Diagenetic Studies Research Articles

A diagenetic study of intrabasaltic siliciclastics sandstones from the Rosebank field

Abstract The Rosebank field contains producible hydrocarbons from Paleocene-Eocene intrabasaltic fluvial and shallow marine siliciclastic sandstone reservoirs. Despite close proximity to volcanic lithologies the siliciclastic sandstones display very good reservoirs properties with permeability in the Darcy range and porosity values ranging from 19 to 23%. Preservation of high quality reservoir properties is the result of low quantities of volcanic minerals in the siliciclastic sandstones and limited element diffusion (e.g. Fe and Mg) from adjacent basalts into siliciclastic reservoir sandstones. The low content of volcanic derived detritus is most likely a result of the main sediment source being situated outside the volcanic terrain. The effect is that formation of clay minerals (smectites and or chlorites) reducing permeability and porosity is low. In contrast to the siliciclastic sandstones, the pore space of the volcaniclastics is occupied by authigenic clays (chlorite/smectite) and carbonates (calcite and Mg-calcite) resulting in poor reservoir quality. This paper demonstrates that high quality reservoirs can be found in intrabasaltic siliciclastic reservoirs. In-depth analyses of core samples show that authigenic phases formed during progressive burial with temperatures up to ∼120 °C. The suite of diagenetic phases in siliciclastic sandstones between wells and the different Colsay units vary and are interpreted to result from differences in pore water chemistry and/or detrital mineralogy. Laumontite was formed as a late stage authigenic mineral restricted to cored Colsay 1 and 3 units in well 213/26-1z and has not been observed in the neighbouring 213/27-2 core, which may be explained by differences in pore water pH or CO2 partial pressure. The occurrence of laumontite is often associated with poor reservoir quality, but due to the patchy distribution of this mineral in the Rosebank reservoir sands it appears not to have affected reservoir quality.

Microfacies and diagenetic studies of the early Eocene Sakesar Limestone, Potwar Plateau, Pakistan: approach of reservoir evaluation using outcrop analogue

The early Eocene Sakesar Limestone of the Salt Range has been investigated in detail for microfacies analysis, depositional modeling, diagenesis and reservoir characterization. This research work was comprised of two outcrop sections, i.e., Nilawahan Gorge Section (NGS) and Katas Temple Sections (KTS) of the Sakesar Limestone, central and eastern Salt Range, Potwar Plateau. This work is mainly concentrated on investigating and evaluating the reservoir quality through depositional and diagenetic fabrics. The depositional, diagenetic and deformational processes are controlling factors of porosity and permeability distribution. On the basis of relative estimated ratio of allochemical constituents and micrite, five microfacies have been recognized. These microfacies are: Benthonic Foraminiferal wackestone, Foraminiferal-Algal wackestone–packstone, Miliolidal-Algal wackestone–packstone, Nummulitic-Assilina Packstone and Alveolina-Algal packstone. On the basis of relative abundance of biota, their associations and the presence of micritic matrix in various microfacies, the Sakesar Limestone is interpreted to be deposited in the proximal inner ramp to middle ramp settings. The Sakesar Limestone is largely affected and modified by various diagenetic events which have destroyed primary nature of reservoir and developed it as prolific secondary reservoir. The paragenetic sequence includes micritization, cementation, dissolution, neomorphism, nodularity, silicification, mechanical compaction, stylolitization, fractures and veins formation. The identified porosity types include intraparticle, intercrystalline, moldic, cavernous and fracture. The visually estimated average micro porosities of the Sakesar Limestone vary between 0.5 and 2.1% in the NGS and KTS. The core plug porosity and permeability of outcrop samples vary between 0.9 and 2.9%. The relation of core plug porosity and permeability has moderate positive correlation coefficient. The fractures and dissolution on microscopic and macroscopic level are the dominant factors that enhance the reservoir potentiality of the Sakesar Limestone.

