In recent years, operators have produced large quantities of hydrocarbon from organic shale reservoirs. Horizontal drilling and multi-stage stimulations, targeting sections with superior reservoir quality (RQ) and completion quality (CQ), have been proven as key to their development. Organic mudstones, often referred to as shales, are fine-grained sedimentary rocks with total organic carbon (TOC) above 2% and typically complex mineralogy, consisting of a mixture of biogenic and terrigenous sources of siliciclastic and/or carbonate debris, affected by diagenetic processes. These rocks are highly heterogeneous especially perpendicular to bedding. Although numerous cores and outcrops have been studied and described, there is no published mineralogy-based classification scheme for organic mudstones. This paper defines a methodology for creating a detailed mineralogy-based description for organic mudstones using inorganic mineralogy, primarily X-ray diffraction (XRD) and geochemical log data.
The proposed classification scheme is based on a ternary diagram created specifically for organic mudstones by using a combination of core- and log-based mineralogical relationships. The primary classes are siliceous mudstone, carbonate-rich mudstone and argillaceous mudstone. Sub-classes are based on relative amounts of these three mineral groups. A mineralogy-based classification may help provide a better understanding of depositional conditions and identify target zones for completion. A common metric for the description of organic mudstones will also facilitate comparison of such reservoirs from different areas, formations, basins, and continents. A secondary objective is to provide a log display that flags other descriptive parameters that impact RQ, CQ and/or operational efficiency. A strong correlation exists between mineralogy and CQ in most U.S. shale plays. The correlation between mineralogy and RQ is not as strong. RQ appears to be driven by both compositional and textural components of organic mudstones. The log displays presented here provide a consistent description of the organic mudstone section and the input necessary for proper decision making when planning a drilling development project.