Figure 1 - uploaded by Howri Mansurbeg
Content may be subject to copyright.
(a) Location map of the onshore and offshore fields of Abu Dhabi. (b) Location map of the investigated wells, in the studied field. The line represents a cross section shown in Figure 2b. The color zones represent the various depositional environments of the Arab Formation.
Source publication
Contexts in source publication
Context 1
... pebble-sized, rounded to sub-rounded micritic intraclasts occur locally. 3. Dolorudstones and dolofloatstones: These dolostones are the least common being encountered in the Arab D3 of a well located to the south and in the Arab D4 of a well to the east (Figure 1b). These dolostones are rich in large moldic pores. ...
Context 2
... study is based on petrographic examination of nearly one thousand thin sections from 16 selected wells ( Figure 1b). These wells cover the entire field, including the crest and flanks of its anticlinal structure. ...
Context 3
... occur in the anhydrite cemented dolostones and cut across the anhydrite crystals, which have replaced dolomite. Thus, stylolitization has occurred subsequent to formation of coarse-crystalline dolomite and anhydrite cement (Figure 10a). The presence of stylolites in dolostones with abundant intercrystalline porosity, which has resulted from the dissolution of intercrystalline anhydrite/ gypsum cements, suggests that dissolution occurred subsequent to stylolitization. ...
Context 4
... presence of stylolites in dolostones with abundant intercrystalline porosity, which has resulted from the dissolution of intercrystalline anhydrite/ gypsum cements, suggests that dissolution occurred subsequent to stylolitization. Bitumen and late calcite cement are encountered along the stylolites (Figure 10b). ...
Context 5
... isotope analyses (Figures 11a and 11b; carbon, oxygen and strontium isotopic data for the Arab carbonates are available from the authors upon request) have provided clues to the conditions encountered during dolomitization and calcite cementation. The δ 18 O VPDB values of fine-crystalline b a dolomite in sabkha-like carbonates vary between -2.9 and -0.1‰ and the δ 13 C VPDB values between +1.5‰ to +3.5‰ ( Figure 11a). ...
Context 6
... isotope analyses (Figures 11a and 11b; carbon, oxygen and strontium isotopic data for the Arab carbonates are available from the authors upon request) have provided clues to the conditions encountered during dolomitization and calcite cementation. The δ 18 O VPDB values of fine-crystalline b a dolomite in sabkha-like carbonates vary between -2.9 and -0.1‰ and the δ 13 C VPDB values between +1.5‰ to +3.5‰ ( Figure 11a). Medium-crystalline dolomite in dolostones has δ 18 O VPDB values between -5.8‰ to -0.7‰ and δ 13 C VPDB values from about +2.1‰ to +3.7‰ (Figure 11a). ...
Context 7
... δ 18 O VPDB values of fine-crystalline b a dolomite in sabkha-like carbonates vary between -2.9 and -0.1‰ and the δ 13 C VPDB values between +1.5‰ to +3.5‰ ( Figure 11a). Medium-crystalline dolomite in dolostones has δ 18 O VPDB values between -5.8‰ to -0.7‰ and δ 13 C VPDB values from about +2.1‰ to +3.7‰ (Figure 11a). Coarse-crystalline dolomite has δ 18 O VPDB values from -6.7‰ to -4.1‰ and δ 13 C VPDB values from +1.0‰ to +3.7‰ ( Figure 11a). ...
Context 8
... dolomite in dolostones has δ 18 O VPDB values between -5.8‰ to -0.7‰ and δ 13 C VPDB values from about +2.1‰ to +3.7‰ (Figure 11a). Coarse-crystalline dolomite has δ 18 O VPDB values from -6.7‰ to -4.1‰ and δ 13 C VPDB values from +1.0‰ to +3.7‰ ( Figure 11a). ...
Context 9
... isotopic signatures in calcitic components can be divided into three main groups (Figure 11b), including: (1) syntaxial rim calcite cement (δ 18 O VPDB from -5.4‰ to -3.5‰; δ 13 C VPDB = +2.0 to +3.1‰); (2) recrystallized peloids and matrix (δ 18 O VPDB from -6.2‰ to - 3.4‰; and δ 13 C VPDB = +1.2 to +2.8‰); and (3) blocky/equant calcite (δ 18 O VPDB from -6.0‰ to -3.4‰, δ 13 C VPDB from +1.7‰ to 3.6‰). The three groups of calcite cements, despite their different formation timing within the diagenetic history of the basin, show overlapping O and C isotopic ratios; the carbon isotopic values are largely similar to those of Jurassic seawater. ...
