A new low-resolution modelling technique, for petrophysical evaluations in complex laminar reservoirs, was applied in the re-evaluation of a mature field in Japan. The reservoir is a laminar clastic formation, with shale, silt and multiple sand types, and with layer thicknesses varying from 2 to 20 cm.
In this formation the shale and silt layers are very conductive, dominating the resistivity logs, meaning that the low conductivity pay sand layers were often overlooked in previous interpretations.
As this is a complex formation, even if a tri-axial resistivity log was available, it would not be possible to use any of the current laminated shaly sand interpretation techniques. Furthermore, in very thin beds, high resolution modelling techniques create uncertainty.
Instead of conventional interpretation techniques, which quantify the presence of various minerals and fluids, this technique uses inverse modelling to solve for volumes of different layer types or rock components. These components are defined in the same way as minerals and fluids in optimizing petrophysical solutions, but the difference lies in the fact that this modelling technique also accounts for the structure of the rock fabric and the effects of thin beds on logging measurements.
Separate saturation height functions are used to determine saturations in each layer type, then the model is verified by computing synthetic resistivity logs and comparing these to the recorded logs.
This technique has a number of benefits, including reducing uncertainty in the results compared with more conventional techniques, identification of laminar from non-laminar intervals and providing results which are easier to upscale for reservoir modelling.
A mature field in Japan was re-evaluated in order to confirm the suspected presence of by-passed pay intervals and improve on the original petrophysical interpretation. The formation is a turbidite sequence which consists of alternating sandstone, siltstone and mudstone layers with thicknesses ranging from 2cm to 2m. There was a large range of porosity in the sandstone layers, noted from core, of between 15 and 28pu.