Cathy Hollis

Diagenetic controls on reservoir properties of carbonate successions within the Albian–Turonian of the Arabian Plate

ABSTRACT The carbonate-dominated Albian to Turonian succession is one of the key petroleum systems of the Arabian Plate. It is dominated by shallow water platform carbonates that were deposited in a shallow epeiric sea on the margins of Neotethys. In general, the reservoirs in this succession have high porosities but exhibit heterogeneous permeabilities. This study reviews published data for the region and attempts to unravel the key diagenetic controls on the porosity and permeability of the reservoirs. The results demonstrate that a spectrum of diagenetic processes created highly heterogeneous multimodal pore networks. Intense boring and micritization of skeletal allochems, differential cementation of a pervasive burrow network and preferential dissolution of aragonitic skeletal allochems are ubiquitous. Locally, particularly on the northern and eastern Arabian Plate, deep-penetrating epikarst can be tied to a differential response to global sea level fluctuation and local tectonism. The development of a peripheral bulge in late Cenomanian–Turonian times, halokinesis, localized influx of channelized clastic material and sub-regional climatic variability contributed to a heterogeneous pattern of meteoric diagenesis across the Arabian Plate. The succession was then buried to up to 10 km during the Late Cretaceous–Tertiary. Where deep-penetrating fault systems were reactivated by Alpine tectonism, flushing by hydrothermal brines resulted in highly localized patterns of hydrothermal dolomitization and leaching, associated with hydrocarbon emplacement.

Permeability and acoustic velocity controlling factors determined from x-ray tomography images of carbonate rocks

Carbonate reservoir rocks exhibit a great variability in texture that directly impacts petrophysical parameters. Many exhibit bi- and multimodal pore networks, with pores ranging from less than 1 μm to several millimeters in diameter. Furthermore, many pore systems are too large to be captured by routine core analysis, and well logs average total porosity over different volumes. Consequently, prediction of carbonate properties from seismic data and log interpretation is still a challenge. In particular, amplitude versus offset classification systems developed for clastic rocks, which are dominated by connected, intergranular, unimodal pore networks, are not applicable to carbonate rocks. Pore geometrical parameters derived from digital image analysis (DIA) of thin sections were recently used to improve the coefficient of determination of velocity and permeability versus porosity. Although this substantially improved the coefficient of determination, no spatial information of the pore space was considered, because DIA parameters were obtained from two-dimensional analyses. Here, we propose a methodology to link local and global pore-space parameters, obtained from three-dimensional (3-D) images, to experimental physical properties of carbonate rocks to improve P-wave velocity and permeability predictions. Results show that applying a combination of porosity, microporosity, and 3-D geometrical parameters to P-wave velocity significantly improves the adjusted coefficient of determination from 0.490 to 0.962. A substantial improvement is also observed in permeability prediction (from 0.668 to 0.948). Both results can be interpreted to reflect a pore geometrical control and pore size control on P-wave velocity and permeability.

The use of burial diagenetic calcite cements to determine the controls upon hydrocarbon emplacement and mineralization on a carbonate platform, Derbyshire, England

Abstract Late diagenetic calcite cements in the Upper Dinantian limestones of the Derbyshire Platform are contemporaneous with both hydrocarbon emplacement and Mississippi Valley-type (MVT) mineralization. Calcite cementation began during the progressive burial of the Derbyshire Platform and the surrounding basins, principally within fractures generated during the waning effects of Upper Carboniferous extension. Six burial calcite cements can be recognized in dilational vein systems. Successive veins contain progressively more mature hydrocarbon inclusions, and calcite cements are intergrown with fluorite, baryte, galena and sphalerite in increasing quantities. Compacting Dinantian-Namurian shales in basins adjacent to the platform offer the most likely sources of fluids, trace elements and hydrocarbons. Fluids entered the platform along major fault systems, and circulated using smaller fracture systems, precipitating calcite. The final phases of calcite cementation and the main phase of MVT mineralization coincided with the onset of the Variscan Orogeny. A model is now established relating fluid flow to Variscan tectonic events in northern Britain.

