Mudstone Intervals Research Articles

Integration of Formation Evaluation Data Overcomes Offshore Coring Challenges

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 195866, “Overcoming Coring Challenges in a New Unconventional Play Offshore by Integration of Formation Evaluation Data,” by Thomas Bradley, SPE, Baker Hughes, and Per Henrik Fjeld and Jonathan Scott, Aker BP, et al., prepared for the 2019 SPE Annual Technical Conference and Exhibition, Calgary, 30 September-2 October. The paper has not been peer reviewed. A well was drilled into a prospective unconventional mudstone play offshore Norway. Two of five coring runs were successful while the rest yielded little to no core recovery. Subsequent investigation of the core substantiated that the coring issues largely had natural causes. This understanding is being applied to two imminent coring operations and has driven selection of drilling, coring, and wireline technology and procedures and is informing casing design. Introduction The Valhall field is in Block 2/8 of the Norwegian North Sea. The primary reservoir is Cretaceous chalk, located at approximately 2400 m true vertical depth. Production of this reservoir began in 1982. Above this reservoir are other formations, including a shallower potential unconventional mudstone reservoir. A production well was drilled targeting the Cretaceous chalk reservoir. As part of the drilling program, logging-while-drilling (LWD) and wireline logs - including LWD density, neutron, gamma ray and acoustic, wireline spectral gamma ray, cross-dipole acoustic, nuclear-magnetic-resonance, and formation-testing and -sampling - were run across the potential mudstone reservoir to appraise the standard petrophysical properties. A conventional core was also attempted; however, recovery was poor, with repeated jams. Coring Challenges During coring operations, significant problems were encountered when coring the mudstone intervals. These challenges are illustrated in Fig. 1, which shows a section of the core from xx14 m, demonstrating the typical condition of the recovered core when still in the core barrel. At the time of operation, poor recovery was attributed to the coring practices used and no further coring was attempted. Formation Evaluation The mudstone interval was not a primary target for the well. Therefore, limited formation evaluation of these intervals was performed, mainly to estimate the petrophysical properties. However, at the time of initial logging, more-advanced formation evaluation data (e.g., images, full waveform acoustic, and nuclear-magnetic-resonance data) were not evaluated fully. A few years after initial operations, the log data were revisited - specifically, detailed interpretation was conducted for all recorded log measurements. On examination of the results over much of the interval, the integration of the conventional and advanced volumetric measurements gave considerable extra value in assessing the petrophysical properties of the mudstone. However, detailed discussion of these results is outside the scope of this paper. In addition to the extra information gleaned from the advanced volumetric interpretation, over certain intervals, some unusual log responses and interpretation results were seen that were deemed worthy of further investigation.

Cyclostratigraphy and astronomical tuning during the Oligocene in the Jizhong Depression, Bohai Bay Basin, northeastern China

The Dongying Formation (Ed) and the first member of the Shahejie Formation (Es1) in the central Raoyang Sag cover about 1705 m-long sedimentary series of terrestrial lacustrine infill. We employed the gamma-ray (GR) log data as a paleoclimatic and paleoenvironmental proxy to conduct the high-resolution cyclostratigraphy research in well W33 and W62. Prominent sedimentary cycles in decameters and meters were revealed by multi-taper (MTM) power spectra, evolutionary fast Flourier transformation (FFT) and correlation coefficient analyses in the Es1 and Ed intervals. The 405 kyr long eccentricity, 125 kyr and 100 kyr short eccentricity, 40 kyr obliquity, and 23 kyr precession can be recognized with a wavelength ratio of approximately 20:5:2:1, respectively. By tuning the 405 kyr filter output in well W33 to the orbital target curve of La2010d astronomical solution, we established an astronomical time scale (ATS) that provided absolute age controls during the Oligocene and shed light on the event correlation among isolated depressions in the Bohai Bay Basin. Sediment accumulation rates (SARs) were derived from the extracted 405 kyr long eccentricity and were tested with an average of 17 cm/kyr. Age boundaries of Ng/Ed1, Ed1/Ed2, Ed2/Ed3, Ed3/Es1 and Es1/Es2 are 23.03 Ma, 24.34 Ma, 25.53 Ma, 29.05 Ma and 33.10 Ma, respectively. Based on the ATS and SARs, we discussed the depositional evolution within age boundaries, cause of high SARs and origin of formation overpressure. After the largest lake transgression, shallow-water deltas and fluvial plains transitionally dominate the central Raoyang Sag with shallowing water depth. Thin interlayers of mudstones and sandstones originate from shallow-water deltaic systems with low sand-to-mud ratios. Under high SARs enhanced by mountain uplift and low sand-to-mud ratios, formation overpressure results mainly from high porosities in under-compacted mudstone intervals.

