Oxfordian reef architecture of the La Manga Formation, Neuquén Basin, Mendoza Province, Argentina

The Neuquén back-arc basin is located on the west margin of the South American platform between latitudes 36° and 40° S. The basin is famous for its continuous sedimentary record from the Late Triassic to Cenozoic comprising continental and marine clastic, carbonate, and evaporitic deposits up to 2.600 m in thickness.The stratigraphical and paleontological studies of the outcrops of the La Manga Formation, Argentina, located near the Bardas Blancas region, Mendoza province (35° S and 69° O) allow the reconstruction of the sedimentary environments of an Oxfordian carbonate ramp, where outer ramp, middle ramp, inner ramp (oolitic shoal), inner ramp margin (patch reef) lagoon and paleokarst were differentiated. The reefs consist of back reef facies and in situ framework of coral boundstones that was formed at the top of shallowing-upward succession.Coral reefs were analyzed by defining coral colonies shapes, paleontological content, coral diversity and taphonomy studies. In some studied sections abundant fragments of gryphaeids, encrusting bryozoans, and isolated sponges provided a suitable substrate for coral colonization; however, other sections show an increase in the proportions of ooids, peloidal and coral intraclasts.The core reef facies is composed of white-grey unstratified and low diversity scleractinian coral limestone dominated by robust and thinly branching corals with cerioid–phocoid growths and massive coral colonies with meandroid–thamnasteroid growth forms.The assemblage is characterized by Actinastraea sp., Australoseris sp., Thamnasteria sp. and Garateastrea sp. Internal facies organization and different types of coral colonies allow to recognize the development of varying framework as well as intercolony areas. A superstratal growth fabric characterizes the coral assemblage. On the basis of coral growth fabric (branche and domal types), the reef of La Manga Formation is considered a typical mixstones. The intercolony areas consist of biomicrites and biomicrorudites containing abundant coral fragments, parautochthonous gryphaeids and another bivalves (Ctenostreon sp.), gastropods (Harpagodes sp., Natica sp.), echinoderms test and spines (Plegiocidaris sp.), miliolids, Cayeuxia sp., Acicularia sp., Salpingoporella sp., intraclasts, ooids, peloids and coated grains.The domal growth forms are probably more protected against biological and physical destruction, meanwhile delicate branching growth forms with very open and fragile framework were more affected and fragmented due to wave action and bioerosion.The reef fabric shows different intervals of truncation as consequence of erosion resulting from coral destruction by storm waves or currents. The maximum flooding surface separates oolitic shoal facies below from the aggradational and progradational coralline limestones facies above. Subsequent sea-level fall and karstification (148 Ma) affected reef and oolitic facies.     

From Alluvial Fan to Playa: An Upper Jurassic Ephemeral Fluvial System, Neuquén Basin, Argentina

The Upper Jurassic fill of the back-arc Neuquén Basin includes a lowstand wedge known as the Tordillo Formation. The studied deposits crop out along a N-S oriented belt that runs parallel to the Andean magmatic arc. They are limited to the south by the east-west oriented positive structure of the Huincul arch formed as a result of Upper Jurassic tectonic inversion. The Tordillo deposits were formed in an arid fluvial-dominated system characterised by systematic downstream changes in architectural style. A gravelly and sandy bedload fluvial system is recognised in the southern upstream sector. The reduced thickness and the coarse grain size suggest steep gradients, excess of bedload supply and a low subsidence rate. Thicker and finer-grained deposits prevail to the north and northeast. They were formed under arid conditions in a wadi-sand flat-playa fluvial system. This distal facies association indicates increased accommodation owing to high rates of subsidence relative to coarse siliciclastic sedimentation rates. These low-gradient deposits are characterised by cyclic alternations of mud-dominated and sand-dominated packages interpreted as high- and low-accommodation systems tracts. The overall fining upward stacking pattern of the Tordillo Formation suggests a change towards higher accommodation rates. This is accompanied by frequent development of soil horizons and darker primary and reworked pyroclastic deposits. These attributes indicate a stronger explosive volcanic activity associated with increased precipitation and high water table emplacement towards the end of the Tordillo lowstand wedge.     

