Mixed Carbonate-siliciclastic Succession Research Articles

Bedding and bedding surfaces in carbonate and mixed carbonate-siliciclastic successions Part 1: a critical evaluation of biases in identification and interpretation across disciplines and schools

Carbonate sedimentary rocks are crucial for understanding Earth's geological history, ecosystems, and climate dynamics. However, their study is complicated by diagenetic processes that can alter original bedding and surfaces, leading to potential misinterpretations, for example of palaeoecological and sedimentological settings as well as the time represented by these surfaces and the overall succession of deposits. This study examines the varied definitions and interpretations of bedding and associated surfaces in carbonate rocks, highlighting differences across geoscientific disciplines and regions. Through a survey, we aim to clarify the usage of the terminology in bedding and associated surfaces to improve communication and reduce misunderstandings within the geoscientific community. The survey results highlight the highly diverse, but restrictive, individual perceptions of the term “bedding” in carbonate rocks. Significant differences exist between disciplines, a researcher's main region of education, and the main region of work. This study underscores the potential for miscommunication and misunderstandings among geoscientists regarding bedding and associated surfaces, emphasizing the importance of interdisciplinary and international exchange to recognize the varied origins of “bedding” in carbonate successions.

Bedding and bedding surfaces in carbonate and mixed carbonate-siliciclastic successions Part 2: processes, identification and implications of diagenetic bedding

Bedding and bedding surfaces in carbonate and mixed siliciclastic-carbonate successions can be diagenetic, not depositional in origin and require interrogation to determine their true origins. This study reviews the different models and implications associated with diagenetic bedding and diagenetic bedding surfaces in such successions. Weathering, especially where strong lithological and mechanical differences occur, can make distinguishing primary and diagenetic bedding more difficult, and criteria are reviewed. Early, shallow diagenetic processes linked to mineral instability of aragonite, mediated by biochemical processes, are capable of producing bedding and bedding surfaces. Zones of early cementation within sediments can produce discrete beds and surfaces exemplified in the formation of limestone-marl alternations (LMAs). Variations in the degree of early cementation (stratified cementation in sense of Bathurst, 1987) in shallow buried argillaceous carbonate sediments, affected by later compaction can produce layering with beds showing little compaction separated by highly compacted, argillaceous fissile interbeds which develop bedding surfaces within those fissile units (“couplets” of Bathurst, 1987). Where such diagenetic limestone beds form intrastratally, as in LMAs, they do not obey the Principle of Superposition, and by virtue of never having been deposited laterally adjacent to their associated deposits, do not comply with Walther's Law of Facies. Bedding surfaces associated with LMAs only become true surfaces when exhumed to produce some types of firmgrounds and hardgrounds.

Compound-clinoform model for tide-influenced deltas in the mixed siliciclastic–carbonate Cisco Group, Upper Pennsylvanian (Virgilian), Eastern Shelf of the Permian Basin, U.S.A.

ABSTRACT Deltas in the Cisco Group contain significant volumes of potential reservoir sandstones in the Upper Pennsylvanian (Virgilian stage) mixed siliciclastic–carbonate succession in the Eastern Shelf of the Permian Basin. While early studies predominantly supported a river-dominated model for these deltas, our findings establish the presence of tide-influenced deltas characterized by the following features: i) predominant heterolithic lithology, ii) diagnostic structures such as flaser bedding, bidirectional ripple cross-lamination, or double-mud drapes, iii) an abundance of interpreted fluid muds, and iv) elongated bars on the subaqueous platform. Through analysis of 290.2 m of core, we identify six facies associations related to tide-influenced deltaic systems in highstand and falling-stage intervals of the proximal Eastern Shelf: delta plain–tidal flat, distributary channel and distributary-mouth bar (delta front) deposits on the subaerial delta; subaqueous delta topset, subaqueous delta foreset, and prodelta deposits on the subaqueous delta. Moreover, two additional facies associations, transgressive shelf deposits and subtidal shallow-water carbonates, record continuous episodic transgressions in the deltaic packages. Well-log cross sections reveal repetitive stacking of deltas, typical of icehouse systems, and sandstone mapping suggests elongated tidal bars forming the interpreted subaqueous delta platforms. These features suggest a compound-clinoform morphology of the Cisco Group deltas akin to modern tide-influenced deltas that have carbonate banks such as the Gulf of Papua. This compound-clinoform morphology has implications for the elongate geometry and isolation of sandstone reservoirs, impacting fluid migration and storage. Moreover, these insights extend beyond the Cisco Group to other Pennsylvanian and Permian levels on the Eastern Shelf, as well as to other deltas in mixed siliciclastic–carbonate systems in icehouse conditions.