Quantitative palynological analysis of the E2a and E2b goniatite biozones (Arnsbergian, Mississippian) in mudstones from the Southern Pennine Basin (U.K.)

We performed a quantitative palynological analysis of Arnsbergian (Namurian, Late Mississippian) mudstone intervals, potentially prospective for unconventional hydrocarbons. While many palynological studies exist on these stratigraphic intervals in the Widmerpool Gulf and the Edale Basin (sub-basins of the Pennine Basin), very few studies perform full statistical analyses. Using the Carsington Dam Reconstruction C3 (Carsington DRC3, Widmerpool Gulf) and the Karenight-1 (Edale Basin) boreholes, we show that the combination of quantitative palynological data and detrended correspondence analysis (DCA) can aid biozonation and additionally, provide paleoecological constraints to the Arnsbergian mudrocks. The studied interval in Carsington DRC3 was assigned to the TK miospore biozone and a hitherto undescribed peak in the fresh water alga Botryococcus was recorded. Using relative abundances of hinterland species, mainly from the genus Florinites, both boreholes could be correlated and a more confident assignment of the TK miospore biozone covering an interval containing goniatite biozone E2b in Karenight-1 was achieved. The techniques used in the current study should be especially valuable for assessing borehole materials where the recovery of macrofossils, like goniatites used as the main biostratigraphic tool in the Namurian, can be very low. Future studies should focus on the same stratigraphic interval from different sub-basins of the Pennine Basin to further assess the applicability of quantitative palynological analysis combined with DCA as a stratigraphic tool for potentially prospective mudstones.

Distinct deep subsurface microbial communities in two sandstone units separated by a mudstone layer

Deep subsurface microbial communities are more abundant in coarse-grained sedimentary rocks such as sandstones than in fine-grained mudstones. The low porosity and low permeability of mudstones are believed to restrict microbial life. Then, it is expected that distinct, isolated microbial communities may form in sandstones separated by mudstones. In this context, the connectivity between microbial communities in different sandstone units can be investigated to infer evolutionary patterns of diversification in space-time, which may potentially contribute with relevant data for analyses of hydraulic connectivity and stratigraphic correlation. In this work, we used high throughput DNA sequencing of a ribosomal 16S gene fragment to characterize the prokaryotic communities found in Permian sandstone samples of the same core that are separated by one mudstone interval, in the Charqueadas coal field, Parana Basin (Southern Brazil). Our samples were collected at ∼300 m deep, in porous sandstones separated by a thick mudstone package. Differences in the bacterial community structure between samples were observed for the classified OTUs, from phylum to genus. Molecular biology might be further applied as a possible tool to help to understand the spatial and temporal distribution of depositional facies, and the efficiency of low permeability rocks to compartmentalize reservoirs. Ongoing studies aim to extend the present investigation into further analyses regarding lateral changes in microbial communities present in the same sandstone units.

Cratonaia novaolindensis gen. et sp. nov. (Unionida, Silesunionoidea) from the Aptian of Brazil (Araripe Basin), and its implications for the early evolution of freshwater mussels

The fossil-rich carbonate deposits of the Aptian Crato Formation, Araripe Basin (Brazil) are one of the main Cretaceous Konservat-Lagerstätten of Gondwana, and have come to fame globally. However, information on fossils from deposits other than the famous laminites of the basal part of the unit is scarce. Herein, we describe the first bivalves of the suborder Silesunionidina Skawina and Dzik, 2011 in the order Unionida Gray, 1854 from South America. The specimens were collected from a 0.3–1-m-thick grey to yellow mudstone interval located 0.3 m above the laminated limestones of the lower part of the Crato Formation at Nova Olinda, State of Ceará. They comprise exquisite composite, internal and external moulds, preserving key anatomical characters. Based on the analysis of muscle scars, hinge and ornamentation, these bivalves are here assigned to a new genus and species, Cratonaia novaolindensis gen. et sp. nov. The presence of a series of small pedal elevator scars linearly arranged on the external wall of the umbonal cavity indicates that this is a member of the suborder Silesunionidina. The new form is the by far youngest representative of this group. Closely related bivalves were previously reported from Triassic deposits of Australia, Africa, Europe and potentially India. Detailed stratigraphic, sedimentological and taphonomic observations indicate that the new taxon thrived in a freshwater lake. The occurrence of Silesunionoidea in the Lower Cretaceous of South America indicates that the condition of the musculature in Mesozoic freshwater mussels needs to be established to assign them confidently at family level.

