ABSTRACT: Diagenesis and Pore Water Evolution in the Keuper (Late Triassic) of the Paris Basin, France

SPOTL, CHRISTOPH, Geologisches Institut, Universitat Bern, Bern, Switzerland

Keuper rocks form a major oil reservoir in the western and central parts of the Paris basin. In the western part of the Paris basin, the sequence is dominated by siliciclastic rocks and locally abundant continental carbonates deposited on an alluvial plain.

Eodiagenetic processes affecting the sediments prior to burial include mechanical clay infiltration, hematite growth, widespread dolomite precipitation (dolocretes), and locally significant karst-dissolution of these carbonate units. Late-stage cementation by quartz, ferroan dolomite/ankerite, barite, and illite occurred after major compaction.

Integration of petrographic, oxygen isotope, and microthermometric data allow the reconstruction of the pore water evolution path through time. The initial groundwater was meteorically derived (~ -7 to -2o/oo SMOW). The isotopic composition of the water evolved toward significantly heavier values (~0o/oo SMOW) during burial and then returned to the present-day value of ~ -4o/oo SMOW. The heavier Oxygen 18 values seen during burial are best interpreted as the result of brine mixing, based on the present-day pore water chemistry in the Keuper reservoir (Br-Cl concentrations) and the high salinities of fluid inclusions in mesodiagenetic cements. Isotope data from different mesodiagenetic cements show that the oxygen isotopic composition of the porewater composition did not change signifi antly during deep burial. These static brines then mixed with freshwater that was introduced into the basin during Tertiary uplift of the basin margin.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)