Quantitative Geologic Analysis of Mancos Shale Core Improves the Geologic Model of the Play

Roberto Suarez-Rivera, Eric Edelman, Patrick Gathogo, and David Handwerger
Schlumberger

Subtle properties in the micro-texture of organic-rich mudstone reservoirs dictate important conditions of hydrocarbon mobility, phase transformations and segregation of hydrocarbon components during production. Understanding these details requires observations at the nano-meter scale. In addition, and in particular for the Mancos shale, the depositional sequences vary at the scale of few millimeters. Therefore, the challenge in descriptive geology is to understand the microtexture at the nano-meter scale, its variability at a millimeter scale, and construct a detailed description of the geologic system along the section of interest, represented by hundreds of feet of core. The challenge is also to describe the core and its geologic variability in a quantitative manner, and be able to propagate the knowledge gained to other core sections, and other cores. In this paper, we describe continuous measurements of strength, thermal conductivity, rock hardness, XRF mineralogy and CT density, at millimeter resolution, and along the length of the core. These measurements allow us to define fundamental rock properties of texture and composition quantitatively. Given that texture and composition are the end result of complex geologic processes of deposition, diagenesis, interaction with micro-organisms, and varying geochemical conditions over time, rock units with similar texture and composition reflect the action of similar geologic processes. And these can be identified, mapped across the core, and subsequently recognized on other core sections or other cores using the same measurements. The identification of millimeter-scale rock units with similar and dissimilar texture and composition give us a powerful tool for understanding the geologic processes that produce them. The thickness and cyclic stacking patterns of these units provide quantitative information of the depositional system and its sequences. In Mancos shale cores, for example, the cyclic nature of the shale and silt sequences is apparent in the measurements. The method also differentiates transitional contacts from abrupt contacts, and provides additional information for developing a geologic model. Results allow us to define the geologic system quantitatively. They also allow us to scale up the variability observed at a millimeter scale, to averaged properties obtained at the sample scale, and used to characterize bulk core properties. It allows us to scale to averaged properties at

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013