Dynamic climate-driven controls on the deposition of the Kimmeridge Clay Formation in the Cleveland Basin, Yorkshire, UK

Abstract. The Kimmeridge Clay Formation (KCF) is a laterally extensive, total-organic-carbon-rich succession deposited throughout northwest Europe during the Kimmeridgian–Tithonian (Late Jurassic). It has recently been postulated that an expanded Hadley cell, with an intensified but alternating hydrological cycle, heavily influenced sedimentation and total organic carbon (TOC) enrichment by promoting primary productivity and organic matter burial in the UK sectors of the Boreal Seaway. Consistent with such climate boundary conditions, petrographic observations, total organic carbon and carbonate contents, and major and trace element data presented here indicate that the KCF of the Cleveland Basin was deposited in the Laurasian Seaway under the influence of these conditions. Depositional conditions alternated between three states that produced a distinct cyclicity in the lithological and geochemical records: lower-variability mudstone intervals (LVMIs) which comprise clay-rich mudstone and higher-variability mudstone intervals (HVMIs) which comprise TOC-rich sedimentation and carbonate-rich sedimentation. The lower-variability mudstone intervals dominate the studied interval but are punctuated by three ∼ 2–4 m thick intervals of alternating TOC-rich and carbonate-rich sedimentation (HVMIs). During the lower-variability mudstone intervals, conditions were quiescent with oxic to suboxic bottom water conditions. During the higher-variability mudstone intervals, highly dynamic conditions resulted in repeated switching of the redox system in a way similar to the modern deep basins of the Baltic Sea. During carbonate-rich sedimentation, oxic conditions prevailed, most likely due to elevated depositional energies at the seafloor by current–wave action. During TOC-rich sedimentation, intermittent anoxic–euxinic conditions led to an enrichment of redox-sensitive and sulfide-forming trace metals at the seafloor and a preservation of organic matter, and an active Mn–Fe particulate shuttle delivered redox-sensitive and sulfide-forming trace metals to the seafloor. In addition, based on TOC–S–Fe relationships, organic matter sulfurization appears to have increased organic material preservation in about half of the analysed samples throughout the core, while the remaining samples were either dominated by excess Fe input into the system or experienced pyrite oxidation and sulfur loss during oxygenation events. New Hg∕TOC data do not provide evidence of increased volcanism during this time, consistent with previous work. Set in the context of recent climate modelling, our study provides a comprehensive example of the dynamic climate-driven depositional and redox conditions that can control TOC and metal accumulations in a shallow epicontinental sea, and it is therefore key to understanding the formation of similar deposits throughout Earth's history.

Chitinozoan biostratigraphy of the Rheidol Gorge Section, Central Wales, UK: a GSSP replacement candidate for the Rhuddanian–Aeronian boundary

AbstractAs part of an international effort to evaluate potential replacement sections for the current Aeronian GSSP, the stratigraphy of the Rheidol Gorge section (central Wales, UK) is being restudied. A total of 28 samples from the Rhuddanian–Aeronian boundary interval at Rheidol Gorge were studied for chitinozoans. This famous Welsh Basin section, its stratigraphy rooted in the work of O.T. Jones and M. Sudbury, comprises a series of laminated black shales with abundant graptolites, alternating with grey mudstone intervals, deposited in anoxic to dysoxic/oxic seafloor conditions, respectively. The exposed section of c. 20 m spans the middle part of the upper Rhuddanian Pernerograptus revolutus Biozone through the lower Aeronian Neodiplograptus magnus Biozone of the lower to mid‐Llandovery Series (Cwmere and Derwenlas formations). Our chitinozoan analysis yielded an assemblage of over 2300 specimens distributed approximately uniformly throughout the entire succession. A total of 16 different taxa are identified, of which 12 remain in open nomenclature, reflecting the poor to moderately intact preservation of the specimens. Based on the presence of key species such as Spinachitina maennili and Conochitina iklaensis, the Rheidol Gorge section is referred to the Spinachitina maennili Biozone. This observation is in perfect alignment with the typical signature of other Rhuddanian–Aeronian boundary successions in the Welsh Basin and in other parts of the world.

