| 1. |
- Khalifa, Muftah A., et al.
(författare)
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QUARTZ AND Fe-DOLOMITE CEMENTS RECORD SHIFTS IN FORMATION-WATER CHEMISTRY AND HYDROCARBON MIGRATION IN DEVONIAN SHOREFACE SANDSTONES, GHADAMIS BASIN, LIBYA
- 2018
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Ingår i: Journal of Sedimentary Research. - : SEPM-SOC SEDIMENTARY GEOLOGY. - 1527-1404 .- 1938-3681. ; 88:1, s. 38-57
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Tidskriftsartikel (refereegranskat)abstract
- Cementation by quartz overgrowths and subsequently by carbonates is a very common global paragenetic phenomenon in sandstones of many basins that is poorly constrained in the literature. Integrated petrography, electron microprobe, stable isotopes, fluid inclusion microthermometry, and Raman spectroscopy have helped unravel the diagenetic conditions, fluid flow, and hydrocarbon migration during the development of quartz overgrowths and later Fe-dolomite cement in the Devonian shoreface sandstones (oil and gas reservoirs), Ghadamis Basin, NW Libya. The high homogenization temperatures of Fe-dolomite (Th = 119 to 140 degrees C) together with its low delta(18) OVPDB values (-17.6% to -13.2%) and saddle-crystal morphology suggest the flux of hot basinal brines. The fluid-inclusion microthermometry shows a shift from NaCl-dominated brines during quartz cementation (16.0 to 17.3 wt.% NaCl eq.) to NaCl and CaCl2-dominated brines during Fe-dolomite cementation (15.6 to 18.4 wt.% NaCl eq.). The latter brines probably circulated through overlying Mesozoic carbonate and evaporite strata before descending deep into the basin. The similarity in T-h ranges for Fe-dolomite and quartz overgrowths, despite the shift in formationwater chemistry suggests precipitation under similar geothermal conditions. The presence of oil-filled inclusions in quartz overgrowths suggests that cementation occurred during oil migration, whereas the presence of methane in inclusions in the Fe-dolomite cement suggests precipitation during gas migration. This study shows that fluid inclusion microthermometry and Raman spectroscopy of the widespread paragenetic sequence of quartz overgrowths followed by carbonate cement are crucial to unravel changes in formation-water chemistry and fluid (including hydrocarbon) migration in sedimentary basins.
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| 2. |
- Mansurbeg, Howri, et al.
(författare)
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Hydrothermal dolomitization of the Bekhme formation (Upper Cretaceous), Zagros Basin, Kurdistan Region of Iraq : Record of oil migration and degradation
- 2016
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738 .- 1879-0968. ; 341, s. 147-162
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Tidskriftsartikel (refereegranskat)abstract
- The common presence of oil seepages in dolostones is widespread in Cretaceous carbonate successions of the Kurdistan Region of Iraq. This integrated field, petrographic, chemical, stable C, O and Sr isotopes, and fluid inclusion study aims to link dolomitization to the origin and geochemical evolution of fluids and oil migration in the Upper Cretaceous Bekhme carbonates. Flux of hot basinal (hydrothermal) brines, which is suggested to have occurred during the Zagros Orogeny, resulted in dolomitization and cementation of vugs and fractures by coarse crystalline saddle dolomite, equant calcite and anhydrite. The saddle dolomite and host dolostones have similar stable isotopic composition and formed prior to oil migration from hot (81-115 degrees C) basinal NaCl-MgCl2-H2O brines with salinities of 18-22 wt.% NaCl eq. The equant calcite cement, which surrounds and hence postdates saddle dolomite, has precipitated during oil migration from cooler (60-110 degrees C) NaCl-CaCl2-H2O brines (14-18 wt.% NaCl eq). The yellowish fluorescence color of oil inclusions in the equant calcite indicates that the oil had API gravity of 15-25 degrees composition, which is lighter than present-day oil in the reservoirs (API of 10-17 degrees). This difference in oil composition is attributed to oil degradation by the flux of meteoric water, which is evidenced by the low delta C-13 values (-8.5 parts per thousand to -3.9 parts per thousand VPDB) as well as by nil salinity and low temperature in fluid inclusions of late columnar calcite cement. This study demonstrates that linking fluid flux history and related diagenesis to the tectonic evolution of the basin provides important clues to the timing of oil migration, degradation and reservoir evolution.
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| 3. |
- El-Khatri, F., et al.
(författare)
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Diagenetic Alterations and Reservoir Quality Evolution of Lower Cretaceous Fluvial Sandstones : Nubian Formation, Sirt Basin, North-Central Libya
- 2015
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Ingår i: Journal of Petroleum Geology. - : Wiley. - 0141-6421 .- 1747-5457. ; 38:2, s. 217-239
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Tidskriftsartikel (refereegranskat)abstract
- Lower Cretaceous meandering and braided fluvial sandstones of the Nubian Formation form some of the most important subsurface reservoir rocks in the Sirt Basin, north-central Libya. Mineralogical, petrographical and geochemical analyses of sandstone samples from well BB6-59, Sarir oilfield, indicate that the meandering fluvial sandstones are fine- to very fine-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 degrees C; <2 km) and mesodiagenesis (ca. >70 degrees 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 intraclasts 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. The development of only limited quartz overgrowths in the braided fluvial sandstones, in which quartz grains are coated by thick illite layers, retained high porosity and permeability (12-23% and 30-600 mD). By contrast, meandering fluvial sandstones underwent porosity loss as a result of quartz overgrowth development on quartz grains which lack or have thin and incomplete grain-coating illite (2-15% and 0-0.1 mD). Further loss of porosity in the meandering fluvial sandstones occurred as a result of chemical compaction (pressure dissolution) induced by the occurrence of micas along grains contacts. Other diagenetic alterations, such as the growth of pyrite, siderite, dolomite/ankerite and albitization, had little impact on reservoir quality. The albitization of feldspars may have had minor positive influence on reservoir quality through the creation of intercrystalline micro-porosity between albite crystals. The results of this study show that diagenetic modifications of the braided and meandering fluvial sandstones in the Nubian Formation, and resulting changes in reservoir quality, are closely linked to depositional porosity and permeability. They are also linked to the thickness of grain-coating infiltrated clays, and to variations in detrital composition, particularly the amounts of mud intraclasts, feldspars and mica grains as well as climatic conditions.
