Naslov
New Insights on the Reservoir Geology, Mineralogy and Geochemistry of the Dentale and Gamba Formations (Early Cretaceous, Gabon) and Their Implications for Regional Geology and Pore-Scale Characterization

Autori
Zanoni, G ; Šegvić, Branimir ; Moscariello, A ; Tranchet, L

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Skup
AAPG Annual Convention and Exhibition

Mjesto i datum
Calgary, Kanada, 21.06.2016

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
no ; key ; words

Sažetak
In this study, we focus on the Early Cretaceous Dentale and Gamba Formations in the onshore part of the South-Gabon sub-basin which represent two of the most prolific oil bearing reservoirs of West Africa. They formed during the syn-rift and transition to post-rift time respectively and they are separated by a 3 million-years unconformity. They both consist of siliciclastic rocks accumulated in fluvial, lacustrine (lagoonal) environments. The new study of core material from these two reservoirs provides unprecedented insight on the mineralogy, geochemistry and thus origin and post-depositional diagenetic processes of these formations. The study includes QEMSCAN petrography, bulk and clay XRD, ICP-MS geochemistry, SEM-EDS study on pore-filling mineralization, O and C stable isotope and cathodoluminiscence. The whole-rock geochemical characteristics show clear distinctions between the two formations allowing for a new chemostratigraphic subdivision of these two reservoirs likely associated with different sand provenance which may be in turn explained by the different development/extension of catchment basin where the clastic sediments were sourced from. The combined QEMSCAN and clay-XRD analyses within the Gamba Formation allowed the detection of the berthierine, a clay mineral forming in fresh-water/brackish conditions. This is consistent with mixed fresh water/marine δ13C isotopic signatures on an intraformational carbonate, confirming the hypothesis of post-rift transitional lacustrine/protected lagoonal systems later flooded by the subsequent mid-Aptian transgression. The clay minerals speciation represented by the occurrence of pore-filling illite-smectite (I-S) and chlorite-smectite (C- S) and their smectitic precursors, provide us with new insights on a differing diagenetic history of the two reservoirs which were likely controlled by the occurrence of different coeval volcanoclastic material mixed with the clastic sediments. This is indicated by the occurrence of preserved small fragments of volcanic glass which in turn represent the first account for active volcanism recorded during Aptian times in the eastern coast of South Atlantic. Overall the findings of this stufy provide new insights both at regional and pore-scale that will assist the future exploration and development of hydrocarbons in these two ‘mature’ reservoirs formations.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

POVEZANOST RADA