Coupled seepage-mechanical modeling to evaluate formation deformation and casing failure in waterflooding oilfields

Waterflooding implemented to enhance oil recovery could concomitantly induce formation deformation and casing failure. The adverse responses should be evaluated to ensure the safety and performance of wells. A coupled seepage-mechanical model is developed to analyze the responses of formation and wells in waterflooding oilfields. The fluid-solid coupling method is employed to capture the flow and deformation in interlayers. The behavior of casing is predicted from the mechanical interaction among the wellbore-cement sheath-casing system. This model is validated by the analytical result. Results indicate that injection-production unbalance causes nonuniform pressurization or depletion in formation. Under pressurization, the vertical displacement along wellbore increases with the increases of axial position and time and displays a sharp rise in mudstone layer. There is an offset in mudstone layer. Casing stress increases under continuous pressurization or depletion, and stress concentration occurs in the mudstone interval. The predicted casing yield situation is consistent with the failure location in oilfield. The incongruous displacement in mudstone is the mechanism of casing deformation. Sustaining injection-production balance and adopting no-cementation operation are the possible strategies for mitigating the adverse effects of waterflooding. The findings can be applied to evaluate and manage formation response and casing integrity in waterflooding oilfields.

Source rock assessment within a sequence stratigraphic framework of the Yogou Formation in the Termit Basin, Niger

A preliminary sequence stratigraphic geochemical investigation was carried out to assess the areal distribution and hydrocarbon generation potential of marine mudstones of the Upper Cretaceous Yogou Formation in the Termit Basin, Niger. A total of three third‐order sequences (YSQ1, YSQ2, and YSQ3, from bottom to top) and nine systems tracts were identified in the Yogou Formation. Mudstones of each sequences are widely distributed throughout the study area with their maximum cumulative thickness of ~600–900 m in the Dinga and Moul depressions. The ΔlogR method was applied to establish a prediction model for predicting the total organic carbon (TOC) content of the Yogou Formation mudstones. The results of this source rock evaluation indicate that thick offshore mudstones with fair to good hydrocarbon generation potential and dominated Type II to Type III kerogen widely occur in YSQ1, YSQ2, and in the LST (lowstand systems tract) and TST (transgressive systems tract) of YSQ3, while the thin lower shoreface mudstone intervals in the HST (highstand systems tract) of YSQ3 are characterized by good to excellent hydrocarbon generation potential, dominated by Type II kerogen. The present‐day thermal maturation level of source beds from the YSQ3, YSQ2, and YSQ1 has overpassed the main zone of oil generation in the depocenter of Dinga Depression, whereas the YSQ3 and part of YSQ2 source beds were still within the oil window in the depocenter of Moul Depression. The organic matter richness and hydrocarbon generation potential of source beds varies within the Yogou Formation because of changes in organic matter source during evolution of Western Africa rift basins and global sea‐level fluctuations. This study provides a practical sequence stratigraphic geochemical framework which will give a better understanding for the geochemical characteristics of source rock and support improved assessment of petroleum resources.

Redefining the Upper Jurassic Morrison Formation in Garden Park National Natural Landmark and vicinity, eastern Colorado: Geology of the Intermountain West

The Garden Park National Natural Landmark (GPNNL) is north of Cañon City, Colorado, and encompasses all of the major historical dinosaur quarries of the Upper Jurassic Morrison Formation in this area. The formation there can be divided into the lower redefined Ralston Creek Member and an upper unnamed member. The Morrison Formation is bracketed below by the J-5 unconformity and above by the K-1 unconformity. The Ralston Creek Member is composed of up to 55 m of arkosic conglomerate, sandstone, siltstone, and gypsum conformably underlying the unnamed member. Fossil fishes previously used to infer a Middle Jurassic age are non-diagnostic. A diplodocid skeleton 4 m above the J-5 unconformity from the west-adjacent Shaws Park, and a radiometric date of 152.99 + 0.10 Ma from the Purgatoire River area demonstrate that the Ralston Creek rightly belongs in the Morrison Formation and correlates with the Tidwell and Salt Wash Members on the Colorado Plateau. The Ralston Creek was deposited in a broad playa complex analogous to those of central Australia and here called the Ralston Creek boinka. Groundwater flux played an important role in gypsum deposition in gypsisols and playa lakes. The overlying unnamed member in the GPNNL can be subdivided on the west side of Fourmile Creek into a lower part composed largely of mudstone with many thin, discontinuous channel sandstone beds, and a thicker upper part containing more persistent tabular sandstone beds; this subdivision does not occur east of Fourmile Creek. Several thin limestone beds occur in the Ralston Creek Member and in the lower part of the unnamed upper member. The limestone contains fresh water ostracods and aquatic mollusks indicating a lacustrine origin. However, these fauna are apparently stunted and the ostracod valves closed indicating periodic hypersaline conditions. All detrital rocks in the Morrison Formation at Garden Park are composed of varying amounts of quartz, potassic feldspar, and the clay minerals illite, smectite, and kaolinite. Mapping of the clay minerals in the unnamed member reflect various paleosols throughout the mudstone interval, including protosols and argillisols. At the top of the formation, a sandstone previously assigned to the Morrison is reassigned to the overlying Cretaceous Lytle Formation based on similar weathering characteristics, mineral content, and fabric. Thus, the K-1 unconformity between the Morrison and overlying Lytle rests on the uppermost occurrence of the Morrison Formation mudstone-sandstone-limestone complex and beneath the blocky, cliff-forming Lytle Formation.

