Coarsening-upward Sequences Research Articles

Re-evaluation of depositional models for the lower Permian Patchawarra Formation, Cooper Basin, South Australia: implications for petroleum exploration

The Late Carboniferous–Triassic Cooper Basin is Australia’s most prolific onshore petroleum province. The lower Permian Patchawarra Formation, which is up to 680 m thick and consists of up to 10% coal, is a major exploration target in the basin. Eighteen cores through the formation have been logged to re-evaluate the existing fluviolacustrine depositional model. The siliciclastics form fining- and coarsening-upward sequences that are 1–10 m thick. They are predominately fine-grained with abundant lenticular bedding, wavy bedding and thinly interlaminated siltstones and clays resembling varves. Granules and pebbles, interpreted as dropstones, are present throughout the formation. Coal beds are up to 60 m thick and rich in inertinite. Other than the coal beds, there is little evidence of the establishment of terrestrial conditions: roots are rare and there are no siliciclastic palaeosols. The siliciclastics are interpreted as the deposits of a large glaciolacustrine system, with the fining-upward successions deposited in subaqueous channels cut by hyperpycnal flows and the coarsening-upward successions deposited as overbank splays between those channels. Hyperpycnal flows may have resulted from sediment-laden cold water emanating from glacially-fed rivers, similar to those seen in many large glacial lakes in high latitudes and altitudes today. Much of the coal is interpreted as the accumulation of peats from floating mires that covered large parts of the glaciolacustrine system at certain time intervals. The high inertinite content of many coals is interpreted as the decay of organic matter within the floating mire. These new interpretations have the potential to enhance reservoir characterisation within the basin.

The geochemical signatures of the Silurian–Devonian succession in eastern Saudi Arabia

This study was performed on 663 sandstone core and cutting samples taken from the Silurian?Devonian succession in five wells, located in eastern Saudi Arabia. This study aimed to correlate the Tawil Formation in the study wells, defining the lower Qalibah/ Tawil and upper Tawil/Jauf boundaries. The Tawil Formation is mostly characterized by clastic-dominated fluvial to marginal marine depositional environments. A chemostratigraphic correlation workflow was established using key element ratios and comprising four zones (C1 to C4) and five subzones (C2-1 to C2-3 and C3-1-C3-2). The chemostratigraphic zonation allowed robust characterization of stratigraphy in the uppermost Qalibah, Tawil, and lower Jauf Formations. Zone C1 is associated in most cases with the Sharawra Formation. Zones C2 and C3 are generally linked with the Tawil Formation, and Zone C4 broadly defines the lower part of Jauf Formation. In detail, the chemostratigraphic zonal boundaries do not precisely match the biostratigraphical and lithostratigraphic boundaries, indicating that the C1-C2 and C3-C4 boundaries are probably time-transgressive. The ratio Zr/Nb was used to model sequence stratigraphy. This ratio can vary in proportion to grain size, in which the element Zr is known to be concentrated in the heavy mineral zircon; while Nb-bearing heavy minerals along with clay minerals, likely illite, produce Nb. The increasing Zr/Nb ratio values are considered to define the coarsening upward or regressive sequences while decreasing Zr/Nb trends can mark fining upward or transgressive sequences.

Cambrian shallow-marine to emergent alkaline volcanism near Ouinguigui (Ougnat inlier, eastern Anti-Atlas, Morocco): Volcanic facies, geochemistry and geodynamic setting

