Parasequence Boundaries Research Articles

Linking dolomitization to sequence stratigraphy: Insights from the Upper Jurassic Arab Formation, offshore oilfield, UAE

This petrographic, petrophysical, and geochemical study revealed that dolomitization in the Upper Jurassic Arab Formation reservoir, United Arab Emirates, can be constrained within 2nd, 3rd, and 4th order sequences. The formation represents a 2nd order regressive sequence/cycle (highstand systems tracts, HST) that was deposited across a carbonate ramp with shoal under hot arid climatic conditions during the Kimmeridgian-Tithonian. Deposition occurred subsequent to maximum marine transgression (maximum flooding surface, MFS) and was accompanied by a progressive increase in the degree of restriction of the connectivity between the inner platform (supratidal, upper intertidal and lagoon) and the open sea. Sporadic dolomitization took place in transgressive systems tracts (TST) of the 3rd and 4th order cycles of earliest stages of the 2nd order regression, below the parasequence boundaries and within bioturbation sites. Subsequent rapid and extended 2nd order marine regression cycles were embossed by trends of progressive increase in salinity, which resulted in concomitant systematic decrease in limestone deposition and increase in intensity of seepage reflux/sabkha dolomitization by seepage reflux of hypersaline brines. Dolomitization is most prevalent in the 4th order HST sequences and was accompanied by occlusion of the moldic and intercrystalline pores in the dolostones by gypsum/anhydrite cement. Dolomitization that was accompanied by limited formation of gypsum/anhydrite in the late 2nd order HST is attributed to reflux of penesaline (salinity saturated with Ca-sulfate) /mesohaline (below Ca-sulfate saturation) brines. The reflux of the latter brines is attributed to smaller extent of relative sea-level falls, i.e., partial restriction of the inner platform. The precipitation of dolomite cement in moldic pores and as thin overgrowths around replacive dolomite is attributed to the flow hot basinal brines (i.e., hydrothermal fluids). This interpretation is supported by the presence of saddle dolomite, along with depleted δ18O values, relative enrichment in 87Sr isotope, and high fluid-inclusions homogenization temperatures. The greater amounts of these post-dolomitization cements in the dolopackstones and dolograinstones caused stronger obliteration of their near-surface isotopic signatures compared to the dolomudstones and dolowackestones.This study demonstrates that linking dolomitization of limestones across carbonate ramps under hot-arid climatic conditions to different orders of changes in the relative sea level and lithology of the succession indicates the involvement of various types of dolomitizing fluids and geochemical conditions, including: (i) domination of seepage reflux of hypersaline and mesohaline/penesaline brines, (ii) dolomitization below the seafloor during marine transgression, and (iii) dolomitization within bioturbation sites.

Variations in extent, distribution and impact of dolomitization on reservoir quality of Upper Cretaceous foreland-basin carbonates, Abu Dhabi, United Arab Emirates

Petrography, petrophysics, and geochemistry of an Upper Cretaceous, foreland-basin carbonate reservoir, Abu Dhabi, United Arab Emirates, are used to constrain the spatio-temporal variations in the extent of dolomitization and its impact on reservoir quality. Dolomitization in highstand systems tracts (HST) is attributed to repeated tidal and evaporative pumping as well as seepage reflux of penesaline brines during restriction of the platform due to 4th and 5th order cycles of relative sea-level fall and particularly below parasequence boundaries. This interpretation is supported by the presence of rare poikilotopic gypsum cement and scattered laths and micro-nodules of calcitized gypsum in the dolomitized peritidal dolostones. Dolomitization along bioturbation sites, which is most common in the transgressive systems tracts (TST), is attributed to the development of suitable localized geochemical conditions (e.g., microbial sulfate reduction and related increase in carbonate alkalinity). Porosity and permeability of the dolostones are strongly controlled by depositional textures of precursor limestones and by subsequent diagenetic evolution. Dolomitization of mudstones, wackestones and matrix-rich packstones has resulted in the formation of micropore-dominated microcrystalline dolostones whereas dolomitization of the shoal grainstones resulted in the formation of coarse-crystalline dolostones with abundant well-connected intercrystalline and moldic macropores. Mesogenetic alterations of the dolostones, which are attributed to the flow of hot basinal brines along steep faults, include dolomite cementation and, subsequently, dissolution and calcitization of dolomite (dedolomitization) and calcite cementation. The lack of systematic differences in porosity and permeability of dolostones between the oil and water-saturated dolostones suggest that most diagenesis occurred prior to completion of oil emplacement and/or reflect the shallow maximum burial depths of the formation (around 1.3 km). This study demonstrates that variations in the distribution and extent of dolomitization within a sequence stratigraphic context across an oilfield should be considered as primary control on the spatio-temporal reservoir lithology and heterogeneity in carbonate successions.

