FLUID HISTORY ANALYSIS — A PROSPECT EVALUATION

Abstract

Fluid history analysis investigates oil migration and basin hydrology which are important factors affecting hydrocarbon charge to reservoirs. A combination of direct observation and measurement at the mineralogical scale and numerical simulations is used.

Fluid inclusions in diagenetic minerals are used to make direct observations about the timing of oil migration and to measure palaeotemperatures. Isotopic compositions of diagenetic minerals are measured for age determination and to identify the sources of ancient pore waters in order to interpret basin hydrology.

Integrated interpretation of time specific and time dependent thermal indicators provides a detailed thermal history of the rocks as input to numerical simulations of oil generation. Hydrocarbon fluid inclusions provide an observational check on the theoretical predictions.

In the Eromanga Basin in south-west Queensland two areas with different oil migration history are identified. On the Jackson-Challum trend there was a pulse of oil migration synchronous with quartz cementation in late Cretaceous time. On the Tintaburra-Bodalla South trend oil migration followed quartz cementation from the mid Cretaceous to the present day. Jackson oil migrated while the isotopic composition of pore water became increasingly oxygen-18 enriched indicating diagenetic modification during slow flow of groundwater. Tintaburra oil migration occurred during the present regime of downdip flow of relatively oxygen-18 depleted groundwater characteristic of a meteoric origin.