Various forms of volcanism occurred during the Middle Cambrian southeast of the Ougnat inlier in the eastern Anti-Atlas of Morocco. This study is the first to examine the formation of these volcanic deposits and interrelationships to the sedimentary host and tectonics on local and regional scales. The emplacement history of the volcanosedimentary succession of Ouinguigui has been characterised by a multi-disciplinary approach of field work, facies analysis of the volcanic rocks and their host and by geochemical analysis of coherent volcanic rocks. Additionally, we introduce a generic facies code system for subaqueous to emergent basaltic successions to make related deposits more comparable. The sedimentary host rock is composed of pre-transgressive conglomerates and sandstones of the Tikirt and Akerouz Member, which are interpreted as deposits of shelf-type fan-delta and shoreface deposits. The transgression is reflected by an abrupt decrease in grain size and the presence of packstones similar to the Micmacca Breccia Member. The post-transgressive clastic deposits of the Feijas Internes Group and Tabanit Group are the main host for the volcanic activity. They are composed of coarsening-upward sequences, which reflect gradual transitions from offshore to shoreface and foreshore facies.The alkaline volcanism comprises four main phases, which are induced by the SW-vergent tilting of a basement block during extensional tectonics. Phase I starts with the emplacement of diatremes within the Jbel Wawrmast Formation and Jbel Afraou Formation reaching an estimated depth of ca. 200 m. They are the result of phreatomagmatic eruptions caused by the rise of basaltic magma into the coastal environment of the host. Phase II is characterised by the emplacement of large volumes of basaltic lava through fissure eruptions and the formation of a lava delta in the eastern part as well as subaqueous to emergent phreatomagmatic volcanism in the middle and western part of the Ouinguigui area. Wave activity led to the formation of volcanogenic conglomerates and sandstones. Phase III and IV are reflected by the emplacement of subvolcanic intrusions consisting of trachy andesites and trachytes.Whole rock geochemical data of 13 samples confirm the alkaline character of the Ouinguigui magmatism, which derived from a garnet lherzolite source of within-plate volcanism.

Occurrences and characterisation of textural and mineralogical heterogeneities in analogue reservoir sandstones: case study of the onshore Central Sarawak, NW Borneo

The plethora of micro (μm)–to–mega (≥ 10 to ≤ 100 m) scale heterogeneities in marginal marine siliciclastic reservoirs makes their petrophysical analysis often cumbersome, and thus yield unrealistic reservoir parameters. Micro-scale heterogeneities typically occur at the microscopic scale, while the mega-scale is observable at the outcrop scale. A good understanding of the possible heterogeneities within sandstone reservoirs can enhance their quality assessment significantly. In this study, heterogeneities in analogue marginal marine sandstones of Nyalau and Balingian formations, NW Borneo, were qualitatively characterised using integrated field geology and geochemical analyses. The outcrops are typically characterised by an upward-coarsening sequence, consisting of sandstone and argillaceous lithounits. The sandstones are moderate- to well-sorted, fine- to medium-grained with varying kurtosis and skewness. Estimated matrix composition in thin section ranges between 15 and 67%, thus classifying the sandstone as feldspathic litharenites to sublithearenite. Skolithos (Ophiomorpha) and Planolites are the commonly observed ichnofacies with bioturbation index of 0 to 90%. Three mineralogical relationships (denoted as MF) were defined based on quartz-feldspar-clay contents. Of these, reservoir units characterised by the MF-1 have better reservoir potential than the other two. Scanning and electron microscopy (SEM) revealed that the secondary pores created from grain dissolutions are partly to completely filled by networks of illite strands, kaolinite/dickite sheets, chlorite flakes, pyrite and very rarely laumontite. An integration of SEM, XRD and mineralogical evolution trend suggests that most of these pore-modifying minerals are of authigenic origin. This research concluded that pore-destroying alterations predominate pore enhancement modifications in the sandstones. Hence, fluid flow parameters and by implication production from the sandstone may be significantly impacted in a negative sense.

The Tajik Basin: A composite record of sedimentary basin evolution in response to tectonics in the Pamir

AbstractInvestigation of a >6‐km‐thick succession of Cretaceous to Cenozoic sedimentary rocks in the Tajik Basin reveals that this depocentre consists of three stacked basin systems that are interpreted to reflect different mechanisms of subsidence associated with tectonics in the Pamir Mountains: a Lower to mid‐Cretaceous succession, an Upper Cretaceous–Lower Eocene succession and an Eocene–Neogene succession. The Lower to mid‐Cretaceous succession consists of fluvial deposits that were primarily derived from the Triassic Karakul–Mazar subduction–accretion complex in the northern Pamir. This succession is characterized by a convex‐up (accelerating) subsidence curve, thickens towards the Pamir and is interpreted as a retroarc foreland basin system associated with northward subduction of Tethyan oceanic lithosphere. The Upper Cretaceous to early Eocene succession consists of fine‐grained, marginal marine and sabkha deposits. The succession is characterized by a concave‐up subsidence curve. Regionally extensive limestone beds in the succession are consistent with late stage thermal relaxation and relative sea‐level rise following lithospheric extension, potentially in response to Tethyan slab rollback/foundering. The Upper Cretaceous–early Eocene succession is capped by a middle Eocene to early Oligocene (ca. 50–30 Ma) disconformity, which is interpreted to record the passage of a flexural forebulge. The disconformity is represented by a depositional hiatus, which is 10–30 Myr younger than estimates for the initiation of India–Asia collision and overlaps in age with the start of prograde metamorphism recorded in the Pamir gneiss domes. Overlying the disconformity, a >4‐km‐thick upper Eocene–Neogene succession displays a classic, coarsening upward unroofing sequence characterized by accelerating subsidence, which is interpreted as a retro‐foreland basin associated with crustal thickening of the Pamir during India–Asia collision. Thus, the Tajik Basin provides an example of a long‐lived composite basin in a retrowedge position that displays a sensitivity to plate margin processes. Subsidence, sediment accumulation and basin‐forming mechanisms are influenced by subduction dynamics, including periods of slab‐shallowing and retreat.