Microporosity evolution in Iranian reservoirs, Dalan and Dariyan formations, the central Persian Gulf

Tight limestones with high amounts of microporosity will be the main targets of future hydrocarbon exploration and production in Middle East. In this paper, microporosity evolution in two Iranian reservoirs, Dalan and Dariyan formations, is investigated. These two formations represent the main hydrocarbon reservoirs in most of the Iranian hydrocarbon fields. Aptian Dariyan Formation is mainly composed of mud-dominated samples deposited in a ramp environment. The upper part of Permian Dalan Formation has the same facies distribution while the lower part is composed mainly of grain-dominated samples. Mud-dominated facies of Dariyan Formation have high porosity with moderate to low permeability. Micrites are rounded with punctic to serrate contacts. In contrast, mud-dominated facies of Dalan Formation have low porosity and permeability. SEM studies revealed that micrite texture has a major effect on final reservoir properties of these two formations. MICP and SEM studies showed that in grain-dominated samples of Dalan Formation, both small and large pore size throats are present. Mud-dominated samples of this formation are composed of micrites with compact-anhedral shape and coalescent contacts. These results combined with diagenetic studies confirmed the role of meteoric dissolution in microporosity development in both formations. In humid conditions of the Aptian time, micrites have been dissolved and a large amounts of microporosity have been developed. In contrast, in dry conditions of Late Permian, the original fabric of the micrites preserved until the late stages of burial. Rounded micrites are present in Dalan Formation where meteoric dissolution had a major effect on porosity development in this formation.

A diagenetic study of the Woodford Shale in the southeastern Anadarko Basin, Oklahoma, USA: Evidence for hydrothermal alteration in mineralized fractures

The objective of this study is to test if external fluids (e.g., hydrothermal) altered the Woodford Shale in the southeast Anadarko Basin and how the diagenesis caused by such fluids, particularly in mineralized fractures, has affected the reservoir quality and mechanical behavior of the Woodford Shale. Two Woodford Shale cores from the Anadarko Basin were sampled to identify diagenetic events, interpret their origin, and determine the diagenetic history of the shale. Thin sections for both cores were analyzed using reflected and transmitted light, cathodoluminescence (CL), and scanning electron microscopy to identify minerals and cross-cutting and textural relationships. X-ray computed tomography was conducted to further characterize fracture networks seen on the petrographic scale. Early diagenesis is dominated by events in the matrix and allochems, with later diagenesis dominated by events associated within fracturing and brecciation. The mineralized fractures and brecciated intervals contain complex mineralogies. Multiple minerals, interpreted to be hydrothermal in origin, are present, including magnesite, norsethite, witherite, gorceixite, potassium feldspar, sphalerite, chalcopyrite, and saddle dolomite. CL in minerals within fractures reveals evidence of evolving fluids and multiple fluid-flow events. Although porosity is present within pyrite framboids, between clay sheets, and as dissolution vugs within fractures and allochems, fracture porosity was destroyed by mineralization that filled the fractures and permeated into the matrix. Vitrinite reflectance data from both cores indicate a correlation between thermal maturity and level of hydrothermal alteration, with core A (0.80% [Formula: see text] [approximately 125°C]) displaying a lower amount of alteration and core C (approximately 1.5% [Formula: see text] [approximately 210°C]) displaying a higher amount of alteration. The extensive faulting present in the southeastern Anadarko Basin likely facilitated the movement of the hydrothermal fluids through the unit. The presence of hydrothermal minerals not only has implications for the thermal maturity and reservoir quality of the Woodford Shale, but it also contributes to the discussion of whether or not shales behave as open or closed systems.

Diagenetic evolution of a shallow marine Kimmeridgian carbonate ramp (Jabaloyas, NE Spain): implications for hydrocarbon reservoir quality

The upper Kimmeridgian outcrops in Jabaloyas (NE Spain) studied here have significant similarity in terms of carbonate ramp morphology and its facies architecture with Member D of the Upper Jurassic Arab Formation reservoirs in the Middle East. Geological models from analogue outcrops enhance or challenge the understanding of multi-scale sedimentological and diagenetic heterogeneities in the subsurface hydrocarbon reservoir models by providing morphometric parameters. For this purpose, conventional diagenetic studies (petrographic microscopy analysis and cathodoluminescence) have been performed to assess the impact of diagenetic modifications through burial. More than 50 thin sections have been used to define a full paragenetic sequence. Additional mineralogical mapping (Qemscan®) on a selection of thin sections serves as a calibration tool for semiquantitative analytic studies of every mineral phases and the proposed porosity evolution. This work unravels the whole paragenetic diagenetic processes and focused on those that occurred right after deposition to shallow burial (eogenesis). High-sequence stratigraphy allows linking these events to variations of the environmental domains that, in fact, respond most likely to sea-level fluctuations. In particular, it provides key parameters to frame the diagenetic overprint such as cementation processes plugging primary interparticle porosity and selective dissolution linked to master bounding surfaces: calcite meniscus cements in vadose zones and selective dissolution processes linked to firmgrounds. Ultimately, they offer valuable trends that would be potentially relevant to decipher and characterize reservoir parameters for the upscaling workflow of fabrics from microscopic to inter-well scale in carbonate hydrocarbon reservoirs.

