Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian to Early Turonian, Middle East)

Shallow-marine microporous limestones account for many carbonate reservoirs. Their formation, however, remains poorly understood. Due to the lack of recent appropriate marine analogues, this study uses a lacustrine counterpart to examine the diagenetic processes controlling the development of intercrystalline microporosity. Late Miocene lacustrine microporous micrites of the Madrid Basin (Spain) have a similar matrix microfabric as Cenomanian to Early Turonian shallow-marine carbonates of the Mishrif reservoir Formation (Middle East). The primary mineralogy of the precursor mud partly explains this resemblance: low-Mg calcites were the main carbonate precipitates in the Cretaceous seawater and in Late Miocene freshwater lakes of the Madrid Basin. Based on hardness and petrophysical properties, two main facies were identified in the lacustrine limestones: a tight facies and a microporous facies. The tight facies evidences strong compaction, whereas the microporous facies does not. The petrotexture, the sedimentological content, and the mineralogical and chemical compositions are identical in both facies. The only difference lies in the presence of calcite overgrowths: they are pervasive in microporous limestones, but almost absent in tight carbonates. Early diagenetic transformations of the sediment inside a fluctuating meteoric phreatic lens are the best explanation for calcite overgrowths precipitation. Inside the lens, the dissolution of the smallest crystals in favor of overgrowths on the largest ones rigidifies the sediment and prevents compaction, while partly preserving the primary microporous network. Two factors appear essential in the genesis of microporous micrites: a precursor mud mostly composed of low-Mg calcite crystals and an early diagenesis rigidifying the microcrystalline framework prior to buria

TEM study of Mg distribution in micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian)

Microporous limestones composed of micrite crystals constitute sizeable hydrocarbon reservoirs throughout the world and especially in the Middle East. However, the crystallization history of micrites is poorly understood. Scanning electronic microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS) studies give morphological and bulk composition information about micrites, but no information exists on the distribution of minor elements inside micrite grains. This study proposes Mg maps obtained with X-ray EDS combined with scanning transmission electron microscopy (STEM) of micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian). Three types of Mg distribution were observed through micrite crystals from five different samples: (1) homogenous Mg concentration, (2) small Mg-enriched areas close to the center of the crystal, and (3) geometric Mg impoverishments near crystal edges and parallel to present crystallographic faces. The homogenous Mg distribution is the most frequent and is found both in microporous and in tight micrites. The second type of distribution showing small Mg-enriched areas inside micrite crystals relatively close to their center comes from a microporous sample located below an emersive surface. These enriched areas may correspond to crystal seeds. The third type of distribution was observed in micrite crystals from another microporous sample situated just below an emersive surface. The Mg-poor zones probably represent overgrowths that precipitated in contact with less Mg-rich meteoric fluid

Genesis and diagenesis of microporous micrites

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

Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian to Early Turonian, Middle East)

Shallow-marine microporous limestones account for many carbonate reservoirs. Their formation, however, remains poorly understood. Due to the lack of recent appropriate marine analogues, this study uses a lacustrine counterpart to examine the diagenetic processes controlling the development of intercrystalline microporosity. Late Miocene lacustrine microporous micrites of the Madrid Basin (Spain) have a similar matrix microfabric as Cenomanian to Early Turonian shallow-marine carbonates of the Mishrif reservoir Formation (Middle East). The primary mineralogy of the precursor mud partly explains this resemblance: low-Mg calcites were the main carbonate precipitates in the Cretaceous seawater and in Late Miocene freshwater lakes of the Madrid Basin. Based on hardness and petrophysical properties, two main facies were identified in the lacustrine limestones: a tight facies and a microporous facies. The tight facies evidences strong compaction, whereas the microporous facies does not. The petrotexture, the sedimentological content, and the mineralogical and chemical compositions are identical in both facies. The only difference lies in the presence of calcite overgrowths: they are pervasive in microporous limestones, but almost absent in tight carbonates. Early diagenetic transformations of the sediment inside a fluctuating meteoric phreatic lens are the best explanation for calcite overgrowths precipitation. Inside the lens, the dissolution of the smallest crystals in favor of overgrowths on the largest ones rigidifies the sediment and prevents compaction, while partly preserving the primary microporous network. Two factors appear essential in the genesis of microporous micrites: a precursor mud mostly composed of low-Mg calcite crystals and an early diagenesis rigidifying the microcrystalline framework prior to burial.status: publishe

TEM study of Mg distribution in micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian)

Microporous limestones composed of micritecrystals constitute sizeable hydrocarbon reservoirs throughout the world and especially in the Middle East. However, the crystallization history of micrites is poorly understood. Scanning electronic microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS) studies give morphological and bulk composition information about micrites, but no information exists on the distribution of minor elements inside micrite grains. This study proposes Mg maps obtained with X-ray EDS combined with scanning transmission electron microscopy (STEM) of micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian). Three types of Mg distribution were observed through micrite crystals from five different samples: (1) homogenous Mg concentration, (2) small Mg-enriched areas close to the center of the crystal, and (3) geometric Mg impoverishments near crystal edges and parallel to present crystallographic faces. The homogenous Mg distribution is the most frequent and is found both in microporous and in tight micrites. The second type of distribution showing small Mg-enriched areas inside micrite crystals relatively close to their center comes from a microporous sample located below an emersive surface. These enriched areas may correspond to crystal seeds. The third type of distribution was observed in micrite crystals from another microporous sample situated just below an emersive surface. The Mg-poor zones probably represent overgrowths that precipitated in contact with less Mg-rich meteoric fluids

Microporous and tight limestones in the Urgonian Formation (late Hauterivian to early Aptian) of the French Jura Mountains: Focus on the factors controlling the formation of microporous facies

Microporous and tight limestones, with contrasting porosity and permeability values directly related to the microfabric of the micritic matrix, outcrop in the Urgonian Formation of the French Jura Mountains. This study investigates the factors controlling the differentiation between the microporous and tight facies, and proposes a diagenetic model for the development and preservation of the microporosity in these limestones. The petrophysical properties are not related to the depositional texture, the petrographical content or the mineralogical composition. However, the tight layers contain indications of emersion (e.g.: bird eyes, keystone vugs, and desiccation cracks). The sedimentation in very shallow conditions up to emersion is confirmed by the covariant more positive values of oxygen and carbon isotopes. The microporous intervals systematically occur a few meters below the tight layers affected by emersion. This position strongly suggests the importance of meteoric water input rapidly after sedimentation in the differentiation between tight and microporous limestones. The diagenetic model proposed for the development and preservation of the microporous facies involves partial early cementation of the interstitial mud, mainly composed of low-Mg calcite crystals (sedimentation during a calcite sea period), inside a meteoric phreatic lens by in situ dissolution–reprecipitation processes (“hybrid Ostwald ripening”). This early cementation partly preserves the original microfabric and intercrystalline microporosity and allows the carbonate sediment to resist compaction during burial. The identification of the conditions favorable to the development of microporosity in these Urgonian limestones may improve the knowledge and modeling of some microporous carbonate reservoir rocks