Formation of detrital clay grain coats by dewatering of deep-water sands and significance for reservoir quality

Abstract

Well-developed detrital clay grain coats are observed in deep-marine sandstones of the Upper Cretaceous Springar Formation of the Vøring Basin in the Norwegian Sea. The detrital clay coats form thin and compact rims on individual sand grains and meniscus-shaped bridges between grains. These well-developed coats are found in high-density turbidites and proximal hybrid event beds with common to pervasive dewatering structures deposited in proximity to the base of a syndepositionally active basin high. Here, in one exploration well, detrital clay grain coats are common throughout a sandstone package 100 m thick. High-density turbidites and proximal and distal hybrid event beds drilled in mid- to distal-fan settings unaffected by seismically resolved seafloor topography show common dewatering features, but have only scattered detrital clay coats confined to individual dewatering pipes or dish structures. Hence, we propose that intense sediment dewatering has the potential to form detrital clay coats in deep-marine sandstones by a combination of elutriation and reorganization of clays during fluid escape from sediment bodies with pore fluid pressures significantly higher than the hydrostatic pressure. In submarine fan systems, deposition of sediment with coeval trapping of large volumes of interstitial pore fluid is most likely to occur where gravity flows undergo rapid deceleration in response to an abrupt decrease in confinement or gradient. Such environments include the channel–lobe transition and settings in proximity to seabed topography. The investigated sandstones are quartz arenites and subarkoses, with minor to moderate volumes of quartz cement (up to 6%). However, strongly to completely quartz-cemented intergranular pore space is observed where detrital clay coats or matrix does not cover quartz grains in the deepest part of the studied formation. Modeling of quartz cementation predicts that most intergranular macroporosity in the lower part of the Springar Formation would be quartz cemented if the sandstones were free of detrital clays. Based on our observations and modeling results we propose that intense sediment dewatering has the potential to form detrital clay coats, which can be important for retaining porosity in deeply buried sandstones and in basins with high present or past heat flow.

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

“…Porten et al. (2019) also observed well‐developed detrital clay coating in certain intervals interpreted to have experienced intense sediment dewatering. Even though the sedimentological results in this study indicate that the Agat Formation was deposited by similar processes in a deep marine environment (Figures 4a and 5), the petrographic characterization suggests that the precursor clay is an inherited clay coating, meaning that it was emplaced prior to final deposition (Wilson, 1992).…”

“…Examples of clay‐coated sandstone units with a coastal origin are well documented in the literature and with deltaic environments being particularly common (Dowey et al., 2012). However, studies of clay‐coated sandstone units deposited by sediment gravity flows into deeper marine settings are rare and the origin of clay coats in such systems has been attributed to sediment dewatering processes at the time of deposition (Houseknecht & Ross, 1992; Porten et al., 2019). In this study, we documented extensive chlorite coating with an inherited detrital texture in gravity flow deposits of the Agat Formation in one well located in the northern North Sea of the west coast of Norway.…”

Bottom Current Modification of Turbidite Lobe Complexes

“…Rapid deposition in combination with the entrained mud causes the formation of local overpressured pore waters that breach through mud rich laminae and develop dish and pillar structures (Lowe, 1975). Locally "flushed" pore space along the undersides of dishes and within pipes facilitate the precipitation of carbonate cement (Southern et al, 2017;Porten et al, 2019). The more massive to cross laminated sandstone indicates deposition by high density turbidity currents along traction carpets that change from grain fallout and tractional deposition (Kneller and Branney, 1995).…”

“…(Lowe, 1982;Prélat et al, 2009;Grundvåg et al, 2014;Marini et al, 2015;Kane et al, 2017). The strong dewatering and large variety of grain sizes suggest a deposition by rapid flow deceleration (Lowe, 1975;Porten et al, 2019) Lobe axis (Fa1)…”

“…With ongoing sediment progradation, highly concentrated turbidity currents enter the confined structure and decelerate along the frontal confinement, which causes frontal erosion and rapid deposition of massive amalgamated high-density turbidites, and entrainment of a wide grain size range (Lf 1) (Lowe, 1982;Soutter et al, 2021). This rapid deposition and large grain-size range promotes the trapping of pore waters along low permeability laminae that eventually burst with increasing fluid pressure, and form the well-developed banding and dewatering structures (dishes and pillars) (Lowe, 1975;Porten et al, 2019). Ongoing infill and erosion along the frontal confinement allows sediment-laden flows to bypass into the next topographic low (Figure 11 t3b).…”