Investigations of Burial Diagenesis in Carbonate Hydrocarbon Reservoir Rocks

Investigations of burial diagenesis are instrumental for hydrocarbon exploration and exploitation. A proper investigation of diagenesis, with the aim to assist in exploration for and exploitation of hydrocarbons, should follow the "6 -Step Process". Step 1 : facies analysis (including establishing the primary porosity and permeability distributions, and the "primary aquastratigraphy" - a term newly defined in this article); Step 2 : petrographic analyses (paragenetic sequence, mapping amounts and spatial distribution of diagenetic phases); Step 3 : geochemical analyses (isotopes, trace elements, fluid inclusions, etc.); Step 4 : burial history and paleohydrology; Step 5: integration with extant data (especially petrophysical data), if available, and Step 6 : modeling (not necessary, but desirable in at least some cases). Diagenesis, at any depth from near-zero to several kilometres, is governed by various intrinsic and extrinsic factors that include thermodynamic and kinetic constraints, as well as microstructural factors. These factors govern diagenetic processes such as cementation, dissolution, compaction, recrystallization, replacement, and sulfate-hydrocarbon redox-reactions. Cementation, dissolution, and dolomitization require significant flow of groundwater (of whatever type and/or salinity, ranging from fresh to hypersaline), driven by an externally imposed hydraulic gradient. Other processes, such as stylolitization and thermochemical sulfate reduction, commonly take place without significant groundwater flow in hydrologically stagnant systems that are geochemically closed. Two effects of diagenesis that are especially important for hydrocarbon reservoirs are enhancement and/or reduction of porosity and permeability. However, these rock properties can also remain essentially unchanged through diagenesis at depths from near-zero to several kilometres. In extreme cases, an aquifer or hydrocarbon reservoir rock can have highly enhanced porosity and permeability because of extensive mineral dissolution, or it can be plugged up by extensive mineral precipitation. SUMMAIRE Les études de diagenèse d'enfouissement sont des instruments essentiels dans les domaines de l'exploration et de l'exploitation des hydrocarbures. Une étude de la diagenèse ayant comme objectif de contribuer à l'exploration et l'exploitation des hydrocarbures devrait suivre le processus suivant en six étapes : Étape 1) L'analyse des faciès (comportant la mesure de la distribution de la porosité et de la perméabilité initiales, ainsi que de l'" aqua-stratigraphie " - terme redéfini dans le présent article; Étape 2) Les analyses pétrographiques (séquence paragénétique, cartographie de la répartition volumique et spatiale des différentes phases diagénétiques); Étape 3) Les analyses géochimiques (isotopiques, d'éléments traces, des inclusions fluides, etc.); Étape 4) L'historique d'enfouissement et la paléohydrologie; Étape 5) L'intégration avec les données existantes (particulièrement les données pétrophysiques), et Étape 6) La modélisation (pas nécessaire mais utile dans certains cas). Qu'il s'agisse de très faibles profondeurs ou de profondeurs de plusieurs kilomètres, la diagenèse est un phénomène qui est déterminé par des facteurs intrinsèques et extrinsèques, incluant des facteurs thermodynamiques et cinétiques, ainsi que microstructuraux. Ces facteurs déterminent des processus diagénétiques comme la cimentation, la dissolution, la compaction, la recristallisation, la substitution, ainsi que les réactions d'oxydoréduction sulfate-hydrocarbures. La cimentation, la dissolution et la dolomitisation suppose la circulation de volumes considérables d'eaux souterraines (peu importe le type et ou la salinité, qu'elles soient douces ou hyper-salines), mobilisés par les gradients hydrauliques ambiants. D'autres processus comme la stylolitisation et la réduction thermochimique des sulfates, se produisent généralement sans apport substantiel en eau dans le contexte de systèmes hydrologiques stagnants et géochimiques clos. La bonification et ou la détérioration de la porosité et de la perméabilité sont deux des effets diagénétiques particulièrement importants dans la caractérisation des réservoirs d'hydrocarbures. Cependant, ces propriétés lithologiques peuvent demeurer presqu'inchangées par la diagenèse qu'elle se produise à des profondeurs faibles ou de plusieurs kilomètres. Dans les cas limites, un aquifère ou un réservoir d'hydrocarbures peut comporter des porosités et des perméabilités qui auront été grandement bonifiées par l'action d'une dissolution minérale importante, ou voir leurs pores colmatés par l'action d'une précipitation minérale importante