Redefining the Upper Jurassic Morrison Formation in Garden Park National Natural Landmark and vicinity, eastern Colorado: Geology of the Intermountain West

The Garden Park National Natural Landmark (GPNNL) is north of Cañon City, Colorado, and encompasses all of the major historical dinosaur quarries of the Upper Jurassic Morrison Formation in this area. The formation there can be divided into the lower redefined Ralston Creek Member and an upper unnamed member. The Morrison Formation is bracketed below by the J-5 unconformity and above by the K-1 unconformity. The Ralston Creek Member is composed of up to 55 m of arkosic conglomerate, sandstone, siltstone, and gypsum conformably underlying the unnamed member. Fossil fishes previously used to infer a Middle Jurassic age are non-diagnostic. A diplodocid skeleton 4 m above the J-5 unconformity from the west-adjacent Shaws Park, and a radiometric date of 152.99 + 0.10 Ma from the Purgatoire River area demonstrate that the Ralston Creek rightly belongs in the Morrison Formation and correlates with the Tidwell and Salt Wash Members on the Colorado Plateau. The Ralston Creek was deposited in a broad playa complex analogous to those of central Australia and here called the Ralston Creek boinka. Groundwater flux played an important role in gypsum deposition in gypsisols and playa lakes. The overlying unnamed member in the GPNNL can be subdivided on the west side of Fourmile Creek into a lower part composed largely of mudstone with many thin, discontinuous channel sandstone beds, and a thicker upper part containing more persistent tabular sandstone beds; this subdivision does not occur east of Fourmile Creek. Several thin limestone beds occur in the Ralston Creek Member and in the lower part of the unnamed upper member. The limestone contains fresh water ostracods and aquatic mollusks indicating a lacustrine origin. However, these fauna are apparently stunted and the ostracod valves closed indicating periodic hypersaline conditions. All detrital rocks in the Morrison Formation at Garden Park are composed of varying amounts of quartz, potassic feldspar, and the clay minerals illite, smectite, and kaolinite. Mapping of the clay minerals in the unnamed member reflect various paleosols throughout the mudstone interval, including protosols and argillisols. At the top of the formation, a sandstone previously assigned to the Morrison is reassigned to the overlying Cretaceous Lytle Formation based on similar weathering characteristics, mineral content, and fabric. Thus, the K-1 unconformity between the Morrison and overlying Lytle rests on the uppermost occurrence of the Morrison Formation mudstone-sandstone-limestone complex and beneath the blocky, cliff-forming Lytle Formation.

