Ihsan S. Al-Aasm

Stable isotope analysis of multiple carbonate samples using selective acid extraction

Selective chemical separation techniques for isotopic analysis of coexisting calcite, dolomite, siderite and magnesite have been investigated in this study. Comparison of reaction rates of all pure carbonate minerals with 100% phosphoric acid at 25° and 50°C demonstrated that a uniformly fine grain size (<200 mesh) is required. Tests with fine-grained (<200 mesh) pure calcite, dolomite, siderite and magnesite reacted at 25°C (calcite) and 50°C (others) show that δ18O of evolved CO2 increases during the course of reaction. This is probably due to a slight kinetic isotope effect associated with rapid dissolution of sub-micrometer carbonate crystallites adhered to the outer surfaces of the very fine-grained crystals. Isotopic analysis of carbonates based on CO2 from incomplete acid reactions is accurate only after minimum reaction times. Conversely, much longer reaction times than the minima are not required. Experiments conducted on variable mixtures of calcite-dolomite, dolomite-siderite and calcite-dolomite-siderite demonstrated the magnitudes of cross-contamination on the inferred isotopic compositions of the individual minerals. The isotopic compositions and magnitudes of their uncertainty can be evaluated for individual carbonates selectively extracted from natural mixtures by variable time-temperature protocol.

Burial dolomitization and dissolution of Upper Jurassic Abenaki platform carbonates, Deep Panuke reservoir, Nova Scotia, Canada

A large gas reservoir was discovered in the previously unproductive Jurassic-aged Abenaki carbonate margin in 1998. Most of the reservoir porosity is developed in dolostones. These dolostones replaced preexisting wackestones, packstones, and grainstones(?) associated with reefal and adjacent depositional environments. Many dolomites were subsequently recrystallized or dissolved, accounting for much of the preserved secondary porosity. Subsequent fracturing helped enhance reservoir permeabilities. Enhanced petrographic techniques established that dissolution of previously dolomitized fabrics generated much of the secondary porosity in these dolostones. Diffused plane-polarized light revealed relict grains and textures invisible with standard microscopic observations. Petrographic and geochemical observations also confirmed that dissolution occurred under deep-burial conditions after incipient pressure solution. Dissolution was not confined to the centers of dolomitized grains, as is commonly seen when remnant calcitic grains dissolve out during the advanced stages of replacement dolomitization. Instead, dissolution was random within relict grains, as isolated dolomite crystals were also variably dissolved. The geochemistry of these dolomites and associated late-stage calcites implied precipitation from basinal hot fluids, as well as hydrothermal fluids. Later diagenetic fluids, either acidic or calcium rich, or perhaps both at different times (based on associated mineralization), seemingly promoted dolomite dissolution. The presence of tectonic fractures and stylolites, helium gas, and faults observed in seismic data implied that dolomitization and subsequent dissolution along the Abenaki platform margin were controlled by reactivated wrench faults tied to basement. On a finer scale, diagenetic fluids moved through fractures and pressure-solution seams. The data collected to date support our contention that the dolomitization and dissolution process, which has created most of the porosity in the Abenaki reservoir, was poststylotization and deeper burial in origin. Given the timing of tectonic activity in the area and its inferred connection to diagenesis, it is probable that at least a part of the diagenetic fluids were hydrothermal in nature.

Distribution of Diagenetic Alterations in Fluvial, Deltaic, and Shallow Marine Sandstones Within a Sequence Stratigraphic Framework: Evidence from the Mullaghmore Formation (Carboniferous), NW Ireland

