Mudstone Lithofacies within a Sequence Stratigraphic Framework: Key to Mapping Shale-Gas Rock Properties in the Horn River Basin, Canada

Lithofacies classifications associate sedimentary characteristics with depositional processes. In petroleum exploration and development, lithofacies classifications are typically tied to porosity and permeability - the two key parameters that control productive capacity. A powerful tool is obtained when the lithofacies classification is constructed within a predictive sequence stratigraphic framework that integrates geological and geophysical data. Such a workflow is commonly adopted by geoscientists working in coarse-grained siliciclastic and carbonate depositional systems.

We show that although depositional processes and environments for fine-grained, organic-rich sedimentary rocks are not as well established, these conventional workflows can be adopted for shale hydrocarbon systems. However, as outlined below, some additional parameters need to be integrated within the workflow including geomechanical data, bio-productivity and elastic properties.

Given the fine-grained nature and characteristic dark color of these rocks, lithofacies can only be confidently classified by fully integrating thin-section observations, core descriptions, wireline log analysis, geochemistry and geomechanical data.

Interpretation requires integration of physical depositional processes, along with an understanding of the controls on bio-productivity and organic matter preservation. Both phyto and zoo-plankton have a profound impact on the resulting depositional sediment. Understanding diagenetic processes is crucial, as it strongly impacts the pore architecture and rock fabric.

Additionally, the lithofacies need to be integrated not only with porosity and permeability characteristics, but also with the elastic properties (bulk and shear moduli) as these have a strong impact on the response of the rocks to fracture stimulation. There is a unique relationship between lithofacies and elastic properties within the Horn River basin mudstones that we have utilized to directly invert lithofacies from acoustic data (both well logs and seismic).

We present a scheme of 10 distinct mudstone lithofacies, fully integrated within a sequence stratigraphic framework, and tied to pore architecture and elastic properties. The framework allowed us to interpret depositional processes and to map the resulting depositional bodies (geobodies) and associated rock properties for vertical and horizontal well placement.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California