Sadoon Morad

The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy

Diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs. Variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability. Linking the types and distribution of diagenetic processes to the depositional facies and sequence-stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity. The heterogeneity patterns of sandstone reservoirs, which determine the volumes, flow rates, and recovery of hydrocarbons, are controlled by geometry and internal structures of sand bodies, grain size, sorting, degree of bioturbation, provenance, and by the types, volumes, and distribution of diagenetic alterations. Variations in the pathways of diagenetic evolution are linked to (1) depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; (2) detrital sand composition; (3) rate of deposition (controlling residence time of sediments at specific near-surface, geochemical conditions); and (4) burial thermal history of the basin. The amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and, therefore, can be predicted in the context of sequence stratigraphy, particularly in paralic and shallow marine environments. Changes in the relative sea level exert significant control on the types and extent of near-surface shallow burial diagenetic alterations, which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs. Carbonate cementation is more extensive in transgressive systems tract (TST) sandstones, particularly below parasequence boundaries, transgressive surface , and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter, bioturbation, and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates, which also enhance the formation of glaucony. Eogenetic grain-coating berthierine, odinite, and smectite, formed mostly in TST and early highstand systems tract deltaic and estuarine sandstones, are transformed into ferrous chlorite during mesodiagenesis, helping preserve reservoir quality through the inhibition of quartz cementation. The infiltration of grain-coating smectitic clays is more extensive in braided than in meandering fluvial sandstones, forming flow barriers in braided amalgamated reservoirs, and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution. Diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars, micas, and mud intraclasts under wet and warm climates, whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers. Turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls, as well as along layers of concentration of carbonate bioclasts and intraclasts. Commonly, hybrid carbonate turbidite arenites are pervasively cemented. Proximal, massive turbidites normally show only scattered spherical or ovoid carbonate concretions. Improved geologic models based on the connections among diagenesis, depositional facies, and sequence-stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies, as well as for CO2 geologic sequestration, but also support more effective hydrocarbon exploration through reservoir quality prediction.

Albitization of detrital plagioclase in Triassic reservoir sandstones from the Snorre Field, Norwegian North Sea

ABSTRACT Detrital plagioclase (An12-An28) in Triassic reservoir sandstones of the Lunde Formation in the Snorre Field, offshore Norway has been partially to completely albitized during burial diagenesis (depths of about 2,500-3,000 m; 75-100°C present temperature). Calcite (CaCO3 > 99 mole %) and kaolinite were formed as by-products of plagioclase albitization. Sodium-rich detrital plagioclase (An2-An11) are usually unalbitized. Detrital K-feldspar is fresh or, rarely, partly albitized. Differences in albitization behavior of detrital plagioclase and K-feldspar are shown to be related to the chemistry of formation waters, reactivity of kaolinite, and temperature. The diagenetically formed albite pseudomorphs are very pure (Ab > 9 ), well-ordered low-albite, dark-luminescent, vacuolized, mostly untwinned, and comprised of numerous euhedral albite crystals. Some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase. The amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable Ca-rich plagioclase. The microporosity formed during albitization is proportional to the anorthite component, which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite. The albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of Na+ from pore solutions.

