Abstract
A subsurface resistivity model is important in hydrocarbon exploration primarily in the controlled-source electromagnetic (CSEM) method. CSEM forward modelling workflow uses resistivity model as the main input in feasibility studies and inversion process. The task of building a shaly sand resistivity model becomes more complex than clean sand due to the presence of a shale matrix. In this paper, a new approach is introduced to model a robust resistivity property of shaly sand reservoirs. A volume of seismic data and three wells located in the K-field of offshore Sarawak is provided for this study. Two new seismic attributes derived from seismic attenuation property called SQp and SQs are used as main inputs to predict the volume of shale, effective porosity, and water saturation before resistivity estimation. SQp attribute has a similar response to gamma-ray indicating the lithological variation and SQs attribute is identical to resistivity as an indicator to reservoir fluids. The petrophysical predictions are performed by solving the mathematical step-wise regression between the seismic multi-attributes and predicted petrophysical properties at the well locations. Subsequently, resistivity values are estimated using the Poupon-Leveaux (Indonesia) equation, an improvised model from Archie’s to derive the mathematical relationship of shaly sand’s resistivity to the volume and resistivity of clay matrix in shaly sand reservoirs. The resistivity modeled from the predicted petrophysical properties distributed consistently with sand distribution delineated from SQp attribute mainly in southeast, northeast, and west regions. The gas distribution of the net sand modeled by 5% and 90% of gas saturation scenarios also changed correspondingly to SQs attribute anomaly indicating the consistent fluid distribution between the modeled resistivity and SQs attribute.
Highlights
Building a reliable resistivity model is an imperative step prior to electromagnetic feasibility studies (Werthmüller et al, 2013)
This paper presented a new approach to model the resistivity distribution of shaly sand reservoirs by incorporating two new seismic attributes derived from rock attenuation property
The resulting resistivity is laterally consistent with the shaly sand distribution delineated from the new seismic attributes, predicted petrophysical properties as well as measured resistivity at the well location
Summary
Building a reliable resistivity model is an imperative step prior to electromagnetic feasibility studies (Werthmüller et al, 2013). Resistivity property modelling has an additional challenge for a shaly sand reservoir due to the low resistivity contrast to its overburden lithology. Archie’s Law mathematically defines the resistivity of clean sand to its water saturation and porosity (Archie, 1942). The Indonesia model (Leveaux & Poupon, 1971) is one of the modified models that has been widely used to define the mathematical relationship between the resistivity of shaly sand to its clay properties. It takes into account the volume and resistivity of the shale matrix to estimate hydrocarbon saturation in shaly sand reservoirs more accurately
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