Fabric-destructive Dolomite Research Articles

Dolomitization and burial history of lower triassic carbonate reservoir-rocks in the Persian Gulf (Salman offshore field)

Shallow marine carbonate sedimentation prevailed during the Early Triassic in the northern part of the Persian Gulf (Kangan Formation; offshore Iran). Associated near surface and (to a lesser extent) burial diagenesis had significant impacts on the evolution of reservoir properties of the Kangan Formation. In some Iranian offshore fields such as Salman, pervasive, early dolomitization of the Kangan carbonate rocks is believed to have played an important role in improving the reservoir characteristics of this unit. Detailed petrographic and geochemical analyses indicate that such dolomitization could be explained by means of the sabkha/seepage-reflux model. Furthermore, three types of dolomites, found in Kangan rocks in the Salman Field, have been described: (1) fabric-retentive, (2) fabric-destructive, and (3) cement. Fabric-retentive dolomites are the most frequent commonly found in the studied field, with respect to the other types. Oxygen and carbon-stable isotopic analyses confirm that the fabric-retentive dolomites and associated anhydrites formed at surface and near-surface conditions, whereas the fabric-destructive dolomite formed at temperatures above 50 °C. The burial history of the Salman Field is divided into three major phases (tectonic quiescence and low subsidence rates, and two active phases with high subsidence rates). The fabric-retentive dolomites are mostly related to the first phase, while the fabric-destructive and dolomite cements are related to the second and third phases including shallow and deep burial, respectively.

Multiple Dolomitization Episodes In Deep-Water Limestones of the Triassic Lagonegro Basin (Southern Italy): From Early Reflux To Tectonically Driven Fluid Flow

Abstract Massive dolomitization does not commonly occur in deep-water carbonates. However, this regional study on outcrops and subsurface samples of Triassic Lagonegro basinal carbonates from the southern Apennines fold and thrust belt (Italy) shows that the evolution of the Lagonegro basin was punctuated by two events of massive dolomitization. These processes resulted in two distinct dolomite types, displaying different petrographic and geochemical features. A medium-crystalline and fabric-preservative dolomite in the northern area (Molise, Campagna, and San Fele–Mount Pierno) contrasts with a medium- to coarse-crystalline and fabric-destructive dolomite in the southern area (from Pignola to the high Val d'Agri). Geochemical data (increase in δ18O with respect to typical Late Triassic seawater values, high Sr and low Mn and Fe content, Sr isotopes consistent with Late Triassic seawater) indicate that the dolomite in the northern area is the result of an early replacement caused by a marine and possibly oxidizing fluid during shallow burial. Dolomitizing fluids likely were expelled through reflux from the Norian Apenninic carbonate platform towards the proximal Lagonegro basin. This reflux was promoted by the Norian peculiar paleoceanographic and climatic conditions in the Tethyan region, and affected only the proximal basinal facies. Dolomites in the southern area display a decrease in δ18O compared to Late Triassic seawater values and show lower Sr, higher Mn and Fe contents, and higher Sr isotope ratios compared to the early dolomites of the northern area. Fluid-inclusion microthermometry reveals homogenization temperatures between 70 and 120°C and salinities of 1.9 to 6.4 wt % NaCl eq. for saddle dolomites. These geochemical signatures indicate that the southern area was affected by late diagenetic burial dolomitization, interpreted to be the result of large scale tectonically driven fluid flow during the second compressional phase characterizing the southern Apennines fold and thrust belt emplacement. The dolomite distribution across the Lagonegro basin (northern and southern) is possibly controlled by the paleogeographic distribution of the facies (proximal vs. distal) as well as by their proximity to thrust faults. The Lagonegro case study demonstrates that basinal successions may undergo multiple dolomitization episodes in a manner similar to carbonate platform successions, from early shallow to deep-burial diagenesis. One of the relevant implications is that in areas of restricted oceanographic circulation, during periods of aridity, reflux dolomitization may extend beyond the platform margin, into deep-water carbonates, having the potential to alter their reservoir quality.

Megafauna—Late Pleistocene faunal extinctions in Australia and the possibilities of a human involvement

Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One of that is microbial boundstone (fabric type 1)—characterised by clusters of dolomite microcrystals which display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is different in the two sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide size-range from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not form via mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by medium and coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).

Early dolomitization of platform carbonates and the preservation of magnetic polarity

Results from a combination of techniques are presented to evaluate the nature of magnetization in shallow‐water platform carbonates which have undergone recrystallization during early calcification and dolomitization. Magnetic grain separates, coercivity spectra, modified Lowrie‐Fuller tests, magnetization efficiency, and magnetostratigraphic constraints indicate that the ultrafine‐grained magnetite is preserved during early burial geochemical regimes, inversion from aragonite/high‐magnesium calcite to low‐magnesium calcite, and even pervasive dolomitization. These single‐domain crystals are thought to occur as interacting multigrain clusters, some of which may exceed 1 μm in diameter. These large clusters may help prohibit magnetic reorientation during diagenesis. Furthermore, during both fabric preserving and fabric destructive dolomitization, the ultrafine‐scale replacement process restricts reorientation of the clusters, thus preserving depositional or early postdeposition magnetic orientation. This early dolomitization (matrix stabilization) may even help protect and extend the subsurface lifespan of the original polarity.