ABSTRACT: Shale Porosity and Organic Maturity: A Graphical Approach to Determination of Primary Migration Efficiency

INAN, SEDAT, and ALAN DAVIS, Pennsylvania State University, University Park, PA

It is widely accepted that primary migration of oil occurs as a separate phase from water. Darcy's Law extended to multiphase fluid flow in porous shales dictates that the ability of oil to flow in presence of water depends on the ratio of its volume to the volume of water-saturated pore space. The critical value of this ratio is known as the saturation threshold under which oil movement is restricted.

In this study, the factors effecting minimum oil saturation such as

organic richness, porosity, organic maturity, burial and heating rates are evaluated. A graphical approach for easy determination of oil expulsion efficiency is introduced.

It is shown that porosity reduction of shales with burial is as important as type of kerogen in determining the nature of hydrocarbons to be expelled from shaly source rocks. Organic maturation modeling for a hypothetical shale unit that has been buried at a range of subsidence rate suggest that oil saturation threshold is never reached where porosity is abnormally high. Thus, generated oil (from type I and/or II kerogen) can not be expelled, instead is cracked into gas within the shale source rocks.

Effective porosity of shales with burial depth is evaluated from pore size distribution and volume of structured water standpoint. Effective porosity evolution in shales is introduced as a two-stage phenomenon where it can be linked to organic maturation through temperature dependency.

Finally, it is suggested that, in considering hydrocarbon phase migration from shales, organic maturity should be evaluated together with shale porosity. Cross-plot of shale porosity and organic maturity is a fast evaluation of primary migration efficiency where TOC content is also available. Graphical approach can probably serve a better purpose than simply measuring bitumen content of shales in migration studies.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)