Establishing an agenda for calcareous dinoflagellates (Thoracosphaeraceae, Dinophyceae) including a nomenclatural synopsis of generic names. Taxon, 57(4): 1289-1303.
Calcareous dinoflagellates are considered to be a monophyletic group of peridinoid taxa that have the potential to produce calcified exoskeletal structures during the life cycle, or that derive from such forms. Frequently, these calcareous bodies are excellently preserved in the fossil record and have received increased attention during the past three decades with regard to their use in biostratigraphy, climate and environmental reconstruction. Fossil and extant taxa have been classified in various, partly concurring, systematic concepts, using character complexes of the theca, cyst wall ultrastructure and archaeopyle/operculum morphology. The significance of such character complexes is briefly discussed in the light of molecular data that have been accumulated during the past decade. Over the years, the number of published taxonomic names has increased, partly due to nomenclatural changes. We propose that the entirety of calcareous dinoflagellates, and non-calcareous relatives derived from them, is accommodated in a single family of the order Peridiniales, the Thoracosphaeraceae, combining the former segregated taxonomic units Calciodinelloideae, a subfamily within Peridiniaceae, and Thoracosphaerales, a separate dinoflagellate order. As a result of a meeting of calcareous dinoflagellate specialists, we outline major subjects that are in need of re-investigation and -evaluation (an Agenda for Calcareous Dinoflagellate Research). In order to contribute to a consistent and stable nomenclature and taxonomy of calcareous dinoflagellates, we list 97 published generic names assigned to known calcareous dinoflagellates in a nomenclatural synopsis, with species names indicating their types and information on type locality and stratigraphy. We evaluate the status of these names—whether validly published and, if so, whether legitimate—, a crucial first step for any revisionary work in the future.
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Enhanced chemical weakening of chalk due to injection of CO2 enriched water. 22nd International Symposium of the Society of Core Analysts, Abu Dhabi, United Arab Emirates.
Chalk deformation and especially the so called water weakening effect induced by seawater injection, which caused a dramatic increase in subsidence rate in the early 90s at the Ekofisk field, has been extensively studied worldwide. Potential CO2 injection into depleted chalk reservoirs will lead to acidification of the formation brine or pore-water itself. Since the solubility of carbonates is strongly pH dependent, enhanced water weakening should thus be expected.
CO2 related chemical weakening of chalk has been experimentally verified at the laboratory of the University of Stavanger. By use of a standard triaxial cell, series of chalk cores were isotropically loaded beyond yield and thereafter left to creep at constant effective stresses between 12 and 14 MPa. All experiments were performed at ambient temperature, and the cores were exposed to alternating flooding with and without CO2 enriched fluids, distilled water, seawater, and/or seawater with 4 times the concentration of SO42-, at flow rates of 0.05 ml/min, i.e., 2 pore volumes (PV) per day, and a backpressure of 10 bar.
Chalk cores exposed to CO2 enriched brines showed an average increase in creep strain rates of 19% compared to CO2 enriched distilled water. Increasing the partial pressure of CO2 in the brine leads to a higher equilibrium concentration of Ca2+. An increased equilibrium concentration of Ca2+ triggers two processes: firstly, dissolution of the calcitic (CaCO3) matrix, and secondly, brine supersaturation with respect to gypsum (CaSO4X2H20). When gypsum is precipitated, more Ca2+ needs to go into solution for the water to be in equilibrium with the core. Thus, the dissolution and water weakening effect of the chalk is not only dependent on the pH, but also the sulphate concentration in the brine.
From our work, it seems that long time storage of CO2 could be challenging in chalk. One may also conclude that CO2 used as an EOR fluid in chalk can trigger significantly increased compaction. The enhanced weakening to be expected when chalk is exposed to CO2 injection will thus be governed by the reservoir conditions – temperature, in situ stresses, CO2 phase, and the fluids in which CO2 dissolves. In order to successfully implement a CO2 flood or storage project in chalk, the aspects related to significant compaction, as well as well design and surface facilities, topics not covered in this study, need to be considered accordingly. If these issues are properly managed, the compaction can result in significantly enhanced oil recovery from chemically induced compaction.