Third-order Sequence Boundaries Research Articles

Sequence lithofacies paleogeography evolution of the Middle Permian Maokou Formation in the northwest margin of the Sichuan Basin

Due to the present tectonic and stratigraphic distribution characteristics in the northwest margin of the Sichuan Basin are complex, which restricts the understanding of sequence lithofacies paleogeography of the Middle Permian Maokou Formation. To investigate the process of tectonic–sedimentary evolution, basin-margin outcrop and intra-basin well and seismic data were used. The results show two structural sequences, namely, SSQ1 and SSQ2, and five third-order sequences, namely, SQ1–SQ5, in the Middle Permian Maokou Formation. SSQ1, with stable formation thickness, shows a slow transgression–fast regression sequence structure. SSQ2 shows a fast transgression–slow regression sequence structure in the area with large thickness and a sustained transgression–fast regression sequence structure in the area with small thickness owing to thickness differentiation caused by tectonic subsidence. Sedimentological analysis shows that sequences SQ1–SQ2 mainly consist of carbonate-ramp sediments, the sedimentary facies of which change from intermediate ramp to outer ramp as the area depressed topographically from south to north. Sequences SQ3–SQ5 mainly consist of rimmed carbonate platform sediments; the platform margin turned up along Shuangyushi–Jian’ge–Yuanba, and the region to the north subsided rapidly to form slope-basin facies. Lithofacies paleogeography in the Middle Permian epoch shows that the northwest margin of the Sichuan Basin was dominated by extensional stresses in northeast and northwest directions. Tectonic activities have a dominant control on the distribution of two structural sequences and the evolution of carbonate platform types in the Maokou Formation, and the sea-level change has a dominant control on third-order sequence boundaries and sedimentary facies migration. Thus, the Middle Permian tectono-sedimentary evolution model of the northwest margin of the Sichuan Basin is established. Research findings may offer new ideas and theoretical support to promising facies exploration in the Maokou Formation in the northwest margin of the Sichuan Basin.

Sequence Stratigraphic Analysis of the Volcanic Rock–Evaporite–Clastic Rock Symbiotic Zone in the Fourth Member of the Shahejie Formation in the South of Laizhou Bay

Laizhou Bay Sag is a typical continental rift lake basin, which is affected by strike–slip activity, salt rock activity, volcanic activity, and paleoclimate change in the Tanlu fault zone, where three major lithologies—volcanic rock, clastic rock, and evaporite—mainly develop. In order to determine the sequence correspondence between the different lithologies in the study area based on drilling, logging data, and seismic data, it is determined that the fourth member of the Shahejie Formation in the southern Laizhou Bay mainly develops three types of sequences: clastic rock type sequences, salt lake type sequences, and volcanic rock/clastic rock type sequences. Two second-order sequence boundaries (SBs3L, SBs4L) and one third-order sequence boundary (SBs4U) were identified, and the principle of three divisions of system tracts (LST, TST, HST) and sedimentary processes in different types of sequences were determined by analyzing the stacking patterns of different sequences. Compared with the previous research results, this study provides a more detailed analysis of the sequence boundary definition of different blocks and different lithologic strata, and it improves the previous general sequence stratigraphic division method. Based on the study of distinct types of sequences, a unified stratigraphic framework is established in the whole zone, and the stratum thickness distribution of the fourth member of Shahejie Formation is smaller in the southwest and larger in the northeast. The study area is generally filled with pyroclastic rocks–evaporite–clastic rocks, changing from fan delta–saline lake filling to braided river delta–freshwater lake–fan delta filling from bottom to top. A sequence stratigraphic model for the comparison of steep slope–gentle slope differences is established to provide guidance for future oil and gas field exploration and development.

Astronomical forced sequence infill of Early Cambrian Qiongzhusi organic-rich shale of Sichuan Basin, South China

Early Cambrian Qiongzhusi Formation has attracted great attention due to shale gas finding. However, this organic-rich shale formation is yet no robust high-resolution stratigraphic sequence framework. Here the evidence on Milankovitch cycles in three wells drilled through Qiongzhusi (QZS) Formation in Sichuan Basin has been reported. The logging data made it possible to develop a floating timeline calibrated to stable eccentricity cycles of 405-kyr within ~10.92 and ~7.34 Myr intervals in intracratonic rift and low bulge of Sichuan Basin, respectively. In this research, astronomical time scale was anchored to the UPb Zircon age of QZS Formation/Maidiping Formation boundary (526.86 ± 0.16 Ma), developing an absolute timescale for considered intervals extending from 526.86 to 515.94 Ma. Average spectral misfit and correlation coefficient analyses were applied to quantitatively measure the fitting of the witnessed sedimentary cycles to astronomical periods and provide a possible sedimentation rate range. A sedimentary noise model was chosen to detect high-resolution sea-level variations under the control of orbital forcing and variations of sea-level were linked to obliquity modulation cycles of about 1.2 Myr. The orbital obliquity modulation cycles witnessed for early Cambrian QZS Formation (~1.2 Myr) were correlated to eustatic variations of sea level generated based on sedimentary noise model (DYNOT and ρ1). Furthermore, correlations between sequence stratigraphy and cyclostratigraphy were discussed, and the third-order sequence boundaries well corresponded to minima ~1.2 Myr orbital obliquity modulation curve and high values of DYNOT. The previous sequences framework was demonstrated to be isochronous and credible.

