Hi Iljaz

An extensional tectonics has taken place  followed by intrusion of   a) salt diapir, alternatively if new magmatism is present in the area any intusion or dyke may have taken place.

Regards,

Lirim Hoxha

Dear Ijaz,

I agree with the answer  given by Hoxa above where an intrusion, igneous or sedimentary (salt or shale diapir)  occurs at depth below the synclinal structure without reaching the surface where it would have been picked by geological mapping. 

In addition, see if there is an unconformity between the anticline below and the syncline above. If there is, the rocks could have undergone different geological histories.That is, the older beds were folded and tectonically stabilized before the deposition of the overlying strata which tectonic processes did not affect the deep seated beds. Probably different sedimentary basins at different times  were superposed. The first option however seems more likely.

Obianuju P. Umeji

What kind of the geophysical data are you referring to?

During university study. our class made a trip to an oil field near Attock, Potwar area, Pakistan. The representative of oil company showed a surface geological map having a syncline type fold exposed in the area. While asking for geophysical survey (seismic survey) he was very pleased on inferring a dome structure from this geophysical data. Now this dome/anticlinal fold is a producing structure.

Apparently, it is a salt or shale diapir within the sedimentary trough as Lirim and Obianuju explained. If you have access to the gravity data, then it would be possible to discriminate between the salt and shale. If the magnetic data is available, then it could show  the presence or absence of the igneous intrusion. Several commercial oil-and-gas accumulations in Texas are known to be associated with the anticlinal structures created by shallow igneous intrusions.

Part of your answer "shale diapir within the sedimentary trough as Lirim and Obianuju explained" is matched the situation here in Pakistan. Cretaceous shale as source rock is there while the reservoir I do not remember perhaps Paleocene limestone. I am interested to see the kinematics. Suggested papers are welcome.

From the website http://www.ogdcl.com/ContentPage?id=u7TY1MjR%2F%2FNFXaIP%2Fx3FAA%3D%3D see also the following:

Sadqal (Gas/ Condensate Field)

The Sadqal Gas Condensate field is located at a distance of 40 Kms in the southwest of Islamabad. The field was discovered in April 1992 and regular production started on 30th June 1993.

I agree with what has already been said, the simplest thing would be to think about crestal collapse above salt cored anticline (e.g. those in the Paradox Basin) or magmatic intrusion, a sort of gravitational collapse.

To clarify it, it could help to study the geometry and symmetry of the syncline at surface, and to compare with other nearby structures confidently related with regional tectonics

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