Abstract: Prediction of Hydrocarbon Quality Through Chromatographic Methods, Golfo San Jorge Basin, Patagonia Argentina

Cardinali, Guillermo - YPF; lnés Labayen - Laqui SRL; Carlos Colo* - YPF

This work presents a new methodology that solves the problem of identifying oil quality evidenced in cuttings from exploratory wells drilled in the Golfo San Jorge Basin, Argentina. These wells drilled through continental deposits of the Bajo Barreal formation (Cretaceous). This formation consists of sandstones that were deposited by ephemereal fluvial currents, channelized and unchannelized, of great areal distribution, with reduced dimensions in the order of 500 to 1000 m wide and 3 to 8 m thick. The evaluation of these deposits presents the great inconvenience of determining the quality of the hydrocarbons present in these reservoirs, At the same time the wells exhibit a great number of reservoirs with evidence of hydrocarbons, mostly liquid, that possess physical and chemical characteristics that are very critical with respect to density and viscosity, for obtaining economic productions. The predictive character of this technique is of great contribution not only in the determination of the beds of major interest to be evaluated, as a function of the type and quality of the fluids, but also in the choice of those zones, that having proven oil with low production, with good quality fluids, to be stimulated hydraulically. This is an important factor that greatly conditions their productivity.

Also it should be stressed that nowadays the productivity of most of the levels drilled by the wells in this basin, depend almost exclusively on hydraulic stimulation to obtain economically producible volume rates. At this point not only is the greater transmissibility obtained by the fracture important, so is the quality of the hydrocarbon to be drained. The technique can be of wide use for the rest of the wells of the field and may be extrapolated to other sedimentary environments in other basins.

The methodology employed in the potential reservoirs drilled is based in the chromatographic characterization of the head space gases and in liquid phase chromatographies of rapid extracts, both obtained from cuttings samples preserved from wells. In the first case the chromatographic analysis is carried out in the gaseous atmosphere of the gas liberated by the cuttings after heating (40° C) and shaking, obtaining an identification and quantification of the saturated hydrocarbons (C1, C2 to C6+) for the second case extraction is by a solvent (dichlormethane) that gives a rapid extract, which after solvent is evaporated goes through a liquid phase, and carbon distribution is determined (C12 to C32). This distribution is intimately related to the type and quality of the fluids present. From the analysis of a series of well samples from various fields, different chromatograms were obtained, which compared with their primary production response and also in some cases, with post fracture production response, were taken to be as masters or types. We call these: chromatographic fingerprints, that characterize different types of fluids present and in the case of oil, its quality. Thus its possible to determine: normal oil, residual oil (not producible) gas and formation water. From the obtention of the finger prints in samples from new wells and the comparison with the ones taken as masters, we can predict the type of fluids and the quality, optimizing evaluation with respect to reservoir productivity.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil