Girardot Basin. Finally, the Late Miocene–Pliocene Andean deformation folds and erodes the strata along the margins of the basin against the Central and Eastern Cordilleras.

Kenn Plateau off northeast Australia: a continental fragment in the southwest Pacific jigsaw

The submarine Kenn Plateau, with an area of about 140 000 km2, lies some 400 km east of central Queensland beyond the Marion Plateau. It is one of several thinned continental fragments east of Australia that were once part of Australia, and it originally fitted south of the Marion Plateau and as far south as Brisbane. It is cut into smaller blocks by east- and northeast-trending faults, with thinly sedimented basement highs separated by basins containing several kilometres of sediment. In the Cretaceous precursor of the Kenn Plateau, Late Triassic to Late Cretaceous basins probably rested unconformably on Palaeozoic to Triassic rocks of the New England Fold Belt. Rift volcanism was common on the northern plateau and was probably of Early Cretaceous age. Late Cretaceous extension and breakup were followed by Paleocene drifting, and the Kenn Plateau moved to the northeast, rotated 30° anticlockwise and left space that was filled by Tasman Basin oceanic basalts. During these events, siliciclastic sediments poured into the basins from the continental mainland and from locally eroding highs. After a regional Late Paleocene to Early Eocene unconformity, siliciclastic sedimentation resumed in proximal areas. In deep water, radiolarian chalks were widely deposited until biosiliceous sediment accumulation ended at the regional Late Eocene to Early Oligocene unconformity, and warming surface waters led to accumulation of pure biogenic carbonates. Calcarenite formed in shallow water on the margins of the subsiding plateau from the Middle Eocene onward. Some seismic profiles show Middle to Late Eocene compression related to New Caledonian obduction to the east. Hotspots formed parts of two volcanic chains on or near the plateau as it moved northward: Late Eocene and younger volcanics of the Tasmantid chain in the west, and Late Oligocene and younger volcanics of the Lord Howe chain in the east. As the volcanoes subsided, they were fringed by reefs, some of which have persisted until the present day. Other reefs have not kept up with subsidence, so guyots formed. The plateau has subsided 2000 m or more since breakup and is now subject solely to pelagic carbonate sedimentation.

Palinología e icnología en la determinación del contacto discordante Cretácico-Terciario en Venezuela suroccidental y su impacto en la exploración por hidrocarburos

Palynology and ichnology in the determination of Cretaceous-Tertiary unconformity contact in southwestern Venezuela and their impact in hidrocarbons exploration. Palynological results from the southwest of Venezuela are given. The material comes from two wells drilled in Barinas state. Integration of results from analyses of marine and terrestrial palynomorphs, and ichnofacies, allows to determine the Maastrichtian-middle Eocene unconformity in well #1 at 9901´5´´ by the presence of Foveotriletes margaritae (Van der Hammen) Germeraad, Hopping & Muller, peridinioid dinocysts and the Glossifungites ichnofacies. In well # 2, the unconformity was established by the presence of F. margaritae, Ariadnaesporites Potonie and Buttinia andreevi Boltenhagen at 10154´6´´.

Potential Eocene and Oligocene stratigraphic traps of the Rockall Plateau, NE Atlantic Margin

Abstract Following thermal uplift during the late Paleocene to early Eocene, the denudation of the subaerial hinterland provided a massive sediment supply that led to the development of a number of large, prograding sedimentary wedge systems flanking the Hatton and Rockall basins. Regional seismic data mapping and borehole data indicate that the wedges are Eocene in age and have a high percentage of coarse clastic material typical of high-energy, fluvial or near-shore marine environments. The prograding wedges have been mapped and can be viewed as large, clastic fairways within which trapping at a number of scales exists. Seismic interpretation suggests that the wedges are present at various stratigraphic levels within the Eocene and are locally separated by unconformities. However, all pre-date the margin-wide late Eocene unconformity (C30), which resulted in subsidence and deepening of the Rockall and Hatton basins. A marine transgression inundated most former land areas, and a marked change occurred in basinal facies; a change from fluvial/near-shore clastic sedimentation to deep-water mud and ooze deposition influenced by bottom-currents. These conditions persisted throughout most of the Oligocene and Neogene and hence provided a seal for potential hydrocarbon-bearing sand-prone Eocene reservoirs internal to the wedge-systems. Additional sealing potential may be provided by shale layers internal to the wedges. Buried Eocene pinchout lobes, submarine fans at the base of basalt scarp faces and Oligocene slump deposits also provide potential trapping mechanisms. High, and probably unacceptable, risks include biodegradation and poor seal development due to the typically shallow depth of burial of the wedges. However, the majority of the wedges should be treated as analogues, with some of the deeper examples providing some scope for consideration as exploration targets. The scale of the prograding wedge play fairway is massive, with volumes measured in tens of cubic kilometres.

Kilometre-scale sandstone intrusions in the Eocene of the Outer Moray Firth (UK North Sea): migration paths, reservoirs and potential drilling hazards

Kilometre-scale sandstone intrusions have recently been documented from the Palaeogene deep-water sequences of the northern North Sea. The distribution of Eocene sandstones within a 126 km 2 3D4C seismic survey located in the Outer Moray Firth was mapped using surface and volume based interpretation of converted wave seismic data calibrated to a number of boreholes. About 35 conical sandstone intrusions, a few tens of metres thick and 1–1.5 km wide, were mapped within the lower Eocene. The intrusions are inclined about 15–20° and crosscut some 100–200 m of lower Eocene mudstones, terminating upward at the Middle Eocene unconformity, below the oil-charged, upper Eocene sandstones of the Alba and Chestnut reservoirs. Individual sandstone intrusions may contain millions of cubic metres of sandstone and often have excellent reservoir properties. Previous studies failed to define a viable migration path for the oil in the Alba/Chestnut reservoirs because they appeared to be completely encased in poorly permeable Eocene mudstones. However, detailed mapping and volume visualization of the converted wave seismic data demonstrates that the upper Eocene reservoirs have suffered post-depositional remobilization and that the conical intrusions provide a viable migration path between the Alba/Chestnut reservoirs and the underlying Paleocene aquifers. Despite more than two decades of exploration activity in the Outer Moray Firth, the widespread occurrence of conical intrusions in the lower Eocene is still not widely recognized, and it is likely that the sandstones may pose a hazard when drilling for deeper targets. However, it is also possible that large conical intrusions that do not connect with overlying sandstones may contain economical quantities of hydrocarbons. Supplementary material: The movie referred to in the article is available at https://doi.org/10.6084/m9.figshare.c.4812948