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| 4. |
- Khalifa, Muftah Ahmid, et al.
(författare)
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Impact of depositional facies on the distribution of diagenetic alterations in the Devonian shoreface sandstone reservoirs, Southern Ghadamis Basin, Libya
- 2015
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738 .- 1879-0968. ; 329, s. 62-80
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Tidskriftsartikel (refereegranskat)abstract
- The middle Devonian, shoreface quartz arenites (present-day burial depths 2833-2786 m) are important oil and gas reservoirs in the Ghadamis Basin, western Libya. This integrated petrographic and geochemical study aims to unravel the impact of depositional fades on distribution of diagenetic alterations and, consequently, related reservoir quality and heterogeneity of the sandstones. Eogenetic alterations include the formation of kaolinite, pseudomatrix, and pyrite. The mesogenetic alterations include cementation by quartz overgrowths, Fedolomite/ankerite, and illite, transformation of kaolinite to dickite, illitization of smectite, intergranular quartz dissolution, and stylolitization, and albitization of feldspar. The higher energy of deposition of the coarser-grained upper shoreface sandstones combined with less extensive chemical compaction and smaller amounts of quartz overgrowths account for their better primary reservoir quality compared to the finer-grained, middle-lower shoreface sandstones. The formation of kaolin in the upper and middle shoreface sandstones is attributed to a greater flux of meteoric water. More abundant quartz overgrowths in the middle and lower shoreface is attributed to a greater extent of stylolitization, which was promoted by more abundant illitic clays. This study demonstrated that linking the distribution of diagenetic alterations to depositional facies of shoreface sandstones leads to a better understanding of the impact of these alterations on the spatial and temporal variation in quality and heterogeneity of the reservoirs.
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| 5. |
- Rodrigues, Luiz Frederico, et al.
(författare)
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The influence of methane fluxes on the sulfate/methane interface in sediments from the Rio Grande Cone Gas Hydrate Province, southern Brazil : A influência dos fluxos de metano na interface sulfato/metano em sedimentos de hidrato de gás do Cone do Rio Grande, Sul do Brasil
- 2017
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Ingår i: Brazilian Journal of Geology. - : FapUNIFESP (SciELO). - 2317-4692 .- 2317-4889. ; 47:3, s. 369-381
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Tidskriftsartikel (refereegranskat)abstract
- Much research has been published regarding the relation between major gas hydrate accumulations and the global carbon cycle. In this context, the determination of the sulfate/methane interface (SMI) depth is of primary importance in order to understand the dynamics of methane flux in the shallow section. This paper identifies the depth of the SMI in sediments based on sulfate and methane concentration profiles in cores recovered in the Rio Grande Cone Gas Hydrate Province, Pelotas Basin, southern Brazil. The shape of methane and sulfate concentration profiles in the sediments can be linked to the local methane flux rate as follows: (i) near linear, high upward-diffusing methane flux coupled with high sulfate diffusion from seawater; (ii) irregular, variable methane flux rates; and (iii) kink-type profile, which is indicative of variable rather than strictly high upward methane flux. The areas in which a high methane flux was identified are spatially associated with gas chimneys in sediments within pockmarks, whereas profiles with low methane flux are present in adjacent areas. These chimneys appear as acoustic blankings in seismic records and can therefore be mapped in subsurface. The wavy-like seismic reflection following the SMI coincides with the occurrence of authigenic carbonate nodules and concretions. In addition, high methane fluxes and the occurrence of concretions and nodules carbonates were correlated by stratigraphic position of the concretions bearing intervals and sulfate profiles
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| 6. |
- Vieira, Marcela Marques, et al.
(författare)
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Origin of holocene beachrock cements in northeastern Brazil : Evidence from carbon and oxygen isotopes
- 2017
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Ingår i: Journal of South American Earth Sciences. - : Elsevier BV. - 0895-9811 .- 1873-0647. ; 79, s. 401-408
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Tidskriftsartikel (refereegranskat)abstract
- This study has utilized carbon and oxygen isotopic data to unravel the origin of Holocene beachrock cements of Rio Grande do Norte State coast, northeastern Brazil. The cements are exclusively constituted of Mg-calcite, with isopachous prismatic rim, equant spar, cryptocrystalline coating or pore-filling, pseudo-peloidal, radial and scalenohedral habits. The delta O-18(VPDB) values of most (93%) of the samples range from -1.8 parts per thousand to +0.5 parts per thousand, which are compatible with precipitation from marine waters. The delta C-13(VPDB) values of these cements are +1.7 to +3.6 parts per thousand (average + 3.05 parts per thousand) suggesting derivation from marine dissolved carbon. In a few cases, the Mg-calcite cement shows low delta C-13(VPDB) (-7.3% o and -7.8 parts per thousand) and delta O-18(VPDB) (-4.4 parts per thousand and -4.3) values, which may indicate precipitation or recrystallization of marine cements under the influence of meteoric waters.
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