Anatase and pyrite are accessory minerals that are routinely found in both carbonate and clastic source rocks. Both are thought to precipitate in marine bottom sediments from trace elements derived from clays and micas, such as biotite. Near the sediment water interface, anoxia created by microbial oxidation of the accumulated organic matter, coupled with incongruent dissolution of biotite or other clay minerals, releases Ti4+ to form anatase, and Fe2+ to combine with sulfide, to form pyrite. X-ray diffraction results suggests the two minerals have a common geochemical origin linked to these processes, as when their concentrations are cross plotted together, they exhibit a positive linear relationship. However, this relationship is difficult to geochemically substantiate using XRD, because anatase is often less than 1 weight% compared to the other minerals composing the rock. This often poses questions over the validity of the value recorded and whether it could be related to the noise associated with the base line of the diffraction pattern. Recent research into development of nanoscale mapping software for resolving and quantifying the mineral fabric of source rocks from scanning electron images has revealed that the mechanisms responsible for the formation of anatase are detectable in the nanoscale rock fabric of organic chalks or marls comprising unconventional reservoirs. This detection helps validate not only the minerals occurrence as recorded from XRD, but also its relationship with pyrite and how these minerals can influence the chemistry of kerogen and ultimately its hydrocarbon generation.
Pyrite's ability to chemically sequester the sulfide created from microbial activity can have profound effects on the kerogen chemistry (Hughes et. al., 1995). Without iron sources to sequester the sulfide generated, the residual sulfur is chemically assimilated into the organic matter accumulated in the bottom sediments. The sulfur-rich kerogen requires less energy to break bonds to generate hydrocarbons, because sulfur can be a free radical which lowers the activation energy (Eglinton, 1990; Vandenbroucke and Largeau, 2007). Thus, the sulfur content of kerogen contributes to differences in hydrocarbon productivity of the source rock reservoir. Source rock intervals containing kerogen with higher organosulfur generate hydrocarbons at lower burial temperatures than other intervals where sulfur is lower, all of which is related to the redox conditions of the original depositional environment (Eglinton 1990; Hughes et al. 1995).