Tectonically controlled evolution of the Yellow River drainage system in the Weihe region, North China: Constraints from sedimentation, mineralogy and geochemistry

This study investigated the evolution of the middle reach of the Yellow River by examining the latest Cenozoic (∼5.0–0.15 Ma) sedimentary history of the Weihe and Sanmenxia basins in the Weihe region of North China. Although the development of the two basins and the evolution of the Yellow River have long been related to the rapid uplift of the Tibetan Plateau, the relationship between the landform evolution and regional tectonics is still poorly understood. Here, we report the results of a detailed study on sedimentation and provenance of the two basins and their spatial and temporal relationships to regional fault development. The results show that the two basins were possibly originally isolated from each other, between ∼5 Ma and ∼2.8–2.6 Ma, but later connected by the Yellow River at ∼2.8–2.6 Ma. This process is recorded in the coarsening-upward sequence of the Weihe Basin strata and the coeval fining-upward succession of the Sanmenxia Basin strata. At ∼1.0 Ma, the Sanmenxia Basin strata developed a coarsening-upward trend, which we interpret as an indication of the formation of the Yellow River in the region. In addition, changes in the drainage system at ∼2.8–2.6 Ma and at ∼1.0 Ma have been recorded as dramatic provenance changes in the sediments of the Sanmenxia Basin.

Facies Analysis and Depositoinal Environment of D-3 Reservoir Sands Vin Field, Eastern Niger Delta

Facies analysis and depositional environment identification of the Vin field was evaluated through the integration and comparison of results from wireline logs, core analysis, seismic data, ditch cutting samples and petrophysical parameters. Well log suites from 22 wells comprising gamma ray, resistivity, neutron, density, seismic data, and ditch cutting samples were obtained and analyzed. Prediction of depositional environment was made through the usage of wireline log shapes of facies combined with result from cores and ditch cuttings sample description. The aims of this study were to identify the facies and depositional environments of the D-3 reservoir sand in the Vin field. Two sets of correlations were made on the E-W trend to validate the reservoir top and base while the isopach map was used to establish the reservoir continuity. Facies analysis was carried out to identify the various depositional environments. The result showed that the reservoir is an elongate , four way dip closed roll over anticline associated with an E-W trending growth fault and contains two structural high separated by a saddle. The offshore bar unit is an elongate sand body with length: width ratio of >3:1 and is aligned parallel to the coast-line. Analysis of the gamma ray logs indicated that four log facies were recognized in all the wells used for the study. These include: Funnel-shaped (coarsening upward sequences), bell-shaped or fining upward sequences, the bow shape and irregular shape. Based on these categories of facies, the depositional environments were interpreted as deltaic distributaries, regressive barrier bars, reworked offshore bars and shallow marine. Analysis of the wireline logs and their core/ditch cuttings description has led to the conclusion that the reservoir sandstones of the Agbada Formation in the Vin field of the eastern Niger Delta is predominantly marine deltaic sequence, strongly influenced by clastic output from the Niger Delta. Deposition occurred in a variety of littoral and neritic environment ranging from barrier sand complex to fully marine outer shelf mudstones.

