Modifications of chalk microporosity geometry during burial - An application of mathematical morphology

The diagenetic evolution of chalk during burial has previously been established and can be summarized as follows: (1) ooze deposition and dewatering (burial < 300 m), (2) lithification via mechanical compaction and grain bridging cementation (300-1000 m), (3) complete cementation due to pressure solution, or porosity preservation thanks to pore fluid overpressure and oil invasion (> 1000 m). Moreover, chalk particles tend to increase in size as a result of diagenesis, i.e. recrystallization or cementation. As a result of compaction and cementation, the shape, size and connectivity of chalk microporosity evolve during burial. However, if the evolution of bulk porosity during burial is well quantified, actual modifications of pore space topology are less understood. Quantifying pore space properties such as pore and throat sizes, aspect ratio and spatial distribution usually require costly instruments as well as long and complex experimental procedures. Instead, this study documents the application of mathematical morphology to investigate chalk microporosity from two-dimensional (2D) scanning electron microscope (SEM). Chalk samples (n = 105) have been collected from onshore quarries and offshore oil and gas reservoirs. Six chalk lithotypes are defined based on their mineralogical composition, i.e. clay and silica content, as well as the average size of chalk particles, and their apparent diagenetic impact, which appears strongly related to the mean grain size. The latter is used as a proxy for the diagenetic overprint, while the burial-diagenetic model offers a framework for the different lithotypes and their respective pore space properties. During intermediate burial (< 1000 m), the effect of mineralogy on pore topology appears more dramatic than after further burial. From the same outcrop previously buried to ca. 700 m, pure chalk yields total porosity of 45-50% whilst clay-rich chalk yields porosity of 35-40%. Moreover, pore and throat dimensions are on average significantly greater in pure chalk (4.15 and 0.73 mu m, respectively) than in clay-rich chalk (2.85 and 0.28 mu m, respectively). In both lithologies however, particles share many similarities, including their size (1.3 mu m), shapes and rare calcite overgrowths, producing a mudstone texture. This suggests that compaction rather than calcite cementation explains the difference in porosity properties. At greater burial depths (> 1000 m), pressure solution and cementation become pervasive and produce large clusters of coalescent calcite crystals (3-10 mu m; mean = 5.13 mu m). Total porosity is strongly reduced as a result (6-11%) while pore body and throat dimensions drop to 1.58 and 0.03 mu m, respectively. A high aspect ratio (88.1) and lower proportion of rough porosity relative to total image porosity suggest that throats are proportionally more affected than pore bodies. Cementation leads to less dendritic, more globular and isolated pores. Where processes such as overpressure and oil charge have helped preserve porosity during deep burial (20-30%), pore and throat dimensions were locked to moderately high values (> 2.0 and 0.1 mu m, respectively), even in silica- and clay-rich chalk. Since the geometry of microporosity exerts a major control on fluid flow, reservoir modelling and recovery predictions could benefit from a better understanding on the geological controls on chalk microporosity. Not only can mathematical morphology quantify pore space properties that are fundamental in pore network and flow modelling, but by being applicable on large datasets, it could provide a new insight into the geological modifications of chalk microporosity during burial.