Exploration challenges along the North Atlantic volcanic margins: the intra-volcanic sandstone play in subsurface and outcrop

Abstract The margins of the North Atlantic rift are covered by an extensive succession of volcanic rocks, with up to 5 km of continental flood basalts, hyaloclastites and interbedded sedimentary rocks. The volcanic succession deteriorates seismic imaging and has hampered petroleum exploration in these areas. Focused research and pioneering exploration activity, however, has improved the understanding and development of new play models in volcanic-influenced basins. In 2004, the Rosebank discovery finally proved that intra-volcanic siliciclastic sandstones of the Flett Formation may form attractive hydrocarbon reservoirs in the Faroe–Shetland Basin. The Kangerlussuaq Basin in southern East Greenland offers a unique opportunity to study the interaction of siliciclastic sediments with lavas and various volcaniclastic units. It is demonstrated that: (1) laterally extensive siliciclastic sedimentary units are present in the lower part of the volcanic succession; (2) the morphology of the lavas controlled variations in sandstone geometry and thickness; and (3) deposition of the interbedded sediments and lavas occurred in a low-relief environment close to sea level. The mineralogical composition of the intra-volcanic sediments is highly variable, ranging from siliciclastic to purely volcaniclastic. Diagenetic studies suggest that the nature of the volcanic component in volcaniclastic sandstones is more important to reservoir properties than the relative concentration.

Microstructural preservation and the effects of diagenesis on the carbon and oxygen isotope composition of Late Cretaceous aragonitic mollusks from the Gulf Coastal Plain and the Western Interior Seaway

The carbon and oxygen isotope compositions of calcium carbonate shells are widely accepted and applied proxies for tracking changes in paleoenvironmental conditions such as temperature, salinity and productivity. In order to accurately interpret isotopic measurements, diagenetic alteration must first be assessed. The occurrence of aragonitic shells is often taken as a first order sign of unaltered material because aragonite at the Earth9s surface is metastable and converts to calcite. However, even specimens that retain an aragonitic composition and are macroscopically well preserved often exhibit subtle to considerable signs of alteration when shell microstructure is examined using scanning electron microscopy (SEM). To determine the textural and isotopic effects of progressive alteration on differing types of shell microstructure, we undertook a comparative study of aragonitic shells from two regions frequently used in paleoenvironmental reconstructions and diagenetic studies -- the Upper Cretaceous of the Gulf Coastal Plain (GCP) and the Western Interior Seaway (WIS). Because the GCP is within a passive margin setting and the WIS has experienced tectonic processes, we also evaluate how diagenetic history affects isotopic composition. Visually well-preserved ammonite, bivalve, and gastropod shells were collected from one locality in the GCP and examined using SEM. To evaluate microstructural preservation, we used a previously published scale based on WIS specimens for nacreous shell material and a complementary scale we developed for crossed lamellar microstructure. The stable carbon and oxygen isotope compositions of specimens across the preservational scale from both regions were analyzed. Isotopic composition does not change systematically with degrading preservation in the GCP specimens, a result that contrasts strongly with patterns in WIS specimens where isotopic composition systematically decreases with decreasing microstructural preservation. Dissimilar tectonic regimes that led to unique post-depositional conditions for each region are invoked to explain this difference. The WIS was a foreland basin subjected to significant meteoritic groundwater flow driven by active uplift, while the GCP was a passive margin that has remained close to sea level, with low hydraulic potential, since the time of deposition. Thus, during diagenesis the WIS shells were exposed to fluids that differed greatly from the seawater in which the organisms grew, whereas diagenetic fluids in GCP settings likely were similar to marine waters. As expected, shells with the best-preserved microstructure provide the most consistently reliable isotopic data for paleoenvironmental reconstructions, but our comparisons show that similar degrees of microstructural alteration can express very different diagenetic shifts in isotopic values.

