Organic geochemical study of the "W" oil-gas field in the Yugoslav part of the Pannonian basin

Abstract

The naphthenic crude oil from the "W" oil-gas field differs from almost all other, paraffinic base oils discovered until now in the Yugoslav part of the Pannonian basin. In order to explain this difference, some geological and organic geochemical investigations were carried out. On the basis of the bulk and biomarker parameters, determined in oil samples taken from 13 producing wells located at different parts of the field, it is concluded that the crude oil throughout the field has an almost uniform composition. However, some differences in alkane content, due to biodegradation and/or water washing were found. In the northeast part of the field where a tilted oil-water layers shows an active hydrodynamism, and where the temperatures are the lowest and pressure gradients the highest, the isoprenoids pristane and phytane were detected in trace amounts-indicating the most intensive biodegradation. In the central part, where a hydrocarbon accumulation exists in a Miocene reservoir, somewhat lower biodegradation was observed. The crude oil from this field has probably a mixed terrestrial-marine origin and is derived from argillaceous source rocks sedimented in marine delta or brackish lakes. Specific maturation parameters confirmed that the "W" crude oil has reached a quite uniform and significant thermal maturation. Considering maturation parameters such as ratios C30 moretane/hopane and C29 αα(20S)/αα(20S)+(20R) steranes, as well as the presence of oleanane, it could be proposed that the "W" oil originates from a Pre-Tertiary source rock, at least an Upper Cretaceous one, unlike most Pannonian basin oils which are Tertiary in age. © 1990.