COMPOUND CLINOFORM MODEL FOR TIDAL-INFLUENCED DELTAS IN THE MIXED SILICICLASTIC-CARBONATE CISCO GROUP, VIRGILIAN (GZHELIAN), EASTERN SHELF OF THE PERMIAN BASIN, U.S.A.

Deltas in the Cisco Group contain significant volumes of potential reservoir sandstones in the Late Pennsylvanian (Virgilian/Gzhelian) mixed siliciclastic-carbonate succession in the Eastern Shelf of the Permian Basin. While early studies predominantly supported a river-dominated model for these deltas, our findings establish the presence of tide-influenced deltas characterized by the following features: (i) predominant heterolithic lithology; (ii) diagnostic structures such as flaser bedding, bidirectional ripple cross-lamination, or double-mud drapes; (iii) an abundance of interpreted fluid muds; and (iv) elongated bars on the subaqueous platform. Through analysis of 290.2 m of core, we identify six (6) facies associations related to tide-influenced deltaic systems in highstand and falling-stage intervals of the proximal Eastern Shelf: lower-delta-plain/embayed tidal flat, distributary channel and distributary mouth bar/delta front deposits on the subaerial delta; subaqueous delta platform, subaqueou s delta foreset, and prodelta deposits on the subaqueous delta. Moreover, two (2) additional facies associations, transgressive shelf deposits, and subtidal shallow-water carbonates, record continuous episodic transgressions within the deltaic packages. Well-log cross sections reveal repetitive stacking of deltas, typical of icehouse systems, and sandstone mapping suggests elongated tidal bars forming the interpreted subaqueous delta platforms. These features suggest a compound clinoform morphology of the Cisco Group deltas akin to modern tide-influenced deltas that have carbonate banks such as the Gulf of Papua. This compound clinoform morphology has implications for the elongate geometry and isolation of sandstone reservoirs, impacting fluid migration and storage. Moreover, these insights extend beyond the Cisco Group to other Pennsylvanian and Permian levels on the Eastern Shelf, as well as to other deltas in mixed siliciclastic-carbonate systems in icehouse conditions.

Concurrence of load-and-flame structures, balls-and-pillows, clastic injectites and shear deformation bands as indicator of seismicity in mixed siliciclastic-carbonate successions (Finale Ligure Basin, Italy)

The present research explores a diverse suite of soft sediment deformation structures (SSDS) combined with clastic dykes and shear deformation bands distributed throughout a Early Miocene-age mixed carbonate-siliciclastic coastal wedge located on the Ligurian coast (NW Italy) of the Mediterranean region. The studied Aquitanian-Serravallian successions of the Finale Ligure Basin are typified by the presence of lithologically heterogeneous units, internally characterized by abrupt changes of lithofacies distributions and associated interpreted depositional environments. The interface between the terrigenous base and the carbonate wedge in many places features SSDS. Here, we report the discovery of a multiform deformational pattern, ranging from small size (cm-to dm-scale) load-and-flame structures, ball-and-pillows and mini-slumpings, to outsized (several meters in height) chaotic contorted and folded stratigraphic intervals, sand intrusions and diapir- or tree-like water escape structures. Many of the SSDS are related to fast injection of overpressurized fluids after sand liquefaction, and occur in close association with disaggregation and phyllosilicate shear bands, and sand dykelets. The stratigraphic distribution and cyclic repetitions of SSDS-prone intervals unambiguously indicates syn-depositional seismicity, thus suggesting that juxtaposition of SSDS, diapiric injectites, and shear deformation bands can serve as a suitable diagnostic criteria for seismite identification in the field. Since these particular assemblages of deformation structures reflect seismic activity in the area, they provide compelling evidence for previously undocumented tectonic episodes in Langhian-Serravallian times, associated with the latest stages of the Liguro-Provençal Basin opening.

