Stress field of a Lower Devonian gas field from borehole sonic and seismic data
|Location||International Geological Congress,oslo 2008|
|Author||Stockhausen, Harald۱; Abdallah, Hussein۱; Baena, Marیa۱; Laurent, Sage۲|
|Holding Date||08 October 2008|
The following research was done in a Lower Devonian gas field, located in northwestern Africa, which has a geological and deformational history that began since Cambrian times.
This paper present data collected from seismic and bibliography that shows the stress field evolution of this particular area, in which a northwest-southeast anticline was formed during Upper Carboniferous till Lower Permian times, as a consequence of the Proto-Tethyan closure (Hercynian Orogeny).
The main purpose of this paper is to introduce a methodology that integrates different acquisition techniques currently used in the oil & gas industry, which in this particular case, helped to understand the stress evolution and find the current stress regime, where the last is important to properly stimulate the wells in this area.
The most important tectonic event in this area happened during Upper Carboniferous till Permian times (Hercynian Orogeny), in which the sedimentary layers in the area were folded, forming a northwest-southeast anticline (where the axis is at approximately 140° Azimuth).
Then during Jurassic times, as Gondwana started to break and the Atlantic Ocean began to open, a series of dolerite events intruded the area during Liassic times. Since then, no other important geological events happened in the area, including the Austrian and Alpine orogeny, which have had poor influence on the deformation of the previous structures or generation of new faults.
Today, the stress regime in the area is governed by the movement of the African plate against the Euro-Asian plate, which can be clearly seen in the wells drilled in the area by several effects such as breakouts, borehole sonic horizontal anisotropy and seismic walk around anisotropy.
In this sense, the Formation Image tools (FMI) and oriented calipers ran in several wells, display breakouts preferentially oriented at 40 to 220° azimuth, which is considered as the minimum horizontal stress (Sh), thus the maximum horizontal stress (SH) is at 130 to 140° azimuth, which coincidently is parallel to the orientation of the anticline, suggesting that the stress in this area had rotated 90° from the Hercynian Orogeny.
The above, had been also corroborated with a new generation 360° borehole sonic data (Sonic Scanner) taken in one of the wells, which shows an anisotropic P-wave behaviour, where the P-wave travel faster in the direction of (SH) and slower in the direction of (Sh). The above was also seen with a walk around seismic, ran in the same well and in another 140 km away, in which vibros set in a circumference of 2000 m radio from the wells, found that P-waves travel faster in (SH) direction and slower in the (Sh) direction, reflecting the far field stress behaviour.
All the above agrees with a broader data set, which is found in the world stress map, which shows the same stress orientation for the entire region.