Constraining Timing of Clay Mineral Authigenesis for Hydrocarbon Exploration and Appraisal

Horst Zwingmann and Andrew Todd
CSIRO ESRE, Kensington, WA, Australia

Gas production from unconventional reserves has increased significantly over the past decades but there remain largely unexplored areas around the world. Many unconventional resources contain fine-grained clay size sedimentary systems. Clay minerals are sheet silicates characterized by their small grainsize (<2 micron), thus requiring specialized separation and analytical methods for isotopic dating (e.g., Clauer and Chaudhuri, 1995). Illite is a general term for the dioctahedral mica-like clay mineral common in sedimentary rocks, especially shales. Illite is of interest to the petroleum industry because it can provide a K-Ar or 40Ar/39Ar isotope date to constrain timing of diagenetic reactions by burial heating or fluid flow events (Clauer et al., 2012). Numerous studies have investigated illite precipitation in the pores of conventional sandstone reservoirs, impeding fluid and hydrocarbon flow and numerous analytical challenges (Clauer et al., 2011; Lee et al., 1985; Franks and Zwingmann, 2010; Zhang et al., 2010).

Investigation of illite authigenesis in unconventional resources comprising shales or fractured granite reservoirs is a relative new approach. Nadeau (2011) summarized clay mineral processes related to energy exploration and showed that the majority of the world’s conventional oil and gas resources are situated within relatively narrow geological intervals. Recent developments in unconventional resources such as shale gas highlight further the need to investigate and constrain the timing of clay mineral formation and transformation processes. Shales comprise normally a mixture of detrital mica and other weathering products with diagenetic illite formed by reaction with pore fluids during burial. Pevear (1999) developed an approach to analyze and distinguish detrital and authigenic illite. K–Ar and 40/39Ar dating of illitic minerals is commonly used in studies of diagenetic series applied to oil and gas prospecting. In spite of a great number of specialized papers numerous analytical problems remain unresolved and mostly related to a misunderstanding of the Argon accumulation process during illitization. Criteria for identifying detrital–authigenic mineral mixtures, crystal ripening, fast precipitation or continuous nucleation-growth processes are discussed using K–Ar data available in the literature (Clauer et al., 2012; Meunier and Velde, 2004; Meunier et al., 2004).

The K-Ar and 40Ar/39Ar dating of diagenetic events in sedimentary and unconventional hydrocarbon systems face methodological challenges that are discussed, focusing on three main applications comprising: (1) deep burial clay mineral authigenesis in sandstones, (2) smectite to illite transition during burial diagenesis in shales and (3) deformation events - brittle fault zone dating (Zwingmann et al., 2010) and influences on trap integrity.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013