SIMPLE STRUCTURES AND COMPLEX STORIES: POTENTIAL MICROBIALLY INDUCED SEDIMENTARY STRUCTURES IN THE EDIACARAN SERRA DE SANTA HELENA FORMATION, BAMBUÍ GROUP, EASTERN BRAZIL

ABSTRACT Microbially induced sedimentary structures (MISS) are abundant in Ediacaran and lower Cambrian successions. However, the relationship between MISS distribution and facies has not been thoroughly explored in Ediacaran–Cambrian successions in South America. This study documents the occurrence of MISS and other potential biogenic structures from the late Ediacaran Serra de Santa Helena Formation in the Bambuí Group of eastern Brazil. This unit overlies the Cloudina-bearing Sete Lagoas Formation and is a mixed carbonate-siliciclastic succession devoid of macroscopic body fossils. Potential microbial structures include wrinkled structures such as “elephant-skin” and Kinneyia-like textures, as well as pustular structures and abundant positive epirelief discoidal structures. Another putative biogenic structure is a mm-wide meandering groove resembling a simple locomotion trail of a small vagile benthic metazoan. Microbial surface textures (i.e., “elephant skin” and Kinneyia-type wrinkles) were mainly observed in heterolithic deposits, usually at the interface between sandstone and siltstone/shale. On the other hand, discs show a facies-independent distribution, observed in heterolithic as well as carbonate and marl deposits. Petrographic analyses of these discs suggest that they have complex origins and some of them may be diagenetic structures. Thus, while facies may have strongly controlled the preservation of MISS-related structures and textures in the Serra de Santa Helena Formation, their abundance and diversity in tidal flat deposits indicate the wide distribution of matgrounds in these shallow marine paleoenvironments. Also, we demonstrate how detailed description and classification of simple features, such as discoidal structures, is an important task for paleoenvironmental reconstruction of marine ecosystems at the Ediacaran–Cambrian transition when the microbially bounded substrates played important roles in the dynamics of coastal environments.

Detailed diagenetic investigation and their impact on the reservoir potential of Upper Cretaceous mixed carbonate-siliciclastic succession, Rakhi Nala Section, Eastern Sulaiman Range, Pakistan

Detailed diagenetic investigation and their impact on the reservoir potential of Upper Cretaceous mixed carbonate-siliciclastic succession, Rakhi Nala Section, Eastern Sulaiman Range, Pakistan

Facies analysis and depositional environment of a late Cambrian mixed carbonate-siliciclastic ramp from the Zard Kuh Mountain (Zagros Basin, south-western Iran)

Mixed carbonate-siliciclastic rocks were particularly widespread during the Furongian (late Cambrian), a critical time interval marked by biotic turnovers, extinction events and carbon cycle perturbations. In this context, the Zagros Basin (SW Iran) is a key area for the study of this time interval due to the well-exposed mixed carbonate-siliciclastic successions. Nevertheless, these are still poorly constrained from a sedimentological point of view. The current study aims to contribute to the palaeoenvironmental reconstruction of the Zagros Basin during the Furongian through high-resolution facies and microfacies analysis of the Tang-e-Ilbeyk section. The latter includes, in ascending order, the upper Mila C Member and Ilbeyk Formation. The facies and microfacies analysis recognized six facies associations, representing different settings of a mixed carbonate-siliciclastic ramp at the north-eastern Gondwanan margin. The vertical stacking pattern of these facies associations revealed five medium-scale Transgressive-Regressive cycles, superimposed by larger-scale sequences. In particular, during the latest Miaolingian (formerly late middle Cambrian) to early Furongian, a climate change from arid to humid conditions caused a major regressive event with the progradation of clastic shorelines and the following pollution and drowning of the Mila C Member's carbonate factory. The latter was replaced by a storm-dominated siliciclastic shelf (Ilbeyk Formation) that, throughout middle/late Furongian, experienced a large-scale transgressive event, culminating with a maximum flooding interval. A significant forced regression occurred in the latest Furongian when shallow water conditions were re-established (uppermost Ilbeyk Formation). The palaeoenvironmental reconstruction of this study allowed correlation with other coeval depositional events at regional (Arabian plate) and global (Laurentia and Baltica) scales, contributing to new constraints for the lower Palaeozoic evolution of north-eastern Gondwana.