Outlines of the Siberian Platform sequence stratigraphy in the Lower and lower Middle Cambrian (Lena-Aidan area)

The development of Cambrian reefal complexes on the inherited pre-Cambrian uplifts determined a subdivision of the Siberian Platform basin into three facies belts: western saliferous basin, eastern open shelf, and transitional reefal belt. Facies shifts which affected all belts emphasised the sequence and parasequence boundaries. The study of the stratotype area of the Russian Lower Cambrian stages (middle courses of the Aidan and Lena rivers) allowed us to distinguish four (third-order?) sequences spanning the Tommotian-lower Mayan interval. The sequence boundaries, except for the base of the Nemakit-Daldynian Stage, are restricted to intrazonal levels rather than to the boundaries between stages and even between zones. Such a relationship between biostratigraphic and sequential boundaries excludes significant chronostratigraphic hiatuses in the Siberian Early Cambrian time scale. This fact together with high fossil content, and good magnetostratigraphic, chemostratigraphic (C and Sr isotopes), and radiometric data show that stable cratons located at low latitudes keep a better record of events serving the chronostratigraphic basis. A special attention should be paid to the fact of the absence of a pronounced correlation between biostratigraphic and sequential events.

Late Holocene to Recent aragonite‐cemented transgressive lag deposits in the Abu Dhabi lagoon and intertidal sabkha

AbstractModern cemented intervals (beachrock, firmgrounds to hardgrounds and concretionary layers) form in the lagoon and intertidal sabkha of Abu Dhabi. Seafloor lithification actively occurs in open, current‐swept channels in low‐lying areas between ooid shoals, in the intertidal zone of the middle lagoon, some centimetres beneath the inner lagoonal seafloor (i.e. within the sediment column) and at the sediment surface the intertidal sabkha. The concept of ‘concretionary sub‐hardgrounds’, i.e. laminar cementation of sediments formed within the sediment column beneath the shallow redox boundary, is introduced and discussed. Based on calibrated radiocarbon ages, seafloor lithification commenced during the Middle to Late Holocene (ca 9000 cal yr bp), and proceeds to the present‐day. Lithification occurs in the context of the actualistic relative sea‐level rise shifting the coastline landward across the extremely low‐angle carbonate ramp. The cemented intervals are interpreted as parasequence boundaries in the sense of ‘marine flooding surfaces’, but in most cases the sedimentary cover overlying the transgressive surface has not yet been deposited. Aragonite, (micritic) calcite and, less commonly, gypsum cements lithify the firmground/hardground intervals. Cements are described and placed into context with their depositional and marine diagenetic environments and characterized by means of scanning electron microscope petrography, cathodoluminescence microscopy and Raman spectroscopy. The morphology of aragonitic cements changes from needle‐shaped forms in lithified decapod burrows of the outer lagoon ooidal shoals to complex columnar, lath and platy crystals in the inner lagoon. Precipitation experiments provide first tentative evidence for the parameters that induce changes in aragonite cement morphology. Data shown here shed light on ancient, formerly aragonite‐cemented seafloors, now altered to diagenetic calcites, but also document the complexity of highly dynamic near coastal depositional environments.

Application of sequence stratigraphic concepts to the Upper Cretaceous Tununk Shale Member of the Mancos Shale Formation, south‐central Utah: Parasequence styles in shelfal mudstone strata

AbstractAlthough sequence stratigraphic concepts have been applied extensively to coarse‐grained siliciclastic deposits in nearshore environments, high‐resolution sequence stratigraphic analysis has not been widely applied to mudstone‐dominated sedimentary successions deposited in more distal hemipelagic to pelagic settings. To examine how sequence stratigraphic frameworks can be derived from the facies variability of mudstone‐dominated successions, the Tununk Shale Member of the Mancos Shale Formation in south‐central Utah (USA) was examined in detail through a combination of sedimentological, stratigraphic and petrographic methods. The Tununk Shale accumulated on a storm‐dominated shelf during the second‐order Greenhorn sea‐level cycle. During this eustatic event, the depositional environment of the Tununk Shale shifted laterally from distal middle shelf to outer shelf, then from an outer shelf to an inner shelf environment. At least 49 parasequences can be identified within the Tununk Shale. Each parasequence shows a coarsening‐upward trend via upward increases in silt and sand content, thickness and lateral continuity of laminae/beds, and abundance of storm‐generated sedimentary structures. Variations in bioturbation styles within parasequences are complex, although abrupt changes in bioturbation intensity or diversity commonly occur across parasequence boundaries (i.e. flooding surfaces). Due to changes in depositional environments, dominant sediment supply and bioturbation characteristics, parasequence styles in the Tununk Shale show considerable variability. Based on parasequence stacking patterns, eleven system tracts, four depositional sequences and key sequence stratigraphic surfaces can be identified. The high‐resolution sequence stratigraphic framework of the Tununk Shale reveals a hierarchy of stratal cyclicity. Application of sequence stratigraphic concepts to this thick mudstone‐dominated succession provides important insights into the underlying causes of heterogeneity in these rocks over multiple thickness scales (millimetre‐scale to metre‐scale). The detailed sedimentological characterization of parasequences, system tracts and depositional sequences in the Tununk Shale provides conceptual approaches that can aid the development of high‐resolution sequence stratigraphic frameworks in other ancient shelf mudstone successions.

