INTEGRATED RESERVOIRS SIMULATION STUDY AND SUBSURFACE FRACTURE NETWORKING ANALYSIS IN CARBONATE RESERVOIRS: POTWAR PLATEAU, PAKISTAN

Abstract

The Balkassar field is located in southern Potwar Basin which is a sub basin of Upper Indus Basin, Pakistan. Tectonically, the study area lies in a compressional regime where reverse faults are observed which make pop up structures. To reduce the risk and problems during drilling a well, the evaluation of reservoir properties is very important 3D seismic cube were interpreted by correlating with well data of Balkassar OXY-01. It appears probable the early Eocene age Chorgali Formation could offer commercial production with lower water cuts from the eastern lobe (yet untapped) of the structure that may have at least 30 million barrels of unrecovered oil. Four horizons in 3D seismic data were marked and pointed out. Two prominent faults were identified and interpreted on the seismic sections. The contours of the horizons (Chorgali, Sakesar and Patala and Khewra formations) are trending north-east to south-west. The contours in the region of the faults illustrate reverse faults forming pop-up structure i.e. north-west and south-east dips and north-east and south-west strike. 3D depth surface models were generated which indicates that the major structure has lower pull out in the course of north-east to south-west with sinking along south-east and north-west directions. The Chorgali Formation (2425m-2490m) were marked and encountered lithologies are composed of limestone and dolomite. The values of GR-log (Gamma Ray Log) shows high values which indicate that the Patala Formation (2605m-2625m) is mostly consist of shale. The Khewra Formation (3050m-3125m) values were relatively ~50% sandstone, ~10% sandy limestone while rest of the formation is shale (high GR-Log values). The targeted reservoirs in Balkassar oil field are Chorgali and Sakesar Formations in well BLK-OXY-01. The overall depth range of the reservoirs zone were marked is in between 2420m to 3100. The reservoir area has a seismic reversal, where the low impedance area indicates the presence of limestone opposite the shale. Now, using an impedance graph model from the well data, an initial map is developed in the time interval between 1300 ms and 1450 ms in which interest is generated. The seismic interpretation and modeling alongside the petrophysical analysis were then quality checked via qualitative rock physics analysis. From the Kdry/Porosity plot, the limestones were generally observed to lie within the 2420-2580m which indicates a high level of compaction. From high impedance values can be observed on a seismic cube of 2600 m, and these values decrease slightly as you go up. This shows that we have Patala shale at 2,600m and as we go up we are approaching the reservoir area at 2,400-2,580m.