Reorganization of sediment dispersal in the Jiuxi Basin at ~17 Ma and its implications for uplift of the NE Tibetan Plateau

The Tibetan Plateau offers an example for understanding the processes of continental collision and subsequent intracontinental deformation. The Jiuxi Basin, located at the northeastern margin of the plateau, bears crucial information on the growth processes of the NE Tibetan Plateau. Combined with published magnetostratigraphic constraints, integration of stratigraphic investigation and detrital zircon geochronological analysis on the sediments of the Cenozoic Hongliuxia section in western Jiuxi Basin suggests a major reorganization of the basin infill process at the period between depositions of the Baiyanghe and Shulehe formations at ~17 Ma. This finding is documented by (1) a generally upward-thinning sequence followed by an upward-coarsening sequence with corresponding changes in sedimentary facies, (2) shifting of the paleocurrent from the south to the north, and (3) shifting of the provenance from the Bei Shan-Kuantan Shan-Hei Shan in the north to the North Qilian Shan in the south. These results suggest that tectonic deformation arrived in the North Qilian Shan by ~17 Ma and lend support to the outward-growth model for the Tibetan Plateau.

Structural setting of the Chanic orogen (Upper Devonian) at central-western Argentina from remote sensing and aeromagnetic data. Implications in the evolution of the proto-Pacific margin of Gondwana

The basement of the Central Andes located in central-western Argentina (31º20′S - 69º22′W) is composed by the Cuyania and Chilenia terranes which were amalgamated to Gondwana in the Early-Mid Paleozoic. Between the Precordillera (Cuyania) and Frontal Cordillera (Chilenia) there are exposures of marine metasedimentary rocks associated with mafic rocks with an E-MORB chemical signature that represent the remnants of an extensional basin developed between both terranes. The stratigraphic features and the distribution of the Early-Mid Paleozoic units along the Western Precordillera were constrained by remote sensing techniques. This allowed us to identify two stages in the evolution of the sedimentary in-fill of the marine basin: an initial stage (Mid-Late Ordovician) marked by widespread extensional tectonics and a finning-upwards sequence interbedded with volcanic-plutonic mafic rocks; and a Late Ordovician?-Devonian where the sedimentation was characterized by the development of coarsening-upwards sequences with low participation of mafic rocks. Flattened parallel folds associated with pre-Andean thrusts have locally a top-to-the SW vergence. These pre-Andean (Late Devonian) structures are the relics of the Chanic orogen whose double vergence is the result of the control exerted by previous structures related to the ordovician rifting. This is constrained by the residual and regional magnetic anomalies which reflect an important correlation between deep and surface structures. We propose the inception of a subduction zone with an eastward polarity on the proto-Pacific margin of Gondwana as the responsible for the compressive geotectonic framework that led to the closure of the Western Precordillera basin during the Late Devonian and the development of the Chanic thick-skinned-dominated orogen.

English

The classification of the sediments into facies and the various environments in which the sediments of the northeastern Voltaian basin were deposited has not been clearly described. Using detailed field mapping together with petrographic studies, six sedimentary facies has been identified; F1 is defined by asymmetric wave formed ripple marks, F2 indicates flute casts on micaceous sandy shales, F3 has bifurcated ripple and climbing ripple marks on argillaceous materials at the middle parts of the scarp, F4 is identified by parallel and cross stratifications observed close to the upper parts of the scarp, F5 has a bipolar herringbone cross beds and planar cross beds on quartz rich sediments at the southern parts of the study area and a distinctive well developed burrows (Skolithos) at the southern half of the study area defines facies F6. The sediments are deposited largely in a shallow marine environment, with subordinate fluvial, flood plain, deltaic, aeolian, tidal flats and shoreface/nearshore environments. Although, there exist a suitable depositional environment for the formation of hydrocarbons, the overlying sediments are not thick enough to give the required temperature and pressure conditions for hydrocarbons development in the northeastern fringes of the basin. Paleosol features evident by pseudokarren, like weathering marking the contact between the middle Oti/Pendjari group and the lower Gambaga group of the study area. The sequence of the area is that of a typical shallowing, thickening and coarsening upward sequence. Key words: Clastic sediments, environments, facies sequence, Gambaga-Nakpanduri, massifs.

