Origin, Accumulation and Production of Natural Gases in the Carpathian Foredeep, Poland
Maciej J. Kotarba
The Carpathian Foredeep is the largest gas basin among all the foredeeps of the Alpine orogenic system in Europe. The Carpathian Foredeep consists of two structural complexes: Paleozoic-Mesozoic and Miocene. Up to the end of 1994, 53 gas fields in Miocene complex and 11 gas fields in the Paleozoic basement have been discovered. Cumulative gas production between 1945 and 1994 has yielded about 75 billion cubic meters from the Miocene strata and about 7 billion cubic meters from the Paleozoic-Mesozoic basement.
Gases in the Paleozoic-Mesozoic basement are variable in both molecular and stable isotope composition [^dgr13C(CH4), ^dgrD(CH4), ^dgr13C(C2H6), ^dgr13C(C3H8), ^dgr34S(H2S)]. Variability of stable isotope ratios and molecular indices suggests complex origin of microbial and thermogenic gaseous hydrocarbons involving migration and mixing. The thermogenic generation of hydrocarbons was a multiphase process and in all the deposits traps were supplied at various times with new portions of hydrocarbons. The hydrogen sulfide was generated during thermochemical reduction of Jurassic sulphates.
Methane which dominates gas deposits in Miocene strata originated from microbial reduction of carbon dioxide. Insignificant quantities of ethane and propane were generated during diagenetic and/or early thermogenic processes. The hydrogen sulfide was generated during microbial reduction of Badenian sulphates. Accumulation of microbial methane was facilitated by high sedimentation rate of alternating clay and sand beds and the presence of methane hydrate formed in the thermodynamic regime of the Miocene basin. Uplift of the Carpathian orogen at the break of Lower and Upper Sarmatian destructed the methane hydrate zone.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California