engleski

Well-log interpretation in terms of depositional environments for "gamma" series, Žutica oil/gas field, Croatia

Spontaneous potential and resistivity log curves combined with dipmeter patterns have been used to identify environment of Gamma series deposition on northern part of Žutica oil/gas field. In this study, three principal shapes of spontaneous potential curve have been determined, i.e. cylinder, bell, and funnel, representing uniform deposition, fining upward sequence, and coarsening upward sequence, respectively. Various curve serrations are interpreted as consequence of interrupted sedimentation. Gamma series was subdivided into seven deposits (g6, g5, g4, g3, g2, gx, and g1) which denote either channel sedimentation characteristics or/and out of channel sedimentation characteristics. Deposits are irregularly distributed on the exploration area, and were deposited as individual depositional environments (bell, funnel, and simple spike SP-curves), and as joint depositional environments (cylindric SP-curves). Five characteristic sedimentation units have been established: deposits g1 to g6 as individual sedimentary units, deposits g2 to g6 as joint sedimentary units, deposits g5 and g6 as joint sedimentary units, deposits g4 and g5 as joint sedimentary units, and deposit g5. Three channelised sedimentary units transect the studied area, and predominant sedimentation occurred within them. They are major oil reservoirs in the area. Diplog patterns also reflect changes in depositional energy, but they show no sign of tectonic activity. Environment of Gamma series deposition is interpreted as shallow, brackish, delta plain with well developed distributary channels extending southeastwards. However, since there are many limitations of standard log measurements, a particular curve shapes may not necessarily denote specific units deposited in a unique environment.

well logging; Žutica oil/gas field; individual depositional environment; joint depositional environment

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