The Upper Jurassic (Kimmeridgian) fluvial–aeolian systems of the southern Neuquén Basin, Argentina

The Kimmeridgian Quebrada del Sapo Formation in the southernmost Neuquén Basin in Argentina represents a succession up to 40 m thick of coarse- to fine-grained fluvial deposits overlain by aeolian deposits. These fluvial–aeolian deposits reflect a significant palaeogeographic change in the basin and are related to a major, tectonically enhanced, relative sea-level fall. The fluvial section is dominated by braided-channel, fine-grained ephemeral, and sheetflood deposits. Aeolian facies are dominated by dune deposits, with minor sandsheet and interdune units. Changes in the nature of both fluvial and aeolian sedimentation within the studied area suggest a regional variability of accommodation/sediment supply conditions. The regional changes of the aeolian succession likely reflect different relative positions within a major erg. In the upwind margin of the erg, a shallow water table promoted water-lain sedimentation in interdune areas, whereas in the central parts of the erg, dry sediment accumulation took place above the regional water-table level. The vertical transition observed in the Quebrada del Sapo Formation, from fluvial to aeolian deposits, may be the result of a local climatic change to drier conditions due to the development of a climatic barrier imposed by growth of a magmatic arc to the west. Alternatively, the vertical transition could be related to a lowering of the water table associated with the compartmentalization of the basin during a period of low sea level.     

Architecture and development of the alluvial sediments of the Upper Jurassic Tordillo Formation in the Cañada Ancha Valley, northern Neuquén Basin, Argentina

The Upper Jurassic Tordillo Formation at Cañada Ancha area, northern Neuquén Basin, Argentina, comprises a multi-stage suit of predominantly alluvial sediments that is heterolithic in nature. In that suit, several lithofacies, architectural elements, and bounding surfaces of different order have been identified and their lateral and vertical distribution characterized. This analysis allowed the differentiation of 3 main units (lower, middle and upper), 20 subunits (C-1 to C-20), and the characterization of their alluvial styles.The lower unit (which comprises subunits C-1 to C-4) is mainly formed by fine- to medium-grained sandstones, which become medium- to coarse-grained towards the top. These sandstones characterize settings ranging from floodplains with isolated, unconfined flows, to more complex, vertically stacked, multi-storey sheet sandstones of braided fluvial systems. The middle unit (C-5 to C-10) is dominated by pale brown-grey fine-to coarse-grained sands and medium size subangular to angular conglomerates, which reflect amalgamated complexes of sandstone sheets and downstream accretion macroforms. Remarkably, this alluvial sedimentation was episodically punctuated by volcaniclastic flows. The upper unit (C-11 to C-20) consists of finer sediments, mainly pink to white fine-to medium grained sandstones and red to green siltstones. Towards the top, bioturbation becomes important, and also the presence of volcanosedimentary flows is noticeable. Fluvial settings include braided sheet sandstones with waning flood deposits evolving to isolated high-sinuosity fluvial systems, with flash flood deposits. At the top of this unit, facies may suggest marine influence.Vertical changes in the fluvial style result from both climatic and tectonic controls. A semiarid to arid climate and the active tectonism linked to the eastward migration of the Andean volcanic arc determined major bounding surfaces, fluvial style evolution and the presence of the volcano-sedimentary deposits. Different stages of high and low subsidence rates has been deduced from the vertical stacking of sediments.     

Tidally dominated, rimmed-shelf facies of the Picún Leufú Formation (Jurassic/Cretaceous boundary) in southwest Gondwana, Neuquén Basin, Argentina

A palaeoenvironmental model for the Picún Leufú Formation (Jurassic/Cretaceous boundary), which crops out in the Neuquén Basin, Argentina, on the southwestern margin of Gondwana, is presented in this paper for the first time. Detailed stratigraphic sections exposed along National Road 40 where it crosses the Picún Leufú Creek (type locality) and in the Cerrito Caracoles area, were examined and sampled. Based on a combination of the sedimentological data obtained (facies/microfacies analysis) and the relationship between benthic macrofaunas and their taphonomic attributes, it is concluded that the formation reflects a tidally dominated, rimmed-shelf setting characterized by prograding bars dissected by channels and thick lagoonal facies with shoal developments. In the Cerrito Caracoles area, where only the basal part of the formation is exposed, it is interpreted to have been deposited in a shallow subtidal marine environment in which shelf margin facies with patch reefs have been recognized.     