ГЕОЛОГО-ПРОМИСЛОВІ ОСОБЛИВОСТІ РОЗКРИТТЯ ТА ВИПРОБУВАННЯ СЛАБОПРОНИКНИХ ГАЗОНАСИЧЕНИХ ВІДКЛАДІВ

The relevance of research is attributed to the scientific justification of the features of disclosure and testing of low-permeable gas perspective deposits. Under the low-permeable gas perspective deposits in the Carpathian region of Western Ukraine, we consider the rocks of the Cretaceous system (based on previous works). Deposits of the Cretaceous system in the cross section of the Carpathian region occupy more than half the volume of the rock mass within the boundaries of the flysch strata, and the bulk in the depth interval 1.5–3.5 km corresponds to the sediments of the Stryiska suite of the upper section of the Cretaceous system. That is, the object of research is quite significant in its volume. In selecting the perforation intervals and methods of secondary action on the productive layer in wells drilled in regions where low-permeable thin-layered flysch formations (thin interlayering sandstones, siltstones and mudstones) are observed, we propose a method by which the choice of methods of secondary action on the layer, the intervals of mudstone is justified. In this case, the gas perspective deposits (due to the increase in the permeability of the mudstone layers during their perforation, and later also during the hydraulic fracturing of the layer) are combined into one significant in thickness gas-bearing exploitation object, which increases the flow of gas into the wells. Target disclosure of the gas horizons and increase of their gas output to optimal geological and economic parameters require the use of new technologies, similar to those used in the extraction of gas from shale rock. The simulation of the hydraulic fracturing process of the sand-mudstone pack of Upper Cretaceous deposits in the range 3260–3330 m in the well 2-Tarasivka with the Meer MFrac program proved an increase of the fixed crack opening (in this case, from 0.02 to 0.22 cm), which will contribute to significant increase in the conductivity of the crack from 668 to 6000 mD·m, and it is an important factor in the extraction of hydrocarbon gas from low-permeable deposits, which will increase the productivity of wells. And this will contribute to a significant increase in the fuel and energy supply of Ukraine by its own hydrocarbon raw material.

The Upper Jurassic Blokelv-1 cored borehole in Jameson Land, East Greenland – an introduction

The Geological Survey of Denmark and Greenland (GEUS) successfully drilled the fully cored Blokelv-1 borehole in the central part of the Jameson Land Basin in East Greenland, targeting the Upper Jurassic, rich source-rock interval of the Hareelv Formation. The borehole achieved 100% core recovery from 1.72 m to a total depth of 233.8 m; the recovered Hareelv Formation section consists of interlayered black, laminated organic-rich mudstones, massive sandstones and heterolithic sandstone–mudstone intervals of the Katedralen Member, and amalgamated massive sandstones of the Sjællandselv Member. The core is of very high quality and has been subjected to an extensive sampling and analytical programme focused particularly on petroleum geological aspects, as presented in the following eight papers in this volume. This bulletin describes an important, previously poorly documented member of the ‘Kimmeridge Clay’ family of prolific petroleum source rocks in the North Atlantic area.

Impact of sequence stratigraphy on static and dynamic reservoir models: examples from the Precipice-Evergreen succession, Surat Basin, Queensland

CO2 storage in the subsurface is a key aspect of climate mitigation. The UQ is investigating whether the Precipice Sandstone and Evergreen Formation in the Surat Basin, Queensland, are an appropriate reservoir-seal pair for the long-term storage of greenhouse gases. However, the Precipice-Evergreen succession remains poorly constrained from a paleo-depositional and stratigraphic standpoint. Studies have mostly applied lithostratigraphy for local correlation, and the understanding of time-stratigraphic relationships across the basin needs development. This has greatly hindered the capacity to construct robust reservoir models and is an active area of research.We utilized core, wireline logs, seismic reflection surveys, and pressure data to compare the dynamic response to various CO2-injection scenarios with contrasting stratigraphic architectures. A lithostratigraphic prediction of reservoir and seal intervals consisted of a layer-cake model of fluvio-deltaic deposits. The models suggest that reservoir layers are laterally well-connected with the gas plume primarily migrating parallel to bedding. In contrast, a sequence stratigraphic arrangement of facies resulted in a more complex architecture, where reservoir and non-reservoir strata cross-cut and intersect one another. The resulting models showed greater reservoir heterogeneity and potentially more complex fluid transmission pathways in both the lateral and vertical directions that could result in slower plume migration and more residual trapping. This is due to the fact that discontinuous mudstone intervals potentially baffle the CO2 plume and may allow for more CO2 trapping within the lower parts of Evergreen succession. The contrasting models show different geological realizations arising from the same dataset, interpreted in different ways than identify where there is uncertainty. They may highlight certain areas of the basin are more conducive to carbon storage than others.Fluid flow is highly sensitive to the stratigraphic arrangement of reservoir and non-reservoir intervals. Refining static and dynamic models using sequence stratigraphy may result in a significant improvement in history matching. Modellers should carefully consider the implications of stratigraphic correlations during static model construction.

