Subsequent Diagenetic Modifications Research Articles

Reservoir Description, Diagenesis and Reservoir Quality of the Deep Marine Sediments in the Propagation Stage of Rift Basin

The exploration and exploitation of the downthrown side of the productive upthrown Pre-rifting reservoirs at the Gulf of Suez marked bring a renaissance to the rift basin Exploration. This work documents the reservoir performance, trap geometry, and structural style of the downthrown deep marine stratigraphic entrapment. In addition, it outlines the requirements necessary beyond the production mitigations and limits. To gain better understanding of Asl Formation (deep marine) reservoir characteristics early in the exploration program, Asl Formation was cored in the north October J-5 well. A continuous 366- foot (112m) core was recovered including the main reservoir interval, which is producing over 12,000 barrel of oil per day. The J-5 Asl Formation has been interpreted as a synthetic, mixed clastic-carbonate turbidity deposit. Three major lithofacies described are recognizable on electric log. These are sandstone, shale and carbonates. The sandstones dominate the lower portion of the core and are the main oil producing section. Sandstone porosity and permeability range from 17 to 27% and 30 millidarcies to 3 darcies, respectively. Compared to the sandstone, the carbonate turbidites have poorer reservoir quality. They dominate the upper portion of the core and are composed of coarse fossil debris, silica framework grains and rock fragments. Horizontal and vertical permeabilities with rock types. Both the sandstone and carbonate sections of the J-5 core have experienced a complex diagenetic history. Deposition of the cored interval was initiated by a convulsive geological event such as flashflood or earthquake. These events would likely generate sufficient energy for mobilization of onshore and/or near shore sediments into multiple gravity flows having regional extent. The reservoir quality is controlled by a combination of primary depositional fabric and subsequent diagenetic modification. This research work can guide future exploration of reservoir-prone deep marine deposits in the Red Sea Rift and provide a reference for the subsurface deep-water reservoir quality analysis in the propagation stage in the Rift Basins.

Automated mineralogy (SEM-EDS) approach to sandstone reservoir quality and diagenesis

Porosity and permeability define the reservoir quality of a sandstone. Porosity and permeability are controlled by primary sedimentary characteristics and subsequent diagenetic modification. Grain size, sorting, detrital mineralogy, and matrix content are defined at the point of deposition. Bioturbation, infiltration, continued alteration of reactive minerals and soft sediment deformation can occur soon after deposition and, together with the primary characteristics, these condition, or define, how a sediment will behave during subsequent burial. Diagenetic modifications include compaction, initially mechanical and then chemical, mineral growth and, in some cases, grain dissolution and development of secondary pores. Porosity and permeability typically decrease as diagenesis progresses. Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) approaches can be applied to study many aspects of sandstone diagenesis including detrital mineralogy, grain size, sorting, grain shape, grain angularity and matrix content. SEM-EDS is also useful for defining quantities and location in the pore network of cements that are mineralogically distinct from detrital grains (e.g., calcite, dolomite, siderite, or anhydrite). SEM-EDS can be useful for studying clay mineral cements, especially when they occur in patches bigger than 5–10 μm. In sandstones, SEM-EDS is not so useful when the cements are mineralogically identical to detrital grains (e.g., quartz cement in quartz sandstones) where additional signals such as cathodoluminescence (CL) may be required. Macro- and meso-pores can be quantified using SEM-EDS but, on its own, it cannot automatically measure micro-porosity as it is below the minimum 1 µm spatial resolution of the approach. Mechanical compaction, a key process that causes porosity-loss in sandstones, is not easily quantified using SEM-EDS. Nonetheless, together with additional routine techniques, SEM-EDS can be used to solve most problems associated with sandstone diagenesis and reservoir quality.

Chemical composition of late Eocene–early Oligocene corals in reef buildups from the Thrace basin, Bulgaria–Greece: Paleoenvironmental implications

