Anomalously High Porosity and Permeability in Deeply Buried Sandstone Reservoirs: Origin and Predictability

Salman Bloch1, Rob H. Lander2, and Linda Bonnell2

ABSTRACT

A significant number of deep (>4 km, or approximately 13,000 ft) sandstone reservoirs, worldwide, are characterized by "anomalously-high" porosity and permeability (p&p). Anomalous p&p exceed the maximum p&p of "typical" sandstone reservoirs of a given lithology, age, and burial/temperature history. The focus of our presentation is on quantification and predictability of three major causes of anomalous p&p: (1) grain coats and grain rims, (2) emplacement of hydrocarbons, and (3) development of overpressure.

Grain coats and grain rims retard quartz cementation, and concomitant p&p reduction, by inhibiting nucleation of quartz overgrowths on detrital-quartz grains. Quantitative diagenetic models can be effective in assessing the impact of coats/rims on p&p preservation in grain coat- and grain rim-prone intervals.

The overall effect of hydrocarbon emplacement on reservoir quality is controversial. However, integration of basin modeling with reservoir quality modeling can quantify, prior to drilling, the potential impact of hydrocarbon emplacement on p&p.

The best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited Tertiary- or Quaternary-aged sandstones that are encased in low permeability lithologies. Under these conditions up to 6% and 14% porosity can be preserved in rigid grain-rich and ductile grain-rich sandstones, respectively. The case for significant porosity preservation associated with fluid overpressures in pre-Tertiary basins is more problematic due to the greater significance of cementation as a cause of porosity reduction, a higher likelihood of breaching the original seal, and uncertainties in the timing of overpressure development.

 

1Texaco Upstream Technology, 3901 Briarpark, Houston, TX 77042, phone: (713) 954-6270, e-mail: blochs@texaco.com.

2Geocosm, Austin, Texas; e-mail: roblander@geocosm.net, lmbonnell@geocosm.net; Visiting Scientists, Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713.