Oil charge preserves exceptional porosity in deeply buried, overpressured, sandstones: Central North Sea, UK

Abstract

Exceptionally high-porosity sandstones are reported from oil provinces worldwide, yet the mechanism of porosity preservation remains controversial. We present strong evidence that the exceptional porosities in the Jurassic Fulmar Formation within the North Sea are the result of early oil charging preventing cementation and chemical compaction in sands with relatively low detrital clay contents, although overpressure and grain coatings do have some effect. The most dramatic evidence that reservoir quality is related to early oil charge is the spatial distribution of the porosity: the maximum porosity of the Fulmar Formation is systematically highest immediately below the top seal, and decreases below this despite uniform sedimentology. Hence, porosity is preferentially preserved where oil first accumulated in these unusually homogeneous overpressured reservoir sandstones, at the top. Oil charge prevents or significantly retards chemical compaction and cementation of a reservoir, preserving porosity. There is independent evidence for early oil charging of reservoirs within the Central Graben, from fluid inclusion studies and K-Ar age dates of authigenic illite. Micro-quartz grain coatings, which have been proposed to preserve porosity during burial, are associated with only 5% extra porosity in sands that exceed the worldwide average sandstone porosity by almost 20%. Edited by J. Poblet and R. J. LisleFold-and-thrust belts occur worldwide, have formed in all eras of geological time, and are widely recognized as the most common mode in which the crust accommodates shortening. Much current research on the structure of fold-and-thrust belts is focused on structural studies of regions or individual structures and on the geometry and evolution of these regions employing kinematic, mechanical and experimental modelling. In keeping with the main trends of current research, this title is devoted to the kinematic evolution and structural styles of a number of fold-and-thrust belts formed from Palaeozoic to Recent times. The papers included in this book cover a broad range of different topics, from modelling approaches to predict internal deformation of single structures, 3D reconstructions to decipher the structural evolution of groups of structures, palaeomagnetic studies of portions of fold-and-thrust belts, geometrical and kinematical aspects of Coulomb thrust wedges and structural analyses of fold-and-thrust belts to unravel their sequence of deformations.

Fluid overpressure as a control on sandstone reservoir quality in a mechanical compaction dominated setting: Magnolia Field, Gulf of Mexico

“…Factors such as the presence of clay mineral coats or microcrystalline quartz coats, early emplacement of hydrocarbon, presence of salt‐related thermal anomalies, mineral dissolution and fluid overpressures can all play a crucial role in preserving anomalous high porosity in sandstones (e.g. Spotl et al ., ; Worden and Morad, ; Taylor et al ., ; Wilkinson and Haszeldine, ; Sathar et al ., ; Nguyen et al ., ). Fluid overpressure, defined as the excess pore pressure above the hydrostatic pressure for a given depth, is commonly encountered in deep High Pressure High Temperature (HPHT) reservoirs (Osborne and Swarbrick, ).…”

The geological risks of exploring for a CO2 storage reservoir

“…But one complicating factor for hydrocarbon reservoirs is the filling of hydrocarbons, which has been suggested to preserve porosity by halting or retarding quartz cementation (Wilkinson and Haszeldine, 2011;Worden et al, 1998). This is shown by higher porosity in oil-bearing sections than the water-bearing equivalents in many oilfields (e.g.…”

A new stratigraphic trap for CO2 in the UK North Sea: Appraisal using legacy information

“…The relationship between porosity preservation and hydrocarbon charging has proved to be controversial (e.g. Wilkinson and Haszeldine, 2011) and is still unresolved.…”