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Sedimentological and paleoceanographic characterization of the source rock intervals in the West-African Atlantic margin
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Author
Andrea Fabbrizzi, Università degli Studi di Perugia, 2015-16
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Academic area
Mathematical and Physical Sciences
Abstract
Several basins generated during the opening of the South Atlantic Ocean were prone to the deposition and preservation of source rock intervals, during the late Cretaceous epoch.
Through the integration of sedimentological data, biostratigraphy, petrophysics and geochemistry, has been characterized the fine-grained sedimentary sequences rich in organic matter, deposited on a West African basin during the upper Cretaceous. Thanks to the experience acquired during the stage at Eni Exploration & Production S.p.a in San Donato Milanese, the multidisciplinary approach used is aimed to built the sedimentological and paleoceanographic scenario of organic-rich horizons deposited during Cenomanian-Turonian, middle Turonian, Coniacian-Santonian and middle Campanian. The correlation of eight well logs allows to describe the late Cretaceous depositional architecture and to study the geochemical characteristics through TOC and Rock-Eval Pyrolysis data, from proximal to pelagic realm. Whereas the source rock maturity follows the normal trend, decreasing upward toward the shallower intervals, the TOC and HI variability are controlled by local depositional features. The kerogen type of the organic-rich facies studied, ranges between type II and III kerogen, oil-gas-prone with some type I samples, purely oil-prone kerogen from the pelagic Coniacian-Santonian interval.
Geochemical inversion techniques show (through generic and maturity molecular biomarkers) that the origin of oils found within Campanian and Cenomanian reservoirs are linked to the Cenomanian-Turonian source rock interval, which is the only mature across the sedimentary sequence analyzed.
The work focuses on a complete source rock evaluation of the Coniacian-Santonian sedimentary event (OAE 3), passing through the identification of the three independent factors which are recognized as main control variables of organic matter accumulation: Dilution, Production and Preservation. Petrophysical logs, thin sections from cuttings, cores and sedimentation rates show that the dilution phase is composed by six parasequences bounded by seven flooding surface (4th order cycle). The Coniacian-Santonian interval is controlled by one 3rd order flooding surface which divides the sedimentary succession in LST phase characterized by lobes turbidites transported from the proximal upper slope environment and HST phase composed by muddy hyperpycnal and hypopycnal flows (the latter are able to reach pelagic realms). The 4th and 5th order cycles and the short-term climatic fluctuations control the deposition of black shales and organic-lean clay respectively during humid/arid phases. Optical kerogen analysis, TOC and Rock-Eval data show that the continental fraction of organic matter is massed mainly in the proximal areas whereas amorphous organic matter, widely present through the studied interval, becomes particularly dominant towards the distal areas. TOC and HI values are influenced by the distance from the paleo-shoreline (which controls the dilution degree and the productivity) and by the climate able to trigger eutrophication pulses during humid period and algal proliferation until pelagic environments. The preservation factor has been studied through the Pristane/Phytane molecular ratio which describes the widespread and prolonged anoxia, in particular during humid climate.
Source Potential Index (SPI) shows fair to good source rock characteristics at the hemipelagic basin and pelagic environment.