Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability

Abstract

A B S T R A C TPorosity and permeability generally decrease with increasing depth (thermal exposure and effective pressure); however, a significant number of deep (Ͼ4 km [approximately 13,000 ft]) sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability. Anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology (composition and texture), age, and burial/temperature history. In sandstones containing anomalously high porosities, such porosities exceed the maximum porosity of the typical sandstone subpopulation.Major causes of anomalous porosity and permeability were identified decades ago; however, quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature. The focus of this article is on quantification and predictability of three major causes of anomalously high porosity: (1) grain coats and grain rims, (2) early emplacement of hydrocarbons, and (3) shallow development of fluid overpressure.Grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital-quartz grains. Currently, prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets. In the absence of adequate empirical data, sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality. Such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability

Spatial variability in depositional reservoir quality of deep-water channel-fill and lobe deposits

“…However, clay coatings (predominantly chlorite) on grains can also act to preserve porosity and permeability by inhibiting the growth of authigenic quartz (e.g. Heald and Larese, 1974;Ehrenberg, 1993;Bloch et al, 2002;Anjos et al, 2003;Dowey et al, 2012). Authigenic quartz growth is accelerated in basins with higher heat flows (e.g.…”

Heterogeneities and Intra Sand-Body Compartmentalization in Late Oligocene Delta-Front Deposit, Niger Delta, Nigeria

“…Such a reservoir quality at depth that range from 3323 to 4120 m suggest that the usual diagenetic processes responsible for permeability and porosity reduction (e.g., compaction, cementation and pressure solution), though might be active for a long time, failed to diminished the quality of the reservoir (Hayes, 1979;Coskun et al, 1993). Bloch et al (2002) adduced some factors that could have preserved the porosity and permeability values which include: (1) Sandstone richness in rigidgrains (i.e., quartz grains). Rigid-grain rich sandstone compact less than ductile-grain (clay and rock fragment) rich sandstone.…”

“…(4) Early hydrocarbon emplacement. Also according to Bloch et al (2002), lower thermal gradients that are normally associated with the rapid burial minimized loss of porosity by quartz cementation through overgrowth. Otherwise, the abundant mica flakes in the studied delta front-mouth bar deposit suppose to act as catalyst for chemical compaction and concomitant release of silica for quartz cementation at grain contact and consequently diminished the quality of reservoir.…”

Diagenesis and reservoir quality evolution of the paleogene sokor1 sandstones in the agadem block, termit basin, eastern Niger

“…Within the depth of 2000 to 2500m, the kaolinite content decreases greatly and the content of chlorite, illite as well as the mixed-layer illite/smectite increases which shown evidence of the transformation belt of clay minerals in Sokor1 sandstones. Therefore, it is predicted that the change of the pore water chemistry and alkaline medium led to the inter-transformation of clay minerals (Salman et al 2002), thus enabling the development of the chlorite and illite and the reducing of kaolinite and smectite content (Fig.16).…”