Systems tracts and their bounding surfaces in the low- accommodation Upper Mannville group, Saskatchewan, Canada

Abstract Sequence stratigraphic interpretations of paralic successions can be challenging, because such settings are commonly controlled by a combination of allogenic controls and autogenic processes, leading to complex depositional architectures. In low-accommodation systems where parasequences are less than 15 m thick, the challenges in recognizing discrete stratigraphic surfaces are exacerbated by limited vertical separation between surfaces or their outright erosional amalgamation, requiring high-resolution facies analysis to resolve their geometries. Herein, we present criteria derived from core data and well logs to differentiate between allogenic and autogenic surfaces in a low-accommodation paralic setting, using the Lower Cretaceous Mannville Group of west-central Saskatchewan, Canada as a case study. The upper part of the Mannville Group (Sparky, Waseca and McLaren formations) displays complex stratigraphic relationships related to base-level changes. Marine flooding surfaces (MFS) and transgressive surfaces of erosion (TSE) of 3rd and 4th order record allogenic changes ( e.g., basin subsidence and/or eustatic changes in sea level), and are used to define parasequence boundaries of strandplain-shoreface successions. Deltaic deposits not only possess these allogenic transgressive surfaces, but autogenic 5th-order flooding surfaces, which are broadly similar in appearance, as well. These autogenic surfaces record responses to channel avulsion or lobe abandonment, separate discrete delta lobes, and display limited lateral extents. Shoreface-related flooding surfaces, by contrast, bound true allogenic parasequences and, therefore, have regional correlatability. The Waseca Formation also possesses surfaces related to base-level fall. To the north, a subtle regressive surface of marine erosion (RSME) marks the base of the falling stage systems tract. Elsewhere, a widespread subaerial unconformity (SU) separates the Waseca into lower and upper members. In most areas, the SU is amalgamated with surfaces generated by later base-level rise. Upper Mannville strata in the study area, therefore, can be separated into parts of two depositional sequences. The main deposits of the lower sequence comprise a highstand systems tracts (HST) in the Sparky Formation. The base of the Lower Waseca Member marks the onset of a transgressive systems tract (TST), associated with widespread shoreline retreat. A 3rd-order maximum flooding surface (MxFS) marks the end of transgression and resumption of normal progradational regression in a highstand systems tract associated with the Lower Waseca Member. Following highstand normal regression, a major base-level fall initiated a subaerial unconformity that marks the base of the upper sequence. Fluvial valley incision led to sediment bypass and deposition of forced regressive and lowstand shoreface and delta complexes of the falling stage systems tract (FSST) and lowstand systems tract (LST), respectively, in the northern part of the study area. Pedogenic modification of subaerially exposed sediment occurred in interfluve areas. Ensuing TST accumulation was initially confined to the tidal-fluvial estuarine infill of the incised valleys and transgressive bay deposits of the Upper Waseca Member. The Upper Waseca is capped by a 3rd-order maximum flooding surface (MxFS), which separates it from the overlying McLaren Formation. This marks the return to regional shoreline progradational regression and corresponds to the highstand systems tract (HST) of the upper sequence.

High-resolution Palaeogene sequence stratigraphic framework for the Cuu Long Basin, offshore Vietnam, driven by climate change and tectonics, established from sequence biostratigraphy

The chronostratigraphy of the Cuu Long Basin, offshore Vietnam to the south of the Mekong Delta, is poorly understood, especially for the Palaeogene succession. This paper utilises a comprehensive biostratigraphic database from 46 exploration wells to establish a high-resolution sequence biostratigraphic framework for the Oligocene and upper Eocene based on the identification of 36 transgressive-regressive depositional cycles. They are shown to be climate-driven and correlate to 406 kyr ‘heartbeat of the Oligocene’ eccentricity cycles. Their occurrence has been rigorously cross checked against seismic across the basin. This re-evaluation requires the repositioning of the Oligo-Miocene boundary in the Cuu Long Basin from the C seismic pick to the BI.1 seismic pick.By integrating sequence biostratigraphic and seismic datasets, the Oligocene can be seen as characterised by two types of depositional sequence. Firstly, thin and repetitive depositional packages, here termed ‘Sequences’ but perhaps better visualised as parasequences, the deposition of which was controlled by cyclically changing climate. Secondly, much thicker packages, each comprising a succession of parasequences separated by unconformities, and which tie to the main seismic divisions within the Oligocene/upper Eocene and were driven by tectonics. The latter are visualised as 3rd order sequences, with the parasequences forming 4th order sequences.Correlating the succession of parasequences to the 406 kyr ‘heartbeat of the Oligocene’ isotope cycles allows accurate ages to be proposed for each parasequence boundary, and in doing so an accurate time framework is established for the basin. The first sediments may have been deposited during the latest middle Eocene (Seismic group F/G) at the same time as or just prior to the initial stage of faulting prior to the opening of the South China (East Vietnam) Sea, with deposition of seismic group E commencing at about 36.6 Ma during the late Eocene. Unconformities reflecting 3rd order sequence boundaries are dated at 33.4 Ma, 29.8 Ma and 27.4 Ma.