Coral whole-rock geochemistry and in situ LA-ICP-MS analyses of coral skeletons were performed on late Eocene–early Oligocene coral reef buildups from the Eastern Rhodope–Thrace region of Bulgaria and Greece. Coral reefs are locally associated with voluminous Oligocene volcanism in the region. The reefs are subdivided into (i) eruption-associated reefs (Krumovgrad); (ii) pre-eruption reefs (Ivaylovgrad); and (iii) Metaxades-Didymotycho reefs from field relations, trace element and rare-earth element (REE) abundances. Coral assemblages are dominated by Cladocora sp., which is accompanied by Porites sp., Colpophyllia sp., Favites sp. and Leptoseris sp. Eruption-associated reefs are characterized by their higher REE content than the lower in all REE contents of pre-eruption reefs showing negative Ce anomaly, and Metaxades-Didymotycho reefs that have lower middle-heavy REE contents compared to previous groups. Trace element and REE geochemistry of the coral skeletons indicates volcanic contribution to seawater, mostly evident in the eruption associated reefs, and contribution from terrestrial input in the site of coral buildup deposition. Contribution from a different source of prior diagenetic nature, along with subsequent diagenetic modification, is inferred. The increase in REE+Y (ΣREY) from pre-eruption to eruption-associated reefs is well correlated with elevated amounts of terrigenous elements like Al and Fe.

Compositional variations in deep-sea gravity-flow deposits. A case study from the Voirons Flysch (Voirons-Wägital complex, Chablais Prealps, France)

Grain-size control is frequently invoked to explain the compositional variation of detrital sediments in fluvial sediments. A similar possibility is here investigated in the fossil deep-sea gravity-flow deposits from the Voirons Flysch (Voirons-Wägital complex, Chablais Prealps). About 270 samples of diverse depositional settings were analysed using the framework composition, including cement and porosity, Mutti facies and grain-size measurements. Six lithofacies have been identified from cluster analysis. They correspond to channel (L1 to L3) and lobe (L4 and L5) depositional settings and to reworked turbidites (L6). Our results emphasize the close relationship between petrography and grain-size which alters the detrital signal from extrabasinal sources. The abundant polycrystalline component of proximal deposits is progressively removed while skeletal grains and additional intrabasinal grains become more abundant in distal deposits. Alongside, porosity is reduced by the simultaneous increase of cement which constrains subsequent diagenetic modifications. Hence, proximal depositional settings keep most of the inherited signal from extrabasinal sources whereas distal deposits will be homogenised. When plotted in provenance-analysis diagrams, the successive depositional settings show distinct fields possibly corresponding to different tectonic settings of the Dickinson model. This compositional variation thus creates a bias in provenance interpretation resulting from both grain-size control and flow mechanisms. Finally, our results emphasize that petrographic data must be handled with care in provenance analyses, especially when comparing various deep-sea gravity flow systems with different grain-size ranges.

Origin of authigenic quartz in organic-rich shales of the Wufeng and Longmaxi Formations in the Sichuan Basin, South China: Implications for pore evolution

Primary sedimentary composition of shales and subsequent diagenetic modifications are key factors in controlling the pore development and preservation in shale gas reservoirs. Thin section examination, X-ray diffraction analysis, scanning electron microscopy (SEM) determination with cathodoluminescence and geochemical analyses were combined to investigate Wufeng and Longmaxi shales of the Sichuan Basin. These experimental methods allowed us to focus on the origin of authigenic quartz in organic-rich shales and evaluate its importance in the evolution of shale gas reservoirs. The siliceous shales in Wufeng and Longmaxi Formations contain up to 60% microcrystalline aggregates of quartz, which are more abundant than silt-size detrital quartz to suggest an authigenic origin. Most authigenic quartz originates from biogenic siliceous dissolution and re-precipitate. Therefore, high excess silica concentration indicates a higher surface productivity with more organic matter (OM) deposition. The abundance of OM pores is positively correlated with the excess-Si content (R2 = 0.77). We suggest that the migrated OM filled in authigenic microcrystalline quartz aggregates is the principal matrix for OM pore development. Authigenic microcrystalline quartz aggregates filled in primary pore space and destroyed primary interparticle porosity. However these authigenic quartz can restrain compaction and preserve the internal pore structure as a rigid framework. Pore space is abundant enough in microcrystalline quartz aggregates for oil and bitumen filling during the oil window. Migrated OM filled the associated pore network in authigenic microcrystalline quartz aggregates would more likely to generate a better three-dimensional OM pore network through secondary cracking and provides favorable reservoir spaces for shale gas.