Taphonomic feedback, paleoecology, and ostracod biostratigraphy of the Oligocene-Miocene carbonate deposits in the onshore Pará-Maranhão Basin, Northern Brazil

The Oligocene-Miocene carbonates of the Bragantina Platform are represented by densely fossiliferous rocks, which overlay the basement of the Northern Brazilian coast. Outcrop-based stratigraphy analysis combined with petrography, taphonomy, paleoecology, and ostracod biostratigraphy was carried out in the eight (8) m-thick of the Neogene mixed carbonate-siliciclastic succession exposed mainly in the Atalaia Beach. This outcrop is the classical exposure of the Pirabas Formation, generally overlaid by the siliciclastic deposits of the middle Miocene Barreiras Formation. The taphonomic signature in the last carbonate beds of this sedimentary succession indicates intense storms as autogenic processes succeeded by rapid allogenic burial by Barreiras’ deposits. These processes caused death, disarticulation, and accumulation of the benthic communities in the shallow platform, forming a coquina substrate (Taphofacies 1). During a low-energy period, shell-gravel-dwelling individuals represented mainly by Turbinella tuberculata and Mercenaria prototypa colonized this carbonate hardground (Taphofacies 2). Forced regression of Barreiras’ siliciclastics caused a fast shift of continental facies over coastal carbonate zones. The rapid burial culminated in the death of benthic communities creating a geochemical environment propitious for the pyritization of shells, leaves, and trunks (Taphofacies 3). The ostracod biozonation formed by the Cytherella stainforthi biozone and by the Quadracythere brachypygaia, Jugosocythereis pannosa, and Neocaudites macertus sub-biozones support the paleoenvironmental interpretation, chronocorrelation with the Caribbean units and confirm the Miocene age for this carbonate succession. Although taphonomic, paleoecologic, and biostratigraphic studies were carried out in a unique occurrence of the Pirabas Formation. These high-resolution stratigraphy analysis allowed the discovery of the details of this burial event that led to the shutdown of carbonate fabric on the Eastern Amazon Coast.

A record of enhanced water cycle in the late Paleozoic icehouse

The late Paleozoic ice age was the longest icehouse of the Phanerozoic with complex terrestrial ecosystems and metazoan life. Simulations of late Paleozoic climate have confirmed the coupled relationship between Gondwanan ice dynamics and paleotropical climate on a multitude of timescales. However, it is still unclear how precipitation in the tropical zone would respond to changes in paleoclimate during the late Paleozoic ice age. Here, we present detailed sedimentological and biostratigraphic works on a late Carboniferous mixed carbonate-siliciclastic succession from the southeastern South China Block to explore sedimentary responses to paleoclimate variations. Conodont and fusulinid biostratigraphy indicate that the Outangdi succession formed during the middle to late Moscovian. Detailed facies analysis suggests that the succession was deposited on a carbonate ramp, episodically punctuated by braid delta sedimentation. Given the relatively stable tectonic setting and well-correlated, coeval siliciclastic deposition in the paleotropical peri-Paleotethys regions, the abrupt increase in siliciclastic influx in the late Moscovian resulted from an enhanced precipitation event associated with the glacial-to-interglacial shift. • An abrupt siliciclastic influx were recorded in tropical peri-Paleotethys regions in the late Moscovian • Water cycle would be enhanced in the paleotropical zone responding to the demise of glacial interval • Anomalous siliciclastic deposits could have been driven by climate factors.

