Pliocene Source Rocks, Miocene Reservoir Rocks and Origin of the Gas Accumulation of the Irma Field (Northern Adriatic, Croatia) Based on Wireline-Logging

Abstract

The Pliocene source rocks of the Middle Adriatic ridge appeared to be thermally immature, according to the geophysical and the geochemicalanalyses, but they may have produced biogene gas. Wireline logging confirmed their broad extent. Correlation between geophysical and geochemical data indicated their distinguishing parameters:GR = 55 - 125 API, U238= 5 - 25 ppm, Rt = 1.5 - 4 Ohmm, ?? = 75-170 us/ft (x 0.306 us/m), Po = 1.6 - 1.9 g/cm3, ¢N = 33 - 39%, SP =-44(-) -56 mV, Corg. = 0.43 - 3.23%, V(%) = 0.47·0.66, K/Th = Illite.These organic matter have been classified as a kerogene type III-II.Characteristic geophysical parameters of the Miocene reservoir rocksare =$ = 22%, R = 0.18m = 1.8 Fr = 15.3, Ro = 2.75 Ohmm, K avg.= 2 1.4 x 10 3 um2. Favorable lithologic- tectonic conditions (overpressure zones, normal faulting) led to gas accumulation in Miocene calcarenites and sandstones. The Pliocene source rocks discordantly overlie Miocene reservoir rocks, separated by the Messinian unconformity. Statistical analyses show that, although the Pliocene shales are thermally immature, they fit into the model of source rock recognition.