Analysis Of Sedimentary Sequences Research Articles

The Geochemical Characteristics and Environmental Implications of the Paleocene–Eocene in the Jiangling Depression, Southwestern Jianghan Basin

Various isotopic and palynological indicators have shown interspersed periods of aridity and humidity for the late Paleocene to early Eocene in central China, so the paleoclimate conditions remain unclear. This research investigates the environmental characteristics of a saline lake in the Jiangling depression, southwestern Jianghan Basin, from the Paleocene to the Eocene, using bulk-rock geochemistry in a 1280 m sediment core. The ratios of FeO/MnO, Al2O3/MgO, and C-value indicate a semi-humid to semi-arid climate in the early–middle Paleocene. There was a rapid shift to a humid climate during the late Paleocene to early Eocene, following a short time of intense dryness. The Eocene climate was arid, but experienced intermittent humidity. The variation trend of the CIA, CIW and PIA was similar to that of FeO/MnO, Al2O3/MgO, and the C-value, so chemical weathering of the surrounding rocks was controlled by climate change. The lake redox conditions in the Jiangling depression from the Paleocene to the Eocene were reconstructed using the ratios of U/Th, Ni/Co, and V/Cr. During humidity and alternations of aridity and humidity, the lake water received external water input, resulting in weak stratification, so the sediments were in oxidizing conditions. During aridity, lakes become endorheic, leading to sediments forming in reduced conditions. The salinity of the lake in the Jiangling depression from the Paleocene to the Eocene was determined through analysis of sedimentary sequences and the trend of the Sr/Ba ratio. In the early–middle Paleocene, lake salinity varied greatly. From the late Paleocene to the early Eocene, lake salinity decreased. In the Eocene, lake salinity increased and halite precipitated, but lake salinity finally decreased due to a humid climate. During the late Paleocene–early Eocene, the occurrence of multiple humid climates in the Jiangling depression were not merely regional effects. The most significant humidity was caused by a global hyperthermal (PETM), which caused a huge increase in precipitation in the whole of East Asia and even in low latitudes around the world.

New insights into the Holocene development history of a Pacific, low-lying coral reef island: Takapoto Atoll, French Polynesia

Low-lying atoll islands are known to be made up of unconsolidated, coral-rich detritus, together with high amounts of foraminifera, derived from both outer-reef and lagoonal environments. While regarded as highly vulnerable to ongoing global changes, these islands are poorly constrained in term of development history. Herein is presented a detailed chronostratigraphic study of Tuamotu atoll islands (French Polynesia, south central Pacific) based on analyses of sedimentary sequences through seven excavations along two transects across both windward and leeward rim areas around Takapoto Atoll. The island accretionary chronology is supported by radiometric dating of 62 coral and molluscan clasts. The sequences range between 3.80 m and 1.20 m in thickness, from the oceanic shoreline lagoonwards. Four lithofacies were identified from sediment composition and texture: a coral (pocilloporid)-rich, gravel-supported, preferentially located in the outermost rim areas; a coral-rich, gravelly sand-supported, locally interbedded with gravel-supported units; a foraminifera (amphisteginid)-rich, sand-dominated, mainly located at the central and innermost rim settings; and an organic-rich, sand facies atop of some sequences. A model of atoll islet formation is drawn up in relation to mid to late Holocene sea-level changes. The foundations of islets (motus), namely conglomerate platforms, started to form with deposition of patchy, rubble spreads over the upper reef-rim surfaces from ca 4,500 yr BP as sea level was about 0.80 m above its present mean level. On these platforms, islets started to accrete not before ca 2,300 yr BP, from isolated depocentres located midway between outer-reef and lagoon margins. At that time, sea level at about +0.60 m above present mean sea level was starting to slowly decrease to its present position. The major growth phases occurred in a context of continued sea-level fall. Islets continued to accrete through concentric ridges mainly until the last 300 years. Accretion was dominantly driven by low-frequency, high-energy wave-surge events. From dating of coral clasts, a number of one to two events by century were identified as having apparently contributed to island formation at Takapoto. Regionally predicted increase in the rates of sea-level rise may have negative impacts on such islands since these have evolved under conditions of falling sea level.

