Fault-controlled dolomitization has significant impact on reservoir properties in subsurface reservoirs. Origin, geometry, and internal reservoir characteristics of such dolomite bodies constrain adequate reservoir modeling and production. Fault-related dolomites in Albian platform carbonates (Basque-Cantabrian Basin, Spain) provide a detailed analog for subsurface reservoirs.Early diagenesis includes marine cements, exposure and phreatic freshwater cements. Dolomite bodies (<900 m thick) are developed along fault-strike; km-scale branches extend from the main bodies parallel to bedding. Dolomitization is pervasive close to faults, but it follows more permeable trends at distance to the fault. Xenotopic, sucrosic and saddle dolomite occur as replacement and cement. A first generation of calcite is associated with dolomite. Dolomitization was followed by stylolites and telogenetic calcite cement.Petrography, isotopes, and fluid inclusions indicate polyphase hydrothermal dolomitization from basinal fluids moving up deep-rooted extensional-transtensional faults. Dolomitization most likely occurred in the Latest Albian ? Early Turonian at shallow depths and max. burial temperatures of ~80�C. Minimal dolomite precipitation temperatures were 75�C-240�C; salinities were up to 22wt.% NaClequiv.Outcrops reflect spatial gradients of decreasing temperature from deeper to higher stratigraphic levels of dolomitization, and away from a fault. Dolomitization occurred from three fluid pulses, which possibly derived from a single parent fluid. Salinity differences between pulses may reflect variable water-rock ratios during leaching of Triassic evaporites. Little thermal interaction with the host rock occurred during fluid movement, consistent with rapid fluid expulsion. Between pulses, the fault conduit was cemented by dolomite and calcite. Calcite likely precipitated from the hydrothermal fluids at lower temperature than dolomite, as a result of Mg exhaustion and/or a drop in fluid temperature at constant Ca/Mg.The diagenetic model provides some predictive rules for fault-related dolomites in subsurface reservoirs. Body geometry can be tied to its position relative to feeder faults. Hydraulic brecciation, dolomite and calcite cementation decrease reservoir properties in immediate vicinity to the faults. At some distance to the faults, dolomitization remains beneficial for reservoir properties through the creation of vuggy, biomoldic and intercrystalline pore networks.
Additional findings of fusulinids and corals from Permian rocks of Western Karakorum and Eastern Hindu Kush are reported. The Lashkargaz Fm. of Western Karakorum (Sakmarian - Kubergandian in age), is redefined in its uppermost part. A new unit of sucrosic dolostone, the Ini Sar Fm., of early Murgabian age in its basal part, is established for a level previously attributed to the Ailak Fm. The Ailak Fm. is confirmed to be Late Permian in its lower part. A rich fusulinid fauna of early Kubergandian age was found in a carbonate nappe previously unidentified, along the Pakistan-Afghanistan border in the Eastern Hindu Kush.
The development and the growth of stromatolites/thrombolites and the relationships with their environments are keys to understand much of the carbonate systems, particularly those of Proterozoic age, encountered in Mauritania, Brazil, Namibia or Siberia.The architecture of the stromatolite types developed in the Members Point Peak and San Saba of the Cambrian Wilbern Formation along the Llano River (Texas) is comparable to those encountered in the field in the Atar Group (Mauritania). Similarities in morphology (size, shape) and structure (pillars and walls), and in microbialite distribution indicate a complex sedimentary system. Regional studies and studies of sedimentary environments in the Llano river area show that detrital input and the ramp setting influenced distribution and spatial evolution of microbialites in the Atar Group. These results are further confirmed by the analysis of outcrops of the Vazante Group (Sao Francisco, Brazil). There, stromatolites of the Conophyton type (C. Metula) show deformation or breaks related to strong hydrodynamism, similar to observations in Shark Bay (Western Australia). The main axes of the stromatolitic domes are parallel to the wave directions or locally parallel to the main current directions.Microbialite distribution and dynamic controls are key parameters to evaluate the potential reservoirs in those specific environments.
