Diamondoid hydrocarbons as maturity indicators for condensates from Southern Indus Basin, Pakistan

Abstract

Diamondoid hydrocarbons have been examined in condensates reservoired in the Southern Indus Basin using GC-MS. Bulk properties reveal that samples are waxy and low sulfur with the exception of Pakhro and Gopang which are nonwaxy. TIC show bimodal distribution of n-alkanes along with high abundance of C 20+ n-alkanes indicating substantial contribution of terrigeneous OM in these samples. CPI close to one is consistent with mature nature of oils. The samples show two ranges of Pr/Ph ratios. Those within the range of 2.2-2.7 reflect marine depositional settings for OM while others with Pr/Ph >3 may have originated from terrestrial OM deposited under marine oxic conditions. The cross plot of Pr/n-C17 versus Ph/n-C18 indicate type III kerogen as main source of OM deposited under marine to marine oxic conditions. The values of diamondoid based maturity parameters, like methyladamantane index 54.1-75.8% and methyldiamantane index 34.9-56.3% indicate high level of thermal maturity corresponding to vitrinite reflectance 1.1-1.6%. No biodegradation is observed in any of these samples as shown by methyladamantanes/adamantane 3.99-5.52 and methyldiamantanes/diamantane 2.16-2.99 and supported by high values of API gravity (45.13°-60.02°) and absence of UCM. © 2013 Shagufta Nasir and Tahira Fazeelat.