Orbital-scale cyclicity and organic matter production in a Permian shallow mixed carbonate-siliciclastic system: Paraná Basin, Brazil

Mixed deposition in shallow seas is highly sensitive to climate variations and sea-level changes, which are the primary controls of high-frequency cycles in the stratigraphic record. The widespread Lower Permian mixed carbonate-siliciclastic succession of the Paraná Basin is composed of rhythmic organic-rich shale interbedded with dolostone. The strata are organized into several orders of magnitude of superimposed asymmetrical and symmetrical facies stacking patterns that developed in a broad epeiric sea, which extended over 106 km2 of Permian Gondwanan land. Detailed sedimentological and stratal stacking analysis allow the hierarchical organization to be identified in large-scale, medium-scale, small-scale, and elementary stacking patterns. The high-frequency depositional cycles compose four 3rd-order sequences. They comprise gray to black shale, marls, and peloidal to micritic dolostone. Due to the shallow water setting, the minimal available accommodation is responsible for the reduced cycle thickness and the overall mixed succession. The rhythmic deposits host highly productive hydrocarbon source rocks. Two widespread, thicker intervals of bituminous shale that develop during 3rd-order sequence transgression events are highly organic-rich oil shale (27 wt%). In addition, thin black shale laminae, which are interbedded with dolostone in high-frequency stacking patterns, are also rich in organic carbon (2–8 wt%). The alternation of high carbonate content and organic matter production was controlled by wet‒dry seasons and was related to total solar radiation maxima and minima. The total solar radiation maxima are translated into elevated carbonate precipitation due to shallower and hypersaline conditions. Total solar radiation minima induce freshwater discharge in shallow and hypersaline environments during wet stages, resulting in water column stratification. The occurrence of kerogen types III and IV, a high gammacerane index, variable Pr/Ph ratios, and C35 homohopane indices indicate this condition. In addition, the input of nutrient-rich terrigenous sediments favor algal blooms, enhancing anoxia in the bottom water, which is responsible for the organic-rich shale facies, as indicated by kerogen types I and II, high hydrogen index values, high C27/C29 sterane ratios, and higher β-carotane values. Based on the hierarchical organization and previous U–Pb SHRIMP zircon ages of volcanic ash layers, it is possible to correlate the observed succession with the global sea-level curve. A time span of 8.0 Ma is assessed for the Irati Sea, with an average of 2.7 Ma for the large-scale cycles. Furthermore, the high-frequency patterns that are compatible with the precession and eccentricity of the short (±100 ka) and major (±400 ka) periodicities of the Earth's orbit are estimated to be 26.6 ka, 135.5 ka, and 400 ka. The high-frequency cycles of organic-rich shales and carbonate that are deposited over thousands of years under long-term sea-level changes and are correlated along the basin suggest climatic control driven by the Earth's orbital perturbation, which is possible in the Milankovitch frequency band.

Reservoir heterogeneity of an Eocene mixed siliciclastic-carbonate succession, northern Pannonian Basin

The quality of any reservoir is strongly controlled by diagenetic processes, especially of those that are comprised of mixed siliciclastic-carbonate sequences. An Eocene mixed siliciclastic-carbonate sequence forms reservoirs with diverse reservoir quality in the Hungarian Palaeogene Basin, which is found within the Pannonian Basin. Late Eocene transgression established extensive shallow-marine depositional environments strongly controlling the early diagenetic processes. Drill cores and cuttings were selected from several boreholes, representing a wide range of lithofacies to reveal the cause for reservoir heterogeneity. Petrographic, stable isotope and fluid inclusion analyses were performed to identify the diagenetic processes that controlled the porosity evolution of the reservoir, The early diagenetic calcite neomorphism caused lateral homogeneity, whereas the early meteoric water incursion enhanced reservoir properties via the development of dissolution pores, which in turn resulted in vertical reservoir heterogeneity. The deterioration of porosity started in the eogenetic realm by kaolinite formation and continued during mesodiagenesis as burial calcite, quartz, dolomite and minor illite cement precipitated in remaining pore spaces. Petrographic evidence, such as kaolinite transformation into dickite and fluid inclusion homogenisation temperatures highlight the role of temperature increase with burial, the effect of changing fluid chemistry during the evolution of the basin is documented by the chemical zonation of dolomite, the variation in salinities of the fluids enclosed fluid inclusions, as well as recurrent and prolonged periods of cementation with calcite, quartz and dolomite. Mesogenetic processes were accompanied by hydrocarbon emplacement and the formation of late-stage pyrite. The propagation and cementation of faults first contributed to increase and later to reduce fluid flow, respectively. These steps collectively resulted in a heterogeneous reservoir, characteristic of the Eocene sequence in the Hungarian Palaeogene Basin.