Sponge-dominated offshore benthic ecosystems across South China in the aftermath of the end-Ordovician mass extinction

Offshore benthic communities were sparse after the end-Ordovician mass extinction. The recent discovery of diverse, abundant sponges in south-eastern China (Anhui and Zhejiang provinces) that flourished during the Hirnantian (Late Ordovician) post-extinction interval raises questions over the extent and ecological significance of the sponge community at that time. This paper dramatically expands the scale of the Hirnantian sponge fauna within South China, with seven new occurrences across 2000 km. These occurrences cross the Ordovician–Silurian boundary interval in the marginal and submerged platform areas of the Yangtze Sea in Jiangsu, Hubei, Hunan, Chongqing and Sichuan provinces. In condensed sections (platform-margin facies), the occurrences are normally represented by extensive spiculite layers, but several sites yield entire, exceptionally preserved body fossils; at Ganggangshan (near Nanjing, Jiangsu Province), such preservation is abundant. The new occurrences are combined with palaeogeographic and tectonic reconstructions to produce a model for the distribution and preservation of the sponge fauna. We argue that the exceptional preservation was due largely to nepheloid layers that were generated during rapid post-glacial transgression that flooded the weathered land surface of the uplifted Cathaysia Block. Nepheloid-layer collapse led to rapid deposition of suspended sediment within the deeper parts of the intra-plate, restricted basin and on adjacent platform margins and slopes. The suspended sediment influx also introduced abundant nutrients to the water column, as indicated by a significant δ13C excursion during the sponge-bearing mudstone interval at Ganggangshan. The combined sponge assemblage was dominant over large areas of the Hirnantian offshore sea floor across South China. We predict that similar occurrences with exceptional preservation will also be found in other continental blocks: abundant articulated sponges may be encountered particularly in restricted basin successions containing rapidly deposited graptolitic mudstones near the Ordovician–Silurian boundary.

Palynology of the Morro do Chaves Formation (Lower Cretaceous), Sergipe Alagoas Basin, NE Brazil: Paleoenvironmental implications for the early history of the South Atlantic

The break-up of Gondwana and the origin of related source rocks and reservoir intervals of the Brazilian coastal basins are being increasingly studied in the last decades. In this context the Sergipe-Alagoas Basin, which lies to the north of the Campos Basin hydrocarbon province, becomes relevant for the completeness of its depositional sequences. This paper presents, for the first time, the detailed palynological record from the Morro do Chaves Formation, located at the InterCement Quarry, near São Miguel dos Campos, Alagoas. The analysis of samples from the mudstone intervals of the unit mainly composed by coquinas has enabled new biostratigraphic, paleoenvironmental and paleogeographic interpretations of the evolution of the South Atlantic. The occurrence of the species Dicheiropollis etruscus, with a vertical distribution ranging from the Berriasian to the Barremian, supports the late Barremian age for the lowermost part of the Morro do Chaves Formation, traditionally interpreted as having been deposited in a lacustrine environment, close to herbaceous plants, surrounded by remote mountainous areas with arboreal vegetation. The occurrence of algae phycomata of the Class Prasinophyceae, associated with Scenedesmus and Botryoccocus, organisms that tolerate some salinity in its uppermost part, together with other biotic and abiotic data, corroborate the possibility of relating the Morro do Chaves Formation to a marine and/or brackish depositional environment, which may have resulted from a marine flooding in the northern sector of the Sergipe-Alagoas Basin during Early Cretaceous times. The results of this work support a paleogeographic model characterized by an episodic input of Tethyan waters towards the north of the South American continent.

A revised Triassic stratigraphic framework for the Arctic Alaska Basin

The Triassic Shublik Formation and the Triassic–Jurassic Otuk Formation are partially age-equivalent lithostratigraphic units that were deposited in the Arctic Alaska Basin (AAB). The Shublik Formation represents proximal deposition within the basin, with episodic siliciclastic input, whereas the Otuk Formation was deposited in the distal part of the basin, with significant intervals of mudstone and chert. Both the Shublik and Otuk Formations have significant intervals of organic-rich mudstone, and the Shublik is a major source rock for northern Alaska hydrocarbon accumulations such as Prudhoe Bay. The revised stratigraphic framework presented herein, based on the integration of lithostratigraphy and biostratigraphy, correlates intervals within these two formations, as well as the Ivishak Formation and the Karen Creek and Sag River Sandstones (which underlie and overlie the Shublik). This stratigraphic framework provides a basis for comparison of proximal and distal parts of the AAB through the Triassic, thus allowing for a more robust understanding of the spatial and temporal variability of lithology and organic richness within this basin. Five transgressive–regressive sequences are defined in the Shublik, based on lithostratigraphy and better age constraints provided by the revised stratigraphic framework. These sequences are age-correlative and recognized in other Arctic basins, implying that they have regional, and perhaps global, significance.