Magneto-biostratigraphy and paleoenvironments of the Miocene freshwater sediments of the Sarajevo-Zenica Basin

The Sarajevo-Zenica Basin of Bosnia-Herzegovina was part of the Dinaride Lake System, a large network of Miocene long-lived freshwater basins in southeastern Europe. The basin contains a thick sedimentary succession of carbonates, coals and mixed siliciclastic deposits that reflects the paleoclimatic and tectonic evolution of the region. In this study, we present novel integrated (magneto-bio)stratigraphic and sedimentological data and reconstruct the paleoenvironmental evolution of the Sarajevo-Zenica Basin during its two main evolutionary phases (thrusting and extension). The basal “Oligo-Miocene” freshwater paleoenvironments are characterized by alternating palustrine, shallow lacustrine and distal fluvial phases. The base level fluctuations are largely controlled by syn-sedimentary pulses of tectonic loading during the final phase of thrusting in the Internal Dinarides. The majority of this succession is considered early Miocene in age, which contrasts with previous Oligocene age estimates. The subsequent extensional phase initiated not later than ~18.4 Ma. This coarsening upward sequence of lacustrine carbonates, silts, sands and conglomerates is correlated between 17.2 and 15 Ma (C5Cr-C5Br) by means of integrated bio-magnetostratigraphy. During this upper extensional phase, subsidence and sediment influx was generally controlled by activity along the basin bounding normal fault, overruling smaller scale climatic influences. We conclude that the existence of the long-lived Sarajevo lake is coeval with other Dinaric and southern Pannonian lakes, and overlaps in time with the Miocene Climatic Optimum. Sedimentation in the Sarajevo-Zenica basin terminated at ~15–14 Ma which concurs with both the end of the climatic optimum as well as the cessation of extension in the Dinarides. These results will help to better quantify the paleoclimatic changes in the Dinaride Lake System as well as the regional tectono-sedimentary events, such as potential migrations of deformation across the Dinarides.

Deglacial sequences and glacio-isostatic adjustment: Quaternary compared with Ordovician glaciations

Abstract Deglacial sedimentary sequences recording the decay and final demise of ice sheets result from intricate interactions between the pattern of ice margin retreat, inherited basin physiography and relative sea-level (RSL) changes. A specific emphasis is here given to the glacio-isostatic adjustment (GIA), which may force postglacial local RSL fall in spite of concomitant glacio-eustatic rise. In this contribution, we characterize a Quaternary deglacial succession emplaced in such a setting, subsequently used as an analogue to interpret an end-Ordovician deglacial record. The Quaternary deglacial succession, tens of metres thick, formed under condition of RSL fall forced by the GIA in c. 10 000 years in the aftermath of the deglaciation. This sedimentary succession consists of a lower, fining-upward sequence representing the backstepping of ice-contact depocentres following the retreat of the ice margin, and an upper, coarsening-upward sequence that relates to the subsequent progradation of a glaciofluvial delta system. A very similar stratigraphic stacking pattern characterizes the Ordovician analogue, suggesting a comparable deglacial sequence. By analogy with the Quaternary succession, this ancient deglacial record would have hence been emplaced under conditions of RSL fall forced by the GIA. Moreover, it must only represent a very short time interval that could be viewed as virtually instantaneous regarding the Late Ordovician glaciation. Such a vision is at odds with commonly accepted interpretations for such successions.

Signal or noise? Isolating grain size effects on Nd and Sr isotope variability in Indus delta sediment provenance

Radiogenic elements, such as neodymium (Nd) and strontium (Sr), are commonly used to decipher the provenance and weathering histories of bulk siliciclastic sediments worldwide, although increasing evidence for diagenetic and sediment transport-driven bias of isotope signatures calls into question the utility of bulk isotope compositions as effective tools. This study evaluates grain size-dependence and variability of Nd and Sr compositions in the Indus delta of Pakistan, documenting both a significant isotopic evolution and coarsening upward sequence during the last deglaciation (~15 ka). Grain size analysis, trace element geochemistry, and Nd and Sr geochemistry are determined for bulk and separate grain size fractions (<63 μm, 63–125 μm, 125–250 μm, >250 μm) from four core sampling sites with similar provenance. Isotopic compositions are controlled by grain size distribution, in turn variably controlled by size fraction mineralogy. Fine-grained bulk sediment Nd isotope compositions are primarily dictated by the finest (<125 μm) sediments enriched in Nd-bearing monazite and allanite, with coarser bulk sediments influenced by mica. The most abundant grain size fraction drives bulk sediment Sr isotope compositions, with the finest grain size fractions indicating strong control by potassium feldspar, mica, epidote, and/or volcanogenic clay minerals. Over the last 15 k.y., resolvable provenance-driven trends are confidently identified in Nd isotope compositions. Long-term trends remain consistent with previous findings although the isotopic shift in delta compositions must be less than previously envisaged (~4 vs. 0.69–1.91 εNd units). Although considerable variability is observed in bulk Sr isotope compositions, <63 μm and 63–125 μm fraction 87Sr/86Sr compositions evolve in parallel through time and show fractionation by ~0.005–0.01 as a result of grain size. Within the isotopically diverse Indus drainage system, bulk isotopic compositions are estimated to deviate on average no than ±1.04 εNd units and ±0.0099 for 87Sr/86Sr values for any sediment as a result of mineralogy, grain size distribution, and analytical error. Isotopic excursions beyond these uncertainties can be unambiguously assigned to either a change in upstream provenance and/or chemical weathering.