Context 10
... of fluid inclusion petrographic and compositional analyses in calcite, dolomite, and anhydrite can be overall summarized as follows: (1) Primary and secondary petroleum fluid inclusions were found in dolomite (DC2) and late anhydrite cements (AN2 and AN3). DC2 dolomite cores contain tiny, yellow to green fluorescent oil inclusions, whereas the rims (DC3) are inclusion free or inclusion poor (Figures 13a and 13b). The presence of oil inclusions in the dolomite core is interpreted to indicate that this dolomite (DC2; Figures 13c and 13d), which is paragenetically earlier than the limpid dolomite (DC3), has been formed during an early phase of oil migration, whereas the limpid, non- fluorescent rim has precipitated during a pause in oil migration. ...
Context 11
... dolomite cores contain tiny, yellow to green fluorescent oil inclusions, whereas the rims (DC3) are inclusion free or inclusion poor (Figures 13a and 13b). The presence of oil inclusions in the dolomite core is interpreted to indicate that this dolomite (DC2; Figures 13c and 13d), which is paragenetically earlier than the limpid dolomite (DC3), has been formed during an early phase of oil migration, whereas the limpid, non- fluorescent rim has precipitated during a pause in oil migration. In addition to fluorescence caused by the presence of oil inclusions, the fluorescence of dolomite core (DC2) suggests precipitation from formation waters rich in dissolved organic acids (Neuweiler et al., 2003). ...
Context 12
... nodular sulfate (AN1) is characterized by aqueous, all liquid inclusions, but in rare cases liquid and vapor phases are encountered. The blocky, intercrystalline (AN2) contains rare oil inclusions whereas fracture- filling anhydrite/gypsum cements (AN3) often contain large oil inclusions (Figures 13e-f Results of compositional analyses have revealed three types of oil inclusions, which display a yellowish, greenish, and bluish fluorescence colors, suggesting that oil migration into the reservoir occurred during stages of progressive increase in organic matter maturation in the source rocks. The yellowish and greenish colored inclusions are indicative of a medium-to heavy oil (API 20-40°), being restricted to the inclusion-rich dolomite cores (DC2.) ...
Context 13
... homogenization temperatures (T h ) and salinities measured in the fluid inclusions for the various diagenetic phases (Figure 14) showed high T h and highly saline fluids in calcite cements in the limestones (C1, C2 and C3). The T h values range from 74.4° to 111.6°C (average 84.1°C) for C1 and C2 cements and from 80.9° to 108.5°C (average 94.5°C) for C3 cement. ...
Context 14
... the presence of two-phase inclusions in the same FIA could indicate that some of the fluid inclusions in this texturally early-diagenetic calcite were thermally re-equilibrated and leaked and re-filled during burial diagenesis. This interpretation is supported by the wide distribution of the microthermometric data (Figure 14). Therefore, the temperatures measured for C1, C2 and C3 are not reliable. ...
Context 15
... inclusion temperature and salinity data (T h = 90.4° to 122.4°C; average 105.9°C and salinity = 16.3 to 23.0 wt% NaCl eq.; average 19.9 wt%) from blocky intercrystalline and moldic pore-filling anhydrite cement in the dolograinstones, and blocky fracture-filling anhydrite in dolostones and limestones ( Figure 14) are similar to those in brines presently occurring in the reservoir (T h = 110.0°C and formation waters salinity of 20.3 wt% NaCl). ...
Context 16
... petrographic observations complemented by geochemical analyses and fluid-inclusion micro- thermometry have allowed us to: (1) construct a paragenesis and establish the formation conditions of the main diagenetic events in the limestones as well as in the dolostones ( Figure 15); the diagenetic regimes are used after Choquette and Pray (1970); (2) unravel the impact of the main diagenetic alterations on reservoir porosity and permeability; and (3) develop a conceptual model linking the distribution of diagenetic alterations and of their impact on reservoir quality to the sequence- stratigraphic framework. ...
