The role of diagenesis and depositional facies on pore system evolution in a Triassic outcrop analogue (SE Spain) (original) (raw)
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Marine and Petroleum Geology, 2002
The Triassic TAGI (Trias Argilo-Greseux Inferieur) fluvial sandstones are the main oil reservoirs in the Berkine Basin, Algeria. None theless, their provenance and diagenesis, and their impact on reservoir quality, are virtually unknown. Samples from the Ourhoud field, representing the Lower, Middle and Upper TAGI subunits, were studied using a combination of petrographic, mineralogical and geochemical techniques. The Lower TAGI sandstones have an average framework composition of Q98.3Fo.6Rl.l and 95% of the quartz grains are mono crystalline. By contrast, the Middle-Upper TAGl sandstones have an average framework composition of QSSJF98RI9 and 79.7% of the quartz grains are monocrystalline. The Lower TAGI quartz arenites derived from Paleozoic siliciclastic rocks, whereas the Middle-Upper TAGI subarkoses originated mainly from metamorphic terrains. This change in provenance is a potential criterion for correlation within the TAG!. Also, this change has contributed to the significantly different diagenetic paths followed by the Lower TAG! quartz arenites and the Middle Upper TAGI subarkoses. Grain-coating illitic clays are abundant in the Lower T AGl, where they exert a critical control on reservoir quality. These clays are interpreted as pedogenic and/or infiltrated in origin and to have had, in part, smectitic precursors. Shallow burial Fe-dolomite cementation was favored in the downthrown block of the field-bounding fault, where it contributed to the poor reservoir quality. Magnesite-siderite cements are multiphase. The earliest generation is composed of Fe-rich magnesite that precipitated during shallow burial from hypersaline fluids with high Mg/Ca ratios, probably refluxed residual brines associated with the Liassic evaporites. Later magnesite-siderite generations precipitated during deeper burial from waters with progressively higher FelMg ratios. Authigenic vermicular kaolin largely consists of dickite that replaced previously formed kaolinite. Dickitization was followed by late-stage illitization related to the dissolution of detrital and authigenic K-feldspar. Quartz, the most abundant cement, was mainly sourced by the pressure-or clay-induced dissolution of detrital quartz and is a critical factor controlling the reservoir quality. Overall, quartz cement is more abundant in the Lower TAGI than in the Middle Upper TAGI, and this increase correlates with a decrease in average porosity. Within the Lower TAGI, quartz cement abundance is stratigraphically very variable, which is in part related to facies controlled variations in grain-coating clay, resulting in major vertical variations in reservoir quality. Anhydrite and barite cements postdate quartz overgrowth. The sulfate necessary for their formation was likely sourced by deep subsurface dissolution of Late Triassic-Liassic evaporites.
The Triassic TAGI (Trias Argilo-Greséux Inférieur) fuvial sandstones are the main oil reservoirs in the Berkine Basin, Algeria. Nonetheless , their provenance and diagenesis, and their impact on reservoir quality, are virtually unknown. Samples from the Ourhoud Feld, representing the Lower, Middle and Upper TAGI subunits, were studied using a combination of petrographic, mineralogical and geochemical techniques. The Lower TAGI sandstones have an average framework composition of Q98.3 F0.6 R1.1 and 95% of the quartz grains are mono-crystalline. By contrast, the Middle-Upper TAGI sandstones have an average framework composition of Q88.3 F9.8 R1.9 and 79.7% of the quartz grains are monocrystalline. The Lower TAGI quartz arenites derived from Paleozoic siliciclastic rocks, whereas the Middle-Upper TAGI subarkoses originated mainly from metamorphic terrains. This change in provenance is a potential criterion for correlation within the TAGI. Also, this change has contributed to the significantly different diagenetic paths followed by the Lower TAGI quartz arenites and the Middle± Upper TAGI subarkoses. Grain-coating illitic clays are abundant in the Lower TAGI, where they exert a critical control on reservoir quality. These clays are interpreted as pedogenic and/or infiltrated in origin and to have had, in part, smectitic precursors. Shallow burial Fe-dolomite cementation was favored in the downthrown block of the feld-bounding fault, where it contributed to the poor reservoir quality. Magnesite-siderite cements are multiphase. The earliest generation is composed of Fe-rich magnesite that precipitated during shallow burial from hypersaline fuids with high Mg/Ca ratios, probably refuxed residual brines associated with the Liassic evaporites. Later magnesite±siderite generations precipitated during deeper burial from waters with progressively higher Fe/Mg ratios. Authigenic vermicular kaolin largely consists of dickite that replaced previously formed kaolinite. Dickitization was followed by late-stage illitization related to the dissolution of detrital and authigenic K-feldspar. Quartz, the most abundant cement, was mainly sourced by the pressure-or clay-induced dissolution of detrital quartz and is a critical factor controlling the reservoir quality. Overall, quartz cement is more abundant in the Lower TAGI than in the Middle-Upper TAGI, and this increase correlates with a decrease in average porosity. Within the Lower TAGI, quartz cement abundance is stratigraphically very variable, which is in part related to facies controlled variations in grain-coating clay, resulting in major vertical variations in reservoir quality. Anhydrite and barite cements postdate quartz overgrowth. The sulfate necessary for their formation was likely sourced by deep subsurface dissolution of Late Triassic-Liassic evaporites.