Sedimentology and Sequence Stratigraphy of Marine to Lacustrine Deltaic Deposits in a Craton Basin and Their Controlling Factors: Shan 2 Member–He 8 Member (Guadalupian–Lopingian, Permian), Southeast Ordos Basin, North China

AbstractThe Shan 2 Member, Shan 1 Member and He 8 Member of the Mid–Late Permian Shanxi and lower Xiashihezi formations, in the southeastern Ordos Basin, together comprise ∼150 m of deltaic deposits. This sequence records an overall evolution from deep marine environment to shallow lake associated with braided river, braided river delta and meandering river delta. Core description, well log interpretation, and stable isotope analysis, including carbon, oxygen and strontium, were conducted to understand the sedimentary evolution of Shan 2 to He 8 Member. The Shanxi Formation, which consists of the Shan 2 and Shan 1 members, is characterized by a tidal‐influenced meandering river delta environment and a higher δ13C value and 87Sr/86Sr ratio and a lower δ18O value. The He 8 Member, the basal part of the Xiashihezi Formation, is featured by a braided river to braided river delta system and a lower δ13C value, 87Sr/86Sr ratio, and a higher δ18O value. Four third‐order depositional sequences separated by five sequence boundaries are determined. Coarsening upward sequences of the Shan 2 Member–He 8 Member indicate a general regression trend, which can be correlated to global sea‐level fall occurring during the Roadian–Wuchiapingian, as also evidenced by previous published zircon U–Pb results. The coal‐bearing sequence (Shanxi Formation) to non‐coal‐bearing sequence (He 8 Member), as well as a decrease of 87Sr/86Sr, suggest a trend from humid to arid climates. A combined effect of sea‐level drop and a small uplift at the end of Shanxi Formation are proposed.

Lithostratigraphy of Tabul Formation and Onshore Geology of Nunukan Island, North Kalimantan

Nunukan Island is located in Northeast Kalimantan Basin, one of prolific basin in western Indonesia. However, this island lies in northern verge of the basin and hydrocarbon exploration were limited before the 1940s. No detail geological fieldworks has been published. This paper attempts to share geological information obtained from few accessible outcrops, particularly from the western part of the island where previous oil drilling were carried out. Most of the island have been built by Tabul Formation, which was also the main objective of reservoir for hydrocarbon exploration. It is composed of claystones, siltstones, and sandstones interbeds, deposited in transitional environment during Late Miocene. They exhibit coarsening upward sequence, the lower part composed of heterolitic sandstones deposited in tidal flats, and the upper part composed of arkosic sandstones deposited in distributary channels and mouthbars. The eastern coastal strip was constructed by Tarakan conglomerates, which have been deposited in fluvio-deltaic environment in Pliocene, unconformably over the Tabul clastics. Those Neogene deposition of paleo-Simengaris Delta apparently deformed and inverted in Plio-Pleistocene, contemporaneously with basaltic volcanism which spread widely over northeastern Borneo, which also observed in the northern half of the Nunukan Island

Application of conventional propagation resistivity logging for formation boundary identification in geosteering