Facies character and skeletal composition of heterozoan carbonates in a high-energy confined embayment (Miocene, Finale Ligure Limestone, NW Italy)

During the early and middle Miocene, diverse photozoan to heterozoan carbonate and mixed carbonate-siliciclastic depositional systems characterized the Mediterranean region at palaeo-latitudes of 30–40°N, during a time of warm climate. These systems ranged from carbonate ramp to rimmed platforms to current-swept flooded incised palaeo-valleys, seaways and straits. This study focuses on the facies character, skeletal biota composition, diagenesis and stable carbon and oxygen isotope signature of a mixed carbonate-siliciclastic succession (lower-middle Miocene Finale Ligure Limestone and underlying Oligocene-lower Miocene siliciclastic deposits) accumulated in a high-energy coastal setting (Finale Ligure, NW Italy) to improve the knowledge on the variety and controlling factors of Miocene heterozoan skeletal carbonates.The 3D reconstruction of the geometry of the early-middle Miocene Finale Ligure basin identifies an embayment, about 35 km2 wide, limited by uplifted and eroded Alpine tectonic units and connected to the open sea through a strait. The Oligocene-Miocene sedimentary succession unconformably overlying the Alpine deformed substrate consists of 14 lithofacies (L1-L14). Besides pockets of karstic breccias (L1) preceding the marine transgression, discontinuous outcrops of glaucony-bearing litharenite, siltstone, conglomerate and wackestone/packstone with planktonic foraminifera (L2-L4) represent the preserved remnants of an eroded marine shelf affected by tectonic uplift and subsequent erosion, approximately in the Aquitanian-Burdigalian, overlain through an angular unconformity by transgressive conglomerates and bioturbated litharenites (L5-L7). The onset of carbonate skeletal production characterizes the overlying compositionally mixed carbonate-siliciclastic Langhian-Serravallian Finale Ligure Limestone succession (up to 100–150 m thick, L8-L14). These cross-bedded skeletal packstone and grainstone/rudstone are composed of barnacles, echinoderms, bryozoans, bivalves, scleractinian and stylasterid corals, and lack halimedacean algae as previously published. They accumulated in a high-energy setting with strong bottom currents, driven by the amplification of tidal and/or storm currents in a confined embayment, promoting the formation of seaward prograding metre-scale subaqueous dunes.Unlike other lower-middle Miocene carbonate systems, the Finale Ligure heterozoan carbonates contain rare coralline red algae and larger benthic foraminifera but are enriched in barnacles, scleractinian and stylasterid corals. The coastal palaeo-environmental conditions, with hard rocky substrates, strong bottom currents, high nutrients and water turbidity, influenced the composition of the skeletal carbonate producers. These distinctive lithofacies character and composition resulted from the confined embayment morphology inherited from the marine flooded Alpine bedrock that was subjected to land runoff driving nutrient input.

Sequence stratigraphy and facies analysis of the NE Gondwana Middle Jurassic inner-platform mixed carbonate-siliciclastic succession, Saudi Arabia

Sequence stratigraphy and facies analysis of the NE Gondwana Middle Jurassic inner-platform mixed carbonate-siliciclastic succession, Saudi Arabia

Evaluation of distinct soft-sediment deformation triggers in mixed carbonate-siliciclastic systems: Lessons from the Brazilian Pre-Salt analogue Crato Formation (Araripe Basin, NE Brazil)

