No abstract

Diagenesis, provenance and reservoir quality of Triassic T AGI sandstones from Ourhoud field, Berkine (Ghadames) Basin, Algeria Also, this change has contributed to the significantly different diagenetic paths followed by the Lower TAG! quartz arenites and the MiddleUpper 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.

No abstract

The Jurassic (hemi)pelagic continental margin deposits drilled at Hole 547B, off the Moroccan coast, reveal striking Tethyan affinity. Analogies concern not only types and gross vertical evolution of facies, but also composition and textures of the fine sediment and the pattern of diagenetic alteration. In this context, the occurrence of the nanno-organism Schizosphaerella Deflandre and Dangeard (sometimes as a conspicuous portion of the fine-grained carbonate fraction) is of particular interest. Schizosphaerella, an incertae sedis taxon, has been widely recorded as a sediment contributor from Tethyan Jurassic deeper-water carbonate facies exposed on land. Because of its extremely long range (Hettangian to early Kimmeridgian), the genus Schizosphaerella (two species currently described, S. punctulata Deflandre and Dangeard and S. astrea Moshkovitz) is obviously not of great biostratigraphic interest. However, it is of interest in sedimentology and petrology. Specifically, Schizosphaerella was often the only component of the initial fine-grained fraction of a sediment that was able to resist diagenetic obliteration. However, alteration of the original skeletal structure did occur to various degrees. Crystal habit and mineralogy of the fundamental skeletal elements, as well as their mode of mutual arrangement in the test wall with the implied high initial porosity of the skeleton (60-70%), appear to be responsible for this outstanding resistance. Moreover, the ability to concentrate within and, in the case of the species S. punctulata, around the skeleton, large amounts of diagenetic calcite also contributed to the resistance. In both species of Schizosphaerella, occlusion of the original skeletal void space during diagenesis appears to have proceeded in an analogous manner, with an initial slight uniform syntaxial enlargement of the basic lamellar skeletal crystallites followed, upon mutual impingement, by uneven accretion of overgrowth cement in the remaining skeletal voids. However, distinctive fabrics are evident according to the different primary test wall architecture. In S. punctulata, intraskeletal cementation is usually followed by the growth of a radially structured crust of bladed to fibrous calcite around the valves. These crusts are interpreted as a product of aggrading neomorphism, associated with mineralogic stabilization of the original, presumably polyphase, sediment. Data from Hole 547B, along with inferences, drawn from the fabric relationships, suggest that the crusts formed and (inferentially) mineralogic stabilization occurred at a relatively early time in the diagenetic history in the shallow burial realm. An enhanced rate of lithification at relatively shallow burial depths and thus the chance for neomorphism to significantly influence the textural evolution of the buried sediment may be related to a lower Mg/Ca concentration ratio in the oceanic system and, hence, in marine pore waters in preLate Jurassic times.

On Leg 79 of the Deep Sea Drilling Project, a Jurassic continental margin was drilled for the first time allowing closer comparison with Jurassic continental margin deposits on land. The Jurassic sedimentary sequence recovered at Site 547 shows many analogies to Tethyan sequences, thus confirming inferences drawn from Mesozoic sequences in the Alpine-Mediterranean area. After initial rifting in the Late Triassic and earliest Liassic, the Sinemurian-Pliensbachian sediments at Site 547 record the first marine transgression in the central Atlantic with a rapid passage from a continental to a basinal and hemipelagic regime: ongoing synsedimentary faulting and the resulting steep submarine topographic gradients are suggested during this time by sediment gravity flow deposits involving both displaced shallow-water and basinal sediments. A mineralogically mixed composition, with a significant contribution of platform-derived carbonate lutum, is similarly suggested for fine-grained slope and basinal deposits. In some cases, complexly organized breccias with evidence for multiple breakage of constituent clasts and postdepositional fracturing in various successive generations place the depositional setting of the Jurassic sediments of Site 547 near or on a submarine faulted slope. Further evidence for an unstable slope environment are probable gaps that are due to nondeposition and/or submarine erosion. Gradual sinking of the margin in Middle and Late Jurassic times is reflected at Site 547 by decreasing sedimentation rates. There is a relative increase in pelagic sediments and components in gravity flow sediments with respect to redeposited shallow-water material. Unstable conditions, however, persisted throughout the Jurassic as suggested by recurrent intercalations of coarse breccias.