Diagenesis in Mesozoic carbonate rocks in the North Pyrénées (France) from mineralogy and fluid inclusion analysis: Example of Rousse reservoir and caprock

Abstract

Fluid-rock interaction is of key interest for deciphering the geodynamic history of complex geological environments. In this paper, we describe the mineralogy and petrography of the Rousse reservoir (the Upper Jurassic “Dolomie de Mano” formation) as well as the reconstruction of its thermal and pressure history. We also investigate the base of the reservoir caprock (the Campanian breccia). A satellite of the giant gas field of Lacq in the south-west of France, Rousse was a gas producer until it moves to a CO2 storage site in 2010. Rousse is located within a deep, isolated, faulted, carbonate Jurassic horst overlain by a 4500 m thick overburden that is composed of a series of turbiditic flysch deposits of Upper Cretaceous (Cenomanian) to Tertiary (Eocene) age. Cores were sampled from two wells, Rousse 1 and 2, and were analyzed by ICP-MS, XRD, SEM, EPMA, TEM, cathodoluminescence and fluid inclusion techniques. Pressure-Temperature (P-T) conditions were derived by modeling data from fluid inclusions trapped in the fracture minerals of the “Dolomie de Mano” formation and the Campanian breccia. In the “Dolomie de Mano”, diagenesis was dominated by three dolomite episodes followed by a calcite stage in fractures. The second and third dolomitizations were associated with fracturing episodes. These dolomites precipitated from low-saline (<1 mol·kg−1 eq. NaCl) hydrothermal fluids at temperatures around 150 °C. These two characteristics exclude dolomitization during a synsedimentary slope brecciation event and also the role of hypothetical secondary brines. In the Campanian breccia, dolomitization is marginal and the main precipitation is calcite. In association, two types of fluid inclusion (liquid water-vapor and vapor) can be distinguished in the two formations and are marked by predominant CH4 and variable CO2 contents. The presence of gas inclusions in the reservoir as in the caprock is indicative of several gas invasions. A P-T modeling of the geologic evolution of the area reveals two periods of fluid circulation: a first stage (150–170 °C, 20 MPa) during the Late Jurassic to Early Cretaceous for the reservoir rock, corresponding to the rifting period of the North-Pyrénées western zone; and a second stage (170 °C, 41 MPa) during the late Eocene period in breccias, corresponding to the Pyrenean compression period. The petrophysical properties of the reservoir, marked by an intense fracture network, were acquired early during an extensional period, whereas the impact of fluids associated with a period of compression appears to have been limited in space to drainage of tectonic origin. Finally, we note significant similarities between the petrographic and fluid signatures of the Chaînons béarnais, in the folded orogenic wedge located south of the North Pyrenean Frontal Thrust, and Rousse, located in the foreland basin to the North. Nevertheless, the temperature recorded in the Chaînons béarnais reached 300–350 °C, much higher than in Rousse due to its position in the vicinity of an over-thinned palaeomargin during the early Cretaceous.