Sequence architecture, slope-break development and geological significance during the P–T transition in the Mahu Sag, Junggar Basin, China

During the P–T transition period (from the Late Permian to the Early Triassic), the Mahu Sag of the Junggar Basin transitioned from a foreland basin to a large depression lake basin, with the development of two sets of unconformities (T1/P and P3w/P2w) and two sets of lowstand deposits, the upper Wuerhe Formation (P3w) and the Baikouquan Formation (T1b) that now host a series of large or giant conglomerate oil fields. For the two third-order sequence boundaries T1/P and P3w/P2w, two types of slope-break zones associated with flexure or erosion have been identified: the flexural slope-break zone controlled by a persistent paleo-uplift that also controls the layer-by-layer “unidirectional onlapping” of strata above the sequence boundary towards the higher part of the slope; and the erosional slope-break zone controlled by the erosional remnants of paleo-mounds, typically characterized by “two-way onlapping” towards the residual mounds or filling along the lower part of the valleys. Both types of slope-break zones control stratigraphy and sedimentary changes, and they encourage the formation of lithostratigraphic traps of up-dip onlapping, lowstand filling, and truncated types. The findings have a significant impact on the exploration of large-scale lithostratigraphic plays controlled by paleotopography.

Relative sea-level changes and organic matter enrichment in the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in the Central Yangtze area, China

Based on the sedimentological and geochemical analysis of total organic carbon (TOC) and major and trace elements in the marine Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in Yichang, Central Yangtze area, paleoredox conditions (indicated by U/Th, V/Cr, Ni/Co, TOC-TS-Fe diagram, and Mo–U covariations), paleoproductivity (indicated by Babio, Ba/Al, P/Ti, Ni/Al, Cu/Al, and Zn/Al), terrestrial detrital influx (indicated by Ti and Al), paleoclimate (indicated by chemical index of alteration and Rb/Sr), and upwelling (indicated by Co × Mn) are revealed within the framework of sea-level changes. The influences of these factors on organic matter enrichment are also discussed. The results identify four third-order sequence boundaries in the Wufeng-Longmaxi Formations; each sequence can be divided into one transgressive systems tract (TST) and one regressive systems tract (RST) according to sea-level changes. The high-TOC interval mainly developed in TST1 (average 3.44%) and TST2 (average 3.68%). The analysis of geochemical indicators shows that during the TST1 depositional stage, the climate was dry and cold, the bottom water was in a stable anoxic condition accompanied by rising sea level, and the seawater had moderate-high paleoproductivity. In the RST1 stage, extensive sea-level fall occurred, and the water was anoxic, with decreased productivity and weakened organic matter accumulation. In the TST2 stage, the sea level rise rapidly, extensive anoxicity and high productivity of the water produced the most ideal environment for organic matter enrichment and the highest TOC. In the RST2 stage, sea level slowly fell, the water column gradually became a stable oxidized environment, productivity was low, the terrestrial detrital influx increased and tended to stabilize, and the climate gradually changed to warm and humid conditions, forming relatively organic-poor shale. The upwelling mainly developed during the period of TST1 to TST2, and ceased in the RST2. The correlation analysis between TOC and the above indicators and the Co × Mn versus Cd/Mo diagram show that TOC is chiefly controlled by paleoredox, paleoproductivity, climatic conditions, and terrestrial detrital influx, indicating that preservation conditions are the crucial factor. Sea-level change played a key role in preservation conditions. In the transgression stages, sea-level rise led to the enhancement of reducing water, which controlled the organic matter enrichment in shale.