Quartz cementation in sandstones

IntroductionQuartz Cementation In Oil Field Sandstones: A Review Of The Key ControversiesThe Origins Of Large-Scale Quartz CementationModelling Quartz Cementation And Porosity In Reservoirs Sandstones - Examples From The Norwegian Continental ShelfRelated Quartz And Illite Cementation In The Brent Sandstones: A Modelling ApproachThe Different Processes Involved In The Mechanism Of Pressure Solution In Quartz-Rich Rocks And Their InteractionsA Test Of Hypotheses Regarding Quartz Cementation In Sandstones: A Quantitative Image Analysis ApproachQuantification Of Detrital, Authigenic And Porosity Components Of The Fontainbleau Sandstone: A Comparison Of Conventional Optical And Combined Scanning Electron Microscope Based Methods Of Modal AnalysesEffects Of Wettability On Quartz Cementation In Oil FieldsExperimental And Field Constraints On The Role Of Silica-Organic Complexation And Silica-Microbial Interactions During Sediment DiagenesisMicrostructure Of Deformed And Nondeformed Sandstones From The North Sea: Implications For The Origins Of Quartz Cement In SandstonePetrophysical And Petrographical Analysis Of Quartz Cement Across Oil-Water Contacts In The Magnus Field, Northern North SeaQuartz Cementation In Cretaceous And Jurassic Reservoir Sandstones From The Salam Oil Field, Western Egypt: Constraints On Temperature And Timing Of Formulation From Fluid InclusionsRegional Loss Of Si02 And Cac03 And Gain Of K20 During Burial Of Gulf Coast Mudrocks USAQuartz Cement: The Millers TaleQuartz Cement Origins And Budget In The Tumblagooda Sandstone, Western AustraliaInfluence Of Uplift And Magnetism On Distribution Of Quartz And Illite Cementation: Evidence From Siluro-Devonian Sandstone Of The Parana Basin, BrazilPolyphased Quartz Cementation And Its Sources: A Case Study From The Upper Palaeozoic Haushi Group Sandstone, Sultanate Of OmanThe Porosity-Preserving Effects Of Microcrystalline Coatings In Arentic SandstonesHigh Temperature Quartz Cement And The Role Of Stylotites In A Deep Gas Reservoir, Spiro Sandstone, Arkoma Basin, USAOxygen Isotope Analysis Of Authigenic Quartz In Sandstones: A Comparison Of Ion Microprobe And Conventional Analytical TechniquesTrace Element Composition Of Quartz Cement

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.

The role of detrital composition and climate on the diagenetic evolution of continental molasses: evidence from the Cambro—Ordovician guaritas sequence, southern Brazil

Abstract The Cambro-Ordovician molassic Guaritas Sequence (Camaqua Basin, southern Brazil) comprises alluvial-fan and braided fluvial sandstones and conglomerates with intercalated aeolian and lacustrine-deltaic deposits and andesitic lava flows. The sediments display a complex detrital composition derived from plutonic/gneissic, acidic volcanic and low-grade metamorphic source rocks. This detrital assemblage was strongly modified by semi-arid continental near-surface diagenesis. Early cementation by hematite, smectite, quartz and calcite, and the relatively limited burial prevented strong compaction and preserved some primary macroporosity in most of the sandstones, whereas the absence of early cements and/or abundance of ductile grains promoted substantial porosity destruction by compaction and the inhibition of further diagenetic modifications. The diagenetic dissolution and replacement of volcanic rock fragments and detrital feldspars by clays and albite changed the original framework composition, as well as the tectonic provenance classification of the sandstones. Detailed quantitative petrographic study allowed the reconstruction of the original detrital compositions and the distinction of six different pathways of diagenetic evolution of the sandstones, attesting to the efficiency of this method for diagenetic modelling and provenance analyses of ancient sandstones.

Alteration of detrital Fe-Ti oxides in sedimentary rocks

Scanning electron microscope study has revealed that in Proterozoic sandstones, shales, and carbonate rocks of the Visingso Group in southern Sweden, detrital Fe-Ti oxides have undergone alteration by dissolution and/or replacement mainly by titanium oxides, hematite (reddish rocks), or pyrite (greenish sandstones, shales, and carbonate rocks). The titanium oxides occur as either poorly crystalline masses, cryptocrystalline aggregates, or discrete, euhedral crystals of anatase, brookite, and rutile that attain a variety of crystal habits. Microprobe analyses have shown that the alteration of ilmenite occurs through several intermediate phases, each successively enriched in titanium and depleted in iron, to an almost pure form of TiO 2 . Some of the iron and titanium which is released during alteration of the Fe-Ti oxides is incorporated in associated clay minerals.

