Jessica Saïag PhD thesis

Thumbnail imageCharacterization of sedimentary and petrophysical heterogeneities of a microporous reservoir: the case of the chalk (Upper Cretaceous, Paris Basin)

Funding: BRGM (bureau de recherches géologiques et minières) and Total

Supervisors: Pierre-Yves Collin (UMR 6282 Biogéosciences, uBFC) and Jean-Pierre Sizun (UMR6249 Chrono-envrionnement, uBFC)

Beginned in october 2013

 

Abstract

Chalk is defined as a microporous reservoir rock. This formation is a prolific hydrocarbon-bearing reservoir in the North Sea and is one of the main aquifers in the Paris Basin. The heterogeneity of chalk in terms of petrophysical properties (both surface and subsurface) is illustrated by porosity ranging from 4 to 52 %, permeability from 0.01 to 100 mD, and P-wave velocity on water-saturated samples from 2.4 to 4.4 km.s-1 (Alam et al., 2011).

In the study area (Normandie, France), taken as an outcrop analogue, 114 samples were collected (Cenomanian – Santonian). As in the North Sea fields, these samples show great petrophysical heterogeneity: total porosity (?) from 6.1 to 46.5 %, very low permeability (K; 0.002 mD) to atypical high permeability (477 mD; without fractures), and P-wave velocity on water-saturated samples ranging from 1.8 to 5.5 km.s-1. However, the origin(s) of this large variation in petrophysical parameters is poorly understood.

Three sedimentary models are defined here for the chalk:
(1) Ramp models, subdivided into argillaceous ramp model, with porosity from 34,4 to 46.5%, and permeability from 0.02 to 3 mD, and carbonate ramp model (mudstone to packstone), with porosity from 26.1 to 46.5%, and permeability from 0.04 to 6 mD,
(2) Contourite model (mudstone to grainstone and micro-packstone), with porosity from 8.3 to 45.5%, and permeability from 0.05 to 477 mD,
(3) Softground to Hardground model, with porosity from 6 to 36.9%, and permeability from 0.002 to 36 mD.
Each model has specific distribution in the porosity-permeability plot.

A microtexture classification of the chalk is proposed, based on SEM observation of four groups of criteria (mineralogical content, biogenic fraction, micritic fraction and cement fraction). From these criteria, two major groups are defined: Pure chalk microtexture and Impure chalk microtexture. The Pure chalk microtexture group reflects the intensity of chalk modification during diagenesis (rate of diagenesis, whether early stage or not). An increase in the diagenetic rate induces a decrease in porosity and an increase in P-wave velocity. Diagenetic transformation induces a decrease in pore-space size, with better grain contact, explaining the decrease in porosity and the improved propagation of acoustic P-wave velocity. For Impure chalk microtexture group, the presence of insoluble particles in the matrix does not affect porosity but decreases permeability, due to the reduction of pore-throat size.

The complex relationships between depositional inheritance and diagenetic transformation lead to variability in chalk petrophysical parameters. Spatio-temporal distribution of depositional facies and associated diagenetic transformation is not random; it depends on sedimentological condition (e.g. climate and distance to shore) and imprint of diagenetic processes (early or late stage). By extrapolating from the combined data on facies and diagenesis, the petrophysical properties of the entire cliff section can be characterised, and used to describe the reservoir architecture of the Chalk.

 

Keywords

chalk, sedimentology, microtexture, diagenesis, petrophysics

 

Jury

Yves Géraud, université de Lorraine
François Fournier, université d’Aix-Marseille
Emmanuelle Vennin, université de Bourgogne
Rudy Swennen, université de Leuven, Belgique
Bruno Caline, Total, Pau – invited
Éric Lasseur, BGRM, Orléans – invited
Pierre-Yves Collin, université de Bourgogne – supervisor
Jean-Pierre Sizun, université de Franche-Comté – supervisor