The Devonian–Carboniferous boundary in Belgium and surrounding areas

The Devonian–Carboniferous boundary is associated with a major extinction event of the Phanerozoic. It was also a time marked by a rapid but short-lasting change in deposition called Hangenberg Event. In the Namur–Dinant Basin the uppermost Devonian (‘Strunian’) deposits recorded a third-order transgression that produced a progressive switch from coastal siliciclastic to proximal mixed deposits with an increase of the carbonate production on the ramp. Hence, the Comblain-au-Pont and lower Hastiere formations are considered as the transgressive system tract, whereas the middle member of the Hastiere Formation is interpreted as the highstand system tract, capped by an erosion surface corresponding to the third-order sequence boundary. Superimposed on these third-order sequences are well-marked orbitally forced precession cycles (wet–dry climate alternations) of c. 18.6 ka, appearing as irregular c. 30–80-cm-thick couplets of limestone and calcareous shale beds. The Hangenberg Black Shale Event is locally present as dark shales that likely spread over the shelf, marking the maximum flooding surface of the sequence. Before and after this event, carbonate facies rich in benthic macrofauna and microfauna continued to develop. The Hangenberg Sandstone Event, appearing as a sandstone bed in pelagic sections, is variously recorded at the base of the Hastiere Formation, either as a sandy siltstone bed in proximal sections or as a horizon with limestone clasts and reworked fossils in more distal settings. The Hangenberg Sandstone Event beds occur sharply in the stratigraphic record and do not correspond to the long sea level fall of a third-order sequence boundary, but most probably to a short out-of-sequence event. The revision of the stratigraphic distribution of major fossil groups pleads for a continuous biostratigraphic succession with no obvious hiatus. The variable development of some micropalaeontological zones at the end of the Devonian is the result of complex eco-biostratigraphic interactions with the environment rather than the reflection of true hiatuses. It is marked by extinctions of Devonian taxa, concomitantly with the end of the reworking produced by the Hangenberg Sandstone Event, most probably immediately below the entry of the conodont Protognathodus kockeli. It is also coincident with the boundary between the foraminiferal zones DFZ7–MFZ1, rugose coral zones RC0–RC1 and between the palynozones LE–VI. After the short-lasting regressive phase of the Hangenberg Sandstone Event, normal depositional settings returned with the deposition of the Hastiere Formation. Hence, the end of the Hangenberg Sandstone Event is proposed as the most natural proxy to pinpoint the Devonian–Carboniferous boundary.

Pore structure response of sedimentary cycle in coal-bearing strata and implications for independent superposed coalbed methane systems

ABSTRACT Coalbed methane (CBM)-bearing systems are often superposed in coal-bearing strata with multiple coal seams. The formation and development of independent superposed CBM-bearing systems is based on the vertical variation of pore structure in coal-bearing strata. This study examines the potential of the pore structure cycle of the intervening rock formation in coal-bearing strata that compartmentalize the CBM system, which, in turn, assist in the production of the coal gas. The pore structure is characterized using lithology, sequence stratigraphy, and mercury injection porosimetry data generated from 25 samples of the intervening formation. The samples originate from a representative drilling operation located in the Bide-Santang basin, western Guizhou, China. We divided the pore structures into 4 types (I, II, III and IV): type I, “well sorted and fine slanting degrees;” type II, “well sorted and coarse slanting degrees with extremely low mercury saturation;” type III, “poorly sorted and uniform distribution” and type IV, “poorly sorted and polarization of micropores and macropores.” Pore structure changes cyclically in accordance with the sedimentary cycle: pores developed poorly within rocks that formed in deep water bodies with low energy and reducing sedimentary environments, such as lagoons and limited subtidal and mire facies; pores are better developed within rocks formed in shallow water bodies with high energy and oxidizing environments, such as supratidal flats, intertidal zones, and distributary channel facies. Third-order sequence boundaries (SB) favor the development of secondary porosity, whereas the maximum flooding surface (MFS) possesses a superior condition for forming regional impermeable barriers between different CBM-bearing systems. The sequence stratigraphic framework limits the variation trend of the pore structure and therefore limits the vertical distribution of independent superposed CBM-bearing systems. The findings are of great significance for enriching the geological theory of multi-seam CBM accumulation, optimizing the co-mining methods and improving the co-mining efficiency of superposed CBM systems.