Soft-sediment deformation structures (SSDSs) are ubiquitous in several depositional sedimentary environments and can be triggered by autogenic- and allogenic-related mechanisms. SSDSs identification in the geological record is more frequent in siliciclastic deposits, from which the most accepted models were developed. Given the fact that carbonate rocks have rapid diagenesis, which results in changes in the rheological behavior of deformational processes in short time, these are excellent facies for exploring past deformational mechanisms in a given sedimentary basin. Here, we describe distinct SSDSs developed in mixed carbonate-siliciclastic successions deposited in an Early Cretaceous lacustrine depositional system (carbonate-dominated) subjected to recurrent epicontinental marine ingressions (siliciclastic-dominated). Deformation occur in some siliciclastic intervals but it is more common in carbonate beds. Facies associations analysis combined with detailed outcrop description of the SSDSs allowed the recognition of seismic- and non-seismic-generated structures. Two km-scale, laterally continuous carbonate beds, characterized by deformation-bearing horizons, are interpreted to be the result of seismic shocks. Conversely, most of the recognized SSDSs cannot be confidentially interpreted as seismites. Autogenic triggers and criteria for accessing the origin of deformation mechanisms in both carbonate- and siliciclastic-dominated settings are alternatively proposed. Assessing such criteria is fundamental for differentiatte the timing and range of deformation and are here discussed in the context of the offshore petroleum reservoirs in the Brazilian Pre-Salt successions, where lacustrine carbonates were probably subjected to a similiar superposition of syn-sedimentary seismicity and later events of salt-tectonics and hydrotermalism.

Silicification, flow pathways, and deep-seated hypogene dissolution controlled by structural and stratigraphic variability in a carbonate-siliciclastic sequence (Brazil)

Fractured and karstified carbonate units are key exploration targets for the hydrocarbon industry as they represent important reservoirs. Furthermore, large water reserves and geothermal systems are hosted in carbonate aquifers. This paper documents the relationships between stratigraphy, structural patterns, silicification, and the spatial-morphological organization of a 3D multistorey cave system developed in a Neoproterozoic mixed carbonate-siliciclastic sequence. We found that the combination of lithology, silicification, fracture patterns (controlled by lithostratigraphic variability), and petrophysical properties control the formation of high or low permeability zones; their distribution was fundamental for the spatial organization of dissolution and the compartmentalization of the resulting conduit system in different speleogenetic storeys. We propose a deep-seated hydrothermal origin for the fluids involved in the main phases of karst formation. Warm and alkaline hydrothermal fluids caused silica dissolution, followed by chalcedony and quartz reprecipitation in pore space and fractures. Rising fluids concentrated along through-going vertical fracture zones in the lower storey, whereas sub-horizontal bedding-parallel fluid flow was focused on sedimentary packages containing highly silicified dolostones (SiO2>80 wt%) characterized by high permeability. The Calixto Cave is an enlightening example for the complex speleogenetic history affecting a mixed carbonate-siliciclastic succession where the combined effect of silicification and hydrothermal karst dissolution can potentially generate high-quality reservoirs.

Sedimentary and reservoir characteristics of an Oligocene-Miocene mixed siliciclastic-carbonate succession in southeast Iraq

The Cenozoic mixed siliciclastic-carbonate succession in southeastern Iraq is an essential and unique reservoir in the region. However, the current understanding of its sedimentology and reservoir characteristics is insufficient. In this study, its sequence stratigraphy, depositional history, and reservoir characteristics are investigated by combining seismic and well log data, core observations, and porosity and permeability measurements from several oilfields in southeastern Iraq. Two types of shallow marine deposits in two third-order sequences are established. Ten facies associations are identified in the mixed siliciclastic-carbonate deposit: the carbonate-dominated type, including sabkha, lagoon, intertidal-subtidal mudflat sandflat, and shoal; the mixed type, including foreshore and shoreface, and; the siliciclastics-dominated type, including distributary channel, interdistributary bay, and crevasse. The lower third-order sequence formed in a siliciclastic delta-dominated shallow marine system, resulting in two patterns of reciprocal mixed deposition, one forming between different system tracts and the other forming between thin carbonate and sandstone layers in the highstand system tract; the factors controlling the deposition of this reciprocal mixed system include allocyclic processes such as sediment supply, sea-level change, and tectonic activities. In contrast, the upper third-order sequence was deposited in a carbonate tidal-dominated shallow marine system, mainly forming a coeval mixed deposition controlled by various hydrodynamic factors. The reservoir quality of the coeval mixed reservoir is facies dependent and diagenesis influenced. The shoreface and parts of the tidal mudflat and lagoon are the favorable reservoir facies; in this reservoir, the strong hydrodynamic depositional environment and the syn-depositional selective dissolution are the advantageous controlling factors. In the reciprocal mixed reservoir, the petrophysical properties of the sandstone layers are better than that of the carbonate layers, causing the carbonate layers to act as interlayers or baffles to some extent. Thus, horizontal well fracturing and vertical wells are recommended for exploiting the coeval and reciprocal mixed reservoirs, respectively. This study helps to better understand mixed siliciclastic-carbonate reservoirs and provides support for oil development in the Middle East region.