ABSTRACT The distribution of diagenetic alterations in the fluvial, deltaic and shallow marine, arkosic to subarkosic sandstones (average Q72F26L02) of the Mullaghmore Formation (Carboniferous, NW Ireland) can be predicted within a sequence stratigraphic framework. Eogenetic calcite (18OPDB = -13.3‰ to -6.5‰, 13CPDB = -3.0 to +3.4‰, and 87Sr/86Sr = 0.706721 to 0.709227) and ferron dolomite (FeCO3 = 8-12 mol%; 18OPDB = -14.2‰ to -7.8‰, 13CPDB = -1.4‰ to -1.0‰, and 87Sr/86Sr = 0.709051 to 0.709167) occur in bioclast-rich, transgressive lag deposits at parasequence boundaries and transgressive surfaces, and in wave-influenced, deltaic, highstand systems tract (HST) deposits. Mesogenetic illite, chlorite, baroque dolomite (FeCO3 = 16 mol%; 18OPDB = -14.2‰ to -12.7‰, 13CPDB = -3.8‰ to -1.0‰), quartz, and calcite (18OPDB = -15.7‰ to -12.5‰, 13CPDB = -5.8‰ to -3.7‰, and 87Sr/86Sr = 0.709016 to 0.709122) were formed mainly in the bioclast-poor deposits, which were not pervasively cemented by carbonates during near-surface eodiagenesis. These deposits include fluvial, incised-valley sandstones of lowstand systems tract (LST), and fluvial-dominated, deltaic sandstones of transgressive systems tract (TST) and HST. Illite is the dominant diagenetic clay mineral in the fluvial, incised-valley sandstones of LST, possibly because of simultaneous albitization of K-feldspars. Conversely, chlorite, dominates in the fluvial-dominated, deltaic sandstones of TST and HST, because of the presence of suitable precursor clays. The integration of diagenesis into sequence stratigraphic framework of clastic sequences should improve the ability to predict the spatial and temporal distribution of diagenetic alterations and related reservoir-quality modifications of sandstone deposits.

Diagenetic Stabilization of Aragonite and Low-mg Calcite, I. Trace Elements in Rudists

ABSTRACT Cretaceous rudists, with thick, multilayered, bimineralic (aragonite and low-Mg calcite) shells, have been studied for the effects of meteoric diagenesis on their textural and chemical attributes. The original fine, compact, prismatic or cellular-prismatic microstructures of low-Mg calcitic layers have been preserved frequently, despite the vagaries of their subsequent diagenetic histories. These layers usually suffered only partial recrystallization, which caused fusion of crystal units, rounding of prism comers, and partial cementation confined to intercrystalline and intraparticle pore spaces. In contrast, the originally aragonitic parts of the shell usually suffered either complete dissolution or, at best, retained some vestiges of their crossed-lamellar or complex crossed-lamella , microstructures. Synsedimentary submarine diagenetic events, such as borings by endolithic fauna, internal micritic and/or peloidal sedimentation, and submarine aragonite and/or high-Mg calcite cements within the primary inter- and intraparticle pore spaces, were coeval with rudist accretion. The subsequent meteoric cementation has been characterized by the formation of bladed and equant ferroan calcites which occluded the residual primary and secondary porosities. Diagenetic stabilization of aragonite to diagnetic low-Mg calcite and low-Mg calcite to diagenetic low-Mg calcite is believed to have been a two-stage process. Initially, Sr and Na have been partially depleted during an early meteoric diagenetic phase (calcitization of metastable phases), with expulsion of Sr at a slower rate than that of Na. This stage is followed by incorporation of more Mn, Fe, and Mg due to precipitation of late ferroan calcite. Stabilization of aragonite causes more pronounced trace-element repartitioning than the stabilization of low-Mg calcite. However, the degree of repartitioning depends not only on mineralogy, but also on the structural buildup of a given shell layer, suggesting that factors such as water/rock ratio (surface kinetics) are of considerable sig ificance for the process of diagenetic stabilization. Rudist skeletal components with preserved original mineralogy and texture have trace-element chemical signatures analogous to Holocene marine bivalves. This similarity argues for comparable chemical composition of the Cretaceous and Holocene seawater, as well as for similar modes of incorporation of trace elements into mollusc shells.