2017
Reservoir quality is mainly controlled by environment deposit type and diagenesis processes. To investigate such subject we usually proceed to microscopic techniques. Absence of outcrops and missing of core samples let us use conventional wireline logs and core lab measurements as primary data. Direct lecture of well logs calibrated by core photos and wellsite description, well logs correlation and cross plots were our main methodology axis. Target of this study is the fluvial Shaly Sandstones of Upper Triassic (Rhaetian) reservoir (abreviated in French; TAGS) located in Toual field-SE of Algeria. The present investigation confirms the absence of clean sandstone levels in the studied reservoir. It detects also the presence of halite and clays (Illite) as cements in sandstone. The influence of illite on permeability was confirmed whereas porosity is less affected.
Journal of Petroleum Geology, 2015
Mineralogical, petrographical and geochemical analyses of representative sandstone samples from well BB6-59, Sarir oilfield, indicate that the meandering fluvial sandstones are fine-to very fined grained subarkosic arenites (av. Q 91 F 5 L 4 ), and that braided fluvial sandstones are medium to very coarse-grained quartz arenites (av. Q 96 F 3 L 1 ). The reservoir qualities of these sandstones were modified during both eodiagenesis (ca. <70 o C; <2 km) and mesodiagenesis (ca. >70 o C; >2 km). Reservoir quality evolution was controlled primarily by the dissolution and kaolinitization of feldspars, micas and mud intraclasts during eodiagenesis, and by the amount and thickness of grain-coating clays, chemical compaction and quartz overgrowths during mesodiagenesis. However, dissolution and kaolinitization of feldspars, micas and mud intraclats resulted in the creation of intercrystalline micro-and mouldic macro-porosity and permeability during eodiagenesis, which were more widespread in braided fluvial than in meandering fluvial sandstones. This was because of the greater depositional porosity and permeability in the braided fluvial sandstones which enhanced percolation of meteoric waters.
Marine and Petroleum Geology, 2021
The Triassic stratigraphy of the UK contains many major reservoir rock units, present in all major offshore and onshore basins, historically resource rich in oil, gas and water. The Ormskirk Sandstone Formation (Sherwood Sandstone Group) and Tarporley Siltstone Formation (Mercia Mudstone Group) represent the main reservoirs found in in one of the UK's earliest producing oilfields at an unusually shallow depth of 30 to 90 m. In this study, environments of deposition present, maximum burial depth, and reservoir quality are evaluated from well Formby-7 using modern analytical techniques. Wireline data and core analysis data were made available by Aurora Energy Resources for the purposes of this study. Formby-7 was continuously cored so that the whole section was logged for sedimentary structures and grain size. Optical analysis of thin sections was conducted, along with SEM-EDS, using thin sections from core plugs. The Ormskirk Sandstone Formation was deposited in a proximal setting by a braided, dryland river with aeolian influence; the Tarporley Siltstone Formation was deposited in a more distal setting on the margins of a playa environment. The study found that the reservoir quality is excellent in the deeper Ormskirk sediments, but relatively poor in the overlying Tarporley sediments. Good reservoir quality present in the Ormskirk Sandstone Formation is linked to early depositional textures, grain size (medium sand) and general lack of pore-filling cements. In the Tarporley, reservoir quality is poor due to the grain size (very fine sand) and the variable presence of pore-filling gypsum associated with playa deposits. Minor feldspar diagenesis (both dissolution and precipitation) occurred in both the Sherwood and Mercia sediments. The small degree of mechanical compaction and lack of any signs of mesodiagenesis suggest that the clastic sediments in the Formby-7 well have not been buried deeper than about 1,000 m. The findings reported here can be used to iii help the understanding of UK Triassic sedimentology and reservoir quality for oil and gas, geothermal energy, CCS (carbon capture storage) and water supply.
