Formation and thermal evolution of insoluble reservoir bitumen in Angolan carbonate reservoirs.
Abstract
Pyrobitumen resulting from the thermal cracking of crude oil is a frequent occurrence in petroleum reservoirs. Despite the detrimental implications of pyrobitumen for the poroperm qualities of the reservoir; little is known about the evolution of pyrobitumen properties as a function of increasing thermal stress. A suite of pyrobitumen-bearing reservoir cores from the carbonate Jurassic Pinda formation in offshore Angola has been studied using geochemical and petrographic techniques (including elemental analysis, Rock Eval pyrolysis, GC, FTIR, XRD, SEM, TEM) in order to characterise the physical, chemical, and optical properties of the pyrobitumen as fully as possible. The 11 core samples contain pyrobitumen at reflectance values ranging from 0.55-2.24%Ro and display varying degrees of solubility in dichloromethane from 98% insoluble bitumen to 45% insoluble, indicating a substantial spread of maturity. However, these indicators of maturity, and further classical maturity indicators such as the H/C and Tmax show surprisingly poor mutual agreement.