Origin of Micropores In Late Jurassic (Oxfordian) Micrites of the Eastern Paris Basin, France

Abstract

Porous micritic facies, either primary chalks or resulting from secondary destructive micritization, can constitute important hydrocarbon or water reservoirs. Characterization of reservoir properties and the understanding of factors which controlled the distribution of porosity are of primary interest to evaluate the prospective reserves. Middle and late Oxfordian limestones of the eastern Paris Basin show several horizons with porosities higher than 20%. The porosity is mainly microporous and located either within secondary micritized grains or in the micritic matrix. Using SEM, cathodoluminescence, as well as confocal microscopy, stable oxygen and carbon isotope ratios, and petrophysical measurements, a scenario for the evolution of the micropores is proposed. Lime mudstones to packstones constitute the majority of the high-porosity facies (HPFs). Inner lagoonal deposits are more micritized and thus more porous than grainstones, and facies rich in leiolitic oncoids and echinoid clasts are less impacted by micritization. Micritization was responsible for an increase of the intragranular porosity in most grain types. During both eogenesis and shallow burial, mineralogical stabilization dissolved aragonitic particles and allowed precipitation of calcite rhombs. This process was probably enhanced below surfaces of subaerial exposure. During burial, Ostwald ripening allowed the growth of larger micrite crystals at the expense of smaller ones during early Berriasian and late Aptian recharges of deep aquifers when the northern margin of the basin was exposed. Overgrowths on micrite crystals were more important in intervals strongly affected by chemical compaction, which favored oversaturation of waters with respect to calcite. In low-porosity horizons (LPFs), the dense micritic texture of oncoids and the monocrystalline architecture of echinoid clasts prevented an intense micritization, while the strong chemical compaction enhanced poronecrosis. Telogenetic fracturing created new fluid pathways that favored inputs of meteoric fluid in porous micrite and allowed the continuation of Ostwald ripening during Cenozoic times. As a whole, mesogenetic inputs of waters undersaturated with respect to calcite in deep aquifers during exposure of basin margins are a more efficient process than early subaerial exposure for enhancing aggrading neomorphism and appearance of microporous micrites. Initial mineralogical heterogeneities also impact the intensity of chemical compaction and thus the stratigraphical distribution of microporous limestones.

Micro‐ and Nanopores in Tight Zechstein 2 Carbonate Facies From the Southern Permian Basin, Nw Europe

“…7). This feature is also observed in Upper Jurassic limestones in the Paris Basin (Carpentier et al, 2015).…”

“…However, relatively few investigations have considered tight, fine-grained carbonate rocks (e.g. Pittman, 1971;Dravis, 1989;Moshier, 1989;Jameson, 1994;Cantrell and Hagerty, 1999;Heasley et al, 2000;Lambert et al, 2006;Richard et al, 2007;Volery et al, 2009aVolery et al, ,b, 2010Bera et al, 2012;Eltom et al, 2013;Loucks et al, 2013;Lucia and Loucks, 2013;Zhao et al, 2014;Carpentier et al, 2015;Barnett et al in press), although these rocks may commonly host migrated or in situ generated hydrocarbons.…”

Classifying chalk microtextures: Sedimentary versus diagenetic origin (Cenomanian–Santonian, Paris Basin, France)

“…This mechanism is known as Ostwald ripening, i.e. the growth of larger crystals at the expense of smaller ones, with dissolution (Fournier et al ., ; Carpentier et al ., ). During cementation, particles grow and come into contact with one another and particle volume increases.…”

Basin‐scale thermal and fluid flow histories revealed by carbonate clumped isotopes (Δ47) – Middle Jurassic carbonates of the Paris Basin depocentre

“…The micrite matrix surrounding the bioclasts record TΔ 47 of 45 ± 3°C, significantly above the 15 to 27°C surface seawater temperature expected for Middle Jurassic time (Picard et al ., ; Lécuyer, ). These high temperatures and the micrite negative δ 18 O carb values (−4·09 ± 0·61‰ – that are too depleted in 18 O compared with Middle Jurassic marine carbonates of Veizer et al ., ) could be inherited from micrites and carbonate mud recrystallization (for example, mineralogical stabilization) occurring under shallow burial conditions (as also suggested by Vincent et al ., , and Carpentier et al ., , for Upper Jurassic micrites in the eastern Paris Basin). An alternative hypothesis lies on the non‐straightforward interpretation of Δ 47 values from bulk samples of very fine‐grained sediments, because they could reflect the natural mixing of micrite with differently sourced material, such as detrital carbonates, and early to late diagenetic cements (refer to Defliese & Lohmann, , for mixing effects).…”