Naslov
Clay minerals from Mesozoic clastic reservoirs of the South Gabon Basin (West Africa) ˗ diagenesis and impacts on regional geology

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

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

Skup
8th Mid-European Clay Conference

Mjesto i datum
Košice, Slovačka, 04.07.2016. - 08.07.2016

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
no ; key ; words

Sažetak
In sandstone reservoir rocks the role of clay minerals has been extensively studied in the last decades because clays readily occlude primary porosity, influence different geophysical parameters of host sandstones and react with drilling fluids, thus affecting various oil recovery practices [1]. The origin of clays in sandstones is complex and multifold, being either detrital or authigenic, formed through a series of diagenetic reactions [2]. In the area of South Gabon Basin in West Africa, several sandstones formations were formed during the opening of the South Atlantic. Two of the most prominent formations therein – Dentale and Gamba Fms. – were reported as most important hydrocarbon-bearing clastic onshore reservoirs in western Africa [3]. Their clay mineral paragenesis is seriously under-researched. In this contribution, we present the first results on the mineralogy and geochemistry of pore-filling and grain coating clays from both formations, discussing their diagenetic evolution and the significance they may have had on the regional geology reconstruction. A total of 30 and 44 samples originating from Gamba and Dentale Formations, respectively were analysed by XRD, SEM, QEMSCAN, and ICP-MS. The studied sandstones showed to be dominated by anisopachous and discontinuous coatings or infills of mixed-layer illite-smectite (I-S), chlorite-smectite (C-S) and berthierine- chlorite (B-C). Such peculiar clay morphology, corroborated by the facies analyses [4], is characteristic for extrabasinal clays entrained into the sediment by clay infiltration, whereas the origin of B-C is linked to the diagenetic conversion of berthierine to tri-octahedral Fe- rich chlorite. Knowing that the composition of infiltrated clays has a decisive impact on the mesogenetic evolution pathways of porous clays, the burial depths of the cores analysed were chosen to conveniently match the transition of eodiagenesis to mesodiagenesis. This enabled to identify the primarily di- and tri-octahedral smectites that are, through a series of mixed- layer intermediates, transformed into illite or chlorite, respectively. In several horizons, however, higher clay abundances (~15–30 wt%) bear signs of pyroclastic material of a twofold geochemical character, conformant to the whole- rock trace-element geochemistry and diagenetic reactions, that led to the preferential formation of I-S and/or C-S. This offers a rare piece of evidence on the Cretaceous regional volcanic activity along the western African ridge at the time of the opening of the South Atlantic Ocean.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

POVEZANOST RADA