Linking shelf delta to deep-marine deposition in reservoir dispersal of the upper Oligocene strata in the Baiyun Sag, the northern South China Sea

Linking shelf delta to deep-marine sediment in the Baiyun Sag (BYS) along the northern South China Sea has led to a great deal of interest, but it remains poorly known. In this paper, we focus on the detailed stratigraphic evaluation and reservoir dispersal of upper Oligocene strata (25.5 to 23.8 Ma, referred to as ZHSQ6), using three-dimensional (3-D) seismic volumes (approximately 2500 km2) calibrated with well-log and core data, to determine internal geometry and sedimentary evolution. Rock velocity estimates from well data show that sedimentary environments may be the most direct controlling factor of wave impedance, reflected polarity and the value of amplitude, which in turn predict the nature of the sedimentary system. Likewise, the analysis of sedimentary sequences reveals that lowstand systems tract and transgressive systems tract consist of inner-shelf deltas. Highstand systems tract, by contrast, contained mid-shelf deltas. A northeast-oriented advantaged longshore current influenced asymmetric sandy ridges with a mixed carbonate–siliciclastic system in the north controlled by paleo-uplift geomorphology. Muddy mass transport deposits (MTDs) and sheet sandstones are both widespread. During falling-stage systems tract, shelf-margin deltas provided coarse grain sediments directly onto the basin floor. Consequently, the volumes of dispersed thick deep-water stacked channels and sand-rich MTDs are significant. The existence of the eastern source clearly increased. Finally, the sediment model for the upper Oligocene Baiyun Sag has been established as an optimal component of source-to-sink systems. This study indicates that sedimentary evolution is controlled by source supply flux and duration, base-level behaviour, topographic gradient and growth faults. Delta front channels and longshore sandy ridges, together with the gully fill, represent the most favourable reservoirs for further hydrocarbon exploration.

A Sedimentological Approach to Upscaling

Optimised upscaling in reservoir simulations requires the construction of realistic petrophysical properties that are representative of the heterogeneity in the sedimentary deposits. Reservoir heterogeneities are controlled by the arrangement of various hierarchies of sedimentary facies and their internal bounding surfaces. The conventional sedimentological approach to reservoir upscaling involves subdivision and ranking of various hierarchies of architectural units and associated bounding surfaces of the reservoir sequence according to their geological significance. This global upscaling approach produces realistic scaled up models that retain both the structural and non-structural heterogeneities of the original sedimentological models. Analyses of sedimentary sequences from various depositional environments indicate that the fractional Levy model can adequately describe the heterogeneity and scaling characteristics of individual genetic sediment sequences in the clastic sedimentary system without further subdividing and ranking of the heterogeneous sequences. The heterogeneous nature of each sedimentary system can be quantified by the Levy index parameter, whereas the maximum upscaling magnitude (or upscaling index) for a particular sequence can be determined from the Levy width parameter plot. Depositional modelling mimics the sedimentary processes in a range of scales and honours hierarchies of sedimentary facies and their bounding surfaces. It can be used effectively for upgridding and upscaling in accordance with the stratigraphic framework and sedimentological models. Both the fractional Levy model and the depositional modelling provide quantitative alternatives to the conventional global sedimentological upscaling approach.

Syntactic Simulation of 1-D Sedimentary Sequences in a Coal-Bearing Succession Using a Stochastic Context-free Grammar

ABSTRACT Syntactic pattern recognition (SPR) provides a new symbolic computing paradigm that has a great potential for application in sedimentologic and stratigraphic theory. In an SPR system, a formal language consists of a set of sentences; each sentence is a string of terminal symbols (or words); a grammar is used to characterize syntactic structures of the language, which tells whether or not a specific sentence belongs to the language. A language represents a class of patterns that are similar to each other in some sense. A sentence from the language then represents an individual pattern of the class. The grammar defines the legality of members of the class in general. There are several key subdomains in this methodology, such as grammar inference, language generation or sim lation, and language parsing. The objective of this paper is to show the possibility of using geological knowledge, obtained by domain experts (here sedimentologists), to construct valid grammars characterizing the vertical structure of different orders of sedimentary sequences, by applying the methodology to the analysis of a coal-bearing succession of the upper Limestone Coal Group, Pendleian (E1) in the Kincardine basin, Central Scotland. First, complete successions from two boreholes are encoded into syntactic strings of lithofacies symbols. Five groups of different high-frequency sedimentary sequences have been recognized. These high-frequency sequences combine to form a lower-frequency sequence. This geological knowledge is used in a heuristic sense to manually construct grammars f r each of the five groups of sequences and for the whole succession. Finally, the constructed grammars, together with a thickness simulation for each lithofacies, are used to generate similar sedimentary sequences to test the validity of the grammars and therefore their further application to a formal analysis of sedimentary sequences. Comparison of simulated sequences with real sequences indicates that SPR is an encouraging technique in the analysis of sedimentary sequences.

