Evaluating the Diagenetic History and Developing a Real-Time Method to Identify Authigenic Carbonate Cements in Organic-Rich Mudrock Reservoirs: A Study in the Eagle Ford, Niobrara, Woodford, and Haynesville Formations

Authigenic minerals are formed from water-rock interactions throughout a formation’s burial history and record valuable information regarding its chemical, thermal, and stress history. Diagenetic cements can negatively impact the porosity and permeability of conventional reservoirs; however, in unconventional reservoirs, significant volumes of authigenic minerals, such as carbonate cement, can increase fracture propensity. Thus, understanding mineral diagenesis is an important step in petroleum reservoir characterization and reservoir evaluation. The industry currently uses several geochemical signals (Si/Al and Si/Zr) to identify authigenic silica phases present in an unconventional reservoir. No such technique has been demonstrated to identify the other authigenic phases commonly present in organic-rich mudrocks, the carbonate cements. Electron probe microanalysis, energy dispersive spectroscopy, and wavelength dispersive spectroscopy were used to document the bulk and trace element (Ba, Mg, Mn, Fe, Sr, and Ca) differences between authigenic and biogenic carbonate in carbonate-rich (Eagle Ford and Niobrara), siliceous-rich (Woodford), and argillaceous-rich (Haynesville) reservoir types, so that chemical proxies may be used to differentiate authigenic carbonate phases using bulk geochemical data. In each reservoir type, a range of thermal maturities were analyzed to evaluate how burial diagenesis influences carbonate cement composition. The thermal maturity of outcrop and subsurface samples extended from the thermally immature window (~0.45% Ro) to the dry gas generation window (~2.5% Ro). This study shows that different generations of carbonate cements have unique and distinguishable compositions which are influenced by the depositional environment, degree of burial, and stage of diagenesis. For reservoirs that are dominated with earliest precipitated marine carbonate cements (Eagle Ford and Niobrara A Marl), contain multiple generations of early and later precipitated cements (Niobrara A Chalk and Woodford), and have very high thermal maturities (Haynesville), a Sr/Ca, Ca-Mg-Sr, and Ca-Fe-Mg proxy can be used to identify carbonate cements, respectively. Regardless if targets are seeking to encounter or avoid highly cemented intervals, these carbonate cement proxies can be applied to XRF data on well cuttings to improve real-time target adjustment plans while drilling.