Seismic Attribute-Aided Characterization of Margin Backstepping and Advance along Isolated Carbonate Sequences, Sirt Basin, Libya

Isolated carbonate platforms are common and contain significant hydrocarbon accumulations, particularly in the tectonically complex Sirt Basin in Libya. This study investigates the margin cyclicity of two carbonate stratigraphic sequences developed on an isolated carbonate platform in the NW Sirt Basin using 3-D post-stack seismic volume and wireline log data. The two sequences (sequences 4 and 5) are bounded by unconformity surfaces from the base and top. Seismic attributes show that each sequence displays a cycle of margin backstepping followed by margin advance for several hundred meters. This study concludes that the margin backstepping and advance are mainly influenced by sea-level changes. A rapid sea-level rise caused the backstepping, whereas slow sea-level rise caused the margin advance.

Changes in Coral Skeleton Growth Recorded by Density Band Stratigraphy, Crystalline Structure, and Hiatuses

Next-generation high resolution brightfield microscopy, x-radiography, and microcomputed tomography (microCT) analyses indicate that coral skeleton high density band (HDB) and low density band (LDB) stratigraphic sequences record dynamic changes in coral growth history. HDB-LDB sequences were studied within three small heads of Orbicella annularis, an ecological keystone species in the Caribbean Sea, collected from the leeward fringing reefs on Curaçao. Results indicate that HDB layers are formed by the thickening of exothecal and endothecal dissepiments, costae, and theca located at the margin and external to individual skeletal cups (corallites). Conversely, septa and columellas located inside individual corallites do not change in thickness. HDB-LDB stratigraphic sequences were laterally traced from the center to the margins of individual coral heads, demonstrating that shifts took place in the trajectory of coral skeleton growth. Normal HDB layers in the center of individual coral heads are formed at the same time (age-equivalent) as surfaces of erosion and no skeleton growth (hiatuses) on the margins of the heads. These hiatus surfaces within HDB-LDB stratal geometries indicate that multiple marine ecological and environmental processes affect the orientation, size, shape, and geometry of coral skeletons during coral growth history. The presence of these hiatus surfaces in other large coral heads would strongly impact sclerochronology and the interpretation of multiple environmental factors including sea surface temperature (SST).

ANALYSIS OF THE TAXONOMIC COMPOSI TION OF THE LLANDOVERIAN AND WENLOCKIAN FOSSILS OF MOLDAVIAN PRIDNESTROVYE ENTERED IN THE ELECTRONIC DATABASE “THE PALAEONTOLOGY OF PRIDNESTROVYE”

Problem statement and purpose. In geological surveys of the territory of the Moldavian SSR from the 1960th to the 1990th, deep horizons of Palaeozoic sedimentary rocks were discovered, the leading place among which belongs to the Silurian deposits. At that time, several dozen species and genera of fossils were identified in the Silurian of Moldavian Pridnestrovye. In 2020, as part of the work financed from the state budget on the theme of the creation of GIS “The Geology of Pridnestrovye”, the laboratory “The Geological Resources” developed an electronic database called “The Palaeontology of Pridnestrovye”. The fauna included in the database of palaeontology was divided into groups corresponding to their palaeontological classes and stratigraphic horizons: “Ordovician brachiopods”, “Silurian tabulates”, “Palaeogene foraminifers”, “Neogene bivalves”, etc. Species and genera of the fauna were linked to specific formations and locations, which provides good opportunity for palaeontological analysis of individual formations, horizons, cores of drilling and outcrops. The purpose of this article is to compile lists of fauna from the Llandoverian and Wenlockian formations of Moldavian Pridnestrovye and to compare their chronology with fossil fauna data on the sameage rocks of Podillia, Estonia and Lithuania. Material and methods. Four survey reports deal that on sheets L‑35-V and L‑35-XII were used as the material. The goal was concerning the sedimentary rocks of Llandovery and Wenlock Series. In the local scheme it corresponds to the Marasesti, Step-Soci, Cioc-Maidan (all in Llandovery), Balti formations and Lower Puhoi subformation (Wenlock). The intervals of the formations in boreholes are composed of organogenic limestones, silty, argillaceous stones, argillites and dolomites. The species and genera of the fauna from these intervals are compared with the biostratigraphic data from Podillia, Estonia and Lithuania. Results. In the stratigraphic sequences of Llandovery and Wenlock of the Moldavian Pridnestrovye, (in most cases) species and genera unique to those divisions of brachiopods, tabulates, stromatoporoids, ostracods, rugosans, and trilobites were identified by specialists; a smaller part of the taxa pass between the lowest formations of the Silurian and the overlying Balti formation. Most taxa have correspondences in the Podillian and Baltic basins; however, in these regions, the species and genera identified in Llandovery or Wenlock of Moldavian Pridnestrovye are mostly transitive between Silurian divisions, up to the Ludlow and Pridoli Series. There is practically no identified fauna in the Lower Puhoi sub-formation of Moldavian Pridnestrovye because of its facial specifics. The species Acidolites lateseptatus (Lindstroem), Altha modesta Neckaja, Longiscula caudalis (Jones) in Llandovery and Pentamerus oblongus Sowerby, Halysites catenularius Linnaeus = Halysites priscus Klaamann in Llandovery and Wenlock Series were the only guideing fauna for the first two Silurian series of Moldavian Pridnestrovye. Due to the attribution of the Muksha sub-formation of Podillia to Ludlow Series, no guiding species of the Wenlock stage was identified in the Balti formation of Moldavian Pridnestrovye.