Division and characteristics of shale parasequences in the upper fourth member of the Shahejie Formation, Dongying Depression, Bohai Bay Basin, China

Shale parasequence analysis is an important part of sequence stratigraphy sudies. This paper proposed a systematic research method for analyzing shale parasequences including their delineation, division, characteristics and origins. The division method is established on the basis of lithofacies. Multi-method analysis and mutual verification were implemented by using auxiliary indicators (such as mineral compositions, geochemical indicators and wavelet values). A typical shale parasequence comprises a lower interval of deepening water-depth and an upper interval of shallowing water-depth (e.g., a shale parasequence including a high-total organic carbon (TOC) shale-low-TOC limy shale). Abrupt increases in pyrite content, TOC value, relative hydrocarbon generation potential ((S1+S2)/TOC), and wavelet values are indicative of parasequence boundaries. The proposed research method was applied to study the upper fourth member of the Shahejie Formation in the Dongying depression, Bohai Bay Basin. Results show that there were seven types of parasequences developed. A singular and a dual structured parasequences were identified. Three factors controlling the development of the shale parasequences were identified including relative lake level change, terrestrial input and transgression. The development of high-TOC (>2%) shale parasequences was mainly controlled by biological and chemical sedimentation. The low-TOC (<2%) shale parasequences were mainly deposited by chemical sedimentation. The diversities of shale parasequences were caused by four major controlling factors including climate, relative lake level change, terrestrial input and emergency (e.g., transgression).

Global sea-level control on local parasequence architecture from the Holocene record of the Po Plain, Italy

Holocene deposits exhibit distinct, predictable and chronologically constrained facies patterns that are quite useful as appropriate modern analogs for interpreting the ancient record. In this study, we examined the sedimentary response of the Po Plain coastal system to short-term (millennial-scale) relative fluctuations of sea level through high-resolution sequence-stratigraphic analysis of the Holocene succession.Meters-thick parasequences form the building blocks of stratigraphic architecture. Above the Younger Dryas paleosol, a prominent stratigraphic marker that demarcates the transgressive surface, Early Holocene parasequences (#s 1–3) record alternating periods of rapid flooding and gradual shoaling, and are stacked in a retrogradational pattern that mostly reflects stepped, post-glacial eustatic rise. Conversely, Middle to Late Holocene parasequences (#s 4–8) record a complex, pattern of coastal progradation and delta upbuilding that took place following sea-level stabilization at highstand, starting at about 7 cal ky BP. The prominent transgressive surface at the base of parasequence 1 correlates with the period of rapid, global sea-level rise at the onset of the Holocene (MWP-1B), whereas flooding surfaces associated with parasequences 2 and 3 apparently reflect minor Early Holocene eustatic jumps reported in the literature. Changes in shoreline trajectory, parasequence architecture and lithofacies distribution during the following eustatic highstand had, instead, an overwhelming autogenic component, mostly driven by river avulsions, delta lobe switching, local subsidence and sediment compaction. We document a ∼1000-year delayed response of the coastal depositional system to marine incursion, farther inland from the maximum landward position of the shoreline. A dramatic reduction in sediment flux due to fluvial avulsion resulted in marine inundation in back-barrier position, whereas coastal progradation was simultaneously taking place basinwards.We demonstrate that the landward equivalents of marine flooding surfaces (parasequence boundaries) may be defined by brackish and freshwater fossil assemblages, and traced for tens of kilometers into the non-marine realm. This makes millennial-scale parasequences, whether auto- or allogenic in origin, much more powerful than systems tracts for mapping detailed extents and volumes of sediment bodies.The Holocene parasequences of the Po coastal plain, with strong age control and a detailed understanding of sea-level variation, may provide insight into the driving mechanisms and predictability of successions characterized by similar depositional styles, but with poor age constraint, resulting in more robust interpretations of the ancient record.

Facies model for a coarse‐grained, tide‐influenced delta: Gule Horn Formation (Early Jurassic), Jameson Land, Greenland

AbstractTide‐dominated deltas have an inherently complex distribution of heterogeneities on several different scales and are less well‐understood than their wave‐dominated and river‐dominated counterparts. Depositional models of these environments are based on a small set of ancient examples and are, therefore, immature. The Early Jurassic Gule Horn Formation is particularly well‐exposed in extensive sea cliffs from which a 32 km long, 250 m high virtual outcrop model has been acquired using helicopter‐mounted light detection and ranging (LiDAR). This dataset, combined with a set of sedimentological logs, facilitates interpretation and measurement of depositional elements and tracing of stratigraphic surfaces over seismic‐scale distances. The aim of this article is to use this dataset to increase the understanding of depositional elements and lithologies in proximal, unconfined, tide‐dominated deltas from the delta plain to prodelta. Deposition occurred in a structurally controlled embayment, and immature sediments indicate proximity to the sediment source. The succession is tide dominated but contains evidence for strong fluvial influence and minor wave influence. Wave influence is more pronounced in transgressive intervals. Nine architectural elements have been identified, and their internal architecture and stratigraphical distribution has been investigated. The distal parts comprise prodelta, delta front and unconfined tidal bar deposits. The medial part is characterized by relatively narrow, amalgamated channel fills with fluid mud‐rich bases and sandier deposits upward, interpreted as distributary channels filled by tidal bars deposited near the turbidity maximum. The proximal parts of the studied system are dominated by sandy distributary channel and heterolithic tidal‐flat deposits. The sandbodies of the proximal tidal channels are several kilometres wide and wider than exposures in all cases. Parasequence boundaries are easily defined in the prodelta to delta‐front environments, but are difficult to trace into the more proximal deposits. This article illustrates the proximal to distal organization of facies in unconfined tide‐dominated deltas and shows how such environments react to relative sea‐level rise.

