Early Diagenesis in Fine-Grained Sediments: Cement Precipitation as a Modifier of Fabrics and Porosity Systems in Fine-Grained Rocks

Joe MacQuaker
ExxonMobil

Early (pre-compaction) diagenesis in fine-grained sediments (mudstones, mudrocks, shales, etc.) directly impacts their porosity systems. These sediment systems are particularly susceptible to fabric modification by diagenesis (cement precipitation and grain dissolution) because their component materials have large surface areas, are commonly composed of either poorly crystalline or metastable materials, and are buried with reductants. Bio-availability of reductants, buried along with other detritus in these fine-grained materials, means that many of the reactions that occur to modify the fabric of these sediments are linked to, either directly or indirectly, microbial metabolic processes. Some of the products of these reactions e.g. carbonate and sulfide cements are well known and related to specific microbial oxidation metabolic pathways e.g. iron reduction, sulfate reduction and methanogenesis. The products of these processes (e.g. concretions and concretionary carbonates) are particularly evident in horizons associated with breaks in sediment accumulation because at these levels there was enough time prior to compaction for sufficient volumes of the reaction products to diffuse to precipitation sites and occlude the majority of the water-filled porosity originally present. In these units diagenetic materials, rather than materials derived either detrital inputs to the basin or production within the basin, dominate rock constituents and profoundly influence the porosity system present. By way of comparison, however, reactions involving silicates and aluminosilicate minerals (e.g. berthierine and kaolinite) in these settings are only poorly known. The importance of these reactions has only recently come to light as electron optical analytical tools become more widely utilized as routine techniques to characterize the variability present within fine-grained rocks. In the light of these comments the aims of this paper are to: (i) illustrate some of the products of early diagenetic reactions in fine-grained sediments paying particular attention to the clay mineral cements that are commonly present but usually not described in detail, (ii) discuss how and when these cements might have precipitated and (iii) discuss what effect their presence might have on the porosity systems of units that now being targeted as unconventional reservoirs.

AAPG Search and Discovery Article #90185 © AAPG Geoscience Technology Workshop, Revisiting Reservoir Quality Issues in Unconventional and Conventional Resources, Austin, Texas, November 12-13, 2013