Relationship Between Fault Systems and Fracturing in Complex Fractured Carbonate Reservoir in the Tunisian Pelagian Block Using Integrated Seismic and Petrophysical Analysis: Implications for Petroleum Exploration and Development

Abstract

ABSTRACT The Abiod carbonate reservoir of the Campanian‐Early Maastrichtian age represents a major oil reservoir In the Pelagian block of eastern Tunisia, forming complex fractured networks that are poorly understood. Given the total absence of geological outcrops of Campanian‐Maastrichtian age in the study area, 2D seismic reflection, geological modelling and well log datasets were integrated to characterise the subsurface geodynamic structures, investigate the petrophysical characteristics, the relationship between fault systems and fracturing and define their implications for petroleum exploration. Analysis of seismic profiles, lithostratigraphic logs, borehole data and geological modelling has shown the prevalence of three major fault networks: northwest–southeast (NW–SE), E–W and northeast‐southwest (NE–SW). These faults have controlled the thickness of the limestones of the Abiod reservoir, favouring highly subsiding NW–SE trending zones where the thickness has exceeded 500 m. Based on responses of the Gamma Ray (GR) and Sonic curves, this formation is made up of four different lithological units (U1–U4). The wireline log analyses concluded that the gross thickness of the Abiod reservoir includes good reservoir property intervals with average effective porosity ranging from 8.6% to 20.6% and average shale volume ranging from 2% to 17%. Furthermore, analysis of the fracturing affecting the Abiod reservoir, based on oil well FMS image logs, has shown the presence of open and partially open fractures that are key to hydrocarbon accumulations. The rose diagrams of fracture directions at the wells and the rose diagram of major fault directions of the map are in perfect agreement with the clear predominance of the NW–SE directions. Beyond the main uplift faults, the structural features that impact the studied area's stratigraphy are divided into three main categories. This study showed that the brittle tectonics play a role in the enhancement of the petrophysical characteristics of the studied limestone reservoir and controlled the charge access of the hydrocarbons into the reservoir. The majority of the fractures affecting the Abiod reservoir are parallel to the large fault and have the same direction of movement, while the others are distributed in a conjugate family close to perpendicular to the large fault and with complementary directions of movement. The combination of geological factors and logging parameters indicates that the tectonic legacy and structural inversions play an important role in the structuring of the cover and the evolution of the petroleum system, which have made this chalk one of the most important oil reservoirs in eastern Tunisia. Finally, the developed 3D structural model suggests favourable locations for high‐potential areas in the studied region and provides insights for the reservoir development.