Journal of Petroleum Geology, 2010
This paper assesses the diagenetic history of potential fluvial hydrocarbon reservoir rocks deposited within incised valley systems of the Lower Carboniferous Marar Formation in western Libya. Outcrop data were collected in the Tinedhan Anticline, located at the southern margin of the Ghadames Basin. Four discrete intervals with channelized sandstones were identified in a section dominated by alternating offshore mudstones and shallow-marine clastics. The incised channels were cut during major sea-level lowstands, and filled by fluvial sandstone packages up to 50 m thick. Fifty-eight samples from four different localities, representing three lowstand systems tracts, were analysed to obtain a statistically meaningful mineralogical and compositional dataset.In addition to burial compaction, three main diagenetic events influenced the reservoir quality of the sandstones. Firstly, early eodiagenesis involved kaolinitization of plagioclase grains. This began before subsequent calcite cementation, probably as a result of flushing by meteoric pore-waters. The deformation of kaolinite during later compaction resulted in the formation of pseudomatrix which further reduced porosity and permeability. Kaolinite is commonly transformed to illite at temperatures above 140°C in the presence of K-feldspar. Although K-feldspar was recorded in the samples, no illite was observed, suggesting that the Lower Carboniferous strata in the study area were not buried in excess of approximately 3.5 km.The second diagenetic phase was the precipitation of calcite cement, present either dispersed throughout the sandbodies or as concretions up to 2 m across, in both cases reducing reservoir quality. The high intergranular volumes (IGV) of calcite-cemented sandstones (ranging between 35% and 40%) suggest that cementation occurred at burial depths of <500 m. Sandstones without calcite cement have lower IGV of between 17% and 25% as a result of mechanical and chemical compaction. Stable C and O isotope analysis of the calcite cement also supports precipitation at shallow burial depths, indicating a meteoric pore-water source for the calcite. The third and final diagenetic stage was partial chloritisation of kaolinite during meso-diagenesis. The elevated temperatures required for this transformation indicate burial to a minimum depth of approximately 2.5 km, which is consistent with the compaction data.Despite these diagenetic effects, the fluvial sandstones have an average porosity of 12%, with a range from 0.5% up to 25%. Permeability measurements on four sandstone samples indicate that the development of pseudomatrix did not reduce permeability significantly.
