Control of variability of primary grain assemblages on the stratigraphic differences in diagenetic processes and products in organic-rich sediments

Abstract

Abstract Diagenesis exerts a key control on the evolution of mineralogical systems in shales, and by extension, on the development of rock properties amenable to unconventional reservoir prospectivity. To develop an understanding of the control of primary grain assemblage on the diagenetic pathways and products in shales, integrated high-resolution petrographic and multiple geochemical studies were conducted on samples from a vertical transect of oil-gas mature (vitrinite reflectance ranging from 0.98 to 2.72%) Whitehill Formation, a shale unit identified as the main potential gas reservoir in South Africa. Results show that the formation comprises three intervals with contrasting primary grain assemblages, including organic matter content, maceral type, and detrital mineral grains, as well as the submillimeter-scale arrangement of these constituents. The lower interval occurs as parallel-laminated black shale dominated by components derived from primary production, such as Tasmanites cysts, colonial algae, radiolarian tests, and amorphous organic matter with up to 5.78% TOC, and depleted in siliciclastic detritus. The mid and upper intervals contain biologically produced components (up to 16.57% TOC) and high quantities of siliciclastic mud dominated by clay minerals and micas, and are organized into thick discontinuous gray laminae. Petrographic data suggest that the high organic content supported a variety of bacterial metabolic pathways, which caused the precipitation of mineral cements in the sediment pore spaces prior to compaction. However, the early diagenetic reactions and products in these intervals display systematic variations that reflect heterogeneity in their primary grain assemblages. The presence of framboidal pyrite throughout the lower interval suggests sulfate reduction as the predominant metabolic pathway. In absence of significant volumes of siliciclastics, the acidity associated with this process was poorly buffered, and the pore waters were reducing. These conditions led to the dissolution of opaline radiolarian tests, which recrystallized within Tasmanites cysts and interstitial pores to quartz. On the contrary, carbon isotopes indicate that organic oxidation in the mid and upper sections was mainly promoted by methanogenic archaea and that the associated acidity was effectively buffered by the detrital aluminosilicates. The resulting bicarbonate alkalinities allowed for the formation of interstitial carbonate cements (dolomite, calcite). Aluminum released during the acid-consumption reactions recrystallized within sheltered and interstitial pores to chlorite. Considering that many other shale successions exhibit similar stratigraphic variability of primary grain assemblages as the Whitehill Formation, this type of study will improve the characterization of shale mineralogical systems and bulk properties amenable to sustainable unconventional exploration.