Integrated workflow for characterizing and modeling a mixed sedimentary system: The Ilerdian Alveolina Limestone Formation (Graus–Tremp Basin, Spain)

This paper proposes an advanced stochastic workflow to jointly model sedimentary facies and diagenesis. The formation of interest is the Early Eocene Alveolina Limestone Formation, which outcrops in the Serraduy area (Graus–Tremp Basin, NE Spain). Ten sedimentary lithotypes representing facies or facies associations of a mixed siliciclastic–carbonate ramp system were identified within the succession. A 3D model describing the depositional architecture is also proposed. The results from the diagenetic study evidenced the occurrence of several successive calcite cements, which were grouped into five diagenetic imprints for modeling. These imprints were then quantified to ease their integration into numerical models. The following step consisted in building a 3D gridded model with seven different modeling units. They were populated using a bi-plurigaussian simulation approach that reproduced both the sedimentary organization and the observed diagenetic imprint distributions. Last, the simulation results were validated referring to paleogeographic and diagenetic conceptual maps.

Diagenetic evolution of Tortonian temperate carbonates close to evaporites in the Granada Basin (SE Spain)

The Granada Basin (SE Spain) is a small basin located in the central part of the Betic Cordillera, structured as such in the late Tortonian and initially connected to the Atlantic Ocean and to the Mediterranean Sea. During the late Tortonian, normal marine conditions prevailed, leading to the deposition of skeletal carbonate sediments on platforms around structural highs. The marine connections were later interrupted, first to the Atlantic Ocean and then to the Mediterranean Sea, and a thick evaporite sequence, marking the transition from marine to continental conditions, was deposited during the latest Tortonian. In this work, the diagenetic evolution of the Tortonian temperate carbonates (TTC), underlying and close to the evaporite bodies, is revealed and discussed. The diagenetic study includes petrographic analyses (conventional petrography, cathodoluminescence, and fluorescence), geochemical analyses (major, minor and trace elements, and δ13C and δ18O stable isotopes), and microthermometry of fluid inclusions. In the TTC, marine diagenetic processes such as micritization and fibrous calcite-cement precipitation and mechanical compaction took place during or just after deposition (Eogenesis). An initial burial event (Mesogenesis 1) is characterized by: 1) stabilization of the temperate-water carbonates by freshwater, and 2) porosity occlusion via precipitation of low-Mg bladed and syntaxial/mosaic calcite cements. The TTC were then subaerially exposed (or got close to the surface) during evaporite deposition and underwent pedogenesis, Mg-smectite infiltration, and pyrite formation (Telogenesis 1). Subsequent brine-related diagenetic alterations, such as dolomitization and silica, halite, and sylvite replacements of carbonate grains occurred during a second burial episode (Mesogenesis 2) concomitant with the Messinian lacustrine deposition, this being followed by chemical compaction (stylolite formation). Finally, the area was uplifted and the TTC exhumed. Microstalactitic (dripstone) and fibre/whisker calcite cement precipitation and extensive dissolution relate to this Pliocene–Quaternary late event (Telogenesis 2). In the study case diagenetic history is closely linked to basin evolution, as diagenetic pathways of carbonate rocks were related to major geodynamic events, including basin restriction leading to evaporite deposition, and several episodes of subsidence and uplift. Up to now, only very few diagenetic studies have attempted to demonstrate this correlation between diagenetic history and basin evolution.

Influence of organic matter on smectite illitization: A comparison between red and dark mudstones from the Dongying Depression, China

Interactions between organic matter (OM) and clay minerals have received considerable attention in previous studies. The influence of OM on smectite illitization has been analyzed primarily in simulation experiments rather than in diagenetic studies. The present study explores the influence of OM on smectite illitization during diagenesis. Thirty red and dark mudstone samples from the Dongying Depression were analyzed. X-ray diffraction (XRD) analyses revealed that the illite percentages in mixed-layer illite-smectite (I-S) of both types of samples were dispersive above 3100 m and more convergent below this depth. The stacking mode of I-S in dark mudstones above 3100 m remained primarily at R0–R0.5 ordering with the average number of layers (Nave) dispersively distributed between 2 and 4.5. In red mudstones, the I-S changed from the R0 to R0.5 mode with the Nave increasing from 2 to 5. Over this range, the smectite illitization in dark mudstones was slower than that in red mudstones. Below 3100 m, the I-S stacking mode of dark mudstones changed from R0.5 to R3 ordering with the Nave increasing sharply from 4 to 8. In red mudstones, the I-S displayed R1.5 and R3 ordering with the Nave varying between 4.5 and 6.5. Over this range, the smectite illitization in dark mudstones accelerated rapidly, whereas the process in red mudstones was retarded. Additionally, the red mudstone samples contained little OM, whereas the dark mudstone samples contained abundant total organic carbon (0.17–4.43%). Thermo-XRD, near-infrared (NIR) as well as mid-infrared (MIR) spectroscopy analyses suggested that the OM in dark mudstones exhibited a significant transition at 3100 m, coincident with the illitization change. Above 3100 m, the smectite illitization in dark mudstones was delayed due to the OM pillar effect in the interlayer spaces of smectite. Below 3100 m, the interlayer OM became varied and desorbed, discharging organic acid. This led to the dissolution of smectite structural layers. Consequently, illitization in the dark mudstone was accelerated. This study revealed that the existence and occurrence of OM could influence the smectite illitization in diagenesis. Further study on the interactions between OM and clay minerals is needed to facilitate our understanding on the mechanism of smectite illitization as well as its geological applications.