Giant middle Eocene bryozoan reef mounds in the Great Australian Bight

This paper reports the discovery of extensive middle Eocene bryozoan reef complexes along the paleoshelf edge of the Great Australian Bight (GAB). The complexes form the earliest carbonate deposit in the GAB, which is the largest Cenozoic cool-water carbonate province on Earth. The bryozoan reef mounds, previously misidentifi ed as volcanic bodies, were deposited parallel to the shelf margin for more than 500 km along strike. Individual reef mound complexes are 60?150 km long, as wide as 15 km, and as thick as 200 m, and dwarf all previously described examples. Superimposed on the distal margin of an underlying Paleocene to mid-Eocene siliciclastic delta complex, the reef mounds provide a critical insight into changing paleoenvironments of the Australo-Antarctic Gulf ca. 43 Ma, coinciding with global and continent-wide climatic and tectonic events. The rapid growth and demise of reef mound?building bryozoans raises new questions regarding the interplay of Southern Ocean opening, ocean currents, and biosphere interactions.

Influence of oxic diagenesis on source potential and lithofacies cyclicity: insight from Cenomanian Natih-B Member intrashelf basinal carbonates, Oman

ABSTRACT Investigation of core and outcrop samples of the Cenomanian Natih-B Member (North Oman) indicates that the different lithofacies present experienced rather different early diagenesis shortly after deposition. Transmitted-light, cathodoluminescence and backscattered scanning-electron microscopy, as well as stable-isotopic, X-ray diffraction and total organic carbon (TOC) analyses were employed to delineate the major controls on the cyclic pattern of early diagenesis and hydrocarbon source potential. The Natih-B intrashelf basinal carbonates are composed of pelagic sediments that exhibit high-frequency cyclicity marked by decimetre-thick lithofacies alternations, mainly between: Lithofacies A compacted, partially bioturbated, skeletal, organic carbon-rich mudstone to wackestone; and Lithofacies B uncompacted, extensively bioturbated, skeletal, sparry-calcite rich wackestone to packstone. Individual units are composed variously of authigenic and biogenic calcite (58.1–97.6%, average 78.5%) and organic carbon (0.3–13.7% TOC, average 3.6%), together with minor quartz, clay, pyrite, dolomite and phosphatic material (fish debris). Lithofacies A contains relatively more organic carbon, clay, pyrite and dolomite than Lithofacies B and constitutes an excellent source rock. Diagenetic textures of Lithofacies A are dominated by compactional deformation of burrow fabrics, faecal pellets and solution seams, in addition to zoned/bright luminescent, non-ferroan sparry and isopachous calcite cement in and around uncompacted foraminifer tests, in an uncemented matrix. In contrast, Lithofacies B does not show any signs of compaction other than microstylolites and is dominated by zoned/dull luminescent, non-ferroan calcite microspar replacement, in addition to pore-filling, predominantly dull-luminescent, non-ferroan, sparry calcite cement. Moreover, Lithofacies B shows evidence of isopachous and meniscus-style cementation, together with geopetal structures and mictritic peloids. Stable-isotopic compositions of both lithofacies were determined from whole-rock samples (δ13C = −0.9 to +0.9‰, average +0.3‰; δ18O = −5.6 to −3.7‰, average −4.8‰) and sparry calcite (both cement and matrix) subsamples (δ13C = −0.6 to +1.2‰, average +0.6‰; δ18O = −5.7 to −3.7‰, average −4.3‰); all results being relative to Vienna Pee Dee Belemnite. These petrographic and isotopic characteristics suggest that the Natih-B abundant calcite cements and replacements were precipitated early, prior to compaction, mainly from ‘normal’ (open, oxic) seawater at slightly elevated depositional temperatures. Some of the slightly negative δ13C values, however, indicate an addition of isotopically light carbon, probably derived from organic-matter oxidation by organisms living in marine pore waters. Based on evidence of extensive seafloor bioturbation and cementation, and their position within the depositional succession, the tops of Lithofacies B (wackestones to packstones) are interpreted as ‘discontinuity surfaces’ that cap shallowing-upward, fifth-order cycles, formed as a function of sediment starvation and increased bottom-current activity during relative sea-level stillstand/turnaround. In contrast, Lithofacies A (mudstones to wackestones) is believed to reflect high organic production coupled with high sedimentation rate and rapid burial. These conditions limited total infaunal colonization and extensive calcite precipitation, and preserved organic matter together with some escape burrows and in-place fauna, suggesting episodic sediment influx when more accommodation was available and seafloor diagenesis was minimized during relative sea-level rises. The relatively higher amounts of pyrite and dolomite in Lithofacies A likely indicate organic-matter degradation by bacterial sulphate reduction in anoxic pore waters during shallow burial.

