Diagenetic Pressure Research Articles

Quantitative evaluation of lateral sealing of extensional fault by an integral mathematical-geological model

To evaluate the lateral sealing mechanism of extensional fault based on the pressure difference between fault and reservoir, an integral mathematical-geological model of diagenetic time on diagenetic pressure considering the influence of diagenetic time on the diagenetic pressure and diagenetic degree of fault rock has been established to quantitatively calculate the lateral sealing ability of extensional fault. By calculating the time integral of the vertical stress and horizontal in-situ stress on the fault rock and surrounding rock, the burial depth of the surrounding rock with the same clay content and diagenesis degree as the target fault rock was worked out. In combination with the statistical correlation of clay content, burial depth and displacement pressure of rock in the study area, the displacement pressure of target fault rock was calculated quantitatively. The calculated displacement pressure was compared with that of the target reservoir to quantitatively evaluate lateral sealing state and ability of the extensional fault. The method presented in this work was used to evaluate the sealing of F1, F2 and F3 faults in No.1 structure of Nanpu Sag, and the results were compared with those from fault-reservoir displacement pressure differential methods without considering the diagenetic time and simple considering the diagenetic time. It is found that the results calculated by the integral mathematical-geological model are the closest to the actual underground situation, the errors between the hydrocarbon column height predicted by this method and the actual column height were 0–8 m only, proving that this model is more feasible and credible.

Potassic alkaline granitoid magmatism in the northern margin of the Tarim Craton: First evidence of a back‐arc extensional environment

The tectonic setting of the northern Tarim Craton during the Palaeozoic is vital to our understanding of the subduction polarity of the paleo‐oceanic plates in the Tianshan Orogen and the accretion history of the south‐western Central Asian Orogenic Belt. We first identified granitoids intruding the Palaeoproterozoic metamorphic rocks in the Kurchu area in the northern Tarim Craton. The zircon LA‐ICP‐MS U–Pb dating of this rock indicates that its crystallization age is 418 Ma, highlighting a late Early Palaeozoic magmatic event. Compared with the contemporaneous granitoids in the southern margin of the Tarim Craton, the Kurchu granitoid has high K2O (6.17–7.25 wt.%) and high alkaline (Na2O + K2O = 9.48–10.56 wt.%) contents. The Rittmann index (σ) ranges from 3.53 to 5.68, and samples plot in the shoshonite series in the K2O–SiO2 diagram, indicating that the rock is a potassic‐alkaline granite (PAG). In addition, this granitoid shows high REE concentrations (264–817 ppm) with significant Eu anomalies and is depleted in Ba, Sr, Nb, Ta, Ti, and P but enriched in Zr and Hf. These geochemical characteristics and high (87Sr/86Sr)i (0.70857–0.70995) and low εNd(T) (−10.67 to −10.24) values of the Kurchu PAG indicate that this rock was derived from the partial melting of the crust. The diagenetic conditions of the Kurchu PAG are high temperatures, as recorded by higher zircon saturation temperatures (855–919°C), low diagenetic pressure, a less shallow emplacement depth recorded by lower Sr (<200 ppm) concentrations, and a low oxygen fugacity recorded by lower Eu* (0.17–0.89), higher Zr (>355 ppm) and Hf (>9.73 ppm) concentrations, and trace element contents of zircons. In various discrimination diagrams, all samples consistently plot in rift‐related areas. These geochemical and diagenetic features suggest that this magmatic event probably occurred in a back‐arc environment. Therefore, there is back‐arc basin occurring in the northern Tarim Craton in the Early Palaeozoic, which provides strong evidence that this region was an active margin probably much earlier than Silurian, necessitating the southward subduction of the South Tianshan Ocean.

REVEALING THE GENESIS OF LIMESTONE-MARL ALTERNATIONS: A TAPHONOMIC APPROACH

Limestone-marl alternations (LMA) are rhythmical successions of carbonate-rich sedimentary rocks. They are often assumed to record cyclic sedimentation linked to Milankovitch cycles. In spite of the importance of LMA for a range of questions in geosciences, it is not unequivocally understood how they originate. The two models explaining their origin both assume carbonate redistribution, either by late diagenetic pressure dissolution amplifying primary depositional differences, or through early diagenetic aragonite dissolution and reprecipitation as calcite, creating LMA even in the absence of primary differences. The latter model is known as differential diagenesis. As both models can imply different interpretations of paleoenvironmental conditions, the identification of the generating process is essential. This study addresses the question how to distinguish the generating process through statistical comparison of taphonomic characteristics of marls and limestones in thin sections by: (1) measuring the relative abundance of originally aragonitic and calcitic components in the fossil assemblages, and (2) by analysis of their orientations. Based on four sets of thin sections from different paleoenvironments from the Upper Ordovician to the Permian, the model of late diagenetic pressure-induced carbonate redistribution is ruled out. The results point towards early diagenetic aragonite dissolution and reprecipitation as calcite as the main diagenetic process generating LMA. Furthermore, the influence of primary sedimentary differences is demonstrated. This approach offers a tool to gauge conditions during sedimentation and a way to assess the systematically poorer preservation of aragonitic components in marine deposystems (aragonite bias) quantitatively.