Reservoir attributes of a hydrocarbon-prone sandstone complex: case of the Pab Formation (Late Cretaceous) of Southwest Pakistan

Links between the architectural elements of major sand bodies and reservoir attributes have been explored in a field study of the hydrocarbon-yielding Late Cretaceous Pab Formation of southwest Pakistan. The lithofacies and facies associations represented in the Pab Formation are the main determinants of its reservoir properties. Thus, thick, vertically connected and laterally continuous sand packets have moderate-to-high mean porosities (10–13 %) in fluviodeltaic, shoreface, shelf delta, submarine channel, and fan-lobe facies associations while deeper shelf and basin floor sand bodies yield significantly lower porosities (4–6 %). Overall, in the Pab arenites, porosity values increase with increasing grain size and better sorting. The varying sand-shale ratios encountered in different sectors of the Pab outcrop are also petrophysically important: Sequences displaying high ratios yield higher bulk porosity values than more shale-rich successions. Diagenetic studies of Pab sandstones reveal that intense mechanical compaction and cementation have reduced primary porosity and reservoir quality. Conversely, dissolution of detrital feldspar grains and volcanic fragments during burial and later uplift of these arenites has generated significant secondary porosity.

Nature and distribution of diagenetic phases and petrophysical properties of carbonates: The Mississippian Madison Formation (Bighorn Basin, Wyoming, USA)

The purpose of this study is to characterize factors controlling the nature and distribution of the diagenetic phases that gave rise to carbonate reservoirs. To do so, a pluridisciplinary approach was carried out, integrating sedimentologic and diagenetic studies on the Madison Formation (Lower Carboniferous, 357–340 My) which is a carbonate reservoir in subsurface.The Madison Formation, outcropping in the Bighorn Basin (Wyoming, USA), is a 340 m thick carbonate series composed of four to seven 3rd-order depositional sequences (S1 to S7) depending of the palaeogeographic location. The first three sequences (S1 to S3) were deposited under arid and warm conditions during Tournaisian times which favoured high accumulation of carbonates leading to a morphological change from a quite flat ramp (S1 to S2) to a wide platform (S3). It also probably favoured the early calcite cementation (isopachous and syntaxial calcite cements) of the subtidal deposits and the early dolomitization (D1) of the supratidal to intertidal sabkha ones. In addition, the very flat profile occurring during S3 was also responsible for the postponed dolomitization (D2) of S1–S2 due to reflux of brines, at various degrees depending on the palaeogeographic location. The deposition of S4 to S7 under humid conditions during Visean times were associated with 1) a decrease of the carbonate accumulation and of the dolomitization; 2) dissolution processes at micro- (pores network) and macro-scales (karst, collapse breccia...) and calcite cementation (C1). The spatial distribution of all these first diagenetic phases acts as a controlling factor on the distribution of the later burial diagenetic phases. Thus, the mesogenetic calcite cements are mostly observed in the secondary porosity created by dissolution (karsts, collapse breccias) or dolomitization. The burial diagenesis of the Madison Formation was characterized by 1) a dolomitization phase (D3) in proximal parts of the platform leading to an increase of porosity and permeability and 2) calcite cementations (C2–C3) in the distal parts of the platform leading to a decrease of these properties.The Madison Formation provides a good spatial representation of sedimentary and diagenetic heterogeneities that may occur in the carbonate reservoirs due to various palaeogeographic locations, climates, burial history and structures.