Sharp‐based marine sandstone bodies in the Mulichinco Formation (Lower Cretaceous), Neuquén Basin,Argentina: remnants of transgressive offshore sand ridges

The interpretation of sharp‐based shallow‐marine sandstone bodies encased in offshore mudstones, particularly transgressive units, has been a subject of recent debate. This contribution provides a multiple‐dataset approach and new identification criteria which could help in the recognition of transgressive offshore sandstone bodies worldwide. This study integrates sedimentology, ichnology, taphonomy and palaeoecology of Mulichinco Formation strata in the central Neuquén Basin (Argentina) in order to describe and interpret sharp‐based sandstone bodies developed in ramp‐type marine settings. These bodies are sandwiched between finer‐grained siliciclastics beneath and thin carbonates above. The underlying sediments comprise progradational successions from offshore mudstones to offshore transition muddy sandstones, grading occasionally into lower shoreface sandstones. The surfaces capping the regressive siliciclastics are flat and regionally extensive, and are demarcated by skeletal concentrations and a Glossifungites suite; they are also marked by sandstone rip‐up clasts, with encrustations and borings on all sides. These surfaces are interpreted as composite discontinuities, cut during a relative sea‐level fall and remodelled during the initial transgression. The overlying transgressive sandstone bodies are 3 to 7 m thick, >4 km long and about three times longer than wide; they are composed of fine‐grained sandstones with little lateral change in grain size. Cross‐stratification and/or cross‐lamination are common, typically with smaller‐scale structures and finer grain size towards the top. Large‐scale, low‐angle (5° to 8°) inclined stratification is also common, dipping at ca 30° with respect to body elongation and dominant currents. These sandstone bodies are interpreted as offshore sand ridges, probably developed under the influence of tidal currents. Intense burrowing is typical at the top of each unit, suggesting an abandonment stage. Final deactivation favoured colonization by epibenthic‐dominated communities and the formation of skeletal‐rich limestones during the latest transgressive conditions. As partial reworking of pre‐existing ridges occurred during this stage, the Mulichinco sandstone bodies are considered the remnants of transgressive offshore sand units.     

Lower Cretaceous ammonites from the Neuquén Basin, Argentina: a Hauterivian Olcostephanus fauna

The olcostephanid fauna of the Olcostephanus (O.) laticosta[formerlyO . (O.) leanzai] Subzone (Agrio Formation) represents the last of four discrete invasions of olcostephanid ammonites into the Neuquén Basin of west-central Argentina. Olcostephanus (O.)laticosta (Gerth) dominates the fauna and is known only from the Neuquén Basin. But the co-occurrence of the distinctive subgenus Jeannoticeras, newly recorded from Argentina, provides a link with the ‘standard’ West Mediterranean sequence and indicates that the O. (O.) laticosta Subzone is probably of mid Early Hauterivian age. This is supported by the discovery of two specimens of the widely distributed late Early Hauterivian speciesO . (O.) variegatus (Paquier) in the overlying Hoplitocrioceras giovinei Subzone. O. (Jeannoticeras) agrioensis sp. nov. is described.     

The role of tidal,wave and river currents in the evolution of mixed‐energy deltas: Example from the Lajas Formation (Argentina)

Many modern deltas show complex morphologies and architectures related to the interplay of river, wave and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed‐energy systems and their reservoir heterogeneities. The Lajas Formation, a sand‐rich, shallow‐marine, mixed‐energy deltaic system in the Neuquén Basin of Argentina, previously defined as a tide‐dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross‐stratified, sheet‐like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean‐water (i.e. very little mud is present in the system), sand‐rich deltaic system, very different from the majority of mud‐rich, modern tide‐influenced examples. The sand‐rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal‐current reworking, which produced amalgamated sandstone bodies in the delta‐front area, and a final stratigraphic record very different from the simple coarsening‐upward trends of river‐dominated and wave‐dominated delta fronts.     