Clay Mineral Cements in Sandstones

Review papers. 1. Clay minerals in sandstones: controls on formation, distribution and evolution: R. H. Worden and S. Morad. 2. Predictive diagenetic clay--mineral distribution in siliciclastic rocks within a sequence stratigraphic framework: J.M. Ketzer, S. Morad and A. Amorosi. 3. Oxygen and hydrogen isotope composition of diagenetic clay minerals in sandstones: a review of the data and controls: S. Morad, R. H. Worden and J.M. Ketzer. 4. Palaeoclimate controls on spectral gamma--ray radiation from sandstones: A. Ruffell, R.H. Worden and R. Evans. 5. Smectite in sandstones: a review of the controls on occurrence and behaviour during diagenesis: J. M. McKinley, R. H. Worden and A. H. Ruffell. 6. Patterns of clay mineral diagenesis in interbedded mudrocks and sandstones: an example from the Palaeocene of the North Sea: H.F. Shaw and D.M. Conybeare, D.M. 7. Cross--formational flux of aluminium and potassium in Gulf Coast (USA) sediments:M. Wilkinson, R.S. Haszeldine and K.L. Milliken. 8. Silicate--carbonate reactions in sedimentary systems: fluid composition control and potential for generation of overpressure: I. Hutcheon and S. Desrocher. 9. Experimental studies of clay mineral occurrence: D. A. C. Manning. 10. Effect of clay content upon some physical properties of sandstone reservoirs: Paul F. Worthington. 11. Quantitative analysis of clay and other minerals in sandstones by X--ray powder diffraction (XRPD): S. Hillier. 12. A review of radiometric dating techniques for clay mineral cements in sandstones: P. J. Hamilton. Chlorite case study. 13. Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin, offshore eastern Brazil: S. M. C. Anjos, L. F. De Ros and C. M. A. Silva. Kaolinite case studies. 14. Origin and diagenetic evolution of kaolin in reservoir sandstones and associated shales of the Jurassic and Cretaceous, Salam Field, Western Desert (Egypt): R. Marfil, A. Delgado, C. Rossi, A. La Iglesia and K. Ramseyer. 15. Microscale distribution of kaolinite in Breathitt Formation sandstones (middle Pennsylvanian): implications for mass balance: K. L. Milliken. 16. The role of the Cimmerian unconformity (Early Cretaceous) in the kaolinitization and related reservoir--quality evolution in Triassic sandstones of the Snorre Field, North Sea: J.M. Ketzer, S. Morad, J.P. Nystuen and L.F. De Ros. 17. The formation and stability of kaolinite in Brent sandstone reservoirs: a modelling approach: Ae. Brosse, T. Margueron, C. Cassou, B. Sanjuan, A. Canham, J.--P. Girard, J.--C. Lacharpagne and F. Sommer. Illite case studies. 18. Illite fluorescence microscopy: a new technique in the study of illite in the Merrimelia Formation, Cooper Basin, Australia: N. M. Lemon and C. J. Cubitt. 19. Geochemical modelling of diagenetic illite and quartz cement formation in Brent sandstone reservoirs: example of the Hild Field, Norwegian North Sea: B. Sanjuan, J.--P. Girard, S. Lanini, A. Bourguignon and E. Brosse. 20. The effect of oil emplacement on diagenetic clay mineralogy: the Upper Jurassic Magnus Sandstone Member, North Sea: R.H. Worden and S.A. Barclay. Glauconite case study. 21. Application of glauconite morphology in geosteering and for on--site reservoir quality assessment in very fine--grained sandstones: Carnarvon Basin, Australia: J.P.Schulz--Rojahn, D.A. Seeburger and G.J. Beacher. Index

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.

Identification of primary Ce-anomaly signatures in fossil biogenic apatite: implication for the Cambrian oceanic anoxia and phosphogenesis

Based on REE analyses of 105 world-wide distributed Cambrian fossil apatite samples and eight late Neoproterozoic phosphate concretions, a close relationship is established between the concentrations of MREE and Ce anomalies. Shale La/SmN ratios of 0.35 is suggested as a critical value for the discrimination of the natural Ce anomaly acquired during pre-burial/early diagenesis by biogenic apatite from an artificial Ce anomaly induced due to the calculation. Ce anomalies in apatites with flat REE distribution and La/SmN ratio >0.35 may be used as indicative of oceanic anoxia.

REE patterns in latest Neoproterozoic–early Cambrian phosphate concretions and associated organic matter

Abstract Analyses of REE were conducted on phosphatic concretions and various types of associated organic matter in eight, key latest Neoproterozoic–earliest Cambrian, inner- to outer-shelf, marine phosphatic successions that are distributed worldwide. A pronounced REE fractionation during diagenetic degradation of the organic matter has been revealed. Selective MREE incorporation and bulk REE contents in some kerogens, organic megascopic fossils and humic acid are compatible with those recorded in associated phosphate. The similarity of REE patterns in apatite and organic matter suggests that early-diagenetic, microbial decomposition of organic matter does not only provide phosphorus and suitable sub-oxic, geochemical conditions, but it is also a likely source of REE enrichment in authigenic apatite formed at shallow depths below the marine water–sediment interface.