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Determining the mineralogy of sedimentary rocks from bulk geochemical analysis and chemofacies modeling

Abstract

Bulk inorganic geochemistry has become an increasingly common method of unconventional reservoir characterization. Most interpretations do not extensively utilize the potential application of geochemical analytical instruments and techniques such as X-ray fluorescence (XRF) or inductively coupled plasma mass spectrometry (ICP-MS) results by focusing on single element trends, associations between major oxides, ratio correlations, and the use of trace element proxies to infer organic material. Using expected relationships mineral assemblages can be predicted. Mineralogy estimates are based on a series of procedural calculations that allocate the total chemistry to molecular proportions of the most predictable major elements to minerals. The mineral assemblages of sedimentary rocks are more complex than those of igneous rocks, with the sediments containing not only minerals derived by physical weathering of igneous protoliths but also other minerals derived from rocks altered by chemical weathering. We propose a new system to estimate mineral proportions in sedimentary rocks from wavelength and energy dispersive XRF inorganic chemistry analysis applied to cuttings samples. Adapting techniques developed for neutron-induced gamma rays have been successfully applied to a variety of sandstone, shale, and carbonate sedimentary formations. Two separate approaches are presented and applied to case studies in carbonate and unconventional shale basins. A deterministic system based on a modified normative mineralogy approach uses criteria of limiting cations to sequentially generate minerals. A stochastic approach is applied to incorporate major and trace elements. To overcome limitations of organic components not reliably detectable from elemental analysis, organic carbon from pyrolysis is considered. Mineralogy models are further enhanced by incorporating organic chemistry from programed pyrolysis including TOC, producible hydrocarbons (S1, S2), maturity, and inorganic carbon (S5). Results are then validated to XRD mineralogy results. Characterizing the mineralogy of sedimentary rock formations is of increasing importance to petroleum exploration and production. The mineral composition of sedimentary formations can influence fractures, faults, porosity, permeability, and hydrocarbon entrapment. Interpretations to evaluate formations is generally constrained to chemostratographic trends which rely on element ratios or relative proxies in a specific formation. Mineralogy models provide an approach to be applied to samples from vertical pilot wells, although the sampling interval from cuttings inherently limits the vertical resolution. Applying geochemical-based mineralogy models to cuttings along the profile of a horizontal well provides a rapid approach to effectively characterize horizontal heterogeneity.