Sedimentary architecture of an ancient linear megadune (Barremian,Neuquén Basin): Insights into the long‐term development and evolution of aeolian linear bedforms

Linear aeolian bedforms are the most abundant bedform type in Earth's dune fields, and are very common in the Solar System. Despite their abundance, the long‐term development of these bedforms and its impact upon the resulting sedimentary architecture in the geological record is still poorly understood. The aim of this paper is to study the exposed record of an ancient linear megadune in order to discuss its development and the factors that impact the sedimentary architecture of aeolian linear bedforms. The outcrops of the ancient Troncoso Sand Sea (Barremian, Neuquén Basin, Argentina) provide a unique opportunity to study a preserved megadune record with an external body geometry that confirms its linear morphology. Architectural analysis reveals significant differences in cross‐stratified set bodies and bounding surfaces’ features and allows for the identification of three architectural complexes within the bedform's record. Analysis of deterministic models, sedimentary body relative chronology and distribution suggest that these architectural complexes result from distinctive phases in bedform development. It also clearly shows that construction of the megadune was achieved by expansion from a core, and that its development was characterized by sustained growth and strong longitudinal dynamics, without net accumulation. This study indicates how sustained bedform growth, rather than accretion, can be a critical factor conditioning linear bedform architecture towards a more ‘classic’ (bimodal bounding surface and cross‐bedding dip directions) concentric sedimentary architecture style. Furthermore, this research reveals how this style of architecture could only be relatively common in the geological record when related to bedform topography preservation.     

Recognition criteria,characteristics and implications of the fluvial to marine transition zone in ancient deltaic deposits (Lajas Formation,Argentina)

The seaward end of modern rivers is characterized by the interactions of marine and fluvial processes, a tract known as the fluvial to marine transition zone, which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the fluvial to marine transition zone are preserved in the rock record, large‐scale outcrops of deltaic deposits of the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) have been investigated. Fluvial–tidal indicators consist of cyclically distributed carbonaceous drapes in unidirectional, seaward‐oriented cross‐stratifications, which are interpreted as the result of tidal modulation of the fluvial current in the inner part of the fluvial to marine transition zone. Heterolithic deposits with decimetre‐scale interbedding of coarser‐grained and finer‐grained facies with mixed fluvial and tidal affinities are interpreted to indicate fluvial discharge fluctuations (seasonality) and subordinate tidal influence. Many other potential tidal indicators are argued to be the result of fluvial–tidal interactions with overall fluvial dominance or of purely fluvial processes. No purely tidal or tide‐dominated facies were recognized in the studied deposits. Moreover, fluvial–tidal features are found mainly in deposits interpreted as interflood (forming during low river stage) in distal (delta front) or off‐axis (interdistributary) parts of the system. Along major channel axes, the interpreted fluvial to marine transition zone is mainly represented by the fluvial‐dominated section, whereas little or no tide‐dominated section is identified. The system is interpreted to have been hyposynchronous with a poorly developed turbidity maximum. These conditions and the architectural elements described, including major and minor distributary channels, terminal distributary channels, mouth bars and crevasse mouth bars, are consistent with an interpretation of a fluvial‐dominated, tide‐influenced delta system and with an estimated short backwater length and inferred microtidal conditions. The improved identification of process interactions, and their preservation in ancient fluvial to marine transition zones, is fundamental to refining interpretations of ancient deltaic successions.     

Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): Implications for provenance and tectonic setting

The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, mineralogical composition of sandstones and pelitic rocks, and major- and trace-element geochemistry of sandstones, mudstones, and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic–Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, which was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of the sandstones and conglomerates was primarily the Andean magmatic arc, located west of the Neuquén Basin. The clay mineral assemblage is interpreted as the result of a complex set of factors, including source rock, climate, transport, and diagenesis. Postdepositional processes produced significant variations in the original compositions, especially the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO₂ contents, variable abundances of K₂O and Na₂O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration, is low to moderate. The major element geochemistry and Th/Sc, K/Rb, Co/Th, La/Sc, and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc, and Th, show that most data lie in the active continental margin field. Geochemical information is not sufficiently sensitive to differentiate the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones.     

First U–Pb SHRIMP age of the Hauterivian stage, Neuquén Basin, Argentina

A high-resolution ion-microprobe (SHRIMP) U–Pb zircon age from a tuff layer intercalated in the ammonoid bearing sedimentary succession of the Neuquén Basin in Argentina provides a robust geochronologic date to add to the absolute ages and to improve the relative chronology of the Early Cretaceous Hauterivian stage. The tuff layer appears interbedded between shales of the upper member (Agua de la Mula) of the Agrio Formation within the Spitidiscus riccardii ammonoid zone (base of the Late Hauterivian) yielding a date of 132.5 ± 1.3 Ma. This date confirms and supports an accurate correlation between the ammonoid biostratigraphy of the Neuquén Basin with the Western Mediterranean Province of the Tethys during the Early Cretaceous and matches with the most recently published time scale. It also casts doubts on the validity of K–Ar ages on glauconite-grains recently reported from the Lower Cretaceous of the Vocontian Basin of France.     