High-resolution chemostratigraphy of Quaternary distal turbidites: a case study of new methods for the analysis and correlation of barren sequences

Abstract The late Quaternary distal turbidites of the NE Atlantic Madeira Abyssal Plain (MAP), represent one of the most intensely studied sequences in the modern deep ocean. More than 160 sediment cores have been recovered from the 68 × 10 3 km 2 of the plain, and existing litho- and biostratigraphies, when integrated with oxygen stable-isotope data for other areas of the NE Atlantic, provide a tight spatial and temporal framework. We have used the MAP as a test-bed to assess the applications of inorganic geochemistry as a sedimentological and stratigraphic tool. Data for 22 major and trace elements were obtained in over 500 samples taken from representative cores. Chemostratigraphic sequences are highly uniform across the plain, and several turbidites have unique geochemical signatures, providing a means of establishing bed-by-bed correlations over distances of > 500 km. Cluster and principal component analysis confirm the statistical validity of empirically derived correlations and groupings. Vertical and lateral geochemical trends within beds document the sedimentological evolution of flows, and may be used to establish palaeotransport pathways. Four compositional groups are interpreted as representing derivations from different source areas: (1) the margins of the NW Africa off Morocco; (2) off Western Sahara; (3) the Canary Islands; (4) the Cruiser-Great Meteor East seamount chains. The study amply demonstrates the viability of chemostratigraphy for the correlation and analysis of sedimentary sequences, providing a stratigraphic resolution and reliability which matches or exceeds that obtainable by other techniques.

A Review of the Development of a Digital Log Database for Petroleum Exploration

Modern petroleum exploration, incorporating routine acquisition of geophysical logs, began in Australia in the mid 1950s. Prior to 1976, data were recorded in analog form. By 1982, digital recording was routine for most logging units, and since 1985 few operators have accepted non-digital records. Approximately 60% of the log data acquired to date in Australia is thus analog. Varying standards of recording practice, evolving hardware and changes in drilling practice have all affected the quality of analog records, and their ease of use. The economics of use are often highly dependent on record quality. In 1981 Wiltshire Geological Services began systematic high fidelity conversion of analog data to digital. Our initial objectives were to develop tools for better and more efficient analysis of sedimentary sequences and basin evolution. By 1985 the developing data set was clearly a major asset. Since then we have built the data set as rapidly as possible whilst maintaining the highest possible data conversion standards. Resulting benefits include: data preservation is assured; data are standardized and enhanced by editing of noise; data reproduction is easier and cheaper; end-user working material is routinely tailored to specific job needs; worker efficiency is dramatically improved. Intangible benefits may arise from better interpretive use of the data in exploration.

Basin Analysis Studies of Lower Paleozoic Rocks, Powder River Basin, Wyoming and Montana: ABSTRACT

The lower Paleozoic (Cambrian through Mississippian) sedimentary rocks of the Powder River basin represent nearly half of Phanerozoic time, yet they remain virtually unexplored in the subsurface. Rocks of the same age in the Big Horn and Williston basins and in the Central Montana trough have produced much oil and gas, as have the overlying Pennsylvanian strata of the Powder River basin. A synthesis of published stratigraphic information, together with a regional analysis of sedimentary sequences, has been undertaken to evaluate the economic potential of the lower Paleozoic formations. The lack of an economic impetus to study these rocks has hampered the development of precise depositional models for these sequences. Furthermore, the depths of prospective beds, as well as long-standing misconceptions about the regional stratigraphy, have also served to restrain exploration. Stratigraphic studies have documented a succession of marine transgressions and regressions on the flanks of a highland in southeastern Wyoming. The highland persisted as a subdued geographic feature through most of early Paleozoic time, until it rose at the end of the Mississippian. Erosion during the Late Silurian and Devonian removed much of the depositional record in the area, but onlap can be demonstrated with relative certainty for Ordovicianmore » and Mississippian rocks. The repetition of sedimentologic features indicates persistent geologic controls in the region and suggests that these paleoenvironments might provide good targets for exploration.« less