The Importance of Vertical Heterogeneities in Unconventional Shale Plays

Wensaas, Lars; Gading, Marita; Løseth, Helge; Boassen, Tony

Unconventional resources such as natural gas in organic-rich shales have become very important exploration and production targets. Shale plays are present in a wide range of geological settings. Shale reservoirs have a large range of compositions, complex micro-structures and some are better producers than others. Some key controls on shale producibility include richness (TOC-content), thickness, thermal maturity, pore types and brittleness.

Outcrop exposures, core images and wireline logs clearly demonstrate a very variable, vertical nature of organic-rich shale source rocks. Their vertical heterogeneity (lithotypes, composition and organic content) is ambient on all scales ranging from the seismic domain (10-100 m) to thin section scale (nm-mm). Visual layering and micro-scale rock fabric variations result in highly anisotropic rock property behaviour that influence both hydrocarbon storage capacity and flow properties. This paper addresses the importance of understanding vertical heterogeneities in shale plays and aims to exemplify tools and methods that are required to characterize the vertical variability of shale reservoirs.

Seismic data can be used to define depositional architecture, stratigraphic framework and lateral heterogeneities. Inverted 3-D seismic data can be used to predict lithology away from well control and highlight potential reservoir sweet spots. Wireline logs are used to estimate in-place resources and mechanical properties (e.g., brittleness) and show vertical variation in parameters such as: clay content, porosity (total and effective), organic content (TOC-content) and saturations. Core (and outcrop) data are essential for calibration of log-derived petrophysical models (porosity, organic content, gas adsorption, saturation, mechanical properties). Key shale reservoir properties vary at all scales. Vertical heterogeneities related to thin beds/laminas and important diagenetic alterations that influence shale mechanical properties require advanced high resolution, scanning electron microscopy techniques. Our work shows that routine sampling and analyses underestimate important micro-scale heterogeneities.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013