Diagenetic imprints on reservoir quality, provenance perceptions and paleosalinity conditions of the Miocene Agbada Formation, Niger Delta Basin: a petrographic and geochemical investigation

Overare, Brume (2024) Diagenetic imprints on reservoir quality, provenance perceptions and paleosalinity conditions of the Miocene Agbada Formation, Niger Delta Basin: a petrographic and geochemical investigation. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The main exploration targets for hydrocarbons in the Niger Delta Basin are within the Agbada Formation sandstone reservoirs. Most studies in the Agbada Formation are skewed towards understanding the petroleum system, and this has led to a paucity of datasets on several important subjects, which is a motivation for this research. This investigation utilizes a multi-technical (Thin-section, SEM-MLA, XRD, XRF, ICP-MS and LA-ICP-MS) approach to evaluate the petrographic attributes, diagenesis and influence of rare calcite cement on reservoir quality, inorganic geochemical signatures and their significance for unraveling provenance and paleosanity conditions prevalent in the Agbada Formation. Weathering proxies (e.g. CIA, CIX, αᴬˡE), paleoclimatic signatures (e.g., Ga/Rb vs. K₂O/Al₂O₃) and elemental ratios (e.g. La/Co, Cr/Th, Th/Co), including REEs signals (LREE/HREE, flat HREE pattern, Eu/Eu*) diagnostic of provenance for the sandstones and interbedded shales suggests moderate to intense weathering, humid paleoclimatic settings and signatures of sediments derived predominantly from felsic sources. Boron-derived paleosalinometric tools (e.g. equivalent boron, Adam's, Landergren and Carvajal's, and Couch's methods), elemental ratios (e.g. B/Ga, Sr/Ba, and S/TOC) and other proxies sensitive to redox (e.g. Cr, Ni/Co, Mo-EF/U-EF) point to mainly low- saline (brackish) settings with considerable signature inputs from both freshwater and marine water (comparatively lesser), and oxic-suboxic conditions for the interbedded shales. The Agbada sandstones (Q₈₈.₅₈ F₉.₉₉ RF₂.₄₃) are classified as feldspatho-quartzose and quartzose sandstones, with a predominance of quartzose sandstones and their diagenetic signatures reflect low degree of compaction and lack of pervasive cementation, with compaction comparatively posing more influence on porosity, excluding some intervals with considerable cementation (siderite and ferroan calcite) where compactional porosity-loss was minimal. The occurrence of ferroan calcite cement (mainly ferroan) is generally small, probably discontinuous and may generally not pose a substantial barrier to fluid flow, but understanding their localized influence may still contribute to building good reservoir models. Petrographic evidence in the sandstones suggests that calcite cement is an early event, but the shale normalized REE + Y patterns appear to reflect non-seawater patterns, indicating possible contamination (Y/Ho = 36.30 ± 1.35) from clastic or detrital grains that depict fluvial/estuarine influence and/or incorporation of particulate matter that can preferentially scavenge LREEs from the overlying water column. A potential concern during reservoir production is the anticipated production of fines and formation damage linked to the occurrence of clay mineral matrix (mostly kaolinitic), but this can be reduced with appropriate reservoir management.

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