Geological Journal, 2019
The Nubia sandstone has a high potential for water and hydrocarbons exploration in its subsurface extensions. The present work aims to study the Nubia sandstone at Gebel Duwi in the central Eastern Desert to characterize its lithostratigraphic setting, microfacies types, and its petrophysical properties. In addition, impacts of mineral composition and diagenetic history of the Nubia sandstone on its storage and deliverability capacities and on its hydraulic and electric pore fabrics will be estimated. The Nubia sandstone is subdivided into three informal members: a lower trough cross-bedded, coarse-grained with little mud braided fluvial member; a middle planar cross-bedded sandstone with ferruginous siltstone intercalations of meandered fluvial member; and an upper gypsiferous coarsening upward clastic coastal to deltaic sediments. The microfacies analysis indicated that six microfacies are present: quartz arenite, subarkose, arkose, sublitharenite, ferruginous greywacke, and ferruginous siltstone microfacies. Several types of diagenetic processes affected the Nubia sandstone; cementation, compaction, and pressure solution are the main reservoir qualityreducing factors, whereas fracturing, dissolution, and leaching out are the most important reservoir quality-enhancing factors. For the present study, helium and water porosities, air permeability, and electric resistivities (for plugs drilled in three perpendicular directions for each block sample) were measured. Petrophysically, the studied Nubia sandstone can be summed up into three petrophysical facies: Facies-1 (quartz arenite and subarkose), Facies-2 (ferruginous subarkose to greywacke), and Facies-3 (sublitharenite and arkose). The slight ferruginous and clay content of the second facies has slightly increased the grain density values and reduced its storage capacity and deliverability. Porosity (∅ He) values which are measured by the helium injection for the first and third facies samples are good to excellent (14.6 ≤ ∅ He ≤ 28.9%), whereas their permeability (k) values are good to excellent (106 ≤ k ≤ 2231 md). The second facies is characterized by relatively less petrophysical potential than the other two facies (av. ∅ He = 19.2% and av. k = 557 md). The hydraulic pore anisotropy (λ k) of the studied three petrophysical facies is isotropic to moderately anisotropic (1.02 ≤ λ k ≤ 1.96), whereas the electric pore anisotropy (λ E) is less intensive (1.11 ≤ λ E ≤ 1.61).
Sedimentary Geology, 2016
The significance of rip-up clasts as sandstone framework grains is frequently neglected in the literature being considered as accessory components in bulk sandstone composition. However, this study highlights the great value of muddy and dolomitic rip-up clast occurrence as: (a) information source about low preservation potential from floodplain deposits and (b) key element controlling host sandstone diagenetic evolution and thus ultimate reservoir quality. High-resolution petrographic analysis on Triassic fluvial sandstones from Argana Basin (T6 and T7/T8 units) highlights the significance of different types of rip-up clasts as intrabasinal framework components of continental sediments from arid climates. On the basis of their composition and ductility, three main types are distinguished: (a) muddy rip-up clasts, (b) dolomitic muddy rip-up clasts and (c) dolomite crystalline rip-up clasts. Spatial distribution of different types is strongly facies-related according to grain size. Origin of rip-up clasts is related to erosion of coeval phreatic dolocretes, in different development stages, and associated muddy floodplain sediments. Cloudy cores with abundant inclusions and clear outer rims of dolomite crystals suggest a first replacive and a subsequent displacive growth, respectively. Dolomite crystals are almost stoichiometric. This composition is very similar to that of early sandstone dolomite cement, supporting phreatic dolocretes as dolomite origin in both situations. Sandstone diagenesis is dominated by mechanical compaction and dolomite cementation. A direct correlation exists between: (1) muddy rip-up clast abundance and early reduction of primary porosity by compaction with irreversible loss of intergranular volume (IGV); and (2) occurrence of dolomitic rip-up clasts and dolomite cement nucleation in host sandstone, occluding adjacent pores but preserving IGV. Both processes affect reservoir quality by generation of vertical and 3D fluid flow baffles and barriers that compartmentalize the reservoir. These findings may provide quantitative useful data for the better understanding of reservoir quality in analogous hydrocarbon-bearing basins such as the Bay of Fundy, Nova Scotia (Canada).
Journal of African Earth Sciences, 2019
Understanding the variability in outcrop reservoir properties may give a guide for subsurface reservoir quality prediction, more significantly for the more stable and resistant reservoir rocks as sandstones, compared to the carbonates. This study was planned to investigate the outcrop lithofacies and reservoir properties of different sandstone lithofacies of the Late Triassic Minjur Formation outcrop, and attempted to develop a relationship between these aspects in outcrop and subsurface settings. Based on field and petrographic analyses, sandstone lithofacies of Minjur Formation were categorized into five types upon their textures. These lithofacies were intensively investigated for porosity and permeability. Variography analyses were applied on composite outcrop lithofacies, and also on porosity and permeability data. A good relationship was observed between porosity and permeability, which coincides with the univariate statistics of each and confirmed by variography analysis. Combination of outcrop and subsurface reservoir properties showed an acceptable correlation between outcrop reservoir properties and data from central Saudi Arabia and Rub' Al-Khali Basin in southern and southeastern Saudi Arabia. This