Diagenesis of a light, tight-oil chert reservoir at the Ediacaran/Cambrian boundary, Sultanate of Oman

ABSTRACT The Al Shomou Silicilyte Member (Athel Formation) in the South Oman Salt Basin shares many of the characteristics of a light, tight-oil (LTO) reservoir: it is a prolific source rock mature for light oil, it produces light oil from a very tight matrix and reservoir, and hydraulic fracking technology is required to produce the oil. What is intriguing about the Al Shomou Silicilyte, and different from other LTO reservoirs, is its position related to the Precambrian/Cambrian Boundary (PCB) and the fact that it is a ‘laminated chert’ rather than a shale. In an integrated diagenetic study we applied microstructural analyses (SEM, BSE) combined with state-of-the-art stable isotope and trace element analysis of the silicilyte matrix and fractures. Fluid inclusion microthermometry was applied to record the salinity and minimum trapping temperatures. The microstructural investigations reveal a fine lamination of the silicilyte matrix with a mean lamina thickness of ca. 20 μm consisting of predominantly organic matter-rich and finely crystalline quartz-rich layers, respectively. Authigenic, micron-sized idiomorphic quartz crystals are the main matrix components of the silicilyte. Other diagenetic phases are pyrite, apatite, dolomite, magnesite and barite cements. Porosity values based on neutron density logs and core plug data indicate porosity in the silicilyte ranges from less than 2% to almost to 40%. The majority of the pore space in the silicilyte is related to (primary) inter-crystalline pores, with locally important oversized secondary pores. Pore casts of the silica matrix show that pores are extremely irregular in three dimensions, and are generally interconnected by a complex web or meshwork of fine elongate pore throats. Mercury injection capillary data are in line with the microstructural observations suggesting two populations of pore throats, with an effective average modal diameter of 0.4 μm. The acquired geochemical data support the interpretation that the primary source of the silica is the ambient seawater rather than hydrothermal or biogenic. A maximum temperature of ca. 45°C for the formation of microcrystalline quartz in the silicilyte is good evidence that the lithification and crystallization of quartz occurred in the first 5 Ma after deposition. Several phases of brittle fracturing and mineralization occurred in response to salt tectonics during burial. The sequences of fracture-filling mineral phases (dolomite - layered chalcedony – quartz – apatite - magnesite I+II - barite – halite) indicates a complex fluid evolution after silicilyte lithification. Primary, all-liquid fluid inclusions in the fracture-filling quartz are good evidence of growth beginning at low temperatures, i.e. ≤ 50ºC. Continuous precipitation during increasing temperature and burial is documented by primary two-phase fluid inclusions in quartz cements that show brines at 50°C and first hydrocarbons at ca. 70°C. The absolute timing of each mineral phase can be constrained based on U-Pb geochronometry, and basin modelling. Secondary fluid inclusions in quartz, magnesite and barite indicate reactivation of the fracture system after peak burial temperature during the major cooling event, i.e. uplift, between 450 and 310 Ma. A number of first-order trends in porosity and reservoir-quality distribution are observed which are strongly related to the diagenetic and fluid history of the reservoir: the early in-situ generation of hydrocarbons and overpressure development arrests diagenesis and preserves matrix porosity. Chemical compaction by pressure dissolution in the flank areas could be a valid hypothesis to explain the porosity variations in the silicilitye slabs resulting in lower porosity and poorer connectivity on the flanks of the reservoir. Most of the hydrocarbon storage and production comes from intervals characterized by preserved micropores, not hydrocarbon storage in a fracture system. The absence of oil expulsion results in present-day high oil saturations. The main diagenetic modifications of the silicilyte occurred and were completed relatively early in its history, i.e. before 300 Ma. An instrumental factor for preserving matrix porosity is the difficulty for a given slab to evacuate all the fluids (water and hydrocarbons), or in other words, the very good sealing capacity of the salt embedding the slab.