Impact of diagenesis on the reservoir quality of tight oil sandstones: The case of Upper Triassic Yanchang Formation Chang 7 oil layers in Ordos Basin, China

The Chang 7 tight oil reservoirs are important oil exploration targets in the Ordos basin. The reservoirs are generally characterized by low porosity, low permeability and strong microscopic heterogeneity. Mineralogical, petrographic, and geochemical analyses have been used to investigate the type and degree of diagenesis and diagenetic history of Chang 7 tight oil reservoirs. The influences of composition, texture and diagenesis on reservoir quality were also discussed in this article. Diagenesis of the Chang 7 tight oil reservoirs was mainly composed of mechanical compaction, grain dissolution and cementation by quartz, carbonates and various clay minerals. Reduction of porosity by mechanical compaction was more significant than by cementation. Eodiagenesis mainly includes (1) mechanical compaction and mechanically infiltrated clays; (2) cementation by calcite, pyrite, and clay minerals; and (3) leaching of feldspars. Mesodiagenesis mainly includes (1) further mechanical compaction; (2) cementation by late stage carbonates; (2) formation of illite and mixed-layered illite-smectite; (3) quartz cements; (4) dissolution of feldspars. The porosity was decreased by compaction and cementation and then increased by dissolution of the framework grains.Primary porosity is higher in sandstones with abundant detrital quartz. Secondary dissolution pores are mainly associated with those feldspar-rich samples. Sandstones which have undergone the most feldspar dissolution are the cleaner (abundant in both detrital quartz and feldspar), better sorted, and coarser-grained samples. Reservoir quality of Chang 7 tight oil reservoirs is also largely controlled by pore occluding cements. The composition and texture have played an important role on intergranular volume and subsequent diagenetic modifications of the Chang 7 tight oil reservoirs. Good quality reservoir intervals are characterized by fine-grained, moderately-well sorted with high percentages of detrital quartz and feldspar but low content of detrital clay and cements.

Constraints on the formation and diagenesis of phosphorites using carbonate clumped isotopes

The isotopic composition of apatites from sedimentary phosphorite deposits has been used previously to reconstruct ancient conditions on the surface of the Earth. However, questions remain as to whether these minerals retain their original isotopic composition or are modified during burial and lithification. To better understand how apatites in phosphorites form and are diagenetically modified, we present new isotopic measurements of δ18O values and clumped-isotope-based (Δ47) temperatures of carbonate groups in apatites from phosphorites from the past 265million years. We compare these measurements to previously measured δ18O values of phosphate groups from the same apatites. These results indicate that the isotopic composition of many of the apatites do not record environmental conditions during formation but instead diagenetic conditions. To understand these results, we construct a model that describes the consequences of diagenetic modification of phosphorites as functions of the environmental conditions (i.e., temperature and δ18O values of the fluids) during initial precipitation and subsequent diagenesis. This model captures the basic features of the dataset and indicates that clumped-isotope-based temperatures provide additional quantitative constraints on both the formational environment of the apatites and subsequent diagenetic modification. Importantly, the combination of the model with the data indicates that the δ18O values and clumped-isotope temperatures recorded by phosphorites do not record either formation or diagenetic temperatures, but rather represent an integrated history that includes both the formation and diagenetic modification of the apatites.

Textural and paleo-fluid flow control on diagenesis in the Paleoproterozoic Franceville Basin, South Eastern, Gabon

The Paleoproterozoic (∼2.15Ga) Franceville Basin, South Eastern Gabon, is a continental sedimentary basin that host unmetamorphosed sediments. This study involve detailed mineralogy, sedimentology, and petrography of the basal sedimentary units of FA and lower FB (FB1) Formations, from the basin margin to centre in relation to mineral paragenesis and fluid flow. The FA Formation conglomerate, sandstone, and mudstone consists lithofacies of mixed fluvial and fluvio-deltaic transitional origin, while the overlying FB1 Formation includes alternating organic rich black shale and sandstone of marine deposit.The medium- to coarse grained fluvio-deltaic quartz arenite in upper part of FA Formation is characterized by pervasive authigenic quartz cementation that reduced the porosity and permeability in the early stage of burial history. This provides a resistant framework for subsequent diagenetic modification and also inhibits fluid flow during burial diagenesis. In contrast, the clay and unstable detrital grains rich fluvial arkosic to sub-arkosic sandstones that escaped early quartz cementation show considerable pressure solution at grain contacts. These arkosics were less porous and permeable when deposited but transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis and subsequent precipitation of authigenic mineral cements in the resulting secondary porosities.From the proximal to distal basin and within sample suites, there is no considerable chemical variation in the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipitation from a near equilibrium homogenous pore-fluid. The predominance of 1Mt illite polytype in most lithologies reflects precipitation of the clay minerals and probably other cements in an environment with high fluid/rock ratio. Dolomite, anhydrite, barite, and Fe-oxides are the main crystallized authigenic minerals aside illite and Fe-rich chlorite clay minerals. The mineralogical assemblages and textural occurrences of the rocks suggest that diagenesis and fluid flow in the FA Formation in the Franceville Basin are mainly controlled by depositional facies.