Previous studies indicate that the polarization horn effect of conventional propagation resistivity (CPR) logs will occur once the logging tool is close to or passes through the boundary between high conductivity contrast layers with a small relative angle. Polarization horns cannot be used to distinguish between the top or bottom boundary. This situation disturbs the application of CPR in geosteering. Considering the cycle sequence features in sedimentary formation, e.g. fining upward sequence, coarsening upward sequence and massive cycle, they can be identified by analyzing the shapes of spontaneous potential (SP) or gamma-ray (GR) logs in logging geologic analysis. Previous studies mostly focused on the ideal medium formation models, which can only represent the massive cycle. Thus in this study, the fining upward sequence and coarsening upward sequence formation models are established, and the logging responses of CPR in the two formation models are simulated under the condition of horizontal well. The simulation analysis shows that polarization horns will either occur at the top boundary in the coarsening upward sequence or at the bottom boundary in the fining upward sequence, if the influence of formation thickness and mud invasion is ignored. The occurrence probability density chart is plotted to determine that polarization horns will occur in what formation resistivity contrast and deviation conditions. In order to identify the top or bottom boundary in sedimentary formation using CPR, a convenient method is established to facilitate geosteering. Firstly, the cycle type of the subject reservoir should be determined by analyzing the SP or GR logs of the offset wells. Secondly, the top boundary in coarsening upward sequence and the bottom boundary in fining upward sequence can be identified by using the occurred polarization horns of CPR logs. Finally, when the tool is close to the boundary, the rough distance from CPR tool to the top boundary (in coarsening upward sequence) or to the bottom boundary (in fining upward sequence) can be obtained. When the tool crosses the boundary, the logging measured depth of the corresponding boundary can be obtained. Two field geosteering examples can verify the two functions of the method, respectively. Based on cost-effective CPR logs, the method contributes to geosteer in sedimentary formation.

Typhoon Haiyan overwash sediments from Leyte Gulf coastlines show local spatial variations with hybrid storm and tsunami signatures

Marine inundation associated with the 5 to 8m storm surge of Typhoon Haiyan in 2013 left overwash sediments inland on the coastal plains of the northwestern shores of Leyte Gulf, Philippines. The Haiyan overwash deposit provides a modern sedimentary record of storm surge deposition from a Category 5 landfalling typhoon. We studied overwash sediments at two locations that experienced similar storm surge conditions but represent contrasting sedimentological regimes, namely a siliciclastic coast and a mixed siliciclastic-carbonate coast. The contrasting local geology is significantly reflected in the differences in sediment grain size, composition and sorting at the two sites. The Haiyan overwash sediments are predominantly sand and silt and can be traced up to ~1.6km inland, extending farther beyond the previously reported <300m inland limit of sedimentation. Sites with similar geology, topographic relief, and overland flow conditions show significant spatial variability of sediment thickness and inland extent. We infer that other local factors such as small-scale variations in topography and the type of vegetation cover might influence the spatial distribution of overwash sediments. The Haiyan overwash deposits exhibit planar stratification, a coarsening upward sequence, a non-systematic landward fining trend, and a sharp depositional (rarely erosional) basal contact with the underlying substrate. Overall, the Haiyan deposits have sedimentologic and stratigraphic characteristics that show a hybrid signature common to both storm and tsunami deposits.

Sedimentological characteristics of the 2015 Tropical Cyclone Pam overwash sediments from Vanuatu, South Pacific

The interpretation of sediments deposited by prehistoric tropical cyclones (TC's) is limited by a lack of modern analogues, particularly in the South Pacific. On 13 March 2015, TC Pam made landfall on Vanuatu, reaching Category 5 intensity with 10-minute sustained wind speeds as high as 270km/h. Three months after landfall, we measured flow height (terrain elevation plus storm flow depth) and inland extent of TC Pam's maximum coastal inundation (composed of astronomical tides, storm surge, and superimposed storm waves), and described the sedimentological characteristics of the TC Pam overwash sediments from trenches and transects at two sites (Manuro and Port Resolution Bay).At Manuro (a mixed-carbonate embayment), the maximum flow height was 5.29m mean sea level (MSL), with an inland extent of 106m. The TC Pam sediments transition from a coarse to medium grained (mean: 1.07 Φ) carbonate sand (≤10cm thick) to pumice (≤18cm thick) that extends 400m inland into Lake Otas. The TC Pam overwash sediments are characterized by a coarsening upward sequence (1.45 to 0.23 Φ) followed by a finer grained eolian cap. At Port Resolution Bay (a volcaniclastic beach, PRB), the maximum flow height was 3.30m MSL (1.51m flow depth), with an inland extent of 117m. The TC Pam overwash sediments transition from a medium grained (mean: 1.76 Φ) volcanic sand (≤44cm thick) to pumice (≤5cm thick) that extends 320m from the shoreline. A subtle fining upward sequence was present in trench PRB2, whereas PRB1, PRB3, and PRB4 contained TC Pam sediments that were laminated and showed little to no vertical gradation in grain size. At PRB, we applied an inverse sediment transport model to reconstruct maximum flow depths using laboratory derived settling velocities and the distance from the berm. The reconstructed flow depths at PRB2 (1.43m), PRB3 (1.36m), and PRB4 (1.34m) compare favorably with the observed estimate (1.51m), illustrating the applicability of the inverse sediment transport model to reconstruct flow depths of prehistoric landfalling TC's.

