Carbonate-Evaporite Sequences of the Late Jurassic, Southern and Southwestern Arabian Gulf
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The carbonate-evaporite sequences of the Upper Jurassic Arab and overlying Hith formations in the southern and southwestern Arabian Gulf form many supergiant and giant fields that produce from the Arab Formation and are excellent examples of a classic reservoir/seal relationship. The present-day sabkha depositional setting that extends along most of the southern and southwestern coasts of the Arabian Gulf provides an analog to these Upper Jurassic sedimentary rocks. In fact, sabkha-related diagenesis of original grain-supported sediments in the Arab and Hith formations has resulted in five distinct lithofacies that characterize the reservoir/seal relationship: (1) oolitic/peloidal grainstone, (2) dolomitic grainstone, (3) dolomitic mudstone, (4) dolomitized grainstone, an (5) massive anhydrite. Interparticle porosity in grainstones and dolomitic grainstones and intercrystalline porosity in dolomitized rocks provide the highest porosity in the study area. These sediments accumulated in four types of depositional settings: (1) supratidal sabkhas, (2) intertidal mud flats and stromatolitic flats, (3) shallow subtidal lagoons, and (4) shallow open-marine shelves. The diagenetic history of the Arab and Hith formations in the southern and southwestern Arabian Gulf suggests that the anhydrite and much of the dolomitization are a result of penecontemporaneous sabkha diagenesis. The character and timing of the paragenetic events are responsible for the excellent porosity of the Arab Formation and the lack of porosity in the massive anhydrites of the Hith, which together result in the prolific hydrocarbon sequences of these formations.Mediterranean Basin
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A review of evaporation rates implies that evaporite models requiring annual evaporation of over 500 cm of seawater may be rejected as imposslble. An examination of laminated sulfates shows that they cannot be annual because that would require annual evaporation many tlmes the physically possible rate. The recognition that Jehresringe, or laminated evapori tes, are non-annual leads to the rejection of Schmalz's (1966) formula that indicates that evaporites rwt be laid down in deep basins. Landes' (1960) Theory of Supersal ine Invading Seas must also fail, for it ultimately requires the,same unrealistic evaporation rates, even though it was formulated . to explain the abnormal proportions of mineral species in evaporite section. Two analyses of the King (1947) and Brlggs (1957) models for the deep-water orlgln of evaporltes show that they will not work; first, if they are placed in hydrostatic equilibrium, for then the water surface will not be horizontal. If the water in them is allowed to go to horizontallty , then the water will not be exposed to evapor- ation long enough to create evaporite-precip itating salinities. Secondly, an analysis of diffusion in these models shows that d iffusion is far more rapid than evaporative concentration, so that a deep water basin could neither de- velop evaporite-producing salinities nor could it maintain them even if it were filled originally with brine of suf- ficient salinity derived from some other source. in view of the inadequacies of all proposed models, I conclude that no deposition of evaporites is possible in deep water and that all evaporites must be products of shallow-water conditions.
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The hydrological regime of the gulf is such that evaporating surface water passes toward the coasts, sinks, and escapes from the gulf by counterflow at lower levels, the highest salinities, both of surface and bottom water, being in coastal areas. This regime, similar to that which must have existed in ancient evaporite basins, is used as a model by which to interpret probable circumstances of evaporite deposition. It is thence argued that in a marine basin in an arid region, introduction of a bar at the entry to the basin or simple overall shallowing without the introduction of a bar may produce similar results in respect to evaporite deposition and distribution. In either case higher grade evaporites will deposit in the more remote coastal areas of a basin contemporaneously with progressively lower grade evaporites toward the point of entry of 9freshening9 oceanic water.
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The new approach on depositional conditions of the Messinian evaporites in Zakynthos Island indicates that the evaporites in the Kalamaki and Ag. Sostis areas were redeposited during the Early Pliocene. They accumulated either as turbiditic evaporites or as slumped blocks, as a response to Kalamaki thrust activity. Thrust activity developed a narrow and restricted Kalamaki foreland basin with the uplifted orogenic wedge consisting of Messinian evaporites. These evaporites eroded and redeposited in the foreland basin as submarine fans with turbiditic currents or slumped blocks (olistholiths) that consist of Messinian evaporites. These conditions occurred just before the inundation of the Mediterranean, during or prior to the Early Pliocene (Zanclean). Following the re-sedimentation of the Messinian evaporites, the inundation of the Mediterranean produced the “Lago Mare” fine-grained sediments that rest unconformably over the resedimented evaporites. The “Trubi” limestones were deposited later. It is critical to understand the origin of the “Messinian” Evaporites because they can serve as an effective seal rock for the oil and gas industry. It is thus important to evaluate their thickness and distribution into the SE Mediterranean Sea.
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This paper, the first of three reviews on the evaporite‐base‐metal association, defines the characteristic features of evaporites in surface and subsurface settings. An evaporite is a rock that was originally precipitated from a saturated surface or near‐surface brine in hydrological systems driven by solar evaporation. Evaporite minerals, especially the sulfates such as anhydrite and gypsum, are commonly found near base‐metal deposits. Primary evaporites are defined as those salts formed directly via solar evaporation of hypersaline waters at the earth's surface. They include beds of evaporitic carbonates (laminites, pisolites, tepees, stromatolites and other organic‐rich sediment), bottom nucleated salts (e.g. chevron halite and swallow‐tail gypsum crusts), and mechanically reworked salts (such as rafts, cumulates, cross‐bedded gypsarenites, turbidites, gypsolites and halolites). Secondary evaporites encompass the diagenetically altered evaporite salts, such as sabkha anhydrites, syndepositional halite and gypsum karst, anhydritic gypsum ghosts, and more enigmatic burial associations such as mosaic halite and limpid dolomite, and nodular anhydrite formed during deep burial. The latter group, the burial salts, were precipitated under the higher temperatures of burial and form subsurface cements and replacements often in a non‐evaporite matrix. Typically they formed from subsurface brines derived by dissolution of an adjacent evaporitic bed. Because of their proximity to 'true' evaporite beds, most authors consider them a form of 'true' evaporite. Under the classification of this paper they are a burial form of secondary evaporites. Tertiary evaporites form in the subsurface from saturated brines created by partial bed dissolution during re‐entry into the zone of active phreatic circulation. The process is often driven by basin uplift and erosion. They include fibrous halite and gypsum often in shale hosts, as well as alabastrine gypsum and porphyroblastic gypsum crystals in an anhydritic host. In addition to these 'true' evaporites, there is another group of salts composed of CaSO4 or halite. These are the hydrothermal salts. Hydrothermal salts, especially hydrothermal anhydrite, form by the subsurface cooling or mixing of CaSO4‐saturated hydrothermal waters or by the ejection of hot hydrothermal water into a standing body of seawater or brine. Hydrothermal salts are poorly studied but often intimately intermixed with sulfides in areas of base‐metal accumulations such as the Kuroko ores in Japan or the exhalative brine deeps in the Red Sea. In ancient sediments and metasediments, especially in hydrothermally influenced active rifts and compressional belts, the distinction of this group of salts from 'true' evaporites is difficult and at times impossible. After a discussion of hydrologies and 'the evaporite that was' in the second review, modes and associations of the hydrothermal salts will be discussed more fully in the third review.
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Anhydrite
Sabkha
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