Geochemical Characteristics and Identification Criteria of effective source rock in Mesozoic Yanchang Formation, Southern Margin of Ordos Basin

The geochemical characteristics of source rock in Mesozoic Yanchang Formation, southern margin of Ordos Basin have not been introduced in literature. In this study, a number of geochemical experiments were conducted to discuss the geochemical characteristics and to establish the identification criteria. It is shown that source rock in the study area can be divided into five types: A1, A2, A3, A4 and B. Based on the detailed oil source correlation, the source rock types in the study area can be identified as: 1) A1, Chang 7 oil shale, is the excellent source rock with large hydrocarbon-generation potential; 2) A2 and A3, mainly Chang 7 dark mudstone, are the effective source rocks with medium hydrocarbon-generation potential; 3) A4 and B, mainly the thin mudstone or carbonaceous mudstone intervals, are the potential source rock with small hydrocarbon-generation potential. [Received: April 22, 2018; Accepted: May 27, 2018]

Orbitally-driven Mid-Burdigalian Coastal Sabkha cycles from the Sivas Basin: Sedimentological, paleontological, and geochemical data

The Early to Middle Miocene period in the Sivas Basin is represented by a thick deposition of mud-dominated marine sediments (Karacaören Formation) in an overall bay-like paleogeographic setting. Previous studies have suggested that halokinesis-induced local tectonics and orbital climatic fluctuations have been the primary controls on depositional evolution. In the present study, we investigated a 60-m–thick Karaman Gypsum Member (KGM), located at the base of the Karacaören Formation, in terms of its sedimentological, paleontological; both palynology and nannofossil biostratigraphy, and geochemical/mineralogical aspects to better characterize climatic controls on its deposition. Nannofossil assemblages indicate that the studied member is confined within the Sphenolithus belemnos Zone (NN3; 18.92–17.97Ma), and is mainly composed of rhythmic alternations of supratidal evaporites and lagoonal/marine mudstones. The palynological data demonstrate a minor, yet consistent, occurrence of megathermic taxa and numerous dinoflagellate cysts in mudstone intervals. The high chemical proxy of alteration (CPA) values and occurrence of kaolinites in the same mudstone intervals further prove deposition under warm and humid subtropical conditions. The perfect match between the environmental cyclicities in the KGM with a benthic δ18O curve justifies a strong control of obliquity/short eccentricity of a 40–100kyr duration, and hints at radical precipitation changes throughout the individual orbital cycles. These precipitation changes are explained by extreme latitudinal precipitation gradients during relatively hot periods before the mid-Miocene Climatic Optimum.

Ground penetrating radar for facies architecture and high-resolution stratigraphy: Examples from the Mesoproterozoic in the Chapada Diamantina Basin, Brazil

Ground Penetrating Radar (GPR) surveys were conducted on Mesoproterozoic eolian, fluvial, deltaic, estuarine, and shallow marine successions in the Chapada Diamantina Basin. The subsurface continuation of facies and facies architecture exposed on road cuts was imaged using the GPR signal of a 400-MHz antenna penetrating 8 m in depth, even with mudstone intervals. Reflection patterns in the GPR profiles that were compared with photo mosaics of outcrops and supporting data from vertical sections and gamma ray logs, reveal sedimentary, stratigraphic, and structural features, such as sedimentary structures, the external geometry of architectural elements, stratigraphic surfaces, folds and tension gashes. The patterns most likely reflect the response from low-weathered, non-porous muscovite-illite-rich mudstone and quartzarenite sandstone in which authigenic and detrital illite and sericite are prevalent clay minerals.Measured vertical sections and radar stratigraphy indicate high-frequency cyclic successions of estuarine and shoreface intervals are present at the base of the Tombador Formation. The shoreface intervals are composed of heterolithic strata and offshore tidal bars deposits. The heterolithic shoreface strata exhibit tabular geometry that can be easily identified throughout the outcrop and in the subsurface. Such intervals represent the end of high-frequency transgressive cycles, and hence they are potential candidates for including the maximum flooding surfaces and for defining genetic sequences. Therefore, GPR proved to be an independent method for studying facies architecture and the establishment of a high-resolution stratigraphic framework even in the Precambrian.