METAMORPHIC SAND /
Granite Reservoir
Here
is a granite/metamorphic example from Indonesia. The reservoir
has a porous granite at the base, metamorphic sandstone above,
topped by conventional sandstone. Porosity is moderately low
throughout but the gas column is continuous. Interbedded shales
(schist or gneiss in the metamorphic interval) are present but do
not act as barriers to vertical flow.
In this case, the mineralogy
was triggered by quantitative sample descriptions, which in turn
were keyed to raw log response to minimize cavings and depth control
issues. Porosity and water saturation were derived from
conventional log analysis methods. The reservoir is naturally
fractured and a fracture intensity curve was generated from
anomalies on the open hole logs. This was compared to the fracture
intensity from resistivity micro image log data.
Quantitative
sample description of mineral composition is shown in track five
(right-hand track). Interpreted lithology is in track four; computed
porosity in track three (middle track). The log analysis porosity
matches core reasonably well (center track) and open hole fracture
indicators (right edge of track one) correspond to resistivity
image log data (left edge of track two).
Metamorphic / Granite example with quantitative sample
description, calculated lithology, log analysis
porosity, saturation, and permeability, with core porosity and permeability
overlay. A production log cumulative productivity curve was
overlaid on a similar curve generated from log analysis
flow capacity (KH). Since this is a gas play, cutoffs are quite
liberal.
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