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The Gabes Gulf located in the South Mediterranean Sea (Southeastern part of Tunisia) is a prolific petroleum-producing area with several oil and gas fields and it's the subject of significant discoveries. The area is affected by intense tectonic events and several stages of hydrothermal activities . During drilling at the MX area in NE of the Gabes Gulf, a magmatic intrusion has been encountered within a reservoir rock (i.e., Douleb Formation). The objectives of the present study are (1) to investigate the impact of the hydrothermal activities on this magmatic intrusion and (2) to characterize the potential modifications in its mineralogy and petro-physical properties that can affect the migration/accumulation of hydrocarbons. To do so we combine optical microscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, micro-Raman spectroscopy, electron microprobe analyses, and petro-physical measurements. Our results show that the initial magmatic intrusion (dolerite) is composed mainly of plagioclase , clinopyroxene , and olivine. Based on petrographic observations, this intrusion subjected to several stages of hydrothermal fluid alterations that modified the primary texture and mineralogy. Albite , calcite , ankerite , dolomite, pyrite , quartz, anatase , kaolinite, and chlorite are the main secondary mineral products. During this alteration process, carbonates, pyrite, and quartz precipitated in vesicles, forming amygdules structure with enveloped zones indicating a fluctuation of fluid chemistry during their formation. In addition, petrographic observations indicate an interaction between feldspars exsolution and metasomatic processes in the studied area. The dissolution mechanisms, as well as the textural modifications in the dolerite induced by the hydrothermal activity, are underlined by the presence of spherulites , pores, and micropores. Such modifications significantly improved the porosity of the dolerite body. Conversely, the new mineralization that filled the fractures and micro-fractures, have reduced the inter-pore connections, and thus reduced its permeability. The present work demonstrates the important role of hydrothermal activity on the petro-physical properties of magmatic intrusions and how it could facilitate the migration/accumulation of hydrocarbons. Our results open the door for further investigations to check the potential presence of hydrocarbons within this doleritic intrusion.

During the evolution of sedimentary basins, clay minerals may record changes in their mineralogy, chemistry, and texture as a response to diagenesis, fluid flows, and tectonic activities. Therefore, studying clay minerals in sedimentary rocks provides important information that may assist to decipher the basin history. The focus of the present study is the Teloua Sandstone Formation (Fm) which belongs to the Tim Mersoi Basin located in the NE of Niger republic (West Africa). The formation is cut by the major Arlit fault which is the largest tectonic structure impacting the Tim Mersoi Basin. The aim of the present work is to use the texture, mineralogy, and chemistry of clay minerals in the Teloua Fm as geochemical markers for burial diagenesis history and fluids flows (i.e., attributed mainly to the Arlit fault). Therefore, three drill holes (DH1, DH2, and DH3) were investigated as a function of their distance from the fault and samples have been collected all along the Teloua Fm (Upper Triassic-Lower Jurassic), at the unconformity surface with the underlaying Moradi Fm (Upper Permian), and at the top of the Moradi Formation. Petrographic observations demonstrate that the sediments of the Teloua Fm have undergone transformations induced by burial diagenesis, brittle deformation, and multiple stages of fluid flows. These transformations are marked by grain compaction, fragmentation, dissolution of detrital minerals, recrystallization of authigenic clay minerals, and modifications in the primary porosity. XRD and SEM results indicate no significant variation in the detrital mineralogy of the three drill holes; Teloua Fm samples are composed of quartz, feldspars (microcline and albite), micas (biotite and muscovite), minor heavy minerals such as hematite, anatase, phosphates (apatite and monazite), and zircon. The authigenic clay minerals observed in the three drill holes are rosette-like tosudite (i.e., a regular interstratified chlorite/smectite), saponite, montmorillonite, and kaolinite/smectite mixed layer. Regarding their vertical distribution, tosudite occurs throughout the Teloua Fm, whereas montmorillonite is mainly present at the top of the formation, and saponite is only observed at the unconformity between Teloua and Moradi formations. The Mg content of tosudite tends to decrease with increasing vertical distance from the basal unconformity and the lateral distance from the Arlit fault. This is interpreted as an evidence of Mg-rich fluid circulations related to the Arlit fault activity that penetrated the formation through the unconformity surface and fractures. This is also evidenced by the enrichment of dolomite at the unconformity when moving closer to the Arlit fault. In addition, the occurrence of both authigenic saponite (at the unconformity) and montmorillonite (upper part of the Teloua Fm) that post-dated tosudite is attributed to another stage of Mg-rich fluid circulation during late diagenesis. Furthermore, late downward circulation of meteoric fluids of low pH are evidenced by the partial alteration and replacement of montmorillonite by kaolinite/smectite mixed layer in the Teloua Fm (at shallow depth of the DH3). These results demonstrate the validity of clay minerals as reliable tools to provide information about fluid/rock interaction in complex geological and tectonic setting.