El origen de la dolomita cebra o bandeada ha sido historicamente discutido. Su presencia en distintos contextos, normalmente asociadas a mineralizaciones de tipo MVT, ha llevado a asociarla mayoritariamente a dichos procesos. Los afloramientos de dolomita cebra estudiados en la playa de San Martin (Santona)
permiten caracterizar su formacion a partir de multiples eventos de fracturacion-mineralizacion ligados a la circulacion de fluidos hidrotermales dolomitizantes. Dichos fluidos migrarian desde zonas de cuenca mas subsidentes hasta las plataformas carbonatadas, dispuestas en bloques tectonicos relativamente
elevados, a traves de la red de fracturas derivada de la intensa actividad tectonica sinsedimentaria que caracterizo el Abiense de la Cuenca Vasco-Cantabrica.
Abstract In Asón Valley hydrothermal dolomite area (Basque–Cantabrian Basin, northern Spain), an overlapping stepover area between two major basement faults, the Cabuerniga and Ruahermosa transtensional faults, was the location for different scales and types of extensive fractures. This fracture mesh affected the Albian Ranero limestone and was formed in a dilational jog, a regional fluid throughflow area. It acted as pathway for overpressured fluids that controlled the dolomite mineralization in the Ranero massif area. The study of synchronous structural features, fluid flow channelling and dolomitization processes indicated their tectonic control. The fluid circulated and concentrated preferentially in more fractured areas with increased permeability, such as extensional chimneys, creating dolomite bodies. Repeated extensive tectonic activity enhanced fracture porosity, promoting overpressured fluid migration and cyclical dolomitization events. The studied fracture pattern suggests the presence of sinistral transtension during the formation of extensional joint-sets that channellized the mineralizing fluid-flow. A gradation in structural features and dolomite facies and textures is visible from the main Pozalagua fault dolomite body to peripheral dolomite bodies as the Breccia body and Ranero megajoints. These differences could reflect a proximal–distal trend from the main fluid-flow area along the Pozalagua fault to the Ranero megajoints.
Microbialite-resedimented dolostone cycles form a prominent part of slope facies for the 2.6-2.5 Ga Campbellrand carbonate platform, Transvaal Supergroup, South Africa. The cycles allow a study of microbialite-environment interaction decoupled from direct influence of relative sea-level variations, which play an important part in shallow-water settings.Microbialite types range from delicate cuspate forms (Sumner 1997) to stratiform and incipient microbialites. Resedimented facies include (a) dolarenites, (b) massive to laminated dolostones, (c) dolomite laminites, and (d) lumpy dolostones with roll-up structures. On a cm- and dm-scale, microbial and resedimented facies are arranged into symmetric or asymmetric microbialite cycles with resedimented facies commonly grading upward into microbialite facies. Cycles are transitional between a microbialite-dominated end member with almost no evidence for clastic carbonate sedimentation, and a sediment-dominated end member where microbialite relief is strongly subdued and clastic carbonate makes up most of the cycle thickness. The microbialite-dominated cycles are remarkably uniform on the platform slope. Systematic variation in cycle architecture leads to dm-scale stacked microbialite cycles, which in turn form the main building blocks of m-thick microbial units. On the scale of decameter-thick sequences, microbial units pass upwards to progressively thicker units dominated by resedimented slope carbonates.Microbialite cycles show a distinct inverse relationship between the development of microbial structures and the abundance of detrital carbonate sediment in cycles. Thus, cyclicity is interpreted as primarily controlled by variations in sediment influx. Additional, although poorly constrained factors, may include changes in community structure and lithification of microbialites via direct carbonate precipitation. Given the deep depositional environment and stratigraphic distribution of cycles, slope cyclicity seems to have been subject to a strong allocyclic control, with external factors changing simultaneously and across most of the slope. On the largest scales, sediment influx was probably controlled by sediment export off the upper slope and, to a lesser degree, the platform margin.
