Oil and Gas

Biography

Biography: Ali Yassaghi

Abstract

With more than 1500 Tcf of gas reserves discovered in more than 25 fields, Gavbandi province of the Zagros, contains about 15% of the world’s proven gas reserves; discovered in fractured Permo-Triassic carbonates, sealed by the thick Triassic evaporates and originated from Lower Silurian highly organic shales. Anticlines located in the region are potentially prospective regarding burial and migration history, as well as the source, reservoir, and cap rock characteristics; and they are suggested for drilling if structural conditions are provided. Subsurface data sets acquired during the recent and successful hydrocarbon exploration in the Gavbandi area showed that the Triassic Dashtak evaporites form an efficient tectonic detachment horizon, decoupling the post-Triassic succession from the Permo–Triassic rocks as reservoirs for gas. The variations of fold geometry and fold crest location across the detachment horizon complicated the selection of the targets for gas at depth. Exploration wells drilled on the crest of several anticlines in the region indicated the rapid thickness variation of the Dashtak evaporites from one anticline to the next. The main objectives of this study are to Impact of Abnormal Fluid pressure within an Evaporitic Cap Rock on the Planning of Gas Exploration Wells in Zagros. To achieve these objectives, a synthesis of well logs, drilling mud weight data as well as depth-converted seismic profiles were analyzed to figure out anomalous high fluid pressure within the Triassic evaporitic cap rock (the Dashtak Formation) and its impact on the geometry of anticlinal traps in the gas rich Gavbandi area located in the south eastern part of the Zagros. The results indicated that the location of anticlinal traps at the depth of Permo-Triassic reservoir is horizontally shifted with respect to surface crest of many anticlines within the Gavbandi area. This Crestal shift across the Dashtak Formation may induced by abnormally high fluid pressure in the ‘A evaporite’ member of the Dashtak Formation, detected in many exploration wells across the area. When fluid pressure increases due to compaction during folding, the higher shaliness could probably cap more fluids and consequently increase the fluid pressure within the Dashtak Formation. Anomalous high fluid pressure decreases internal friction and shear strength of rock units and facilitates fracturing and faulting within the Dashtak Formation and consequently cause crestal shift of anticlinal traps. This should be taken in consideration when planning a new exploration well in Gavbandi area, in order to prevent out of trap drilling.