Geochemical
Controls on Petrophysical Responses and Properties
Davies, Sarah1, Tim Brewer1,
Joe Macquaker2, Mike Lovell1 (1) University of Leicester,
Leicester, United Kingdom (2) University of Manchester, Manchester, United
Kingdom
Petrophysical responses, such as nuclear
and electrical measurements, are used to help constrain our petrophysical
understanding against a geological model. Petrophysical properties such as
porosity, saturation and permeability, define the storage and flow potential of
fluids. The mineralogy and geochemistry of the rock matrix contribute to both
the responses and the properties, but also affect the fluids.
Fine grained siliciclastic rocks
(particularly shales) that comprise seals and sources differ from coarser
siliciclastic reservoir rocks. Their more diverse mineral assemblages and
chemical compositions produce complex systems that can be extremely
heterogeneous. A fine grained nature combined with a clay-rich composition
means the matrix has a large surface to volume ratio with significant total
porosities, but small effective porosities, with little free water as it is
bound. This reduction in available pore space has significant consequences for
predicting saturation and permeability from petrophysical responses in these sedimentary
rocks. This effect depends very much on the clay mineralogy and consequently
the petrophysical properties of mudstones are dominated by their mineralogy and
chemical variations. Clearly to understand the petrophysics of mudstones, and
hence source rocks, fluid migration pathways and baffles, it is important to
decipher: (a) the petrophysics of the minerals that build such rocks (e.g.
clays, pyrite, quartz, zircon, monazite), and (b) how these minerals,
particularly clays, affect the pore space (e.g. through cation exchange
capacity) and bound water.
We examine the nature of the mineralogy
and chemistry of mudstones and the control these exert on both the
petrophysical responses and properties.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California