Geomechanical characterization of mud volcanoes using P-wave velocity datasets

Abstract

Mud volcanoes occur in many petroliferous basins and are associated with 8 significant drilling hazards. To illustrate the type of information that can be extracted 9 from limited petrophysical datasets in such geomechanically complex settings, we use 10 P-wave velocity data to calculate mechanical properties and stresses on a 2D vertical 11 section across a mud volcano in the Azeri-Chirag-Guneshly field, South Caspian Basin. 12We find (a) that the values of the properties and stresses calculated in this way have eruptions (Lerche & Bagirov 1999; Kopf et al. 2009; Contet & Uterseh 2015; Hill et al. 50 2015), and (e) controls on the geochemistry of the erupted fluids (Azzaro et al. 1993; 51 Mazzini et al 2009; Bristow et al. 2000; Feseker et al. 2010; Oppo et al. 2014). In 52 offshore areas, numerous multi-scale near-surface geological studies have been 53 performed to mitigate the risks to seabed facilities that are associated with mud volcano 54 activity and its accompanying hazardous phenomena, such as the presence of shallow 55 gas, slope failure and pockmarks (Hill et al. 2015; Contet & Unterseh 2015; Unterseh & 56 Contet 2015). Yet the extent to which drilling in such zones has to be avoided because of 57 mud volcano related risks remains unclear. 58Among the challenges posed by the complicated geology in and around mud volcanoes 59 is the prediction of local pore fluid pressures which has significant implications for 60 drilling (e.g. borehole blowouts and instability). Understanding these manifestations of 61 3 localized fluid flow from a geomechanical perspective requires an analysis of the fluid 62 and pressure distribution, the deformation history, the distribution of fractures, and the 63 state of stress around the mud volcano. This, in turn, requires comprehensive 64 petrophysical datasets and sophisticated data analysis. However, within these 65 geomechanically complex areas there remains value in adopting a simpler 66 reconnaissance-type approach in order to identify targets for more detailed 67 investigation and key features that require a better understanding. 68In this paper, we use P-wave velocity data available in the public domain to estimate the 69 mechanical properties and stresses on a 2D vertical section across a mud volcano 70 structure located in the Azeri part of the Azeri-Chirag-Guneshly (ACG) field, South 71Caspian Basin (SCB). The aim of the study is to determine whether useful 72 geomechanical information can be extracted from such a limited dataset. 73 Geological setting 74 Regional geology 75The South Caspian Basin, offshore Azerbaijan (Fig. 1a) (Fig. 84 1b). Along the northern margin of the basin, anticlinal structures developed within the 85 NW-SE trending Absheron-Balkhan deep-seated structural uplift, which is the offshore 86 extension of the Caucasus fold belt (Fig. 1a). 87The sedimentary succession in the basin (Fig. 2) The lacustrine shales act as major pressure seals within the basin (Javanshir et al. 102 2015). 103Within the South Caspian Basin ...

Seismic geomechanics of mud volcanoes

“…and 5 make up the main body of the thesis, of which chapter 3 is published (Gulmammadov et al 2017) and chapters 4 and 5 are intended for publication following peer review.…”

“…Chapter 3 is a paper entitled "Geomechanical characterization of mud volcanoes using p-wave velocity datasets", which addresses the 2D geomechanical analysis of a mud volcano from the Azeri-Chirag-Guneshly field in the offshore South Caspian Basin. This paper is published in Geological Society's Special Publication entitled "Geomechanics and Geology" (Gulmammadov et al 2017) and is authored by Gulmammadov, R., Covey-Crump, S. and Huuse, M.…”

“…Review of studies performed over the structure has been compiled by Gulmammadov et al (2017) and is also addressed in Chapter 3.…”

“…However, mud volcano caldera geometry can be used as an indicator of the regional stress field, when the geometry of the caldera is controlled by the tectonic regime or the shape of the fluidised substrate beneath the caldera. Fluid chambers of mud volcanoes and their surface expressions have tendency to be elongate parallel to ℎ and normal to (Bonini, 2012;Gulmammadov et al 2017).…”

“…Data utilised in these studies include hydrographical (sub-bottom profiler, multibeam echosounder and side scan sonar), geotechnical (drop coring and box coring), geophysical (3D and 2D reflection and velocity seismic), satellite and gravimetric records(Kadirov et al 2003;Kadirov et al 2005;Evans et al 2007;Roberts et al 2011a; Antonielli et al 2014;.Sediments recovered from erupted fluids on the surface indicate that they derive from the Oligocene and Early Miocene age Maykop Formation, a kilometre thick sequence composed of organic-rich mudstonesEvans et al 2006). The mechanical stratigraphy of the sequence is characterized by an average porosity of 11 %, permeability of 1 nD and geothermal gradient of 21 °C km -1(Buryakovsky et al 2001;Gulmammadov et al 2017). Geochemical analysis of the fluids expelled at the surface implies that they originate primarily from the deeper Miocene and older marine mudstones(Abrams & Narimanov 1997; Mazzini et al 2009).…”