The dynamic behavior of shallow marine reservoirs: Insights from the Pliocene of offshore North Trinidad

The Pliocene reservoirs of the North Coast Marine Area (block NCMA-1) comprise shallow marine shoreface to shelf sandstones (up to 30 m [98 ft] thick) and interbedded shelfal mudstones organized into stacked upward-coarsening parasequences (ca. 25–70 m thick). Production behavior is strongly conditioned by the interplay of subregional sand distribution and aquifer connectivity, and intrafield reservoir architecture complexity, commonly in the form of clinoforms. The M2 parasequence is a well-connected reservoir unit and forms part of a late highstand systems tract with a strongly progradational architecture. Large-scale reservoir connectivity can be demonstrated by pressure communication in producing fields up to 10 km (6 mi) apart. The widespread nature of the sand also links the reservoir to a significant aquifer that provides pressure support during field depletion. In contrast, clinoforms that dominate the M2 reservoir architecture at a variety of scales reduce lateral connectivity. Wells situated in individual sand bodies show significant pressure interference during production, whereas wells that straddle bounding clinoforms (possible parasequence boundaries), although in pressure communication, show more subtle pressure interference. In the Chaconia field, clinoforms that bound lobate sand bodies in the M2 reservoir have also apparently slowed the onset of water cut within the field by several years and therefore increased reserves recovery. Similarly, in the M6 reservoir of the Ixora field a significant bounding clinoform has partitioned the field through much of its production life, but production data are now suggesting breakdown and late-stage pressure support across the same clinoform. The M6 parasequences form part of a lower quality early highstand systems tract and are more discontinuous than their M2 counterpart due to well-developed bounding clinoforms. As such they are not characterized by large active aquifers and do not benefit from such prominent long-term pressure support. The M4 reservoir is an isolated sand-body complex that forms part of a high-frequency lowstand complex. It shows similar stratigraphic heterogeneity to that displayed in the M2, with variations in pressure decline exhibited within individual wells of the same reservoir unit. Its isolated geometry means it is not connected to a regional aquifer and is therefore undergoing volumetric decline with no water breakthrough.

Diagenetic Evolution of Selected Parasequences Across A Carbonate Platform: Late Paleozoic, Tengiz Reservoir, Kazakhstan

Abstract Predicting the diagenetic modification of reservoir quality in carbonate fields is poorly understood but of paramount importance to reservoir characterization. The diagenetic processes and current pore system are reported for two parasequences from the platform center to the margin of the Tengiz reservoir (∼ 16 km); the late Visean (Lvis13 cbs 900) and Bashkirian (Bash2.5 MSF 350) parasequences are separated by 8 million years. Syndepositional fibrous cement (> 10% rock volume) and aragonite skeletal molds developed in both parasequences. On subaerial exposure, brown soil-related calcite, and meniscus and pendant cements were emplaced in both parasequences. The application of CL petrography to these rocks for the first time has revealed the depositional components and early marine components are altered developing a mottled pattern implying that their original chemistry is not preserved. The principal calcite cement shows a complex growth pattern by CL that is unreported from any other calcite cement. CL is also used to divide the main cement into stage 1, nonluminescent, and stage 2, orange to brown luminescent, calcite. Stage 1 calcite shows vadose meniscus and pendant fabrics followed by uniform phreatic euhedral terminations. Stage 1 cement is patchily distributed at a millimeter scale; 28% to 40% in the Visean and 0.5% to 22% in Bashkirian grainstones. Stage 1 cement is more abundant at the center of the platform in the late Visean than at its margin. Abundant stage 1 calcite creates a framework that prevents later compaction preserving a high porosity at the center of the platform. This sweet spot is reinforced by the deposition of late-stage bitumen cement that occupies 8% rock volume at the margin of the platform but is almost absent at its center. Stage 1 δ13C values (−2.4‰ to +1.7‰) are derived by dissolution of host rock, and their δ18O values (−6.3‰ to −3.7‰) are compatible with precipitation from meteoric or mixed marine-meteoric water. Depositional micrite and micritic grains are transformed by dissolution–precipitation processes into porous microrhombic calcite not by Ostwald ripening. Microrhombic calcite is ubiquitous in the Tengiz icehouse, dispelling the myth that this fabric is restricted to greenhouse epochs. At ∼ 300 million years before significant burial the study grainstones were patchily cemented and dissolved to reach peak porosity; their transformed texture set up flow paths that controlled the distribution of later diagenetic products. Reservoir compartmentalization did not occur through extensive hardground formation at this time as occurs in some greenhouse sequences, but ash bands at parasequence boundaries were turned into tonsteins and wackestones were compacted perhaps sufficiently to influence vertical flow. CL examination of stage 2 calcite that adds 0.5% to 16% to rock volume reveals that its precipitation was episodic and switched sites as the pore network became blocked through precipitation and compaction and was opened by new fractures. A change from acute to equant habit occurred during stage 2 precipitation, causing the change from fibrous to equant crystal fabric. The δ18OPDB values of stage 2 calcite (−3‰ to −9‰) are linked to precipitation from an 18O-enriched fluid that evolved to ∼ +6‰ SMOW at temperatures of 60°C to 120°C, using the equation of Kim and O'Neil (1997). Many open macropores are lined with small cement crystals that CL shows have all stages of growth present and are terminated by euhedral surfaces. These macropores are only partly filled because of cement inhibition, not because they were dissolved during burial, as was previously claimed. Late-stage dissolution did create some macropores and rounded micrite crystals, but its effect on overall platform porosity is small. The CL examination of Tengiz platform calcite cement has recognized many diagenetic events that cannot be recognized without CL. The new Tengiz diagenetic textures recognized here are probably present in other carbonate platforms, especially those from icehouse epochs.

