ABSTRACT: Reservoir Delineation and Estimation of Producibility of Hydrocarbons by Organic Geochemical Logging

Vaughn D. Robison

In highly fractured heterogeneous reservoirs, such as found in the Monterey Formation of California, routine logging runs are commonly inconclusive as to delineation of the hydrocarbon-bearing reservoir intervals. Organic geochemical well logging provides a means to identify and estimate the producibility of such intervals throughout the penetrated section.

Various geochemical indices used in source rock assessment can be used to identify the presence of anomalously high levels of free hydrocarbons downhole. Many can be derived from Rock-Eval pyrolysis data of cuttings and core samples which is traditionally used as a screening tool for source rock assessment. The data from Rock-Eval can be used to identify potentially productive horizons. Well profiles of the S₁/TOC and KTR ratios highlight intervals that contain reservoired hydrocarbons, while depressed values of T_max can be used to indicate high concentrations of asphaltenes.

Estimations of the producibility (API gravity) of the reservoired crude may also be made by analyzing the cuttings samples using a modified pyrolysis gas chromatographic technique. Resolution of oils with API gravities of approximately 20-25° or less is possible. The recognition of these heavy and extra-heavy oils prior to drill-stem testing is economically important. The application of these techniques to studies from offshore California will be presented.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990