Structural modeling and volumetric reservoir estimations of the cretaceous derdere and karababa reservoirs in the Çemberlitaş oil field, southeastern Turkey

Abstract

The Adiyaman region is located in the frontal belt of the southeastern Anatolia fold and thrust belt in Turkey. The region is in the northwestern continuation of the Zagros fold-thrust belt and contains several small-to-medium size oil fields. These oil fields are characterized by structural traps located in east-west trending anticlinal structures along the frontal belt. One of these oil fields is the Çemberlitaş oil field which produce from fractured and dolomitized reservoirs of the Karababa and the Derdere formations of the Cretaceous Mardin group carbonates. This study focuses on delineating the structural geometry and evolution of the Çemberlitaş Anticline and the reservoir characteristics in the Çemberlitaş oil field based on interpretation of available 2-D seismic reflection profiles, conventional wire-line well log data, and check shot surveys. The subsurface geometry suggests that the production in the oil field is from the hanging wall anticlines within the blind leading imbricate thrust system caused by Cretaceous break-forward sequence of thrusting which involves Karababa and Derdere formations. Within the study area, there are thrust faults branching from a horizontal detachment surface in a break-forward sequence of thrusting which is suggested by decreasing dip angle of thrust faults from north to south. The subsurface geometry also demonstrates the presence of high-angle Cretaceous normal faults to the south of the leading-edge thrust of the Cretaceous contractional structures. These normal faults were most likely formed during the Cretaceous subduction due to flexural bending of the subducting slab. Three strike-slip faults are identified in the subsurface as the tear faults of the thrust system developed contemporaneously with the north–south contractional deformation in Late Cretaceous. They align parallel to the tectonic transport direction, and separate thrust sheets into different thrust blocks. Break-forward sequence of thrusting in the study area developed asymmetric hanging wall anticlines and footwall synclines and resulted in structural thickening of the Cretaceous Derdere and Karababa formations at overlapping parts of the thrust sheets. These areas present ideal production well locations due to their structural thickening and possibly fault sealing. Therefore, production possibility is higher at these overlapping thrust sheets due to increasing net pay zone thickness and volume of reservoirs. Thrust blocks which were separated from each other with tear faults caused compartmentalization of the reservoirs into productive and non-productive blocks. It is suggested here that these tear faults may be acting as lateral seals in the Çemberlitaş oil field. Recoverable original oil in place in the Çemberlitaş oil field was estimated into three different volumes according to porosity values obtained from well logs. Low estimate (P90) (Ø≥ 9%), high estimate (P10) (Ø≥ 4%) and best estimate (P50) (Ø≥ 6%) are estimated at 10,817,098 STB, 33,769,395 STB, and 19,157,391 STB respectively, while total oil remaining in the field was estimated at 5,649,127 STB.