Paleoearthquakes Constrained by Stratigraphic Sequences of Different Drainages Since Late Pleistocene: A Case Study Along the Gulang Fault, NE Tibetan Plateau

Paleoearthquake data obtained from fault trenching are essential for rebuilding the rupture history and understanding the rupture behavior of active faults. However, due to the lack of attention to stratigraphic sequences, the usual multiple trench constraining method may result in uncertainties of paleoearthquake sequences. In this study, we proposed an improved constraining method to generate stratigraphic sequences from multiple trenches of different drainages to obtain a paleoearthquake sequence of the Gulang fault. Single-trench stratigraphic sequences were built up by nineteen trenches excavated along the fault. Based on stratigraphic characteristics, we found the strata sedimented around the fault were derived from five drainages. The single-trench sequences were divided into five drainages to establish the composite sequence of multiple trenches through the correlation of stratigraphic units. Meanwhile, we used high-quality event indicators to pick out very likely earthquakes. Coupled with the dating samples, the events were used to determine the earthquake horizons in the composite sequence and to constrain the numbers and ages of events in each drainage. After combining the event sequences, six paleoearthquakes were determined along the Gulang fault since the late Pleistocene. Their occurrence timings are 13,700–10,400, 10,400–10,200, 8,560–7,295, 5,825–4,810, 4,285–3,200, and 2,615–2,240 a B.P. And their different rupture scenarios indicate that the fault might be composed of two rupture segments.

Possible hydrogeological and thermal conditions of the Quilmes Tectonic Trough (Buenos Aires Province, Argentina): a working hypothesis

The proposal of the Quilmes Tectonic Trough (Fosa tectónica de Quilmes – FQ) as the extension of the southern end of the Santa Lucía basin in Uruguay and its connection to the Salado basin in Argentina suggest the existence of a large sedimentary volume capable of housing a new aquifer on the La Plata River coast. However, the sedimentary volumes that form the FQ are hidden under a thick, recently deposited cover, and thus, there is a lack of studies on the nature of this formation. Nevertheless, the Uruguayan section of the Meso-Cenozoic depocenter of the Santa Lucía basin has been more thoroughly studied for hydrocarbon exploration, which enabled us to estimate the equivalent tectosedimentary characteristics in the FQ. In the Uruguayan territory, three aquifer systems of the Santa Lucía basin are exploited: the Raigón (Plio-Pleistocene) aquifer, which is the most important source of groundwater for various uses in the south-central region of Uruguay, and the Mercedes (Upper Cretaceous) and Migues (Lower Cretaceous) aquifers, which are also used, albeit to a lesser extent, for drinking water, irrigation, and industrial purposes. The Migues aquifer, the least known of the three, shows a variable depth ranging from 100 to 1500 m and considerable stratigraphic sequences of porous and permeable sandstones. These sandstones provide the aquifer with very good qualities as a reservoir rock; as such, the Migues aquifer has been studied for its potential natural gas reserves and geothermal and water resources. Accordingly, if the same sequences with equivalent sedimentary and hydrogeological qualities are present in the FQ, similar aquifers with interesting properties may remain unidentified along the Buenos Aires coast beneath the intensely explored Puelches, Pampeano and Paraná aquifers. In conclusion, specific exploratory activities may prove the existence and quality of these hydrogeological resources, the regional slope toward the southwest from the Uruguayan outcrops, upwelling or semiupwelling conditions and even geothermal energy associated with the deepest cretaceous aquifers.