Albian–Cenomanian–Turonian carbonate-siliciclastic systems of the Arabian Plate: advances in diagenesis, structure and reservoir modelling: introduction

This special issue contains a thematic set of papers arising from a conference entitled ‘Second Arabian Plate Geology Workshop: Albian–Cenomanian–Turonian Carbonate-Siliciclastic systems of the Arabian Plate’, organized by the EAGE in Abu Dhabi in January 2010. Full conference abstracts are published in GeoArabia (2010, Vol. 15, No. 1). The meeting followed a very successful First Arabian Plate Geology Workshop in Oman in 2008, which focused upon Barremian to Aptian systems (van Buchem et al . 2010). A key recommendation from that meeting was that subsequent workshops should be broadened to consider the impact of stratigraphy on patterns of diagenesis and, ultimately, on reservoir performance. Consequently, the Albian–Cenomanian–Turonian Workshop concentrated not just on unification of stratigraphic nomenclature, but also on the relationship between sedimentation, structural style, diagenesis and their impact on hydrocarbon entrapment and production. The Albian–Cenomanian–Turonian interval is one of the most important petroleum systems on the Arabian Plate. Large volumes of hydrocarbon are hosted within clastic (mostly Albian) and carbonate (largely Cenomanian–Turonian) reservoirs in Oman, United Arab Emirates (UAE), Qatar, Kuwait, Iran and Iraq. The fine-grained siliciclastic sediments of the early Albian form regional seals to the underlying Aptian reservoirs, whilst the organic-rich mudstones deposited in Albian and Cenomanian intrashelf basins (e.g. Natih B Formation, Oman; Kazhdumi Formation, Iran) are important hydrocarbon source rocks. Compressional tectonism, initiated in the latest Cenomanian to early Turonian with the partial closure of Neotethys, and completed in the early Tertiary, was responsible for the formation of the main hydrocarbon traps in the region and for triggering hydrocarbon maturation and migration. Margin-proximal areas (i.e. Zagros Simply Folded belt in Iran and Iraq) are characterized by the development of short wavelength, often asymmetric anticlinal traps, whilst areas distal to the margin (i.e. Mesopotamian foreland basin of Iraq and Arabian Plate in Kuwait, Qatar, Saudi …

Burial diagenetic evolution of the Lower Carboniferous (Dinantian) of the southern margin of the Askrigg Platform and a comparison with the Derbyshire Platform

In order to predict the style and impact of post-depositional modification of carbonate successions, well-studied and accessible outcrop analogues are invaluable. The Lower Carboniferous (Dinantian) carbonate platforms of the Pennine Basin of northern England have a long history of investigation. As such, they offer the potential to evaluate the mechanisms and timing of fluid flux during extensional tectonism, post-rift basinal subsidence and inversion. This study concentrates upon the diagenetic evolution of the late Dinantian of the southern margin of the Askrigg Platform of North Yorkshire and a comparison with published data from the age-equivalent Derbyshire Platform. A pattern of consistent, diagenetic modification during early diagenesis is evident, but key differences occur in the burial realm. On both the southern margin of the Askrigg Platform and the Derbyshire Platform, patterns of dolomitization, hydrocarbon emplacement and mineralization can be determined on the platform that reflect the diagenetic evolution of the adjacent basins. However, within the study area of the Askrigg Platform, there is only local evidence for a fault/fracture control on the migration of Mg-enriched, hydrocarbon-bearing fluids. In contrast, on the Derbyshire Platform, burial diagenesis is intimately associated with NW–SE- and NE–SW-trending faults and fractures. Data suggest that pervasive cementation in the marine and meteoric realm occluded matrix porosity in both areas, such that fluid migration was almost entirely fracture controlled. With the localization of structural deformation along the Craven Fault Zone, and a low abundance and density of open fault/fracture networks, circulation of fluids on to the southern margin of the Askrigg Platform was inhibited, however. Furthermore, the presence of local aquifers in the Craven Basin may have led to fluid expulsion from the basin during early burial.