A late Jurassic pterosaur (Reptilia, Pterodactyloidea) from northwestern Patagonia, Argentina

A small to medium-sized pterodactyloid pterosaur (wingspan approximately 1.10 m) from the Upper Jurassic (middle-late Tithonian) marine deposits of the Vaca Muerta Formation of Patagonia (Los Catutos area, central Neuquén Province, Argentina) is reported. The specimen lacks the skull but constitutes a nearly complete postcranial skeleton, which includes cervical and dorsal vertebrae; a few thoracic ribs; both pectoral girdles; the left pelvic girdle; a proximal right wing (humerus, ulna, and radius) and metacarpal IV; a left wing that lacks only wing phalanx four; and both hindlimbs, the right one without the foot. Ontogenetic features suggest that the new fossil corresponds to a relatively mature individual, probably a subadult. Observed characters support its assignment to the Archaeopteroactyloidea, a basal clade within the Pterodactyloidea. This specimen is the second pterosaur from Los Catutos and the most complete Jurassic pterosaur so far known from South America.     

River‐dominated,shelf‐edge deltas: delivery of sand across the shelf break in the absence of slope incision

Shelf‐edge deltas record the potential magnitude of sediment delivery from shallow water shelf into deep water slope and basin floor and, if un‐incised, represent the main increment of shelf‐margin growth into the basin, for that period. The three‐dimensional complexity of shelf‐edge delta systems and along‐strike variability at the shelf edge in particular, remains understudied. The Permian–Triassic Kookfontein Formation of the Tanqua Karoo Basin, South Africa, offers extensive three‐dimensional exposure (>100 km²) and therefore a unique opportunity to evaluate shelf‐edge strata from an outcrop perspective. Analysis of stratal geometry and facies distribution from 52 measured and correlated stratigraphic sections show the following: (i) In outer‐shelf areas, parasequences are characterized by undeformed, river‐dominated, storm‐wave influenced delta mouth‐bar sandstones interbedded with packages showing evidence of syn‐depositional deformation. The amount and intensity of soft‐sediment deformation increases significantly towards the shelf edge where slump units and debris flows sourced from collapsed mouth‐bar packages transport material down slope. (ii) On the upper slope, mouth‐bar and delta‐front sandstones pinch out within 2 km of the shelf break and most slump and debris flow units pinch out within 4 km of the shelf break. (iii) Further down the slope, parasequences consist of finer‐grained turbidites, characterized by interbedded, thin tabular siltstones and sandstones. The results highlight that river‐dominated, shelf‐edge deltas transport large volumes of sand to the upper slope, even when major shelf‐edge incisions are absent. In this case, transport to the upper slope through slumping, debris flows and un‐channellized low density turbidites is distributed evenly along strike.     

Notes on fossil remains from the Early Cretaceous Lohan Cura Formation, Neuquén Province, Argentina

In this contribution, we describe an association of fossil remains collected in the locality of Cerro de Los Leones, approximately 8 km southwest of the town of Picún Leufú, Neuquén Province, Argentina, from different levels of the Albian Cullín Grande Member, Lohan Cura Formation. The fossils include pelecypodan (Neocorbicula dinosauriorum, Neocorbicula pehuenchensis, and genus indet.) and gastropodan (Physa wichmanni) molluscs, a mesoeucrocodylian, possible pterosaurs, theropods, and sauropods. The sedimentary environment at Cerro de los Leones is characterized by fluvial deposits corresponding to a sandy meandering fluvial system. The most fossiliferous levels (located in the lower part of the exposed sequence) were developed in a distal flood plain and flood basin areas. Sedimentological and paleopedological features suggest semi-arid conditions during the deposition of the Cullín Grande Member. Despite the fragmentary nature of the specimens described here, the new information provided from the Albian assemblages of the Lohan Cura Formation at the Cerro de Los Leones locality provides a broader panorama of the composition of terrestrial faunas during the Early Cretaceous in Patagonia.     