To establish a sequence stratigraphy in lacustrine rift basin: A 3D seismic case study from paleogene Baxian Sag in Bohai Bay Basin, China

The varied range usually corresponds to different levels of sequence boundaries in the lacustrine basin. The correspondence between the unconformity of the Paleogene and the sequence boundaries are studied in this article, based on an integrated analysis of 3D seismic reflections, wire logs and cores. Fifteen third-order sequence boundaries are identified and the Paleogene of Baxian sag can be divided into one first-order sequence, 3 s-order sequences and fourteen third-order sequences. Four types of sequence architectures are identified: fault slope break type, fault bench-slope break type, weak fault slope-bend slope type, fault steep slope-gentle slope type. We try to set ideas and processes suitable for the study of the sequence of lacustrine rift basins. It is believed that the sequence boundaries and orders in the continental fault lacustrine basin are comprehensive responses to tectonic activities and time. The tectonic activity is not only the main contributor to the accommodation, but also control the difference of spatial and temporal inheritance characteristics and fault spatial combination, thus controlling the combination of the depositional systems under the sequence frame and adjust the spatial dispersion characteristics of the sediment. Therefore, the sequence stratigraphic units of the lacustrine rift basin are highly consistent with tectonic evolution, stratigraphic and sedimentary cycles.

Sequence stratigraphy of the lacustrine rift basin in the Paleogene system of the Bohai Sea area: Architecture mode, deposition filling pattern, and response to tectonic rifting processes

The Bohai Sea area is a complex lacustrine rift basin characterized by multistage rifts, polycycle superimpositions, and multiple genetic mechanisms. We recognized three types of sequence boundaries from the Paleogene strata of offshore Bohai, including 2 first-, 3 second-, and 10 third-order sequence boundaries. The third-order wedges, composed of the low-stand system tract (LST), lake expansion system tract (EST), and high-stand system tract (HST), are significant for hydrocarbon exploration. The sequence stratigraphic patterns vary in different periods. We discovered that the LST and EST are mainly developed in the rifting stage, whereas the HST is developed in the subsidence stage. Sequences developed in different depressions present significant variations, but the migration of the depocenter from the margin to the center during basin evolution can still be clearly recognized. Vertically, episodic tectonism plays an important role in controlling the formation of sequence boundaries, sequence structural features, and sequence architectural patterns. However, various rift basin marginal tectonics control the lateral sequence architectures. Because each structure belt is characterized by a unique sequence framework due to the strong influence of basin rifting processes, we establish a corresponding sequence stratigraphic model for the specific tectonic environment using process-based sequence stratigraphic methods.

Coupled stratigraphic and U-Pb zircon age constraints on the late Paleozoic icehouse-to-greenhouse turnover in south-central Gondwana

Abstract The demise of the Late Paleozoic Ice Age has been hypothesized as diachronous, occurring first in western South America and progressing eastward across Africa and culminating in Australia over an ∼60 m.y. period, suggesting tectonic forcing mechanisms that operate on time scales of 106 yr or longer. We test this diachronous deglaciation hypothesis for southwestern and south-central Gondwana with new single crystal U-Pb zircon chemical abrasion thermal ionizing mass spectrometry (CA-TIMS) ages from volcaniclastic deposits in the Paraná (Brazil) and Karoo (South Africa) Basins that span the terminal deglaciation through the early postglacial period. Intrabasinal stratigraphic correlations permitted by the new high-resolution radioisotope ages indicate that deglaciation across the S to SE Paraná Basin was synchronous, with glaciation constrained to the Carboniferous. Cross-basin correlation reveals two additional glacial-deglacial cycles in the Karoo Basin after the terminal deglaciation in the Paraná Basin. South African glaciations were penecontemporaneous (within U-Pb age uncertainties) with third-order sequence boundaries (i.e., inferred base-level falls) in the Paraná Basin. Synchroneity between early Permian glacial-deglacial events in southwestern to south-central Gondwana and pCO2 fluctuations suggest a primary CO2 control on ice thresholds. The occurrence of renewed glaciation in the Karoo Basin, after terminal deglaciation in the Paraná Basin, reflects the secondary influences of regional paleogeography, topography, and moisture sources.

The complex transgression and regression history of the northern margin of the Palaeogene Tarim Sea (NW China), and implications for potential hydrocarbon occurrences

Abstract The Tarim Basin is an epicontinental marine basin that developed during the Palaeogene in the eastern margin of the Paratethys Ocean. Palaeoclimate models suggest that sea-level fluctuations were the main driving force causing the Palaeogene aridification of Asia's interior and the increase in monsoon intensity. The Palaeogene succession in the northern margin of the south-western Tarim Basin is composed of five lithofacies associations in which two third-order sequences and three third-order sequence boundaries are present that correspond a complex history of trans- and regressions. Sequence 1 formed in an epicontinental sea during the Palaeocene and early Eocene, whereas Sequence 2 represents a transitional stage from marine to terrestrial conditions during the late Eocene and Oligocene. Part of this development was synchronous with the development along the southern margin of the south-western Tarim Basin, but the transition from marine to continental along the northern margin of the south-western Tarim Basin did not take place until the late Eocene, when the sea retreated westwards. The present contribution can help reconstruct the Palaeogene palaeogeography of the Paratethys regions and may also facilitate paleoclimate modelling for central Asia. It also suggests that there is a good hydrocarbon potential in the western and southern parts of the south-western Tarim Basin.