An integrated paleomagnetic, rock magnetic, and geochemical study of the Marcellus shale in the Valley and Ridge province in Pennsylvania and West Virginia

AbstractAn integrated paleomagnetic, rock magnetic, and diagenetic study of the Devonian Marcellus Subgroup from threefolds in the Valley and Ridge province in Pennsylvania and West Virginia indicates that the unit contains an intermediate temperature chemical remanent magnetization (CRM) with south‐southeast declinations and shallow negative inclinations residing in pyrrhotite and a CRM with more southerly declinations and shallow positive inclinations residing in magnetite. Rock magnetic results confirm the presence of pyrrhotite and magnetite. Tilt tests indicate that pyrrhotite and magnetite CRMs are syntilting to posttilting, and paleopoles for both CRMs are similar and plot on the Permian part of the apparent polar wander path for North America. The Marcellus Subgroup has a complex paragenetic sequence which includes bitumen deposition and vein formation. The magnetite and pyrrhotite CRMs formed in the Permian, probably due to burial diagenetic processes and not orogenic fluids. The base of the unit contains the highest total organic content (TOC) values and highest magnetic intensities, both of which decrease up section. This connection between TOC and magnetic intensity suggests that the magnetite formed as a result of the maturation of the organic matter although more work is needed to test this hypothesis. The pyrrhotite may have formed as a result of thermochemical sulfate reduction involving organic matter during the production of methane in this gas reservoir. The similar Permian CRM ages may be explained by thrusting which resulted in rapid burial of the Marcellus Subgroup as it passed through the oil window and into the gas window quickly producing magnetite and pyrrhotite, respectively.

Application of handheld energy-dispersive X-ray fluorescence spectrometry to carbonate studies: opportunities and challenges

In situ rapid quantitative geochemical measurements of carbonate using handheld XRF.

Shallow burial dolomitization of an Eocene carbonate platform, southeast Zagros Basin, Iran

ABSTRACT Here, a case example of a dolomitized Eocene ramp setting from the southeastern Zagros Basin is documented and discussed in the context of published work. This is of significance as well-documented case examples of Eocene dolomitized inner platforms are comparably rare. The same is true for detailed diagenetic studies from the Zagros Basin in general. Three measured field sections were combined with detailed petrographic and geochemical analyses and four main dolomite types were defined. The most significant dolomite type is present in the form of a volumetrically significant occurrence of meter-thick beds of strata-bound dolostones. These dolomites are characterized by near-stoichiometric composition, fabric-retentive and fabric-destructive textures, subhedral to anhedral in shape and most being in the tens-of-microns range. Dolomite δ18O (averaging -2.6‰) values are depleted relative to that expected for precipitation from Eocene seawater (averaging 0‰), while δ13C (averaging -0.1‰) values are within the range of Eocene seawater values (averaging 0.5‰). Dolomite Type II and III 87Sr/86S values from 0.7079 to 0.7086 are somewhat elevated with respect to Eocene seawater (0.7077 and 0.7078). Based on these data, it is suggested that moderately evaporated seawater, via shallow seepage reflux, acted as agent for the initial dolomitization process. Subsequently, early diagenetic dolomites were recrystallized during shallow burial to variable degrees. The absence of volumetrically significant evaporitic deposits indicates that the salinity of porewater during dolomitization was beneath the threshold limit for gypsum precipitation. In addition, ascending saline fluids from deep-seated salt diapirs might have affected dolomitizing fluids.

A fluid inclusion study of diagenetic fluids in Proterozoic and Paleozoic carbonate rocks, Victoria Island, NWT

AbstractDespite the presence of known economic resources in Canada's Arctic archipelago, Victoria Island remains understudied. This study addresses the fluid history and economic potential of two major carbonate units on Victoria Island by integrating fluid inclusion microthermometry with SEM‐EDS analysis of evaporate mounds. Three cements containing fluid inclusion assemblages (FIA) occur in the Neoproterozoic Wynniatt Formation: saddle dolomite, brown dolomite and calcite, in paragenetic order. The two dolomite‐hosted cements have average homogenisation temperatures (Th) for FIAs (n = 3) of 108°C (saddle) and 101 and 116°C, but metastability precluded determining salinities; most calcite‐hosted fluid inclusions are too small and/or necked to obtain Th values, but rare larger inclusions have salinities from 1.7 to 0.4 wt. % NaCl equiv. SEM‐EDS analysis of evaporate mounds indicates the fluid changed from an early K‐rich (saddle dolomite), to a later K + Na (brown dolomite), and finally Na‐rich (calcite), which suggests mixing of two end‐member fluids (i.e. Na‐rich and K‐rich). Dolostone of the lower Paleozoic ‘Victoria Island formation’ contains two cements: early quartz and late dolomite. Quartz‐hosted FIAs (n = 2) have an average Th value of 126°C, and salinity of 23.2 wt. % NaCl equiv., whereas FIAs (n = 3) in dolomite have average Th values of 109, 116 and 124°C; metastability precluded determining salinity. Evaporate mound analysis for the cements indicates evolution from a Na‐rich to a Na + K fluid through interaction with reservoir rocks. A reduced, metal‐rich fluid was present during quartz precipitation, as implied by the presence of pyrite framboids along growth zones and nanoparticles of barite and sulphide minerals (Zn, Cu and Pb) in evacuated inclusions, which suggests the area may have potential to host base‐metal mineralisation. Importantly, distinguishing different fluid compositions in both of the case studies would not have been possible without evaporate mound analysis and therefore the results emphasise integrating this technique into diagenetic studies.