Context 17
... in general, is still a topic of considerable controversy and debate (Warren, 2000;Machel, 2004). The close association of anhydrite/gypsum beds, nodules, and cements with dolostones of the Arab Formation suggests that dolomitization occurred by evaporative increase in salinity and density of seawater, which was promoted by arid climatic conditions (Adams and Rhodes 1960;Machel, 1986;Kaufman, 1994;Gill et al., 1995;Meyers et al., 1997;Jones and Xiao, 2005;Swart et al., 2005 from evaporation of seawater in a lagoon, which was semi-isolated from the open sea by a shoal barrier (Figure 16). The removal of Ca 2+ from seawater through the precipitation of sulfate beds, nodules, and cements in the evaporative settings of the Arab Formation, which results in increase in the Mg 2+ /Ca 2+ in porewater and lagoon brines, may have promoted dolomitization (Tucker and Wright, 1990). ...
Context 18
... 87 Sr/ 86 Sr ratio of the fine-crystalline dolomite in sabkha-like carbonates (i.e. associated with patches and/or nodules of anhydrite/gypsum) ranges from 0.70701 to 0.70729 (Figure 12a). These values are similar to those of Jurassic seawater indicating that precipitation occurred from marine pore waters with no evidence of involvement of meteoric fluids. ...
Context 19
... the fluid inclusion T h in conjunction with the range of δ 18 O composition for various generations of dolomite and the dolomite-water fractionation equation (Land, 1983), the δ 18 O V-SMOW composition of the precipitating fluid for coarse-crystalline limpid dolomite would have varied between +3.5‰ and +6.5‰; while the δ 18 O V-SMOW composition of the precipitating fluid for medium-crystalline dolomite ranges from +2.5‰ and +5.5‰ (Figure 17a). In contrast, the earliest type of dolomite shows lower T h and drastically different salinities. ...
Context 20
... salinity ca. 14-18 wt% NaCl equivalent) suggests a mesogenetic origin (Figure 14). This would mean that meteoric water flux could have occurred during uplifting caused by the obduction of Oman ophiolites (Ali and Watts, 2009). ...
Context 21
... maintainance of the undersaturation state of meteoric waters would be difficult to achieve taking into account the enormous buffering capacity of the thick carbonate successions. Using T h and δ 18 O VPDB values of C4 calcite cement, and employing the calcite-water oxygen isotope fractionation equation (Freidman and O'Neil, 1977), it is inferred that cementation occurred from pore waters with δ 18 O SMOW values of +3.5 to +6.5‰, which are typical values for basinal brines in evaporative settings (Figure 17b) (Egeberg and Aagaard, 1989) and resembles those obtained for the dolomite cements. The textural mesogenetic origin of calcite suggests that TSR is a most likely explanation. ...
Context 22
... cross plot of porosity versus permeability, with different groups based on various extents of dolomitization (replacive and cement types) in the Arab Formation carbonate rocks (Figure 18) shows that dolomitization has varied patterns of impact on reservoir quality resulting in wide ranges of porosity and permeability. The plot further reveals that there is a significant positive correlation between porosity and permeability for the whole data set. ...
Context 23
... spatial and temporal distribution of eogenetic alterations in Arab D and C members has been linked to the various systems tracts, which include the highstand systems tracts (HST), transgressive systems tracts (TST), and lowstand systems tracts (LST; or lowstand/ramp margin wedge LSW/ RMW), and key sequence-stratigraphic surfaces, including the transgressive surface (TS), maximum flooding surface (MFS), sequence boundaries (SB), parasequence boundaries (PB), and parasequence set boundaries (PSB). A conceptual model for the distribution of diagenetic alterations versus sequence stratigraphy has been developed (Figure 19). ...
Context 24
... diagenetic alterations encountered include dolomitization of mudstones, wackestones and packstones (Tucker, 1993). Dolomitization in the Arab D5-D3 occurs locally in limestone-dominated successions, whereas dolomitization is widespread throughout the Arab D2 and C, presumably denoting a trend of progressive, significant fall in the relative sea level, which would result in evaporative development of lagoonal brine (Figure 16). High frequency oscillation of relative sea level has presumably been encountered during the overall trend of fall in the relative sea level, which could account for the presence of interbedded limestones and dolostones in the Arab D2 and, particularly the Arab C ( Figure 16). ...
Context 25
... in the Arab D5-D3 occurs locally in limestone-dominated successions, whereas dolomitization is widespread throughout the Arab D2 and C, presumably denoting a trend of progressive, significant fall in the relative sea level, which would result in evaporative development of lagoonal brine (Figure 16). High frequency oscillation of relative sea level has presumably been encountered during the overall trend of fall in the relative sea level, which could account for the presence of interbedded limestones and dolostones in the Arab D2 and, particularly the Arab C ( Figure 16). ...