Lower Cretaceous ammonites from the Neuquén Basin, Argentina: The Hauterivian genus Spitidiscus

Spitidiscus is a widely-distributed Hauterivian genus that briefly invaded the Neuquén Basin, Argentina, in mid Hauterivian times, well after it first evolved in the west Tethyan area. Its appearance in Argentina is linked with a globally-significant mid Hauterivian sea-level rise. This is marked in the basin by a sharp facies change in the Agrio Formation, from non-marine sandstones of the Avilé Member to marine sediments, often laminated black shales, with Spitidiscus at the base of the overlying Agua de la Mula Member. Our extensive field work has shown that Spitidiscus occurs across the whole basin, where it is represented by two species, Spitidiscus riccardii Leanza and Wiedmann and Spitidiscus kilapiae sp. nov. For most of its vertical range Spitidiscus occurs alone, characterising the S. riccardii Zone. But at the top of its range it is joined by the first crioceratitid ammonites, their appearance marking the base of the Crioceratites schlagintweiti Zone.     

白云凹陷北坡珠江组陆架边缘三角洲前缘砂体储层特征及控制因素

采用铸体薄片、岩石薄片、扫描电镜和压汞实验、物性分析等测试资料,对珠江口盆地白云凹陷北坡珠江组陆架边缘三角洲前缘砂体储层特征及控制因素进行研究。结果表明,该储层主要为灰色中细粒岩屑砂岩,成分成熟度、结构成熟度均较低。储集空间以次生孔隙为主,其次为剩余原生粒间孔、原生孔隙等。储层具有排驱压力较高、配位数较高、孔喉半径小和孔喉分选性较差的孔隙结构特点,为中-低孔、中-低渗型储层。储层发育受沉积相和成岩作用的复合控制,水下分流河道砂体储集物性明显优于其它微相砂体。成岩作用过程主导储层物性变化,砂岩储集体中的原生储集空间因机械压实作用、胶结作用而大幅降低,后期溶蚀作用产生较多的次生储集空间,尤其是长石的溶蚀极大改善了本区储层物性,导致储集体中溶蚀性次生孔隙为主要的储存空间。     

鄂尔多斯盆地侏罗系直罗组砂岩发育特征

根据岩性旋回结构,在全盆地范围将直罗组划分为上、下两段。分段制图表明,直罗组上、下段地层厚度展布规律与全组厚度展布规律基本一致,均表现为西厚东薄;上段地层较下段厚。下段砂岩层数少、单层厚度大;而上段砂岩以层多、层薄为特点。直罗组砂岩总体成熟度较低,区域制图和碎屑组分等反映周缘存在多个源区。砂岩粒度分布表现为河道和分流河道等沉积特征。直罗组砂岩发育多种反映陆上河流沉积环境的层面和层理构造。综合分析认为,在今残留盆地范围内,直罗组下段以辫状河、曲流河沉积为主,东北部可见三角洲平原沉积。上段以辫状河、曲流河、三角洲和湖泊沉积为主。直罗组下段辫状河道砂岩具有良好的渗透性、连通性和成层性,是砂岩型铀矿和石油的良好储层。
     

Cretaceous stratigraphy and palaeoenvironments of the Southern Petén Basin, Guatemala

The Cretaceous deposits of the Southern Petén Basin, an oil-producing province, are located to the south of the Yucatan Platform and to the east of the Chiapas Carbonate Platform of Mexico. The succession in the southern part of this basin has been studied both in wells and at outcrop by microfacies analysis. It is composed of 5000 m of shallow marine carbonates and evaporites with a few thin layers of pelagic limestones rich in organic carbon and planktic foraminifera deposited during peak transgressions or maximum flooding. The sedimentation of this thick succession was not continuous, and the section is punctuated by subaerial erosional bounding surfaces and a single hardground which marks the final drowning of the carbonate platform during the late Maastrichtian. New age determinations have been obtained on the basis of planktic and benthic foraminifera from the Aptian to Santonian Cobán Formation, D, C, B and A Members. Cobán C (Albian) and B (Cenomanian) Members are the present oil reservoirs. The rudist and alveolinid limestones of late Campanian and Maastrichtian age referred to the Campur Formation in the Southern Petén Basin are here reassigned to the Angostura Formation, as recognized in the Chiapas area. The Actela Formation is defined here to encompass the limestone breccia deposits that occur at the Cretaceous–Tertiary boundary and extend into the Parvularugoglobigerina eugubina Zone of early Danian age. The D, C, B and A Members of the Cobán Formation and the Angostura Formation represent second order transgressive and regressive trends in a passive margin area where the deposits indicate a succession of various environments, including fluviatile, salinas, shallow marine carbonate platform, outer shelf and intrashelf basin. High subsidence rates, sea-level changes and tectonic uplift controlled the sedimentation.

