PII S0016-7037(99)00121-0 Basin scale evolution of formation waters: A diagenetic and formation water study of the Triassic Chaunoy Formation, Paris Basin R. H. WORDEN, 1, * M. L. COLEMAN, 2 and J-M. MATRAY 3 1 Environmental Engineering Research Group, The Queen’s University, Belfast, BT7 1NN, Northern Ireland, UK 2 Postgraduate Research Institute for Sedimentology, The University of Reading, Whiteknights, Reading, RG6 6AB, UK 3 ANTEA/STO groupe BRGM, 3 Avenue C. Guillemin, BP 6119, 45061, Orle ´ans, cedex 2, France (Received September 25, 1997; accepted in revised form February 26, 1999) Abstract—Formation waters and their movements within basins are commonly attributed with responsibility for patterns of cementation and porosity-loss within reservoirs and aquifers. It is thus important to understand when and how waters move in the subsurface. We have studied the evolution and movement of formation water in the Triassic Chaunoy Formation of the Paris Basin, NW Europe to define the way in which the water has evolved and to interpret water movement patterns using mineral isotope and fluid inclusion data in conjunction with detailed formation water analyses. The Chaunoy Formation is cemented with different types of dolomite, calcite and quartz cement. We have studied the evolution of waters in terms of oxygen, carbon and strontium isotopes and salinity. Connate waters were meteoric in origin but strongly influenced by the proximity of a playa lake. Input of palaeo-meteoric water, which entered the Chaunoy via eastern halite bodies, resulted in highly saline formation water for the majority of the Chaunoy Formation from about 100 Ma until (and for some time after) maximum burial. Saline waters spread into the aquifer initially taking a west-north west trajectory and then switching to a west-south west pattern in the Eocene. Following the Alpine orogeny and localised uplift of the basin margin, the southern portion of Chaunoy received fresh, low salinity, meteoric water. It is likely that formation water was lost from the Chaunoy in a more central part of the basin, via sub-vertical cross formational flow to the Mid Jurassic Dogger Formation, to accommodate the influx of fresh water. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION Water is a major component of sedimentary basins. Waters are trapped at the time of sedimentation and are thought to evolve due to a number of hydrodynamic and thermodynamic factors in terms of chemistry and isotopes. Waters that are free to move under appropriate hydrodynamic conditions, as opposed to such as electrostatically-bound water on clay surfaces, are known as formation waters. Formation waters are thought to move great distances in sedimentary basins over geological time in perme- able sedimentary units (e.g., To ´th and Corbet, 1987; Long- staffe, 1988). Consequently, formation waters are thought to be important for the movement of mass (e.g., Mullis and Haszel- dine, 1995) and the movement of formation waters is conven- tionally assumed to be an important process in the diagenesis of limestones and sandstones. While the movement of fluid and the precipitation of minerals is thoroughly documented for spatially localised mineral deposits and where the relevant fluids are still in place (Longstaffe, 1988), it is often inferred without proof in describing the evolution of rock properties in ancient sedimentary basins. Basin margins are often accessible for field studies and can provide large quantities of rock for examination. However, waters from such samples are unlikely to represent anything other than modern meteoric water. Basin centres are usually only accessible following oil exploration; the material available is limited to produced fluids and sparsely spaced four inch diameter cores. Although formation water associated with this rock will have significance in terms of basin evolution, it is just the latest of a possible succession of compositions of pore water. The best method to understand the evolution and significance of water movement is to examine formation waters in a given unit on a basin scale for a long period of basin history. For the earlier part of the history it is necessary to examine evidence of fluid evolution and movement left behind in the rock using techniques such as stable and radiogenic isotopes and fluid inclusion analysis. We have combined a study of present day formation waters from widely-spaced exploration wells in a sedimentary basin with a study of the rock record. After con- structing thermal histories for each well, we have converted fluid inclusion temperatures into ages and thus quantified the evolution of the isotope and fluid inclusion salinity data. Whilst these approaches have been used before individually, they have not been integrated previously on a basin-scale. The novelty of this study lies in the facts that a number of wells across the length and breadth of a basin were examined, that the data from the rocks were converted into specific ages and that formation water and rock data were fully integrated. We have used material and data from Upper Triassic (Car- nian) Chaunoy Formation sandstones from the Paris Basin in France to show that the basin margins evolved differently from the basin centre and that meteoric invasion is strictly limited in its influence in the basin centre. We will show that formation water evolution must be considered in terms of both hydrody- namic and water-rock interaction. Finally, we will show that waters show little evidence of significant change of composi- tion in the basin centre for tens of millions of years despite basin inversion and uplift. *Author to whom requests for reprints should be addressed (r.worden@qub.ac.uk). Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 17, pp. 2513–2528, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 2513