Paleobathymetry and sequence stratigraphic interpretations from benthic foraminifera: Insights on New Jersey shelf architecture, IODP Expedition 313

Integrated Ocean Drilling Program (IODP) Expedition 313 drilled three holes (Sites M27, M28, and M29; 34–36 m present water depth) across a series of prograding clinothems from the inner continental shelf of the New Jersey (USA) margin, a region that is sensitive to sea-level change. We examined 702 late Eocene to Miocene samples for benthic foraminiferal assemblages and planktonic foraminiferal abundances. We integrate our results with lithofacies to reconstruct paleobathymetry. Biofacies at all three sites indicate a long-term shallowing-upward trend as clinothems built seaward and sediment filled accommodation space. Patterns in biofacies and lithofacies indicate shallowing- and deepening-upward successions within individual sequences, providing the basis to recognize systems tracts, and therefore sequence stratigraphic relationships in early to early-middle Miocene sequences (ca. 23–13 Ma). The clinothem bottomsets and the lower portions of the foresets, which contain the thickest parts of clinothems, yield the deepest water biofacies. Shallower bio facies characterize the sequences in the upper portions of the clinothem foresets and on the topsets. Topsets are characterized by transgressive (TST) and highstand systems tracts (HST). Foresets contain lowstand systems tracts (LST), TSTs, and HSTs. Flooding surfaces mark parasequence boundaries within LSTs, TSTs, and HSTs. Superimposed on the long-term trends, short-term variations in paleowater depth are likely linked to global sea-level changes indicated by global oxygen isotopic variations.

The Coalbed Methane (CBM) Potential and CO2 Storage Capacity of the Santa Terezinha Coalfield, Paraná Basin, Brazil – 3D Modelling, and Coal and Carbonaceous Shale Characteristics and Related Desorption and Adsorption Capacities in Samples from Exploration Borehole CBM001-ST-RS

This study investigates the CBM generation and CO2 storage capacity of the coal-bearing Permian age Rio Bonito Formation in the Santa Terezinha Coalfield, Paraná Basin, Brazil. In order to assess the volumetrics of the coal seams a fourth-order sequence stratigraphic framework of the coal-bearing succession was established based on parasequence boundaries. A 3D geometric model based on data from 36 well logs was developed, and enabled subdivision of the study area into regions with very different CBM potential, due to variable cumulative coal thickness, depths and presence/absence of diabase. Strata of the coal-bearing Rio Bonito Formation were penetrated in well CBM001-ST-RS at a depth of 575.70 to 638.35 m, from which twelve coal seams and carbonaceous shales, ranging in thickness from 0.15 to 1.77 m, were retrieved for analysis. Gas desorption values range from 0.3 to 2.2 cm3/g. Gas composition is dominated by methane (&gt; 94 Vol%). Gas isotopic analyses suggest a mixed biogenic and thermogenic origin. Vitrinite reflectance indicate a rank of high volatile A bituminous coal, with semi-anthracite occurring in contact with igneous rocks. The majority of the samples has vitrinite group contents &gt; 50 Vol%, mineral matter-free basis. Ash yields range from 42.1 to 92.8 wt.%, whereas sulphur contents range from 0.1 to 7.7 wt.%. Volatile matter and calorific values (dry ash-free) range from 37.6 to 79.4 wt.%, and 2352 to 8107 kcal/kg, respectively. Mineral composition of the samples is dominated by quartz and kaolinite, whereas major element analyses show that SiO2 is most abundant. The 3D model allowed volumetric calculation for the coal seams and formed the basis for calculations of gas content and CO2 storage capacity. According to the 3D model the four coal-bearing parasequences of the study area contain a coal volume on the order of 3212 × 10 6 m3 suggesting a resource base of 5.5 × 109 m3 of methane associated with the coal seams and carbonaceous shales of the study area. The methane and carbon dioxide sorption capacities show a high positive correlation with TOC content. The coal seams appear to be undersaturated, since sorption capacity is significantly higher than the desorbed gas volume. The CO2 sorption capacity at mean reservoir pressure was up to 7 times higher than the methane sorption capacity. Based upon sorption capacities and reservoir conditions, a storage potential of 13.8 Gt CO2 in coal seams and carbonaceous shales of the study area is estimated.