Reservoir Heterogeneity of an Eocene Mixed Siliciclastic-Carbonate Succession, Northern Pannonian Basin

Reservoir Heterogeneity of an Eocene Mixed Siliciclastic-Carbonate Succession, Northern Pannonian Basin

Early Carboniferous (Mississippian) intertwining sediment dispersal network across the Laurentian cratonic interior

Early Carboniferous (Mississippian) strata across the Laurentian cratonic interior record the transition from a stable carbonate platform, supporting a broad epicontinental sea, to a series of widespread, mixed carbonate-siliciclastic successions. As the southern terminal sink, the Mississippian succession preserved on the southern Midcontinent provides evidence and constraints for sediment dispersal patterns. This paper presents the first comprehensive U–Pb detrital-zircon geochronological record of all known Mississippian sandstones from the southern Ozark Dome of southern Missouri and northwestern Arkansas. This study identifies the provenance of these Mississippian sandstones and establishes their sediment dispersal networks across the cratonic interior. Results show that a major provenance shift occurred between Lower and Upper Mississippian strata. The Lower Mississippian Bachelor Sandstone was sourced from the Laurentian craton, while Upper Mississippian and basal Pennsylvanian sandstones were sourced through recycling of upper Proterozoic and lower Paleozoic sandstones within the Laurentian craton to the north, the unroofing of the Appalachian orogenic belt from the east-northeast, and potentially some direct contribution from uplifted Precambrian basement along the Transcontinental Arch to the northwest. By combining previously published detrital zircon data from Laurentia, we propose that the Mississippian sedimentation reflects an intertwining dispersal system associated with multiple sources. Sediments derived from all three potential sources were merged and transported southward across Laurentia.

Fracture stratigraphy and oil first migration in Triassic shales, Favignana Island, western Sicily, Italy

This study aims at evaluating the control exerted by fracture stratigraphy and diagenetic processes on oil first migration through an outcropping, mixed carbonate-siliciclastic succession. The present work included results of sedimentological, paleontological, mineralogical, petrographic, structural, and microstructural analyses carried out on organic-rich shales exposed at the Favignana Island of Sicily, Italy. The analyses focus on Upper Triassic yellowish siltstones and greyish laminated dolomitic limestones, which form a 10's of m-thick succession exposed along the westernmost portion of the Sicilian fold-and-thrust belt. The studied succession deposited in a coastal lagoon associated to a wide carbonate platform, in which anoxic bottom conditions allowed the preservation of the organic matter forming catagenetic patches and veins/fractures infill. In fact, two orthogonal fracture sets perpendicular to bedding are pervaded by organic matter. They are hence interpreted as structural elements that affected the paleofluid circulation and oil migration within the Triassic source rock. These two orthogonal sets form the background fracture network predating the formation of three other sets infilled mainly with calcite cements. The latest fracture set also includes barite cements implying a hydrothermal origin (<200 °C), as supported by the R1 and R3 mixed-layer Illite-Smectite (I–S) stacking. Data are consistent with a burial depth of the sedimentary succession of about 3 ± 0.5 km. In light of these considerations, the study area might represent an outcropping stratigraphic and structural analogue of the source rocks buried in the offshore of western Sicily and northern Tunisia.