High-resolution passive seismic tomography for 3D velocity, Poisson’s ratioν, and P-wave qualityQ_Pin the Delvina hydrocarbon field, southern Albania

Abstract

We have studied using traveltimes of P- and S-waves and initial seismic-pulse rise-time measurements from natural microearthquakes to derive 3D P-wave velocity VP information (mostly structural) as well as P- and S-wave velocity VP/VS and P-wave quality factor QP information (mostly lithologic) in a known hydrocarbon field in southern Albania. During a 12-month monitoring period, 1860 microearthquakes were located at a 50-station seismic network and were used to obtain the above parameters. The data set included earthquakes with magnitudes ranging from –0.1 to 3.0 R (Richter scale) and focal depths typically occurring between 2 and 10 km. Kohonen neural networks were implemented to facilitate the lithological classification of the passive seismic tomography (PST) results. The obtained results, which agreed with data from nearby wells, helped delineate the structure of the reservoir. Two subregions of the investigated area, one corresponding to an oil field and one to a gas field, were correlated with the PST results. This experiment showed that PST is a powerful new geophysical technique for exploring regions that present seismic penetration problems, difficult topographies, and complicated geologies, such as thrust-belt regions. The method is economical and environmentally friendly, and it can be used to investigate very large regions for the optimal design of planned 2D or 3D conventional geophysical surveys.

Reservoir Structure and Wastewater‐Induced Seismicity at the Val d'Agri Oilfield (Italy) Shown by Three‐Dimensional Vp and Vp/Vs Local Earthquake Tomography

“…Variations in lithology and fracture density seem to be also the primary drivers of significant V p and V p / V s changes defined within the IAP. Very high V p anomalies (6.4–6.5 km/s) detected both in the upper and lower part of the IAP can be ascribed to dolomitic rocks based on laboratory velocity measurements (Trippetta et al, ) and passive seismic tomography of carbonate reservoirs (Tselentis et al, ). The upper anomalies associated with very high V p / V s (> 2.0) can be related to tectonic slices incorporated in the core of ramp anticlines and formed presumably by Jurassic‐Lower Cretaceous sequences of fractured dolomitic limestone and dolostone with high pore fluid pressure (label c in Figure b).…”

“…Our results show that high‐resolution LET can provide valuable information on the reservoir structure and properties that are complementary to those obtained from seismic reflection profiling. Such a combination of passive and active seismic imaging can be particularly effective in case of complex hydrocarbon plays, such those in thrust belt regions, where reflection seismic is often hampered by unfavorable conditions (Tselentis et al, ). The relevance of using high‐resolution LET to the comprehension of wastewater injection seismicity is evident from our study that is likely the first of its kind in published literature.…”

“…High-resolution local earthquake tomography (LET) has proven useful to characterize geothermal sites and hydrocarbon reservoirs especially in geologically complex areas where seismic reflection exploration is challenging (Calò & Dorbath, 2013;Julian et al, 1996;Tselentis et al, 2011;Vanorio et al, 2005;Zhang & Lin, 2014;Zhang et al, 2009). High-quality V p and V p /V s LET images help define the geometric and petrophysical properties of reservoirs, while 3-D earthquake locations allow pinpointing active faults.…”

“…At increasing depths, lower V p /V s values are expected because the progressive increment in lithostatic pressure tends to close pores and fractures. In hydrocarbon and geothermal fields, low V p /V s values can characterize gas-bearing pressurized formations or liquid-bearing depleted zones (Dvorkin & Nur, 1996;Gritto & Jarpe, 2014;Gunasekera et al, 2003;Mavko et al, 2003;Tselentis et al, 2011;Vanorio et al, 2005;Wang et al, 1998;Zhang & Lin, 2014).…”

Application of Cross-hole Seismic Tomography in Characterization of Heterogeneous Aquifers

“…Cross-hole seismic tomography which involves measuring travel times of seismic ray paths between two or more boreholes in order to derive an image of seismic velocities between the wells offers an approach for characterizing geological heterogeneities at a high resolution. Its application range from near surface aquifer studies, geothermal, to deep oil and gas reservoirs (Sainkov et al 2005;Tselentis et al 2011). Becht et al, (2007), evaluated the use of cross-hole seismic tomography for high resolution aquifer characterization.…”