Linking diagenesis to sequence stratigraphy in fluvial and shallow marine sandstones: Evidence from the Cambrian–Ordovician lower sandstone unit in southwestern Sinai, Egypt

By integrating diagenesis and sequence stratigraphy, the distribution of diagenetic alterations and their impact on reservoir quality was investigated within a sequence stratigraphic framework using the fluvial and shallow marine sandstones in the Cambrian–Ordovician succession of southwest Sinai. The perographic and geochemical analysis of the studied sandstone revealed that the eogenetic alterations display fairly systematic spatial and temporal distribution patterns within the lowstand system tract and transgressive system tract, as well as along the sequence stratigraphic surfaces (i.e., sequence boundaries, transgressive surfaces and parasequence boundaries). During relative sea-level fall, percolation of meteoric waters through sandstones of the LST and below sequence and parasequence boundaries resulted in extensive dissolution of detrital grains and formation of kaolinite, authigenic K-feldspar and feldspar overgrowths as well as formation of mechanical infiltrated clays around the detrital grains. During relative sea-level rise, invasion of marine water into the sandstones as a consequence of landward migration of the shoreline, as well as low sedimentation rates encountered in the TST, resulted in the formation of glauconite, apatite and pyrite. Development of pseudomatrix, which was formed by mechanical compaction of mud intraclasts, is mostly abundant along transgressive surfaces and parasequence boundaries of the TST, and is related to the abundance of mud intraclasts in the transgressive lag deposits. The types and extent of eogenetic alterations have an important impact on the distribution of the mesogenetic alterations, including the formation of quartz overgrowths and dickite. Distribution of mesogenetic quartz overgrowths in the sandstones was controlled by the distribution of mechanically infiltrated clays and the presence of eogenetic cement. Sandstones that remained poorly cemented during eodiagenesis and that have thin or discontinuous infiltrated clay rims around the detrital grains were cemented during mesodiagenesis by quartz. The absence of extensive eogenetic cements in the sandstones suggested that the partial deterioration of porosity was mainly due to mechanical compaction. Partial transformation of kaolinite to dickite, which indicates neomorphic change to a better-ordered and more stable crystal structure at the elevated temperatures during mesodiagenesis, is partially a function of distribution of kaolinite during eodiagenesis. The conceptual model developed in this study shows the diagenetic evolutionary pathways in the reservoir sandstones within a sequence stratigraphic context, which in turn provides some insights into the controls on reservoir heterogeneity.

Early Miocene shell concentration in the mixed carbonate-siliciclastic system of Kutch and their distribution in sequence stratigraphic framework

The Early Miocene succession of Kutch represents a mixed carbonate-siliciclastic depositional system. The carbonate part of the succession, characterized by high abundance of shallow marine benthic fauna, hosts typical shell concentrations (also referred as shell beds). The thickness of shell concentrations vary in scale from 5 cm to 100 cm and are separated by poorly fossiliferous to barren silty shales/siltstones. Based on taphonomic and sedimentological observations, shell concentrations are classified as lag, event, composite/multi-event and hiatal types. The occurrence of different types of shell concentrations in the background of sequence stratigraphic framework is the main theme for the present study. Overall, the shell concentrations occupy the middle part of the sequence i.e. upper part of the transgressive systems tract and lower part of the highstand systems tract. Lag concentrations are found in the lower part of the sequence while composite concentrations, the major contributors in the sequence, occur in upper part of the transgressive systems tract and in the lower part of the highstand systems tract. The hiatal concentrations are associated with maximum flooding surface while the position of event concentrations is independent of sequence stratigraphic framework. The shell concentrations occupy marine flooding surface or marine ravinement surface, thus mark parasequence boundaries.

Local tectonic control on parasequence architecture: Second Frontier sandstone, Powder River Basin, Wyoming

The Cenomanian Second Frontier sandstone, one of the major producing units of the Frontier Formation in the Powder River Basin, Wyoming, is a basin-isolated sand body that pinches out to zero thickness in both seaward and basinward direction. The Second Frontier has previously been viewed as a single, relatively homogeneous, wave-dominated sediment succession. Our high-resolution study, integrating detailed facies relationships in outcrop, core, and well logs, shows that the Second Frontier sandstone comprises an offlapping parasequence set, formed from seven regionally mappable wave-dominated parasequences. The parasequences are bounded by minor flooding surfaces that represent previously unrecognized potential flow barriers or baffles. Parasequence boundaries are oriented obliquely to the well-defined regional north-northwest–south-southeast–elongated trend of the unit, and successive parasequences offlap to the south in an along-strike direction. The seaward pinch-out of the Second Frontier sandstone is depositional in nature, as illustrated by well-preserved healing-phase deposits, whereas the basinward pinch-out is caused by marine truncation over a tectonically uplifted area. Parasequence architecture was strongly affected by syndepositional tectonic movement, which determined both (1) the distribution of the entire parasequence set, forming a depositional remnant, and (2) the position and shape of parasequences internal to the remnant. The Second Frontier is thus interpreted as a depositional remnant of a once more extensive wave-dominated deltaic shoreface complex.

