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.