Algorithms and Models for Smart Well Planning With Emphasis on Trajectory Optimization

Well planning is a time-demanding procedure, which integrates knowledge and experience of various field professionals, as drilling engineers, pipe manufacturers, drilling mud specialists, geologists, etc. According to OG21 report (1), time spent for planning a well is on average 2–3 months. Drilling engineers have to design a well at minimum costs maintaining wellbore integrity at any time. The wellbore should also allow for maximum production from the reservoir and minimum tortuosity for successful casing running and completion. Fortunately, in the well-explored geological areas, where stratigraphic sequences and corresponding geopressures are known, the main objective can be narrowed down to optimal trajectory design. Optimal wellbore trajectory considers at least three criteria: shortest possible path, collision avoidance and longest possible contact with reservoir. Due to large number of modules involved, well planning requires a holistic approach. At the same time, due to complex interaction between the modules and respective physical models, this task implicates a high level of precision. This paper presents a development of an in-house well planning simulator, which integrates all essential well planning modules in a smart way. Central part of the simulator is represented by a trajectory planning and optimization module, which is based on minimization of wellbore length and dogleg severity. Constraints related to anticollision are also included. Introduction of smart optimization techniques for wellbore trajectory has a real potential of saving time and efforts by providing engineers with multiple options, which satisfy the aforementioned constraints. Our ultimate goal is to automate wellbore planning to the largest possible extent by developing smart optimizers for other vital modules within the well planning simulator.

Unique Path Method of the Pinch-Out Profile Based on Unified Stratigraphic Sequence

Pinch-outs refers to the gradual thinning of the thickness of the sedimentary layer laterally until there is no deposition and are a major topic of modern research on the automated drawing of geological profiles. The rapid development of smart geological systems imposed an urgent need for high-speed, accurate methods to plot pinch-outs. However, because of their complexity, excessive number of branch paths, low rendering speed, and poor reliability in the case of large-scale data, the existing pinch-out drawing methods are inadequate and cannot satisfy the modeling needs of large-scale geological projects. To resolve these problems, based on unified stratigraphic sequences, this paper proposes a unique path method for drawing pinch-out profiles by converting the principle of plotting of pinch-outs into controlling the appearance of stratigraphic boundaries, and a high-speed and reliable method for drawing pinch-out in digital profiles is also proposed. The proposed method is successfully applied to drawing geological profiles for an urban geological project in East China, and greatly reduces the complexity of the method without reducing the drawing accuracy. Compared with those of other methods, the speed and reliability are significantly improved. Therefore, the unique path method for drawing pinch-out profiles based on a unified stratigraphic sequence proposed in the writers’ previous paper effectively avoids the excessive branch paths, slow speed, and insufficient reliability of the existing methods and provides effective and reliable support for the rapid drawing of profiles in smart geological systems.

Permo-Carboniferous Climate Change: Geochemical Evidences from Lower Gondwana Glacial Sediments, Rangit Valley, Sikkim Lesser Himalaya, India

The Rangit Gondwana Basin of Sikkim in the lesser Himalaya witnessed a wide span of climate change during the Permo-Carboniferous period. The principal objective of the present study is to document this Permo-Carboniferous climate change in the form of a geochemical signature preserved in the siliciclastic facies of the Rangit Pebble Slate Formation. The stratigraphic sequences of the Rangit Gondwana Basin are categorically well defined and subdivided into upper and lower sequences on the basis of their depositional environment. The lower sequences of the Rangit Gondwana basin are comprised of massive diamictite with large stromatolitic dolomite boulders and dropstone embedded in the coarser sandstone which indicate the cold glaciomarine environment of deposition, whereas upper sequences consist of repeated alternate beds of sandstone, black shale and coal seam with a regular interval depicting the fluvial and deltaic environment of deposition. The enrichment of SiO2, Al2O3, TiO2, MnO, MgO, and K2O indicates that these sediments were mostly derived from felsic rock source areas. Chemical Index of Alteration (CIA) and Index of Compositional Variability (ICV) CaO + Na2O + K2O/Al2O3, and SiO2 vs. (Al2O3 + K2O + Na2O) values suggest that the sediments maturity and paleoclimatic environment deposition of the sediments of lower sequences was cold and semi-humid whereas the deposition of sediments of upper sequences was warm and humid. The A-CN-K ternary plot and CIA vs ICV binary plot also indicate and verify that the source areas were subjected to prolonged intense chemical weathering from low to high grade due to shifting of cold to warm humid paleo-climatic condition.