MATURITY OF POTENTIAL CRETACEOUS SOURCE ROCKS AT THE ISIS FIELD, OFFSHORE EASTERN TUNISIA: INFLUENCE OF HYDROTHERMAL FLUIDS

Abstract

An increase in source rock thermal maturity is in general linked to burial‐related heating according to the regional geothermal gradient, but maturities may also locally be influenced by high‐temperature hydrothermal fluids or igneous intrusions. In the present study of the Isis field located in the Gulf of Gabes (offshore Tunisia), we combine an analysis of organic matter maturity indicators and clay mineral signatures to constrain possible fluid/rock interactions and to define controls on the maturity of potential source rocks. Cuttings samples were collected from source rock intervals in the Cretaceous Bahloul (Cenomanian – Turonian) and underlying Fahdene (Albian – Cenomanian) Formations at the PM borehole, and detailed organic geochemical and clay mineralogical analyses of source rock samples and extracts were carried out. Samples from the Bahloul Formation (2381 m to 2400 m) consist of black to dark grey claystones and globigerinid limestones. Those from the Fahdene Formation (2400 m to 2700 m) comprise alternating claystones and chalky limestones containing globigerinids including Ticinella primula. Both source rock intervals have similar mineralogical compositions consisting of calcite, quartz, albite, anorthite, minor anatase, pyroxene and pyrite. The clay mineralogy of the formations is composed of abundant smectite (two generations), subordinate kaolinite and minor illite and/or mica. The Fahdene source rock contains organic matter consisting of mixed kerogen Types II/III; Type II OM is present in the Bahloul Formation.At the borehole location, the source rock intervals in the Fahdene and Bahloul Formations have been affected by hydrothermal fluids which are derived from an unconformably overlying basalt succession which is >300 m thick. Based on bulk mineral assemblages, the Bahloul Formation has been modified by the hydrothermal fluids more significantly than the underlying Fahdene Formation. Analyses of the clay minerals compositions indicate that interactions between the hydrothermal fluids and the host rocks were very rapid and were localized within fractures and micropores. The Bahloul Formation source rocks have reached an advanced stage of thermal maturity (overmature or end‐ oil window) compared to the thermally immature source rocks in the Fahdene Formation.The results show that the main control on source rock maturation at the studied well was maximum palaeotemperature and this may locally have been influenced by short‐term hydrothermal and magmatic heating as well as by the regional geotherm. Clay minerals were less affected by hydrothermal heating compared to organic matter indices; this may be because mineral components are slower to react to short‐term higher temperature conditions than organic matter.Thus magmatic activity and hydrothermal fluids may locally accelerate the thermal maturation of source rocks even at shallow depths. The study also shows that a combination of clay mineral characterization and organic matter indicators can be useful for the assessment of thermal maturity in potential source rocks.