Paleoclimate and tectonic controls on the depositional and diagenetic history of the Cenomanian–early Turonian carbonate reservoirs, Dezful Embayment, SW Iran

Integrated facies and diagenetic analyses within a sequence stratigraphic framework were carried out on mid-Cretaceous Sarvak carbonate reservoirs in five giant and supergiant oilfields in the central and southern parts of the Dezful Embayment, SW Iran. Results of facies analysis indicate a homoclinal ramp-type carbonate platform for this formation with the frequencies of different facies associations in six wells reflecting their approximate position in the sedimentary model. Diagenetic studies indicate periods of subaerial exposure with different intensities and durations in the upper Sarvak carbonates producing karstified profiles, dissolution-collapse breccias, and thick bauxitic-lateritic horizons. Sequence stratigraphic interpretations show that the tectonic evolution of the NE margin of the Arabian Plate (Zagros Basin) during Cenomanian–Turonian times shaped the facies characteristics, diagenetic features, and strongly influenced reservoir formation. Reactivation of basement-block faults and halokinetic movements (related to the Hormoz salt series) in the middle Cretaceous, resulted in the development of several paleohighs and troughs in the Dezful Embayment hydrocarbon province. Movements on these structures generated two and locally three disconformities in the upper parts of Sarvak Formation in this region. The paleohighs played an important role in reservoir evolution within the Sarvak Formation in three giant-supergiant oilfields (including Gachsaran, Rag-e-Safid, and Abteymour oilfields) but where these structures are absent reservoir quality is low.

Sedimentological and diagenetic study of the Early Middle Miocene Jeribe Limestone Formation in selected wells from Iraq northern oilfields (Ajil; Hamrin; Jadid; Khashab)

Five subsurface sections covering the entire length of the Jeribe Limestone Formation (Early Middle Miocene) were studied from four oilfields in northern Iraq. It is hoped to unravel this formation microfacies ; depositional environment; diagenetic attributes and their parental processes; and the relationship between these processes and the observed porosity patterns. The microfacies were found to include mudstone, wackestone, packstone, and grainstone, which have been deposited respectively in open platform, restricted platform, and edge platform which represent the lagoonal environment, while the deposits of the lower parts of the Jeribe formation especially in well Hamrin- 2 reflect a deeper fore slope environment. By using the lithofacies association concepts, the depositional model of the Jeribe Formation was built. From a reservoir point of view, the formation suffered from two groups of diagenetic processes. The first one includes the porosity destructive ones such as cementation; compaction; mechanical degradation; anhydritization; and silicification. The second group include porosity enhancers ones which to include dissolution; and dolomitization.

Sedimentological and diagenetic study of the Early Middle Miocene Jeribe Limestone Formation in selected wells from Iraq northern oilfields (Ajil; Hamrin; Jadid; Khashab)

Five subsurface sections covering the entire length of the Jeribe Limestone Formation (Early Middle Miocene) were studied from four oilfields in northern Iraq. It is hoped to unravel this formation microfacies ; depositional environment; diagenetic attributes and their parental processes; and the relationship between these processes and the observed porosity patterns. The microfacies were found to include mudstone, wackestone, packstone, and grainstone, which have been deposited respectively in open platform, restricted platform, and edge platform which represent the lagoonal environment, while the deposits of the lower parts of the Jeribe formation especially in well Hamrin- 2 reflect a deeper fore slope environment. By using the lithofacies association concepts, the depositional model of the Jeribe Formation was built. From a reservoir point of view, the formation suffered from two groups of diagenetic processes. The first one includes the porosity destructive ones such as cementation; compaction; mechanical degradation; anhydritization; and silicification. The second group include porosity enhancers ones which to include dissolution; and dolomitization.