High resolution linkage of channel-coastal plain and shallow marine facies belts, Desert Member to Lower Castlegate Sandstone stratigraphic interval, Book Cliffs, Utah-Colorado, USA

AbstractThe Campanian Desert Member and Lower Castlegate Sandstone in the Book Cliffs of east-central Utah to western Colorado, USA, has served as a foundational data set in the development of sequence stratigraphy. Contrary to previous work, no third-order sequence boundaries are recognized. These were originally thought to partition the neighboring coastal plain and shallow marine facies belts into separate systems tracts, unlinked in time or space. In contrast, adjoining channel-coastal plain and shallow marine facies belts are genetically-, temporally-, and spatially-related. Evidence includes the (i) synchronous, strongly progradational stacking patterns within each facies belt, (ii) gradational and conformable transitions between adjoining facies belts, accentuated by the ubiquity of flat-topped, rooted foreshore sandstones passing upwards into carbonaceous-rich-mudstone-dominated coastal plain, (iii) parasequence-scale interfingering of coastal plain-channel and foreshore-shoreface deposits, with channels, white caps and coals embedded within stacked shoreface parasequences, (iv) regional correlation of coals and flooding surfaces, and (v) near orthogonal paleocurrent relationship between channels and shorelines. Terminal channels incise into proximal foreshore-shoreface sandstones in most Desert-Castlegate parasequences. Incisions are generally confined to the parasequence in which the channels are nested, rarely cutting deeper. These shoreface-incised channels are cut and filled at a parasequence-scale, and are bounded above by the same flooding surface that caps each foreshore-shoreface package. The ubiquity of ascending regressive shoreface trajectories and near absence of descending regressive trajectories that intersect depositional slope argues against any significant sea level fall. Increased rates of sediment supply, driven by autogenic and/or allogenic processes, likely generated the strongly progradational Desert-Castlegate great tongue of sandstone.

Integration of static and dynamic data and high-resolution sequence stratigraphy to define reservoir architecture and flow units within a ‘super giant’ gas condensate and oil field, Kazakhstan

Abstract The Upper Serpukhovian (Mississippian) interval of the Karachaganak reservoir comprises stratified platform interior carbonates, which are characterised by relatively high porosity and matrix permeability (maximum 23.8% and 2198 mD, respectively), and comprise high frequency (fourth-order) glacio-eustatic cycles. Based on static (geological) and dynamic (well test and production) data, this cyclicity exerts a strong control on gas migration pathways, and reservoir quality is predictable within a third- and fourth-order sequence stratigraphic framework that defines two discrete flow units. The contribution to flow made by fourth-order cycles within flow units is dependent on their position within third-order transgressive or highstand systems tracts. An upper flow unit comprises thick (up to 40 m) fourth-order cycles within a third-order transgressive systems tract. These fourth-order cycles contain large scale, clinoformal bedforms, can be correlated for up to 6 km, and are the main pathways of injected gas through the Upper Serpukhovian reservoir. A lower flow unit comprises fourth-order cycles (within a third-order highstand systems tract) that are thinner, and less laterally continuous, than those in the upper flow unit, and contribute far less to horizontal gas flow. Pressure data indicate that vertical permeability baffles are developed at the third-order sequence boundary that separates the two flow units (linked to diagenetic alteration during prolonged subaerial exposure), and also within the low porosity, mud-rich, transgressive bases of thick fourth-order cycles of the upper flow unit, that formed during periods of increased accommodation space across the platform top during third-order transgression. The scale at which this data set is observed is critical to understanding the dynamic behaviour of the reservoir. Relatively small differences in the composition of microfacies of cycles within both flow units do not explain the variations in dynamic data. This variation is better explained within the broader context of the sequence stratigraphic framework as defined from analysis of cycle stacking patterns. Thicker high frequency cycles deposited in increased accommodation space on the platform top during third-order transgressive intervals are key to developing thick intervals of net pay.