Use of NanoSIMS in the search for early life on Earth: ambient inclusion trails in ac. 3400 Ma sandstone

Ambient inclusion trails (AIT) are enigmatic microtubular structures created by the migration of mineral crystals through a lithified substrate. The decomposition of organic material has been suggested as the driving force for the crystal migration, but has yet to be rigorously tested. AIT may hold potential as a biosignature for investigating early life on Earth if the associated organic material can be shown to be biological. This paper attempts to test the formation mechanism and biogenicity of AIT from the c . 3400 Ma Strelley Pool sandstone of Western Australia using NanoSIMS technology. In doing so, we demonstrate the unique ability of the NanoSIMS to combine sub-micron scale imaging with in situ chemical and isotopic data, thereby enhancing our ability to evaluate the biogenicity criteria for Archaean microstructures. Enrichments of a suite of major elements (C, N, P, S) and trace elements (Co, Fe, Ni, Zn), often associated with biological processes, are found within several AIT in this sandstone. C and N enrichments are most common along AIT margins, and correlate with depletions of Si, O, Ca and Mg, indicating that this material is indeed organic in nature. δ 13 C values of this carbonaceous material average −26‰. Petrographic observations show that some of the AIT occur in the centre of detrital sandstone grains where they were sealed from later fluid flow and therefore preserve primary Archaean (bio)geochemistry. In contrast, AIT found around the outer edges of sandstone grains may contain more recent organic material introduced by later fluid migration and are an unreliable biosignature. Using the petrographic and geochemical data a multi-stage model for AIT formation and subsequent diagenetic modification is proposed. The possible sources of the primary organic material are discussed and we conclude that the data are consistent with a biological origin for these AIT. An abiogenic origin is more difficult to sustain but cannot yet be completely excluded for AIT in general.

Morphologic and Mineralogic Transitions From Opal-A to Opal-CT in Low-Temperature Siliceous Sinter Diagenesis, Taupo Volcanic Zone, New Zealand

ABSTRACT The opal-A to opal-CT silica phase transformation in New Zealand's siliceous hot-spring deposits (sinter) is accompanied by recurring textural changes at the micron scale. Recognition of these changes is essential to extract paleohydrological, paleoenvironmental, and paleobiological signatures from ancient hydrothermal systems, and to recognize diagenetic overprints upon primary depositional signals. We examined 39 samples of siliceous sinter from the low-temperature (< 35° C) palisade microfacies at Orakei Korako and Te Kopia geothermal fields, Taupo Volcanic Zone, New Zealand, in three different postdepositional environments (least-disturbed, heat-affected, weathered). Friable to indurated samples were collected from modern, presently silicifying microbial mats in the outflow of nearly neutral alkali chloride springs, to sinter 3,500 years old. The progressive mineralogical change from noncrystalline opal-A to paracrystalline opal-CT was traced by X-ray powder diffraction and scanning electron microscopy. One diagenetic sequence with two morphological pathways, spherical and smooth (polymeric and monomeric deposition, respectively), was identified at the micron scale. Both pathways ultimately lead to formation of typical, opal-CT bladed lepispheres. Along the spherical pathway, initial morphological restructuring includes formation of circular holes (< 0.1 to 1.0 µm in diameter) in opal-A spheres (< 3.0 µm in diameter). These changes correlate with a shift in the maximum intensity of the opal-A X-ray scattering broadband, from 4.0 A to 4.09 A, the characteristic position for opal-CT. An early transitional diagenetic stage is indicated in both sphere and smooth silica pathways by development of hexagonal platelets and a composite opal-A/-CT scattering broadband that is sharp-peaked in comparison, and centered at 4.09 A. A still sharper-peaked, composite opal-CT/-A X-ray diffraction band follows, which has developed an incipient tridymite shoulder; it is found in samples that display both hexagonal platelets and incipient fuzzy opal-CT lepispheres. At this late transitional stage, patchy replacement textures are visible at the macroscale. Only with the appearance of numerous, well-developed, opal-CT bladed lepispheres is a typical, sharp-peaked (4.09 A), opal-CT X-ray trace evident, with a well-defined tridymite shoulder. This diagenetic process can produce either porous or vitreous fabrics, depending on the degree to which infill of silica has reduced porosity and increased density of the deposit. This study provides a textural-mineralogical context for understanding silicification of microbial communities in geothermal settings and their subsequent diagenetic modifications. These modifications continue long after geothermal activity has ceased, with little or no burial, and occur in horizons or localized patches of a given deposit. Moreover, postdepositional conditions, such as heat overprinting or weathering, influence the diagenesis of siliceous sinter by increasing its maturation rate.

