Role of authigenic clay minerals in preserving primary porosity and permeability of the sandstone hydrocarbon reservoirs

Abstract Among different diagenetic alterations, quartz cements are the foremost porosity and permeability destroying cement in deeply buried (>2 km) sandstones. Clay minerals are also known to commonly reduce reservoir quality of sandstones, however, detailed diagenetic studies, has suggested that in some diagenetic situations, the authigenic clay minerals not only do not reduce the reservoir quality, but they can help in preserving primary porosity and permeability of sandstones. Based on this research, if during the eodiagenesis, the clay minerals, occur as well-formed, thick, and continuous clay coatings on grains, they inhibit formation of quartz cements, especially overgrowths, during mesodiagenesis. This results in preserving primary porosity and permeability, leading to high reservoir quality in deeply buried sandstones. Different studies show that among different authigenic clay minerals, chlorite is the first most and illite is the second most abundant and important clay minerals in sandstones. Investigation about the conditions of formation and extension of clay coating minerals in sandstones help us in prediction and recognition of strata with high reservoir quality for hydrocarbon exploration. Keywords: Diagenesis; Sandstones reservoir quality; Grain coating clay minerals; Chlorite; Illite   Introduction Prediction of the reservoir quality based on sedimentary and diagenetic processes within sedimentary basins is a curtail component for hydrocarbon exploration and risk assessment. Diagenetic alterations can reduce, increase or preserve primary porosity of the sediment and sedimentary rocks. Formation of authigenic clay minerals during diagenesis are commonly known to reduce reservoir quality. However more detailed petrographic studies of deeply buried sandstones have shown that under some diagenetic conditions, the clay minerals could play a different role, which is the subject of his study. This condition is mainly related to the clay minerals occurring as the form of grain coatings. In this paper, the different types of grain coating clays, source and mechanism of their development and their effects on reservoir quality of sandstones has been studied.   Material and Methods In the last two decade, the role of authigenic clay minerals was subject of numerous studies and many papers have been published based on the case studies accordingly. In this paper, based on reviewing more than 100 papers and comparing two case studies, the circumstances in which clay minerals could help increase the reservoir quality of deeply buried sandstones is demonstrated. The case studies includes the Late Cretaceous Lower Tuscaloosa sandstone reservoir in the USA and the Cambrian-Ordovician Lower Sandstone Unit in Egypt. The sandstone intervals of the Lower Tuscaloosa Formation are high quality reservoir zone in subsurface of the Mississippi Interior Salt Basin, with average porosity of 25%, and average permeability of 50 md while the Lower Sandstone Unit in Egypt have very low porosity and permeability. The role of authigenic clay minerals, as the most important factor in construction of the reservoir quality, has been compared in these two case studies.   Discussion of Results & Conclusions During diagenesis, the primary porosity may be reduced, mostly by compaction and cementation. Among all the diagenetic cements, quartz cement and specially quartz overgrowths are the most important and common alterations that could reduce or totally destroy the porosity and permeability of the deeply buried sandstones. If the authigenic clays form as well-formed, thick, and continuous clay coatings on grains in shallow to medium depth burial, they inhibit formation of quartz overgrowths in deep burial. Quartz cementation can be inhibited by the presence of grain coating clays, because quartz needs clean substrates to form and quartz crystal cannot nucleate on or through the coatings. This results in development of high reservoir quality in deep burial. By these processes, the well-formed, thick and continuous chlorite coatings in the Lower Tuscaloosa Formation inhibited formation of quartz overgrowth, resulted in high porosity and permeability after deep burial, whereas the Lower Sandstone Unit, without any clay coatings on the detrital grains, have been cemented by quartz overgrowths. Type and amount of the clay mineral coatings depend on the sandstone composition and pore-water chemistry. Based on the literature, among all different authigenic clay coatings, chlorite and illite are the most common grain coating clay minerals in the sandstone reservoirs. Availability of depositional and/or early diagenetic precursor clay minerals, primarily berthierine and odinite are important factor in development and type of the chlorite coats (Odin1988; Ehrenberg 1993; Aagaard et al. 2000; Dowey et al. 2017). To a lesser extent, magnesium and iron rich smectite tend to be chloritized during diagenesis (Chang et al. 1986; Anjos et al. 2003). Authigenic chlorite coats are usually iron-rich, but magnesium-rich chlorite has also been documented in sandstones (Pittman et al. 1992; Ehrenberg 1993; Ajdukiewicz et al. 2010). Chlorite coats usually are composed of relatively small chlorite crystals that form perpendicular to the detrital grain surface. Chlorite coatings around framework grains can prevent quartz cement growth by blocking silica from nucleating on the grain surface. This results in preserving primary porosity and permeability of sandstones during late diagenesis. Authigenic illite can form by various processes including transformation of a clay precursor (smectite, montmorillonite and/or mixed-layer illite-montmorillonite), direct precipitation from pore-waters and replacement of other minerals (e.g. feldspar). However availability of precursor infiltrated clays, especially smectite, is the main factor in development of illite coats in shallower depth burial (Worden and Morad 2003). The authigenic illite usually occurs as irregular flake with lath-like projections. These projections may be relatively short or they may develop into curled fibrous projections up to 10 µm long, as fibrous and filamentous crystal habit, which extend into intergranular pore spaces. Unlike the chlorite and flake crystal habit of illite coatings, which lead to development of both high porosity and high permeability in deep burial, the fibrous and filamentous crystal habit of illite would reduce the pore and throat space, thus significantly decreasing reservoir quality, especially permeability (Wilson and Pittman 1977; Pittman et al. 1992; Wilson et al. 2014). In conclusion, chlorite coatings, which are the most common and effective clay coats in sandstones, can prevent quartz cement growth by preventing silica from nucleating on the grain surface, resulting in preservation of primary porosity and permeability. Illite coatings could also prevent formation of quartz overgrowths. However, due to the fibrous and filamentous crystal habit of illite, illite could cause permeability deterioration in sandstone reservoirs.