Sequence stratigraphy control on fossil occurrence and concentration in the epeiric mixed carbonate-siliciclastic ramp of the Early Permian Irati Formation of southern Brazil

The Early Permian Irati Formation records sedimentation of a mixed carbonate-siliciclastic, storm-influenced ramp in a shallow and restricted epeiric sea. It is formally divided in the dark grey mudstones of the lower Taquaral Member, and alternating dark grey mudstones, bituminous black shales, and carbonates of the upper Assistência Member. The formation crops out from central to southern Brazil on the eastern border of the intracratonic Paraná Basin. Fossils of mesosaurid reptiles, pygocephalomorph crustaceans, and subordinate paleonisciform fish form dense concentrations within carbonate tempestites, interpreted as mass mortality events caused by storms. The present study describes the Passo do São Borja outcrop and five new locations, along with 11 drill cores of the complete Irati interval in the Rio Grande do Sul state, applying methods of facies analysis and sequence stratigraphy. Three fourth-order sequences within the Irati Formation were recognized, termed sequences 1, 2, and 3 (S1, S2, S3). S1 is broadly coincident with the Taquaral Member, and S2 and S3 with the Assistência Member. The upper two sequences are composed of a lowstand systems tract (LST), a transgressive systems tract (TST), and a highstand systems tract (HST), whereas the lower sequence lacks a LST. Sequence boundary SB2 was interpreted as coincident with a third-order sequence boundary. Mesosaurid and pygocephalomorph concentrations occur only in S3. Fossil occurrences and concentration were found to be strongly controlled by facies and sequence stratigraphy. Paleonisciform fish fossils are considerably more common at the early HST of S2 and S3, resulting from balanced sedimentation to accommodation rates. Pygocephalomorphs compose intraclasts concentrated in grainstone proximal tempestites of the LST in S3. They are the result of mass mortality events and background bioclastic input, enhanced by firmground formation (hiatal concentration) and subsequent storm reworking. Mesosaurids are also concentrated in grainstone tempestites of the LST, but in cores they were mostly found in distal tempestites of the TST, becoming more common towards the maximum transgressive surface (MTS). Their accumulation is also the result of mass mortality and background sedimentation, enhanced by sediment omission (hiatal concentration) of a severely sediment starved TST (culminating in a condensed section near the MTS), and subsequent storm reworking.

Depositional Cycles in a Lower Cretaceous Limestone Reservoir, Onshore Abu Dhabi, U.A.E.

AbstractThe upper reservoir zone of the Lower Cretaceous Kharaib Formation (46–54 m thick in the studied wells) is regarded as the upper portion of a third-order depositional sequence comprising higher-order cycles. Whereas the third-order sequence interpretation is clearly supported by the upward-shoaling trend of the reservoir zone, relationships defining the component cycles have not previously been documented and are the focus of the present study. Core descriptions from four wells in a single oilfield reveal little evidence of facies changes or trends of facies patterns indicative of high-frequency depositional cycles. Cycle boundaries could possibly be represented by the repetitive pattern of coarse beds (rudstone and floatstone) 0.1–2 m thick, commonly having sharp basal contacts and gradational upper contacts with enclosing packstone to wackestone. Because the coarse beds do not appear correlative between wells, however, we prefer the alternative interpretation that they reflect episodic storm events which locally redistributed detritus, sourced from a patchwork of low-relief lithosomes, across the flat surface of the epeiric Kharaib platform–lagoon. Although the existence of high-order eustatic fluctuations during upper Kharaib deposition is well established, low-amplitude variations in water depth may not have touched down on the sea floor to significantly affect sediment textures in contrast with the dominant storm signal.Reservoir sub-zones used for production operations, but previously suggested to be fourth-order parasequence sets, are defined by dips in porosity-log profiles, reflecting thin (approximately 1 m) intervals of increased stylolite frequency. These boundaries are thus diagenetic in character, but their correlation over tens to hundreds of kilometers indicates an underlying depositional control. We suggest that the link between sea level and diagenesis is depositional-clay content, which facilitates stylolitic dissolution. Profiles of bulk-rock alumina analyses in the studied cores show subtle indications of higher clay content at the sub-zone tops. Much greater clay peaks mark the third-order sequence boundaries, resulting in the “dense” (very low porosity) zones above and below the studied reservoir zone and the increased stylolite frequency in the upper and lower several meters of the zone. Possible factors promoting clay influx across a carbonate shelf during falls in sea level include increased stream gradients and more humid climate.

