Seismic for shales

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Seismic technologies

Anisotropy

Azimuth

AVAz

AVO

Post-stack attributes

Fracture detection

Surface curvature Volume curvature Coherency Semblance Variance Ant tracking

Brittleness

Cluster analysis

Eccentricity

Reliability

Elastic inversion

LMR inversion

Shear wave splitting

Pore pressure prediction

Principle stress estimation

Differential horizontal stress ratio

Shale gas fields

What is shale?

Microseismic

Shales and mudrocks contain roughly 95% of the organic matter in all sedimentary rocks. However this amounts to less than 1% by mass in an average shale.

Geomechanical: Shales are fined grained sedimentary rocks in which clay minerals form the load bearing phase. This contrasts with sandstones where the load is transmitted via grain to grain contacts.

Petrophysical: many geoscientists use GR to determine clay mineral volume, however the difference between neutron porosity & density porosity is usually a better indicator of clay content (water?). Sonic may allow determining the transition from grain-supported sands to clay-matrix supported shales as a function of increasing clay content.

Clay minerals

hydrous aluminum phyllosilicates: flat hexagonal sheets (flakes) which are weathering products. Clay minerals are common in all fine grained sedimentary rocks such as shale, mudstone, and siltstone.

kaolinite, montmorillonite, smectite, illite, chlorite.

Shale vs mudstone: shale is characterized by breaks along thing alminae or parrallel layering or bedding less than 1 cm in thickness. Mudstones are similar in composition but do not show this same fissility. Mudstones are comprised of 1/3 to 2/3 silt particles, whereas claystones are comprised of less than 1/3 silt. Siltstones are similar in particle size but less clay (greater than 2/3 silt).

Color of shales

The redux state of shale rocks can be seen in it's color. Red is oxidizing conditions, greens are reducing conditions. black shales result from the presence of greater than one percent carbonaceous material indicative of a reducing environment (reduction in O2). Red shales indicative of ferric oxide (e.g hematite). Brown shales indicative of iron hydroxide (goethite), yellow limonite, greens, biotite, illite, chlorite (micaceous minerals).

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