Distribution of pore pressure and fracture pressure gradients in the paleozoic sediments of Takouazet field, Illizi basin, Algeria

• Published in: Journal of African earth sciences (1994) Vol. 164; p. 103778
• Main Authors: Baouche, Rafik, Sen, Souvik, Boutaleb, Khadidja
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.04.2020
• Subjects: Abnormal pressure; Fracture pressure; Overburden; Pore pressure; Takouazet field; Well log analysis; Overburden; Takouazet field; Abnormal pressure; Well log analysis; Pore pressure; Fracture pressure
• ISSN: 1464-343X; 1879-1956
• DOI: 10.1016/j.jafrearsci.2020.103778

A comprehensive pore pressure and fracture gradient (PPFG) characterization is a basic requirement for subsurface geomechanical modeling. This study deploys indirect approaches to estimate subsurface pressure profiles in the Mesozoic and Paleozoic successions of eastern Illizi basin, Algeria. Amoco exponent has been modeled for pseudo density generation and overburden gradient has been deciphered from a composite density profile. PP calculated from sonic and resistivity logs have been calibrated and validated with direct downhole measurements. Results indicate normal pore pressure regime in the Mesozoic and Devonian sediments. Disequilibrium compaction induced abnormal pore pressure in Silurian marine shales with a maximum gradient of 12.7 PPG. Top of geopressure has been marked at Silurian top (2050m). Pore pressure drops sharply from Silurian to Ordovician sediments, across the glacial unconformity (2332 m). Fracture gradients have been interpreted by Eaton's Poisson's ratio based model and Mathews & Kelly's effective stress ratio based approach. Based on the vertical distribution of subsurface pressure, effective vertical stress, a safe mud window has been recommended for optimum drilling fluid design. •A comprehensive PPFG model has been presented by integrating the geophysical logs and direct downhole measurements.•Pore pressure from sonic and resistivity logs indicate Silurian marine shales are abnormally pressured.•Devonian and Ordovician reservoir sandstones reveal hydrostatic pressure condition.•Suitable mud window has been inferred based on the interpreted PP and FG.