Sedimentological and petrographic analysis of drill core FC77‐1 from the flank of the central uplift, Flynn Creek impact structure, Tennessee

AbstractDrill core FC77‐1 on the flank of the central uplift, Flynn Creek impact structure, Tennessee, contains 175 m of impact breccia lying upon uplifted Lower Paleozoic carbonate target stratigraphy. Sedimentological analysis of this 175‐m interval carbonate breccia shows that there are three distinct sedimentological units. In stratigraphic order, unit 1 (175–109 m) is an overall coarsening‐upward section, whereas the overlying unit 2 (109–32 m) is overall fining‐upward. Unit 3 (32–0 m) is a coarsening‐upward sequence that is truncated at the top by postimpact erosion. Units 1 and 3 are interpreted as debris or rock avalanches into finer sedimentary deposits within intracrater marine waters, thus producing progressively coarser, coarsening‐upward sequences. Unit 2 is interpreted to have formed by debris or rock avalanches into standing marine waters, thus forming sequential fining‐upward deposits. Line‐logging of clasts ranging from 5 mm to 1.6 m, and thin‐section analysis of selected drill core samples (including clasts &lt; 5 mm), both show that the Flynn Creek impact breccia consists almost entirely of dolostone clasts (90%), with minor components of cryptocrystalline melt clasts, chert and shale fragments, and clastic grains. Cryptocrystalline melt clasts, which appear isotropic in thin section, are in fact made of exceedingly fine quartz crystals that exhibit micro‐Fourier transform infrared (FTIR) and micro‐Raman spectra consistent with crystalline quartz. These cryptocrystalline melt clasts are the first melt clasts of any kind to be reported from Flynn Creek impact structure.

Source rock characteristics, depositional setting and hydrocarbon generation potential of Cretaceous coals and organic rich mudstones from Gombe Formation, Gongola Sub-basin, Northern Benue Trough, NE Nigeria

The ongoing exploration activities at the Maiganga coal mine uncover some missing gaps in the stratigraphy of the Late Cretaceous Gombe Formation. Therefore, there is a need to further investigate this sequence, including the organic facies variations and distributions of the coals, mudstones and sandy-shaly sediments in the Gombe Sandstone (Formation), Gongola Sub-basin, Northern Benue Trough of Northeastern Nigeria. Geochemical and petrographic methods were used to assess the organic matter source input, paleodepositional conditions, thermal maturity and hydrocarbon generation potential of the analysed Maiganga and Yaya-Ngari samples. Field observations showed four coal seams interbedded with mudstones and shales deposited in coarsening upward sequence of a deltaic environment. Evaluation based on the source rock analyses (SRA) reveals that Maiganga sedimentary facies is dominated by terrestrial source input of Type III kerogen. This is supported by the biomarker study suggesting land plant origin for the organic matter with minor lacustrine and marine influences, in Maiganga, that grades into predominantly mixed organic matter input of a transitional (terrestrial to marine) environment in the Yaya-Ngari area. This shows a progressive movement towards marine source input. Paleodepositional conditions of organic matters were sub-oxic to oxic in Maiganga, while the mixed organic matters in Yaya-Ngari were deposited in sub-oxic conditions. Based on organic petrology and bitumen extraction data, the Maiganga coal was distinctly observed to be immature of pre-oil generation window, while the Yaya-Ngari is in early oil generation window. These analysed sediments are mainly gas prone based on the dominance of Type III kerogen, although the presence of Type II/III kerogen supported by relatively higher HI may indicate gas/oil potential for the Maiganga coal. On the other hand, Type IV kerogen dominated the Yaya-Ngari samples. This suggests that these shales are gas prone or do not possess any hydrocarbon generation potential as indicated by the very low HI values.