Lithofacies, petrography and reservoir characterization of Tobra formation, postwar plateau, upper Indus basin, Pakistan (T-1453) (MFN 8627)

Abstract

The Pe1mian Tobra Fonnation is composed of glacial, glacio fluvial and fluvial deposits of the Gondwanaland in Upper Indus Basin of the Punjab Platform. Three lithofacies have been distinguished for this formation from the studied sections of Tobaar village, Nilawahan Gorge and Choa Saidan Shah road sections. These include the deb1is flow deposits of matrix supported massive gravel (T I), clast supp011ed massive gravel (T2) and massive sandstone facies (T3). Petrographic studies reveal that the sandstone of Tobra Fom1ation is feldspathic litharenite. MonocJystalline quartz is abundant in the sandstone and plagioclase is higher than K-feldspar in the sandstone. The provenance studies reveal that the sandstone is mostly recycled orogenic (collisional orogeny) with a few samples also showing craton interior provenance. The sandstone was deposited mostly in semi-arid to semi-humid climatic conditions. The petro physical analysis is done on Fimkassar-04 and Bhangali-01 wells in the Postwar Plateau. In Fimkassar-04, the results show that Tobra Formation is not a good reservoir based on low volume of clean, low resistivity values (between 7.4 ohm.m to 140 ohm.m) and high values of density and neutron logs. The GR log varies between minimum 70APJ and maximum 226 API for Fimkassar-04 producing lo w volume of clean. In Bhangali-0 I, however, the fonnation can probably be productive. The true resistivity calculated using DT and Faust's equation is 0.05 ohm.m. An Sw curve shows that hydrocarbons are present at depths of 4000 111 to 40 I 5 111 in Tobra Formation in Bhangali-01 well. The hydrocarbon saturation is around 78% at these depths