An integrated workflow for characterizing gas potential: Axios-Thermaikos basin (Greece)

Abstract

The Axios-Thermaikos basin in northern Greece is a sedimentary composite depocenter that developed tectonically during the Neogene. It has been considered promising for hydrocarbon resources, with a proven offshore gas field. Several geological and geophysical surveys, including drilling, were conducted in previous decades. However, a complete model of the basin subsurface has yet to be constructed. This paper seeks to understand the hydrocarbon prospectivity of the onshore Axios-Thermaikos basin by analyzing various geophysical (gravity, magnetic, seismic reflection) data and integrating the geophysical model with other available data. Our approach started with a qualitative analysis of the gravity and magnetic data to extract the structural and lithological trends controlling the basin formation and development. Magnetic data were further used to map the interface between the sedimentary pile and the basement. Seismic data were interpreted using different attributes and mapped in detail the geologic contacts and the tectonic regime of the basin. The density variations in depth were investigated using two-dimensional forward modeling constrained by seismic data. As a result, the 2D gravity model displays the interfaces between the formations and the main seismic-scale faults. Based on the seismic and stratigraphic data from wells, preliminary three-dimensional models were built showing the stratigraphic and tectonic regimes, including the presence of structural hydrocarbon traps, and the different depositional environments that have acted in the basin through time. The mapped tectonic structures in the onshore Axios-Thermaikos basin might also be useful for other activities of economic importance, such as gas storage and CO₂ sequestration.

Appendices

Appendix

A. Additional figures

See Figs. 10 and 11

B. Additional equations

Equation 4

Instantaneous phase (Chopra and Marfurt 2007a, b):

$$\varphi \left(t\right)={\text{arctan}}\left(\frac{\mathfrak{I} F\left(t\right)}{\mathfrak{R} F\left(t\right)}\right) ,$$

(4)

where F(t) is the seismic trace, $\mathfrak{R}$ F(t) is the real part of the seismic data and $\mathfrak{I}$ F(t) is the imaginary part of the complex trace.

Equation 5

RMS amplitude (Chopra and Marfurt 2007a, b):

$${x}_{{\text{RMS}}}=\sqrt{\frac{1}{n}\sum_{i=1}^{n}{{x}_{i}}^{2}},$$

(5)

where x is the amplitude of the seismic trace and n is the total number of amplitudes (or traces).

Equation 6

Variance (Chopra and Marfurt 2007a, b):

$${\text{var}}\left(t,p,q\right)=\frac{1}{J}\sum_{j=1}^{J}{\left[{u}_{j}\left(t-p{x}_{j}-q{y}_{j}\right)-\langle u\left(t,p,q\right)\rangle \right]}^{2} ,$$

where J is number of the seismic traces, t, p, and q are the different variables of the seismic trace, and the mean $\langle u\left(t,p,q\right)\rangle$ is defined as:

$$\langle u\left(t,p,q\right)\rangle \equiv \frac{1}{J}\sum_{j=1}^{J}{u}_{j}\left(t-p{x}_{j}-q{y}_{j}\right) .$$

(6)

Equation 7

Sequential indicators simulation (Alabert and Modot 1992; Deutsch 2006):

$$i~\left( {\varvec{u};k} \right) = ~~\left\{ {\begin{array}{*{20}c} {1,} & {{\text{if}}~{\text{category}}~k~{\text{prevails}}~{\text{at}}~{\text{location}}~u} \\ {0,} & {{\text{otherwise}}} \\ \end{array} } \right.,~~k = 1,~ \ldots ,~K.$$

(7)