Pore pressure and stress regime in a thick extensional basin with active shale diapirism (western Mediterranean) Ferm ´ ın Fern ´ andez-Ib ´ añez and Juan I. Soto ABSTRACT The Alboran Sea in the Mediterranean is a back-arc basin de- veloped during the Miocene by extensional collapse within an arc-shaped orogen. A major depocenter (>10 km [>6.2 mi]) is located to the west of the basin (West Alboran Basin [WAB]) and contains a diapiric province with overpressured shales and mud volcanoes. Seismic and well data are used to analyze the evolution of the shale structures in the northern margin of the WAB and to estimate the in situ stress tensor. Geomechanical modeling suggests a present-day normal faulting stress regime along the northern WAB, where the maximum horizontal stress is parallel to the coastline. Pore pressure shows a hydrostatic gradient down to 2000 m (6561 ft), where the top of the re- gional pore pressure ramp is located. Undercompaction is the dominant mechanism generating overpressures in sediments shallower than 5000 m (16,406 ft). At greater depths, thermal mechanisms impose an excess of pore pressure on the sediments feeding the diapirs. This framework is used to discuss the con- tribution of thermally generated pressures to the triggering of shale diapirism. Increasing thermal pressures in the deepest confined units cause tensile failure of the overburden and subsequently promote mud withdrawal and injection in the overburden. The magnitude of the overpressure conditions the vertical ascent of shale. The more mature structures reuse pre- existing normal faults in their ascent toward shallower basin levels. Results provide insights into the current discussion about the triggering factors behind shale diapirism. They also help to explain the differences between shale structures and those shaped by salt tectonics. AUTHORS Ferm ´ ın Fern ´ andez-Ib ´ añez ~ Departamento de Geodin ´ amica and Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada - Consejo Superior de Investigaciones Cient´ ıficas, Av. Fuentenueva s/n, 18071 Granada, Spain; present address: ExxonMobil Upstream Research Company, 22777 Springwoods Village Parkway, Spring, Texas 77389; fermin.fernandez@ exxonmobil.com Ferm´ ın Fern ´ andez-Ib ´ añez received his Ph.D. in 2007 from Granada University before working for GeoMechanics International and ExxonMobil Development Co. Ferm´ ın has experience on integrating geomechanics and structural geology. Over the past four years he has worked on naturally fractured reservoirs. He is currently working at ExxonMobil Upstream Research Co. Juan I. Soto ~ Departamento de Geodin ´ amica and Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada - Consejo Superior de Investigaciones Cient´ ıficas, Av. Fuentenueva s/n, 18071 Granada, Spain; jsoto@ugr.es Juan I. Soto is a professor in geodynamics at Granada University, Spain. He holds a Ph.D. in structural geology from Granada University and conducted research at University of Oxford and Rice University. He has worked in extensional settings and in salt and shale tectonic processes, focusingonthestructural interpretation of two-dimensional–three- dimensional seismic data and the relationships between deformations and sedimentary processes. ACKNOWLEDGMENTS We acknowledge the suggestions made by the reviewers Christopher K. Morley and Liangmiao (Scott) Ye, together with the associate editor Richard H. Groshong, which have substantially improved this contribution. The authors thank IHS and Schlumberger for maintaining various academic agreements to use KingdomSuite and Petrel, and GeoMechanics International for providing access to their software. This research has been made possible thanks to the data set Copyright ©2017. The American Association of Petroleum Geologists. All rights reserved. Manuscript received November 15, 2015; provisional acceptance February 18, 2016; revised manuscript received May 22, 2016; revised manuscript provisional acceptance May 27, 2016; 2nd revised manuscript received June 8, 2016; final acceptance July 13, 2016. DOI:10.1306/07131615228 AAPG Bulletin, v. 101, no. 2 (February 2017), pp. 233–264 233