Geologia Croatica, Vol. 50 No. 1, 1997.
Original scientific paper
Pliocene Source Rocks, Miocene Reservoir Rocks and Origin of the Gas Accumulation of the Irma Field (Northern Adriatic, Croatia) Based on Wireline-Logging
I. Vulama
Abstract
The Pliocene source rocks of the Middle Adriatic ridge appeared to be thermally immature, according to the geophysical and the geochemical analyses, 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, Dt = 75 - 170 micros/ft (x 0.306 micros/m), rsr = 1.6 - 1.9 g/cm3, fN = 33 - 39%, SP = -44 (-) -56 mV, Corg. = 0.43 - 3.23%, Vot(%) = 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 rocks are: f = 22%, Rw = 0.18, m = 1.8, Fr = 15.3, Ro = 2.75 Ohmm, K avg. = 21.4 x 10-3 microm2. 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.
Keywords
Pliocene source rocks; Miocene reservoir rocks; Messinian unconformity; Wireline logging; Geochemical analysis; Discriminant function; Overpressure zone; Biogene gas; Middle Adriatic ridge; Croatia
Hrčak ID:
16674
URI
Publication date:
30.6.1997.
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