Forming mechanism of the Ordovician karst carbonate reservoirs on the northern slope of central Tarim Basin

The Ordovician karst carbonate reservoirs on the northern slope of central Tarim Basin are important oil and gas exploration targets in the basin, but their dissolution mechanisms are in controversy. In this paper, based on the integrated study of sedimentation, sequence and reservoir, together with microscopic analysis and macroscopic seismic data analysis, the carbonate karst reservoirs in the study area were divided into three types: dissolved pore-cavity limestone reservoir, pore-cavity dolomite reservoir and fracture-cavity siliceous reservoir, and their forming mechanisms were discussed respectively. Some findings were obtained. First, dissolved pore-cavity limestone reservoirs are distributed in the upper Yingshan Fm and Yijianfang Fm of the Ordovician vertically, while pore-cavity dolomite reservoirs are mainly developed in the Penglai Fm and lower Yingshan Fm of the Ordovician with great thickness. Second, dissolved pore-cavity limestone reservoirs were formed by karstification on the third-order sequence boundary (lowstand tract), while pore-cavity dolomite reservoirs were formed by deep burial dolomitization controlled by karstification on the third-order sequence boundary, both of which are distributed in the highstand tract below the third-order sequence boundary. Third, siliceous reservoirs are developed under the control of faulting, as a result of reworking of deep hydrothermal fluids along faults to the limestone, and the siliceous reservoirs and their hydrothermal solution fracture-cavity systems are distributed near faults. It is further predicted that, in addition to the three types of reservoir above, platform-margin reef-flat reservoirs are developed in the Ordovician on the northern slope of central Tarim Basin.

Kimmeridgian hardground-sequence boundary from the Mesozoic margin of the Holy Cross Mountains (central Poland): implications for the evolution of the northern Tethyan carbonate shelf

The sedimentary succession in central Poland records significant changes in facies at the turn of the Planula and Platynota zones in the Upper Jurassic, expressed by the drowning of the ramp-type platform and development of an extensive isochronous marl horizon. The topmost level of the marl horizon is a regional hardground, which is interpreted as the third-order sequence boundary Kim 1. In some areas, the hardground was eroded and is only preserved as bored and encrusted clasts. The composition of the borings may indicate that colonization and recolonization of the clasts took place in an extremely shallow water environment. The overall low level diversity of clast-encrusting organisms and their occurrence on both sides of clasts indicates frequent overturning and high current activity. However, other extrinsic factors, such as salinity fluctuations, may have been involved. The final redeposition and burial of the clasts were related to subsidence through widespread reactivation of Paleozoic faults. Comparison with Middle Oxfordian–Lower Kimmeridgian sequences of central and southern Poland indicates that the carbonate ramp morphology and paleoenvironmental conditions were periodically subjected to significant modification during phases of extensional tectonics, with the development of fault-controlled intra-platform ridges and basins. These features may have been the NE continuation of parallel swells and basins developed in the southern part of the carbonate platform adjacent to the Tethys.

Half-year results of Johnson Matthey for the six months ended 30 Sep 2016

Most shallow marine sections described from the Lower Triassic formed in mixed carbonate-siliciclastic systems. Shallow marine limestones from an isolated carbonate platform, the Great Bank of Guizhou, in the Nanpanjiang Basin, South China, provide a unique opportunity to examine a pure peritidal carbonate depositional system. The platform had a low-relief bank profile in the Early Triassic with oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and pelagics, debris-flow deposits and turbidites on gentle basin-margin slopes. Interior strata are 265 m thick beginning in the lowermost Triassic with Renalcis biostromes, followed by lime mudstone, oolite, and cyclic peritidal limestone.The cyclic limestone is Olenekian in age, 164 m thick, and contains up to 83 cycles (parasequences). Parasequences are 0.2–7.4 m thick and typically shallow upward from subtidal oolite grainstone and skeletal packstone facies followed by Renalcis reef mounds or biostromes and capped by intertidal flaser-bedded ribbon rock. The skeletal packstone and reef mounds contain a low-diversity biota of cyanobacteria, echinoderms, bivalves, gastropods, lingulid brachiopods, spirorbids, and ostracodes. Ribbon rock contains alternating lime mud and fine peloidal packstone and grainstone laminae, scour surfaces, ripple cross lamination, lime-mud drapes, and minor prism cracks. Parasequence stacking patterns exhibit gradual increase and decrease of parasequence thickness, defining a third-order sequence boundary and several 4th-order parasequence sets that correlate between interior sections. Parasequence stacking patterns similar to those of Lower Palaeozoic green-house sequences and different from ice-house sequences.The Lower Triassic platform-interior parasequences represent an anachronistic facies in that they are strikingly similar to the common Renalcis-mound, ribbon rock parasequences of the Lower Palaeozoic and different from other Permo–Triassic parasequences (e.g. Capitan shelf parasequences or Loferites). Facies similarities such as Renalcis mounds and flaser-bedded intertidal ribbon rock reflect anomalous oceanic conditions resulting in low biodiversity and low intensity of bioturbation after the end-Permian extinction. Similarities in parasequence stacking patterns reflect low-amplitude, high-frequency sea-level fluctuations resulting from green-house conditions common to the Early Palaeozoic and Early Triassic.