Landmark‐based shape analysis of the archaic Homo calvarium from Ceprano (Italy)

The Ceprano calvarium represents one of the most important sources of information about both the dynamics of the earliest hominid dispersal toward Europe and the evolution of the genus Homo in the early-to-middle Pleistocene. In this paper, the midsagittal vault profile and the 3D frontal bone morphology of Ceprano are investigated comparatively, using landmark coordinates and Procrustes superimposition. In fact, despite the fact that the skull appears partially distorted by diagenetic pressures (thus precluding a comprehensive landmark-based analysis), some aspects of the overall morphology are suitable for consideration in terms of geometric morphometrics. The midsagittal profile shows an archaic shape, comparable with the H. ergaster/erectus range of variation because of the fronto-parietal flattening, the development of the supraorbital and nuchal structures, and the occurrence of a slightly larger occipital bone. By contrast, the frontal bone displays a derived 3D shape that, mostly because of the widening of the frontal squama, appears comparable with the Afro-European variation of the Middle Pleistocene (i.e., H. heidelbergensis/rhodesiensis). Taking into account the unique morphological pattern displayed by Ceprano, its role as a link between early Homo and the Middle Pleistocene populations of Europe and Africa is not falsified. Thus, when aspects of the Ceprano's morphology are described within the analytical framework provided by geometric morphometrics, the relationships between Ceprano and the subsequent Afro-European fossil record are emphasized, suggesting the occurrence of an ancestral stock of H. heidelbergensis/rhodesiensis that is properly represented by the Italian specimen.

Diagenetic Pressure Seal Formation in the Deep Tuscaloosa Trend, Gulf Coast: I. Textural, Compactional, and Chemical Constraints

Diagenetic Pressure Seal Formation in the Deep Tuscaloosa Trend, Gulf Coast: I. Textural, Compactional, and Chemical Constraints

Stoichiometric saturation tests of NaCl1− xBrx and KCl1− xBrx

Stoichiometric saturation is examined as a possible control on Br contents of halite and sylvite during precipitation from binary salt solutions of NaCl-NaBr and KCl-KBr, respectively. Experimental data at 25°C, assumed to represent Stoichiometric saturation, were used to predict mole fractions of NaBr in halite and KBr in sylvite in thermodynamic equilibrium with fluids as a function of aqueous activity ratios of Br −:Cl −. The predictions are based on the additional assumption that the aqueous activity product of the major salt component in the precipitated salt was independent of the trace Br content in the salt lattice. The extension of the predictions to diagenetic pressures and temperatures is discussed. The predicted equilibrium Br content of halite at initial halite saturation of evaporating seawater is in close agreement with that computed from the distribution coefficient of Lutz (1975), measured in slowgrowth single crystal experiments. Fluid recrystallization of halite and sylvite at near-surface temperatures is predicted to generally deplete the Br contents in the solids. Bulk Br contents in halite in cap-rock of Gulf Coast salt domes generally agree with those predicted by the recrystallization of halite in the presence of evaporative-concentrated seawater. At a constant solution composition, increasing temperature results in increasing the equilibrium Br content of halite, making less efficient Br depletion in halite by recrystallization.

Multiple Parallel Microstylolites and Early Diagenetic Pressure Solution in Chalk: ABSTRACT

Pressure-solution phenomena, including both early-stage microstylolites and late-stage macrostylolites, are locally common in chalk. It is now well known that Upper Cretaceous chalks of northwestern Europe exhibit a wide range of stylolitic development in association with nodular fabrics and hardgrounds, generally in Thalassinoides-rich facies. It is not widely recognized, however, that virtually uncemented chalk commonly exhibits extensive microstylolitization of remarkably unique character. Multiple parallel horizontal microstylolites are well-developed in homogeneously white non-nodular pure-calcitic chalks of Denmark and elsewhere. They can be seen, however, only when samples are treated with contrast-enhancing methods, such as the application of light oil, to increas the visibility of small-scale primary and secondary structures in the sediment. The microstylolites are nearly planar and some are slightly wavy, but almost none are zigzag or sharply spiked. They occur in fairly evenly spaced sets that render a finely laminated appearance to the rock upon close inspection. Typically, each dark lamina is only about 10-50 µm thick, and they are spaced approximately 100-500 µm apart. The lamination planes obviously are diagenetic and not primary, because they are imprinted on top of a totally bioturbated ichnofabric; the tiny dissolution seams cut straight through some burrows and are diverted around others. The relationship of these planar microstylolites to healed hairline microfractures and to various trace fossils, especially Zoophycos, suggests that the microstylolites are very early diagenetic in origin, postdating he burrows but predating or coinciding with the microfractures. End_of_Article - Last_Page 252------------