Linking the Variation of Sediment Accumulation Rate to Short Term Sea-Level Change Using Cyclostratigraphy: Case Study of the Lower Berriasian Hemipelagic Sediments in Central Tunisia (Southern Tethys)

High-resolution magnetic susceptibility and % CaCO3 records (5 to 10 cm sampling interval) are used to track astronomical cycles from a Lower Berriasian record from central Tunisia. Six hundred and twenty two samples were measured for magnetic susceptibility and carbonate content as paleoclimate proxies for the detection of potential Milankovitch cycles. Elemental data using X-Ray fluorescence analyses was acquired from 19 samples to prove the reliability of the MS signal on recording the past paleoclimatic changes. We performed multiple spectral analyses and statistical techniques on the magnetic susceptibility signal, such as Multi-taper Method, Evolutive Harmonic Analysis, Correlation Coefficient, Time-optimization, and Average Spectral Misfit to obtain an optimal astronomical model. The application of these spectral analysis techniques revealed a pervasive dominance of E405-kyr and e100-kyr cycles showing that the climate turnover across the early Berriasian—middle Berriasian seems to had been governed by the long and short orbital eccentricity cycles. The identification of Milankovitch cycles in the record also allowed to propose a floating astronomical timescale of the studied section, with ~4 long eccentricity cycles (E405) extracted, which points to a duration estimate of ~1.6 Myr with an average sediment accumulation rate (SAR, after compaction) of 2.77 cm/kyr. The inferred floating ATS was tuned to the La2004 astronomical solution. In addition, we applied the DYNOT and ρ1 methods for seal-level change modeling to reconstruct a local eustatic profile which matches the previously published local and global eustatic charts.

Petrographic study of oil migration on the example of Imilorskoye field (Kogalymsky petroleum region, Western Siberia)

Subject. This paper presents the results of studies of the mechanism of primary oil migration in the boundary sediments of the late Jurassic (Bazhenov formation)–early Cretaceous (Achimov pack) on the example of the well 431Р of the Imilor field. Materias and methods. Based on the working hypothesis of fluid fracturing as the main mechanism of primary oil migration, using optical-petrographic analysis, supplemented by geochemical research methods, systems of interconnected microcracks in the Tithonian-lower Berriasian and early Valanginian deposits were studied. Results. It is established that the investigated cracks occurred in three stages: 1) formation of primary sedimentary-lithogenetic fissures as a result of dehydration of clays in the stage of diagenesis and hydraulic fractures in the implementation of the Achimov of terrigenous rocks; 2) partial healing of cracks with secondary minerals in the catagenesis; 3) restoration of patency of the cracks in the generation of large amounts of free hydrocarbons of protopetroleum. It is shown that the migration of hydrocarbons generated by the formation occurred both within the Bazhenov formation itself and through a system of interconnected cracks in the contact zone of the Bazhenov formation with the Achimov formation. Conclusion. A detailed study of the mechanism of oil migration allows us to expand our understanding of the reservoir properties of the oil column, which in the future will allow us to forecast the reservoir properties of the Bazhenov formation including in the zones of anomalous sections (for example, in the Kogalym region).

Biostratigraphy and microfacies of the pelagic carbonate formations in the Yavorets section (Tithonian–Berriasian), Western Balkan Mts, Bulgaria

Calpionellid study has provided new evidence of early and late Tithonian age of the top of the Gintsi Formation, and of late Tithonian and early Berriasian age of the Glozhene Formation in the Yavorets section (Western Balkan Mts, Bulgaria). The calpionellid Chitinoidella, Praetintinnopsella, Crassicollaria, and Calpionella zones have been documented in successive order. Three calcareous dinocyst zones – Colomisphaera tenuis, Colomisphaera fortis and Stomiosphaerina proxima, have been determined in this lower Tithonian to lower Berriasian interval. From the base upwards, the following microfacies have been recognized: Saccocoma (Gintsi Formation, lower and upper Tithonian), Globochaete alpina, and calpionellid (Glozhene Formation, upper Tithonian and lower Berriasian). The base of the Berriasian has been traced at the explosion of the uniformly shaped spherical variety of Calpionella alpina. Evolutionary lineages of species of the genus Calpionella are discussed, as well as the vertical distributions and abundance peaks of crassicollarians. The calpionellid zones described herein are correlated with coeval zonations from the Western, Central and Eastern Tethyan domains. The regional correlation with previously studied sections of Tithonian/Berriasian pelagic carbonates in the Western Balkan Mts revealed a transition to hemipelagic deposition of the limestone-marl succession of the Salash Formation and/or sandstone accumulation during the middle to late Berriasian (Elliptica and Simplex calpionellid subzones) due to unstable conditions of the sedimentary environment. From the west to the east in the Western Balkan Unit (i.e., from the Rosomač section in eastern Serbia to the Sarbenitsa, Bov and Yavorets sections in the Iskar River Valley area), there is a trend of slight progressive deepening of the basin. This is manifested in the occurrence of redeposited shallow-carbonate-platform microfossils in the west to greater thickenesses of the Gintsi and Glozhene formations and occurrence of sandstone channel deposits in the east.