Stem/Aem Evidence for Preservation of Burial Diagenetic Fabrics in Devonian Shales: Implications for Fluid/rock Interaction in Cratonic Basins (U.S.A.)

ABSTRACT Fabrics, microstructures, and compositions of authigenic illite-rich clays in Devonian intracratonic basin shales were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning transmission electron/analytical electron microscopy (STEM/AEM) methods in order to relate the extent of clay diagenesis to the timing of mudstone-system closure and to reconcile conflicting radiogenic and paleomagnetic ages obtained from these shales and associated limestones and bentonites. Authigenic illite in these Devonian shales is similar to post-transition illite-rich mixed-layer illite/smectite (I/S) in Gulf Coast and other young basin mudstones that have formed through replacement of detrital smectite or smectite-rich I/S without subsequent diagenetic modification. The clay matrix is composed of subparallel intergrowths of illite-rich crystals (30-200 A thick) with low-angle grain boundaries and defects at layer terminations. Contrast in some lattice fringe images is consistent with R1-, R2-, and R3-ordered, mixed-layer I/S. Electron diffraction patterns indicate partially disordered (1Md) stacking sequences with significant turbostratic stacking and mixed layering. The illite-rich material is K- and Aldeficient, and Si-enriched, relative to ideal muscovite. Preservation of burial diagenetic, authigenic, mixed-layer I/S microstructures in these Devonian shales implies that they have remained effectively closed systems following the smectite-to-illite transformation. Later Alleghenian (ca. 300 Ma) tectonic or fluid-flow events recorded by authigenic minerals in bentonites and carbonate lithologies throughout the U.S. Midcontinent are apparently not recorded in these Devonian shales. Preservation of diagenetic fabrics indicates that radiogenic isotope dating of cratonic basin shales should yield syndepositional to early burial diagenetic ages corresponding to the timing of I/S authigenesis.

Diagenetic Influences on Reservoir Characteristics of Terry Sandstone, Spindle Field, Colorado: ABSTRACT

The Upper Cretaceous Terry sandstones of Spindle field, Weld County, Colorado, were deposited as offshore bars in a shallow-marine environment. The Terry Sandstone Member of the Pierre Shale is presently buried at depths of 4500-5000 ft (1375-1525 m). The Terry sandstone reservoir was diagenetically created by selective dissolution of calcite cement and detrital material in downdip sands. Hydrocarbon entrapment occurred as a diagenetic/stratigraphic trap through preservation of cement updip and by shale boundaries laterally and vertically. The arkosic litharenites of the Terry sandstone have experienced four major stages of diagenesis: (1) early compaction and calcite cementation, (2) cement and grain dissolution and renewed compaction, (3) chlorite authigenesis, and (4) late illite and smectite authigenesis. In the study area, samples which have experienced dissolution of calcite exhibit porosities of up to 20%. However, permeabilities are low (0.1-11.0 md) due to the presence of pore-lining chlorite and the development of a pseudomatrix related to postdissolution compaction of glauconite pellets. The authigenic chlorite and glauconite pseudomatrix reduced porosity and blocked pore throats. Semilogarithmic plots indicate a logarithmic decrease in permeability with increased chlorite and glauconite content. Terry sandstone reservoir quality has been strongly modified by the diagenetic processes of cementation and compaction. Thesemore » diagenetic processes resulted in the destruction of primary porosity, the formation of secondary porosity, and the subsequent partial occlusion of secondary porosity. Variations in reservoir properties and production potential within the sandstones can be ascribed to both primary lithologic contrasts and subsequent diagenetic modifications.« less

Formation of Peloids in Plant Rootlets, Grand Cayman, British West Indies

ABSTRACT Spherical to elliptical peloids, averaging 80 µm in length, occur inside plant rootlets that penetrate limestones of the Pleistocene Ironshore Formation on Grand Cayman. The peloids, which are associated with desiccated fragments of mites, are formed of minute calcite platelets and fragments of algal and/or fungal filaments bound by organic-rich material. The calcite platelets are morphologically similar to platelets that occur in the epidermal cells of the surrounding rootlets. The peloids probably originated as fecal pellets produced by mites which mined into the softer cores of the roots. In so doing, they recombined the calcite platelets, algal/fungal filaments, and organic material into coprogeneous material. Subsequent diagenetic modification produced peloids formed of mic ite that are morphologically identical to peloids found in many caliche units.