Abstract

Les formations crétacées du sud-est du Bassin du Petén, région pétrolière du nord du Guatemala, sont situées au Sud du Yucatan et à l'Est du Chiapas au Mexique. Ces sédiments ont été étudiés à la fois dans deux puits et à l'affleurement par l'analyse des microfaciès. Cet ensemble sédimentaire est composé de 5000 m de carbonates de plate-forme de faible profondeur et d'évaporites, dans lesquels s'intercalent de minces couches de calcaires pélagiques riches en matière organique et en foraminifères planctoniques. Ces calcaires pélagiques se sont déposés lors des maximums de transgressions. La sédimentation de cette épaisse série n'est pas continue et est ponctuée par quelques surfaces d'érosion sub-aérienne et une surface durcie qui souligne l'ennoyage définitif de la plate-forme carbonatée au Maastrichtien terminal. De nouvelles datations ont été obtenues sur la formation Cobán et ses membres D, C, B et A qui s'étagent de l'Aptien au Santonien. Les membres C (albien) et B (cénomanien) constituent les réservoirs productifs. Les calcaires à rudistes et à alveolinidés du Campanien supérieur–Maastrichtien ont été appelés à tort au Petén formation Campur. Ils doivent, selon nous, être attribués à la formation Angostura comme au Chiapas voisin, où affleurent des faciès comparables et de même âge. Une nouvelle formation est créée (formation Actela) pour les brèches calcaires de la limite Crétacé–Tertiaire et de la Zone à Eugubina du Danien inférieur. Les membres D, C, B, A de la formation Cobán et la formation Angostura représentent des cycles transgressifs–régressifs de deuxième ordre. Sur cette marge passive du Petén, les dépôts correspondent à des environnements variés comme des terres émergées soumises à une altération karstique et à des dépôts fluviatiles, des salinas, des plates-formes carbonatées marines, des shelf externes et des bassins intra plate-forme. La sédimentation a été, ici, contrôlée par une forte vitesse de subsidence, les variations du niveau marin et des soulèvements tectoniques.     

鄂尔多斯盆地东胜地区直罗组砂岩的地球化学特征与物源分析

沉积岩物源分析对揭示盆地砂体展布、物质组成、储集层性质及砂岩型铀矿床铀源等科学问题具有重要的意义。对取自鄂尔多斯盆地东北部8个钻井中的中侏罗统直罗组砂岩进行了详细的岩石学和地球化学研究,试图鉴别岩石的源区和形成的构造环境。研究表明,砂岩以中粗粒杂砂岩为主,其w(SiO₂)值及w(Al₂O₃)/w(SiO₂)值中等到较高,w(K₂O)值相对较高,Fe₂O₃、MgO、TiO₂含量较低,暗示碎屑物质的富长英质、贫镁铁质特征。Nb、Ta、Ti、P等不活动元素的强烈亏损和K、Rb、Sr、Ba等LILE的富集,共同指示源区具有明显的俯冲带的特征;而w(TFe₂O₃+MgO)-w(TiO₂)、w(TFe₂O₃+MgO)-w(Al₂O₃)/w(SiO₂)、w(Th)-w(Sc)-w(Zr)/10、w(Th)-w(Co)-w(Zr)/10等判别特征则表明砂岩源区岩石可能形成于大陆弧-活动大陆边缘环境。而且,在球粒陨石标准化分配模式中,LREE强烈富集,HREE无明显分异、有着较平坦的曲线,Eu无异常或表现为弱的正、负异常;在PAAS标准化曲线中,轻重稀土均无明显分异,仅Eu表现为明显正异常。本区直罗组砂岩与华北北缘显生宙闪长岩-花岗闪长岩有着很强的亲缘性。