Reconstructing fluid history: an integrated approach to timing fluid expulsion and migration on the Carboniferous Derbyshire Platform, England

Abstract Galena-sphalerite-baryte-fluorite mineralization and non-economic reserves of liquid hydrocarbon and bitumen are hosted on the Derbyshire Platform within Lower Carboniferous limestone which was deposited during Variscan back-arc extension. The limestone also hosts a sequence of burial calcite cements, precipitated dominantly in crosscutting and refractured extensional vein systems, which typically follow Caledonian-Variscan trends. The calcite cements are often intergrown with galena, sphalerite, baryte and fluorite as well as bitumen, whilst epifluorescence reveals hydrocarbon inclusions. Paragenetic relationships, in conjunction with geochemical results, permit definition of a precise timing for mineralization and hydrocarbon emplacement, and modelling of the source, composition and migration pathways of the mineralizing fluids.

Examining the interplay of climate and low amplitude sea-level change on the distribution and volume of massive dolomitization: Zebbag Formation, Cretaceous, Southern Tunisia

During the Cretaceous, a humid global climate, calcitic seas, high relative sea‐level and low amplitude changes in relative sea‐level largely prevented large‐scale dolomitization in many carbonate successions. However, the well‐exposed shallow‐water carbonate sediments of the Upper Albian–Lower Turonian Zebbag Formation on the Jeffara Escarpment, southern Tunisia, are pervasively dolomitized. This study considers why dolomitization was so widespread in this region during a period of Earth history when platform‐scale dolomitization is rare. Marine conditions were established in the Upper Albian, evidenced by stacked upward‐shallowing packages of shallow subtidal to peritidal carbonate sediments in the basal Rhadouane Member. A gradual increase in the volume of subtidal sediments in the Cenomanian Kerker Member, culminated in deposition of laterally extensive marls, during maximum flooding of the platform in the Lower Turonian. The overlying Gattar Member was then deposited in shallower water as relative sea‐level fell. The entire Zebbag Formation is pervasively replaced by stratabound, fabric‐retentive, dolomite, except within the marl at the top of the Kerker Member, which is only partially dolomitized. Petrographic textures indicate dolomitization largely post‐dated marine cementation and platform emergence but pre‐dated chemical compaction. Slightly more positive oxygen isotope signatures, slightly elevated concentrations of Sr and a near‐absence of evaporites are consistent with dolomitization by reflux of mesohaline sea water. An upward‐decrease in major element concentrations and higher 87Sr/86Sr compared to Upper Cretaceous sea water suggest that basal, Albian siliciclastic beds acted as aquifers facilitating dolomitization by fluxing fluids offshore. Dolomitization is interpreted to have resulted from multiple fluxes of sea water over periods of 0·5 to 2·5 Ma. The unusually high volume of dolostone for a platform of this age most probably reflects deposition within an arid climate belt, where an efficient reflux system was facilitated by basal, permeable siliciclastic strata.

The control of basin evolution on patterns of sedimentation and diagenesis: an example from the Mississippian Great Orme, North Wales

The Mississippian North Wales Platform is located on the margins of the East Irish Sea Basin and has been little studied over the last 30 years. The exposed Visean limestones provide new insights into the deposition, porosity evolution, distribution of dolomitization, and Pb–Zn and Cu mineralization on the North Wales carbonate platform. This is of relevance to the characterization of fault-related dolomite hydrocarbon reservoirs and age-equivalent Mississippi Valley-type mineral deposits. In particular, the study demonstrates the intimate relationship between sedimentation, basin-scale tectonism and post-depositional fluid flux. Depositional cyclicity is marked, with metre-scale upward-shallowing cycles in which pervasive marine and meteoric calcite cements occlude matrix porosity and syndepositional fractures. Consequently, subsequent burial diagenetic replacive dolomitization is matrix selective and cements are primarily restricted to fractures. Seven phases of dolomite are defined based on texture and cathodoluminescence petrography, with phases D1–D3 as the most volumetrically significant. Dolomite phases D0–D2 are matrix replacive, cross-cutting stratigraphy and locally fingering along beds for several metres. Dolomite phases D3–D7 are hosted by faults and fractures and also line vugs. Evidence of telogenesis is recorded where burial diagenetic products are post-dated by calcite cements precipitated from meteoric fluids. Dolomitization probably occurred during the Mississippian and continued into the Pennsylvanian. Pb–Zn mineralization is also interpreted to have occurred during the Pennsylvanian, associated with Variscan tectonism. Overall, the North Wales Platform displays a more complex paragenesis than age-equivalent platforms in the Pennine Basin, owing to multiple phases of burial and exhumation. The study demonstrates the importance of linking burial history to detailed field and petrographical data to understand and predict the spatial and temporal controls on diagenetic processes and products within syn- and post-rift sequences.