Cathodoluminescence in Calcite and Dolomite and Its Chemical Interpretation

Most carbonate petrologists consider Mn2+ and Fe2+ to be the only trace elements responsible for cathodoluminescence in carbonates. However, luminescence in carbonates is caused or inhibited by a number of trace elements. The main activators in calcite and dolomite are Mn2+ , Pb2+ and several rare earth elements. For Mn-activated luminescence in these minerals, the main sensitizers are Pb2+and Ce2+, and the main quenchers are Fe2+, Ni2+, and Co2+. Non-sensitized Mn-activated luminescence in calcite and dolomite occurs at a minimum concentration of 20-40 ppm, perhaps as little as 5 pm. Pb2+ and Ce2+ can sensitize Mn-activated luminescence at levels as low as 10 and 20 ppm, respectively. Fe2+, the most abundant quencher, begins to quench Mn-activated luminescence at about 35 ppm. Ni2+, Co2+ and Fe3+ quench at even lower concentrations. The quencher concentrations necessary for extinction of Mn-activated luminescence have not been determined with sufficient accuracy, and probably depend on the quencher/activator ratios. It may be misleading to assign a specific activator or quencher element to a carbonate crystal on the basis of the luminescence colour without spectroscopic measurement. Several elements can interact to produce a certain luminescence colour that is a mixture of different emission peaks. These peaks are distinctive for certain activators and/or sensitizers and may be used to identify these elements. The variable cathodoluminescence of diagenetic carbonates is commonly used to infer the pH and redox potential of diagenetic environments by means of pH/E, diagrams that contain only Mn and Fe as cations. This is permissible only if elements other than Mn and Fe are insufficiently abundant to be effective. Other important processes that lead to significant luminescence variations in diagenetic carbonates are closed- and open-system partitioning, clay mineral and organic matter diagenesis and variations in trace element supply. Considering the multitude of parameters that determine and influence the luminescence of carbonates, environmental and stratigraphie interpretations of diagenetic carbonates on the basis of their cathodoluminescence should be undertaken with extreme caution

Facies and diagenesis of the Upper Devonian Nisku formation in the subsurface of central Alberta

The Nisku Formation in the Alberta subsurface consists of bank facies, reefal facies, and basinal/slope facies along the Outer Shelf. The bank facies was not previously recognized, and is here designated the Dismal Creek Member. Most buildups are coral-bearing mudmounds.The Nisku Formation was affected by more than twenty diagenetic processes, most notably by dolomitization and anhydritization. The buildups were partially lithified in shallow phreatic environments, and some were subaerially exposed. Dolomitization took place at depths of about 300 to 1000 m by fluids that were derived mainly from the underlying Ireton Formation. Most of the anhydrites formed during the last stages of and/or after dolomitization. After oil emplacement, thermochemical redox reactions between hydrocarbons and sulfates resulted in partial removal of anhydrite in the deepest buildups, and the formation of 'dead' oil, sour gas, replacive calcite, saddle dolomite, celestite, and native sulfur. Diagenetic changes after maximum burial were very minor

Unusual polygenetic void and cave development in dolomitized Miocene chalks on Barbados, West Indies

Barbados provides an unusual case of polygenetic cave development within dolomitized chalks and marls of the Miocene Oceanics Group. These diagenetic processes are driven by a succession and interplay of tectonic uplift, fracturing, hypogene fluid injection, overprinting by mixing zone diagenesis, and mechanical and biological erosion in the current littoral zone. The significance of the voids and caves within the chalks on Barbados are: 1) these appear to be the first dissolution caves documented in dolomitized chalk, and 2) these features show a polygenetic origin documenting the diagenetic changes in lithology that allowed the development and preservation of these cave types

Mineralogy, Nucleation and Growth of Dolomite in the Laboratory and Sedimentary Environment: A Review

Dolomite [CaMg(CO3)2] forms in numerous geological settings, usually as a diagenetic replacement of limestone, and is an important component of petroleum reservoir rocks, rocks hosting base metal deposits and fresh water aquifers. Dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of Ca2+ and Mg2+ cations interspersed with CO32− anion layers normal to the c-axis. Dolomite has R3 symmetry, lower than the (CaCO3) R3c symmetry of calcite primarily due to Ca-Mg ordering. High-magnesium calcite also has R3c symmetry and differs from dolomite in that Ca2+ and Mg2+ ions are not ordered. High-magnesium calcite with near-dolomite stoichiometry (≈50 mol% MgCO3) has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high-magnesium calcite. Many dolomites display some degree of cation disorder (Ca2+ on Mg2+ sites and vice versa), which is detectable using transmission electron microscopy and X-ray diffractometry. Laboratory syntheses at high T and P as well as studies of natural dolomites show that factors affecting dolomite ordering, stoichiometry, nucleation and growth include temperature, alkalinity, pH, concentration of Mg and Ca, Mg to Ca ratio, fluid to rock ratio, mineralogy of the carbonate being replaced, and surface area available for nucleation. In spite of numerous attempts, dolomite has not been synthesized in the laboratory under near-surface conditions. Examination of published X-ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near-ambient conditions by microbial mediation are unsubstantiated. These laboratory products show no evidence of cation ordering and appear to be very high-magnesium calcite. Elevated-T and elevated-P experiments demonstrate that dolomite nucleation and growth always are preceded by very high-magnesium calcite formation. It remains to be demonstrated if microbial-mediated growth of very high-magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high-temperature experiments