Abstract: The aim of this study has been to enlighten complicated structural relationshisps of the rock units, to,establish true sequence of the rocks, present and to evaluate the rezervoir area as a potential reservoir lake and also,discuss problems relating to water leakage, In the reservoir area mainly there are theree groups of rock namely a metamorphic the old basement in NE, overlying carbonates in the middle and, the ophiolites on top. They form a thick sequence of homocline dipping NW-This homocline which are made up of the rocks of different origine appears to have beenformed by emplacement of the ophiolite in the region. Base of the sequence, namely Paleozoic sir basement complex consist of phyllites and metaquartzite contacts between the main rock groups, e.g between the basement and the carbonates,and, the carbonates and the ophiolites are tectonically formed graduel transition. The present complicated distributionof these rocks and their relationships appear to have been controlled by tectonic rearrengments. It is the tectonic rearangements of the region causing the present distribution of the rock groups have also yielded, their lensings interculatings so on, Thus, formed an imbricated structure with clearely spaced thrust sheets. The younger strata of the Tertiary sedimentary rocks transgresivly overly these sequence. Second phase of tectonic transport from the N broughtthe region a sequence of flat-lyng carbonates over the continental Miocene detritials. The limestone of jurassic-çretaceous age known as the Andırın limestone which occurs in the topographycal highs and platforms as thin clips of thethrust. In the piedmont plain younger plato basalts and allivium cover large areas. In the reservoir area, the Ceyhanriver form a narrow and steep valley and is surrounded by high mountain range with the peaks reaching....m at Kayadüldül. The area in the north of the Bani axis is occupied by impermeable rocks of the sir basement complex, Therefore this part of the reservoir does not seem to bear any potential water escape problems. The axis of sır, DüzkesmetBani dams are also plonned on the basement rocks. Thus problems of water escape is not expected. Berke A and Bdams are on the Berke limestone which is sandwiched between the two impermeable units. It is concluded as a resultof detail investigation that no future water escape is forseen in these dams. However, water escaping problems ofeach axis cases should be considerd separetly from the reservoir problems. No surface expression of a deep karst whichmight have occured beneath the maximum water hight has been seen.