Climate change signature of small-scale parasequences from Lateglacial–Holocene transgressive deposits of the Arno valley fill

Despite recent report of short-term cyclicity from Lateglacial–Holocene deposits of several coastal plains worldwide, no precise documentation of the key factors controlling cyclic facies architecture has been made available by previous work. Detailed sedimentological analysis of a continuously-cored borehole, around the town of Pisa, in western Tuscany, provides evidence for the occurrence of three high-frequency, transgressive–regressive cycles within the post-Last Glacial Maximum (LGM) transgressive succession (13–8 cal. kyr BP) of the Arno incised-valley fill. These cycles, which are bounded by lateral equivalents of marine flooding surfaces, are 8–12 m thick and correspond to small-scale parasequences. Micropalaeontological (foraminifers and ostracods) investigations based upon differentiation of eight microfossil associations, allow to refine the stratigraphic framework, emphasizing subtle changes in palaeosalinity across parasequence boundaries. Diagnostic changes in vegetation patterns, driven by opposite climate conditions, enable precise documentation of parasequence development as a function of climate change. Pollen spectra invariably show expansions of broad-leaved forests at parasequence boundaries, suggesting that rapid shifts to warmer climate conditions accompanied episodes of rapid sea-level rise. In contrast, stillstand phases saw the development of cold-temperate communities (upper parts of parasequences), suggesting transition to temporary colder climate conditions. Reconstruction of parasequence architecture on the basis of adjacent stratigraphic data, combined with palaeoclimate characterization and radiometric dating enable identification, within the transgressive Arno valley body, of three major “regressive” pulsations that are tentatively correlated with the most important cooling events of the post-LGM period. The sedimentary response to these short-term phases of climatic cooling is clearly documented by episodes of widespread coastal-plain and bay-head delta progradation, leading to partial estuary infilling and temporary establishment of continental environments in the proximal and central sectors of the valley.

Lithofacies and depositional history of Midale carbonate-evaporite cycles in a Mississippian ramp setting, Steelman-Bienfait area, southeastern Saskatchewan, Canada

Abstract Carbonate-evaporite cycles in the Mississippian (Osagean) Midale Beds of southeastern Saskatchewan were deposited over a shallow homoclinal ramp along the northern margin of the Williston Basin. Fourteen lithofacies, including nine limestone facies (C1 to C9), two dolostone facies (D1 and D2), and three anhydrite facies (A1 to A3), were identified from cores (46 wells) and geophysical well logs (750 wells) in the Steelman-Bienfait area. These lithofacies represent a range of subtidal to supratidal settings and are interpreted to be related to four transgressive-regressive parasequences (S1 to S4, in ascending order). Paleosols and microkarst features (T1) characterize parasequence boundaries and provide evidence for subaerial exposure. Oolitic grainstones and packstones were deposited as shore-parallel, shoals and barrier islands in the S2 and S3 parasequences along the western margin of the Steelman and Bienfait oilfields. Paleotopographic hinge-zones were an important control on the distribution of shoals, lagoons, algal marsh sediments, salinas and algal/ostracod mounds. Abrupt thickness changes and lateral, carbonate-evaporite facies transitions were observed across the hinge-zones. Facies distributions in the Midale Beds were related to sea level fluctuations, paleotopography, and the availability of accommodation space. Minor sea level fluctuations in this shallow ramp setting resulted in major lateral shifting of facies, significant migration of the paleocoastline, and periodic exposure of the upper to middle ramp environments, producing stratigraphic traps as exploration targets, in addition to the traditional sub-crop “plays” in the study area.

Diagenetic alterations related to marine transgression and regression in fluvial and shallow marine sandstones of the Triassic Buntsandstein and Keuper sequence, the Paris Basin, France

The distribution of diagenetic alterations in Triassic fluvio-deltaic, quartzarenitic to sublitharenitic, lowstand systems tract (LST) sandstones of the Grès á Voltzia Formation, anastomosing fluvial, quartzarenitic transgressive systems tract (TST) sandstones of the Grès á Roseaux Formation, and shallow marine, quartzarenitic to sublitharenitic, TST sandstones of the Grès Coquiller Formation, the Paris Basin (France), can be linked to transgression and regression events, and thus to the sequence stratigraphic context. Near-surface eogenetic alterations, which display a fairly systematic link to the depositional facies and sequence stratigraphic framework, include: (i) cementation by meteoric water calcite ( δ 18O=−8.9‰ and δ 13C=−9.1‰) in the fluvio-deltaic, LST sandstones, (ii) cementation by mixed marine–meteoric calcite ( δ 18O=−5.3‰ to −2.6‰ and δ 13C=−3.9‰ to −1.3‰) and dolomite ( δ 18O=−4.6‰ to −2.6‰ and δ 13C=−2.9‰ to −2.3‰) in the foreshore, TST sandstones and below parasequence boundaries (PB), and transgressive surface (TS), and in the shoreface, TST sandstones below maximum flooding surfaces (MFS), being facilitated by the presence of carbonate bioclasts, (iii) dissolution of detrital silicates and precipitation of K-feldspar overgrowths and kaolinite, particularly in the fluvio-deltaic, LST sandstones owing to effective meteoric water circulation, and (vi) formation of autochthonous glauconite, which is increases in abundance towards the top of the fluvio-deltaic, LST sandstones, and along TS, and in the shoreface, TST sandstones, by alteration of micas owing to the flux of seawaters into the sandstones during transgression, whereas parautochthonous glauconite is restricted to the TS sandstones owing to marine reworking. Mesogenetic alterations, which include cementation by quartz overgrowths and illite, display fairly systematic link to fluvio-deltaic, LST sandstones. This study has revealed that linking of diagenesis to transgression and regression events enables a better understanding of the parameters that control the spatial and temporal distribution of diagenetic alterations in sandstones and of their impact on reservoir quality evolution.