SHALLOW SEISMICITY AND FLUID EXPLOITATION IN THE NORTHERN BURGOS BASIN (NUEVO LEÓN, MÉXICO)

Abstract

This paper examines the relationships between recent shallow seismicity and exploitation of fluids in the northern Burgos Basin where cumulative seismic events recorded in the State of Nuevo Leon reach a total of 304 earthquakes between 2006 and 2016. In detail, 2 to 5 yearly events occurred from 2006 to 2011; but a later remarkable increase was evident as follows: 89 in 2012, 69 in 2013, 75 in 2014, 31 in 2015 and 27 in 2016. This behavior doesn’t match the random fluctuations from natural seismicity rates. A statistical analysis allowed us to determine that the sequence of earthquakes after 2011 could be related to the activity of exploratory wells in the Burgos Basin, which were drilled down to the Pimienta (Upper Jurassic) and Agua Nueva (Upper Cretaceous) shale gas plays. The epicenters located in the State of Nuevo Leon, in the municipalities of China, General Terán, Montemorelos and Los Ramones, were associated with the Upper Jurassic Pimienta and Upper Cretaceous Agua Nueva shale gas fields. Only 17 earthquakes had magnitudes ranging from 4.0 - 4.5 Richter  magnitude and those were associated with the exploratory wells Anhelido-1, Arbolero 1, Batial-1, Durian-1, Kernel-1, Mosquete-1,  Neritas-1, Nuncio-1, Serbal-1 and  Tangram-1. The hypocenters correspond to the depth at which the Pimienta and Agua Nueva Formations lie; hence, sharp changes in the minor shock frequencies were considered as indicators of induced seismicity related to hydraulic fracturing for fluid extraction. The scatterplot of the frequency and magnitude of events for 2009-2014  shows slopes between -7.0963 to -1.1538 that were considerably more negative than the   natural seismicity values which span from 0.75 to 0.9.  The slopes for 2012, 2013 and 2014 are negatives (-7.0963, -0.3656 and -0.1333), respectively. These dramatic changes in increasing of the minor shock sequences in the Burgos Basin allow us to be considered as indicators of induced seismicity due to fluid exploitation. This  interpretation is based on the frequency and magnitude of shocks which achieve values  of hydraulic fracturing-induced   earthquakes associated with anthropogenic fracking,  similar to other seismicity data obtained in different parts of the world where this Technique is applied.