Diagenesis and Reservoir-Quality Evolution of Fluvial Sandstones During Progressive Burial and Uplift: Evidence from the Upper Jurassic Boipeba Member, Reconcavo Basin, Northeastern Brazil

The reservoir quality of fluvial sandstones of the Upper Jurassic Boipeba Member, Reconcavo basin, northeastern Brazil, is highly heterogeneous and controlled by eodiagenesis under semiarid climate, mesodiagenesis during burial to a depth of 3500 m, and telodiagenesis due to local uplift. Eodiagenesis resulted in mechanical compaction, calcite cementation, clay infiltration, and limited grain dissolution, whereas mesodiagenesis resulted in the precipitation of calcite cement and quartz over growths, intergranular quartz-grain dissolution, chloritization and illitization of smectite, and albitization of feldspars. Sandstones continuously buried at maximum burial depths of about 1600 m (T = 65°C) since 125 Ma display a relatively greater degree of mesogenetic modifications and, on average, poorer reservoir quality than sandstones that were buried deeper (2100 m, T = 75°C) prior to uplift, but only since 13 Ma. Uplift, which affected the sequence along the western border of the basin, has resulted in telogenetic dissolution of framework silicates and formation of kaolinite. Relatively good reservoir quality in the deeply buried sandstones occurs when (1) the grains are coated with a thin layer of chloritized infiltrated smectite, (2) there is little or no pseudomatrix, and (3) there are widely scattered patches of eogenetic calcite cement that supported the framework of sandstones against compaction.

Chemical Stabilization of Low-Mg Calcite: An Example of Brachiopods

ABSTRACT Trace-element study of brachiopod shells from the Upper Ordovician Ellis Bay Formation (Anticosti Island, Quebec, Canada) shows that they underwent only a slight (<=20%) diagenetic equilibration with meteoric waters despite 450 million years of postdepositional history. This, in contrast to large trace-element repartitioning observed in their enclosing rocks, attests to the relative stability of low-Mg calcite even within the realm of meteoric waters. Paleozoic brachiopods apparently exerted a biological control on the distribution of Na and Mg in their low-Mg calcitic shells, preferentially incorporating the former and discriminating against the latter trace constituent.

Subaerial exposure and meteoric diagenesis of the Cenomanian-Turonian Upper Sarvak Formation, southwestern Iran

Abstract The Sarvak Formation (Cenomanian–Turonian) forms one of the main reservoir rocks in many oilfields in southern Iran. Extensive lateral and vertical facies variations as well as effects caused mainly by the subaerial exposure associated with the regional Turonian unconformity have resulted in variable porosity and permeability. Dissolution affected the entire upper part of the Sarvak Formation and is the most important process related to subaerial exposure. Brecciation, development of palaeosol and formation of bauxite deposits are also limited to the upper few metres of the top of the Sarvak Formation and indicate warm and humid climatic conditions. Subaerial exposure had varying effects on the diagenesis depending on its duration, palaeotopography and the availability of meteoric water. The δ18O and δ13C values obtained from calcitic matrix, various generations of calcite cements and calcitic rudist shells in the Upper Sarvak overlap to a large extent, indicating their equilibration with fluids of similar isotopic composition. Negative δ18O values (i.e. −6.6‰ to −1.7‰) suggest a significant meteoric component. More 18O-depleted values (e.g. −12.3‰) obtained from late calcite cements indicate their precipitatation from warm fluids. Positive δ13C values (i.e. 0.00‰ to 3.4‰) in the various carbonate phases reflect values of seawater coeval with an Oceanic Anoxic Event and later modified by meteoric waters.

