Page 365 - Geology and Geochemistry of Oil and Gas

326 APPENDIX C

Oncolite An accretionary carbonate particle composed of
a particulate nucleus surrounded by a cortex of
algae and entrapped sediment and/or precipitated
cement.
Onlap Progressive onshore migration of the updip
terminations of sedimentary beds within a
comformable sequence of rocks. Cf: Offlap.
Ooid An accretionary, sand-size carbonate particle
composed of a particulate nucleus surrounded by a
laminated cortex of microcrystalline calcium
carbonate; oolite is the term commonly used for rocks
composed of ooids. Partial syn: Oolite, Oolith.
Ooze Soft, soupy mud generally composed of at least
30% skeletal remains of pelagic organisms
(calcareous or siliceous), the remainder being clay
minerals.
Overbalance The extent to which the hydrostatic pressure of the
mud column exceeds formation pressure.
Overburden Section of rocks overlying a given stratum or strata.
Overburden pressure Total pressure, p t , exerted on a reservoir by the weight
of the overlying rocks and fluids. It is balanced by the
pore pressure, p p , plus the grain-to-grain stress, p g (or
effective pressure, p e ), p t ¼ p p +p e .
Overpressuring/ Porous rocks characterized by greater than normal
overpressured reservoirs hydrostatic pressure resulting, for example, from
undercompaction due to rapid sedimentation.
Oxidation Process in which a given substance loses electrons or a
share of its electrons.
P
Packer Mechanical device set in the casing (attached to the
tubing) to prevent communication between the tubing
and annulus.
Packing Three-dimensional arrangement of particles in a rock.
Packstone Muddy, but grain-supported carbonate rock textural
type.
Paleoenvironment Ancient depositional (or diagenetic) environment.
Paleogeomorphic Term used in reference generally to a buried
landscape; in reference to hydrocarbon reservoir traps
in or along certain buried landscape features.
Paleokarst Buried or relict karst.
Paleosol Fossilized soil.
Paraffin Denotes a solid, waxy material.
Paraffins Group of saturated aliphatic hydrocarbons with
general formula C n H 2n+2 .