2D seismic interpretation and petrophysical analysis of Minwal block, upper Indus basin Pakistan (P-1082) (MFN 4002)

Abstract

Minwal is situated in the Punjab province of Pakistan. This thesis contains the study of 2D seismic reflection data interpretation of selected seismic lines (MN-02, MN-04, MN-05 and MN-11) of Minwal, Potwar Plateau and Upper Indus Basin, for delineation of subsurface structures favorable for hydrocarbon accumulation. Two prominent reflectors (yellow and red) were marked on the basis of their reflection, which represent Chorgali Formation and Sakesar Limestone respectively. Reflectors were identified with the help of well top data, after conversion of time to depth with the help of average velocity. Two-way time (TWT) from the seismic sections and average velocities were used to create TWT and depth contour maps. Pop-up structure bounded by two thrust faults was observed from the time and depth surfaces of two formations. Chorgali Formation is of Early Eocene age and consists of shale and limestone. The Sakesar Limestone is of Early Eocene and consists of nodular limestone with chert. The Patala shale of Paleocene age is the source rock but Precambrian Salt Range Formation also contains some potential. Reservoir rocks include fractured limestone of Chorgali formation and Sakesar Limestone. Khewra Formation of Cambrian age is the main potential reservoir. Clays and shales of the Murree Formation act as traps. Petrophysical analysis shows two zones, Zone 1 for Chorgali formation and Zone 2 for Sakesar Limestone. In Zone 1 average of shale is 20%; saturation of water is 21%; effective porosity is 0.17(v/v); fracture porosity is 0.4(v/v) and shale volume is 45%. Whereas in Zone 2 average value of shale is 55%; saturation of water is 100%; effective porosity is 0.26(v/v), fracture porosity is 0.007(v/v) and shale volume is 32%. Hence Zone 1 seems to be a better reservoir because it has less shale volume, good effective porosity and low saturation of water. Whereas in Zone 2 shale volume is high, effective porosity is low and saturation of water is 100%. In Zone 1 maximum value of hydrocarbon saturation is around 90% whereas average hydrocarbon saturation is 20%. In Zone 2 hydrocarbon saturation shale volume, effective porosity and saturation of water the Zone 1 seems to be more promising hydrocarbon prospect zone.