Upper Cenomanian-Lower Campanian Derdere and Karababa formations in the Çemberlitaş oil field, southeastern Turkey: their microfacies analyses, depositional environments, and sequence stratigraphy

The frontal belt of the southeastern Anatolia fold-thrust belt in Turkey contains several small to mid-size oilfields, producing from carbonate reservoirs of the Cretaceous Mardin group. Many of these fields are found along narrow, asymmetrical anticlinal structures, associated with the formation of the fold-thrust belt. The Cemberlitas oil field in Adiyaman, southeastern Turkey, is one of the most important oilfields in the region. It produces from the Upper Cretaceous Derdere and Karababa formations of the Mardin group. We have conducted a detailed study of the microfacies, depositional environments, and sequence stratigraphy of the Karababa (Coniacian-lower Campanian) and Derdere (mid-Cenomanian-Turonian) formations in the oil field. Eight microfacies in the Karababa and Derdere formations have been identified; the microfacies in the Karababa formation are 1) mollusk-echinoid wackestone/packstone, 2) dolomitic planktonic foraminifera wackestone, 3) planktonic foraminifera bearing wackestone/packstone, and 4) phosphatic-glauconitic planktonic foraminifera bearing wackestone. The microfacies in the Derdere formation are 5) lime mudstone, 6) bioclastic wackestone/packstone, 7) medium-coarse crystalline dolomite, and 8) fine crystalline dolomite. These microfacies suggest that the Derdere formation was deposited in lagoonal to shelf depositional environments and the Karababa formation was deposited in a deep to shallow marine intrashelf basin. Two third-order sequence boundaries of late Turonian and early Campanian in age have been recognized in the reservoir interval. Depositional sequences contain transgressive and highstand systems tracts. These sequences are compared with those in other regions to differentiate the local, regional, and global factors that controlled sedimentation within the Cemberlitas oil field area.

Upper Cretaceous Sequence Stratigraphy of the Rock Springs Uplift, Wyoming

Three scales of sedimentary cyclicity are observed in the Campanian to Maastrichtian-aged strata of southwestern Wyoming based on sedimentary stacking patterns, facies successions, paleontological data and geometric criteria from outcrop, and subsurface data. The largest scale of sequence stratigraphic interpretation is of 10-15 million years duration per cycle and is designated as second order. Second-order cyclicity is represented by profound sequence boundaries and maximum-flooding surfaces that are related to regional tectono-subsidence drivers. The 2nd-order sequence boundaries at the base of the Ericson Formation (Moxa Unconformity) and the base of the Fort Union Formation (Laramide Unconformity) locally erode thousands of feet of section on discrete structural highs, but also show evidence of uplift on a broader scale. Both unconformities terminate long successions of marine shoreline progradation. Second-order maximum flooding surfaces are present within thick marine mudstone successions of the Baxter-Niobrara and Lewis formations. These intervals represent the deepest water deposits in the Upper Cretaceous and are associated with important regional seals and source rocks. The large-scale tectono-cyclicity is composed of an aggregate of smaller-scale, 3rd- and 4th-order sequences and their component systems tracts. In addition to the second-order sequences described above, third-order sequence boundaries are interpreted at the base of the Blair Formation, base of the Chimney Rock Member of the Rock Springs Formation, and at the base of the Canyon Creek Member of the Ericson Formation. Third-order maximum flooding surfaces are in the middle Blair Formation, middle Black Butte Member of the Rock Springs Formation, and in the upper part of the Rusty Member of the Ericson Formation. At least 7 additional 4th-order sequences can be interpreted within these successions. The higher-order cycles are shorter in duration and smaller in magnitude (i.e., thinner and with a smaller degree of change in environments or bathymetry within a cycle). The sequence stratigraphic interpretation approach is observationally based and consistent across these scales. However, finer-scale sequence stratigraphic interpretations, especially at the 4th-order scale, are subject to additional subjectivity. An important challenge is to separate apparent cyclicity related to factors such as local to regional shifting of depocenters (autocyclicity) from regionally correlative cyclicity (allocyclicity) within the high-frequency sequences. Examples of this are provided, most clearly for the lower portion of the Blair Formation. Such considerations are important economically, as this is the scale that controls sandstone reservoir and mudstone seal architecture within petroleum fields. Standard systems tracts criteria using parasequence stacking patterns are extended to non-marine strata by use of the degree of amalgamation (net/gross) of the fluvial sandstones. This approach is demonstrated to be plausible via physical ties to shoreline systems using regional correlations and paleontology, and via estimates of accommodation change from geohistory analysis.