Karst-controlled diagenesis and reservoir development: Example from the Ordovician main-reservoir carbonate rocks on the eastern margin of the Ordos basin, China

The Ordovician Majiagou Formation contains the main reservoir of the Ordos Central gas field in the Ordos basin. The producing zone at the top of member 5 consists of carbonate rocks modified during a long period of subaerial exposure and karstification from the Late Ordovician to the middle Carboniferous. On the eastern margin of the Ordos basin, the porosity of the exposed carbonate rocks of this unit ranges from 0.5 to 15.1%, and permeability ranges from 8%, and permeability ranges from <0.1 to 5 md. The main reservoir porosity is a dissolution-enhanced vuggy porosity, associated with dolomite. The carbonate rocks show great heterogeneity, reflecting the varying effects of karstification in creating and modifying porosity. Petrographic and geochemical analyses of various components in these carbonates provided evidence for depositional and diagenetic processes. The reservoir carbonates were deposited in shallow and restricted hypersaline environments and were later modified by karstification and burial diagenesis. Dolomitization appears to have resulted from mixing of marine and meteoric waters and probably occurred in both shallow and deep burial settings. Cementation by calcite also occurred in both shallow and deep environments, under different hydrodynamic conditions. Both depositional settings and diagenetic processes, such as leaching by meteoric water, paleokarstification, dolomitization, and cementation, controlled reservoir development. The outcrop and subsurface samples show similar petrographic features, porosity types, and geochemical characteristics, but the exposed section of the formation shows evidence of more alteration by meteoric water.

Origin and characterization of hydrothermal dolomite in the Western Canada Sedimentary Basin

Abstract Regional and localized extensive fluid flow events may have occurred during tectonic thrusting, sediment loading, uplift andcompression in the Western Canada Sedimentary Basin (WCSB). These fluids were responsible for the formation of sediment-hosted ore deposits; petroleum migration and dolomitization affecting the majority of Devonian and Mississippian carbonate reservoirs. The timing and origin of these fluid flow events remain a controversial issue. Pre-, syn- and post-Laramide fluid flow events have been invoked in the literature based on a multitude of paleomagnetic, geochemical and other evidence. The composition and evolution of ancient sedimentary fluids have been successfully reconstructed using the techniques of stable and radiogenic isotopes and fluid inclusion analyses. In this contribution, hydrothermal dolomites from several reservoirs in the WCSB will be discussed to show how hydrothermal fluids could have been instrumental in the formation of dolomite. These fluids were mostly focused through fractures and faults generated early and late in the diagenetic history of the basin. These dolomites occur in both Devonian and Mississippian carbonates from Alberta and British Columbia. Earlier hydrothermal incursions occurred in a shallow burial enviromnent during the Tournaisian or Visean, while later hot fluids were generated during and post-Laramide tectonic event. The composition, temperature and salinity of the hydrothermal fluids produced during these two main events were very different. Earlier, hydrothermal event is characterized by higher salinity, lower temperature values and variable isotopic signatures than later event.

Chemical and Isotopic Constraints for Recrystallization of Sedimentary Dolomites from the Western Canada Sedimentary Basin

Mississippian shoal carbonates of Western CanadaSedimentary Basin are important hydrocarbon hosts.Dolomitization plays a major role in the evolution ofreservoir porosity in these carbonates. This processvaries across the basin and reflects, in part, divergentsources and chemistry of pore fluids. Dolomites fromseveral petroleum reservoirs were analyzed formineralogical, geochemical and isotopic variation. Thedata clearly demonstrate the progressive and complexrecrystallization of dolomite during shallow and deepburial in modified marine, meteoric and burial fluids.These data include: change in crystal size,stoichiometry, cathodoluminscence characteristics,stable oxygen and carbon isotopic shifts and changesin radiogenic Sr isotopic composition. However,regional geology, tectonic history and fluid flowevolution play important roles in the diageneticimprints and the degree of recrystallization.Early microcrystalline dolomite formed in normalmarine and evaporative conditions in Mississippiancarbonates from Western Canada Sedimentary Basinhave undergone variable degrees of recrystallization, frompristine dolomite akin to Holocene sabkha dolomitewith preserved mineralogical and chemical attributesto highly recrystallized mesodolomite, however stillnonstoichiometric, but with highly altered chemicalsignatures. Careful attention should be made to localgeology, hydrodynamics and fluid flow when investigatingdolomite recrystallization in sedimentary basins.