Chlorite coating patterns and reservoir quality in deep marine depositional systems – Example from the Cretaceous Agat Formation, Northern North Sea, Norway

Abstract

Sediment gravity flows transport large volumes of sand and clay minerals into submarine systems, which store some of the world's major reserves of oil and gas. However, knowledge about grain‐coating clay mineral formation and its role in preserving reservoir quality in deep marine settings is poorly documented. Here we present a case study on the Agat Formation, a deep marine deposit interpreted as a series of turbidites, using a multimethod approach including petrographical, petrophysical and sedimentological data. This study investigates the occurrence and origin of chlorite coating and demonstrates how extensive chlorite coating substantially affects reservoir quality. The presence of green marine clay pellets suggests an initial shallow marine origin and sedimentological evidence reveals that the sediments were later remobilized by gravity flows and deposited at their present location. We suggest that the precursor clay coating was emplaced prior to sediment remobilization because of the presence of clay coating on grain contacts and all detrital components, the continuous nature of coating and the lack of clay bridges between the grains. Therefore, the origin of chlorite coating in deep marine environments may be recognized using the characteristic properties of inherited precursor clay coating. Chlorite coating thickness varies between an upper and lower sand unit, with an average of ca. 4.5 µm and ca. 24 µm, respectively. Permeability is significantly reduced in the interval with exceedingly thick chlorite coating but shows only a subtle decrease in helium porosity. This study enlightens the importance of crucially evaluating porosity in sandstones with thick chlorite coating using a multimethod approach. The results from this study can be useful in future exploration endeavours in the area and in other deep marine systems with a similar setting worldwide.

Impact of Grain-Coating Clays on Porosity Preservation in Paleocene Turbidite Channel Sandstones: Nelson Oil Field, UK Central North Sea

“…Thus, understanding the processes of formation of quartz cement is crucial for successful prediction of porosity in quartz-rich sandstones exposed to high-temperature diagenesis [17]. Clean, clay-free sandstones are susceptible to intense quartz cementation during burial [6,8,28,31], thereby destroying intergranular porosity. In contrast, sandstones with clay coatings can preserve anomalously high porosity during deep burial by preventing quartz cementation.…”

“…Authigenic quartz overgrowth is often considered as the most significant diagenetic cement and the major control on RQ in deeply-buried, quartz-rich sandstones [18][19][20][21]. However, clay mineral coatings on sand grains are widely reported for preservation of anomalously high porosity in deeply buried sandstone reservoirs, by arresting the development of ubiquitous, porosityoccluding quartz overgrowths [22][23][24][25][26][27][28][29]. Additionally, numerous published studies have documented the formation of grain-coating clays (e.g., chlorite) in coastal sandstones, with deltaic and estuarine depositional environments being particularly common [30,31].…”

“…Studies on RQ evolution of deep-water turbidite sandstones are significant for two main reasons: (1) Deep-water turbidites are important hydrocarbon exploration targets, and as hydrocarbon exploration focus shifts towards more-challenging, poorly-understood deep-water sandstones, the need to include RQ in pre-drill assessment is even more important [1,28]. (2) As hydrocarbon production attains maturity stage in, for instance, almost all major oil and gas field of the axial Forties Fan system [32], the Forties Sandstone Member can be a potential site for geologic carbon capture and storage (CCS).…”

Origin of Grain-Coating Chlorite and Implications for Reservoir Quality in the Triassic Deep-Buried Volcaniclastic Sandstones, Central Junggar Basin, Northwestern China

“…24 The quality of grain-coating chlorite, characterized by coat coverage and coat thickness, is closely related to lithofacies types and directly influences the degree of quartz cementation and reservoir quality. 16,31,32 Different lithofacies correspond to varying clay mineralogy and fluid dynamic conditions, typically resulting in higher grain-coating chlorite content and better quality under strong hydrodynamic conditions. 24,28,31,34,35 Currently, some studies have analyzed the formation mechanisms of grain-coating chlorite under different hydrodynamic conditions.…”

“…Some propose that under conditions of pore fluids rich in Fe and Mg during diagenesis, grain-coating chlorite can develop through precursor-driven recrystallization induced by heat or through in situ transformation and alteration of precursors, and grain-coating chlorite formed by these two mechanisms exhibits distinct morphological differences . The quality of grain-coating chlorite, characterized by coat coverage and coat thickness, is closely related to lithofacies types and directly influences the degree of quartz cementation and reservoir quality. ,, Different lithofacies correspond to varying clay mineralogy and fluid dynamic conditions, typically resulting in higher grain-coating chlorite content and better quality under strong hydrodynamic conditions. ,,,, …”