| Popis: |
Microporous limestones made of rhombic to sub-rhombic low-Mg calcite crystals generally smaller than 4 µm (micrites) account for many carbonate reservoirs, especially in the Middle East. However, despite their substantial economic interest, the genesis of microporous limestones is poorly understood. The main factors involved in the development of the intercrystalline microporosity, as well as the timing of this development, remain a matter of debate. In order to determine the factors and the conditions of formation of microporous limestones, five different studies were conducted: 1) bibliographic inventory of the microporous carbonate formations in the Middle East and comparison of their stratigraphic occurrence with calcite/aragonite seas periods and the relative position of sea-level, 2) study of the lacustrine microporous and tight micrites of the Madrid Basin (Late Miocene, Spain), 3) investigation of the marine microporous and tight limestone alternations in the Urgonian Formation (late Hauterivian to early Aptian, France), 4) description of core sections composed of microporous and tight limestones from the A reservoir of the Mishrif Formation (Cenomanian to early Turonian, Mesopotamian Basin) and an outcrop laterally equivalent to these cores in the Natih Formation (Cenomanian to early Turonian, Oman), and 5) analysis of the Mg distribution inside micrite crystals by using scanning transmission electron microscopy (STEM) combined with X-ray energy dispersive spectroscopy (X-ray EDS). This multi-approach permitted to observe important similarities in the different study objects, to highlight the main factors responsible for the development of the intercrystalline microporosity and to propose a diagenetic model explaining the formation of these microporous limestones. The mineralogical composition of the precursor mud must be dominated by low-Mg calcite crystals. Aragonite and high-Mg calcite muds constitute unstable sediments that transform during diagenesis into low-Mg calcite limestones. On the contrary, muds made up of low-Mg calcite crystals are able to resist a moderate diagenesis and can thus partly preserve their primary microfabric and intercrystalline microporosity. The formation of microporous limestones implies an early cementation of the precursor carbonate mud mainly made up of low-Mg calcite crystals rapidly after sedimentation. In the ionically active zone of a meteoric phreatic lens, the dissolution of the most unstable crystals (aragonites and high-Mg calcites coming from the disintegration of organism tests and the smallest low-Mg calcites) leads to the precipitation of calcite overgrowths around the most stable micrite crystals (the largest low-Mg calcites). The process was named “hybrid Ostwald ripening”. This early and moderate cementation rigidifies the original microporous framework before burial, while partly conserving its microfabric with intercrystalline microporosity, and allows the precursor carbonate mud to resist compaction. In conclusion, two main factors are essential to create microporous limestones: 1) a precursor mud mainly composed of low-Mg calcite crystals, and 2) an early cementation of the precursor mud before burial to allow the sediment to resist compaction and to partly conserve its original microfabric with intercrystalline microporosity. |
