Formation Evaluation: Shaly-sand Formation
1. Would it remain feasible to confirm the detection of a ‘commercially sweet onshore dry gas field’ - drilled vertically - using water-based mud - to a total depth greater than 4 km – by resistivity logs and DST?
2. How easy to retrieve the petro-physical properties (porosity, permeability, water saturation & net reservoir thickness) and its associated production forecast - when the target formation in the stratigraphy of the area pertains to a shaly-sand (silts and clays with thin turbidite sands)?
Can we comfortably use the traditional approach, whereby the properties are derived from well-logs (formation density; NMR; resistivity; neutron; sonic; & dual combinations of resistivity, neutron and sonic logs); formation pressure tests; or, cores through formation evaluation, or combined cores with wireline logs and/or formation pressure test analysis – when the total depth exceeds 4 km (particularly, in the absence of acquiring cores in the target interval)?
3. How do we generally account those properties (such as water saturation, porosity, formation water salinity & temperature [Horner plot of BHT]) which remain to vary vertically (depth) and laterally (depending on lithological-type and texture with the degree of sorting, compaction, cementation and dissolution) – pertaining to a shaly-sand formation?
4. Can we the use the traditional approach of using NMR/formation-testers/cores for deducing permeability values for a shaly-sand formation?
5. How critical will be the role of shale volume towards estimating net reservoir thickness deduced from well logs with cutoff values of shale volume, porosity/permeability & water saturation in a shaly-sand formation?
6. In a shaly-sand formation, in the absence of layer-wise data of petro-physical properties, to what extent, the interpretation of layer-wise petro-physical properties would really help in obtaining input to a rock property model in order to build a reservoir simulation model?
Whether such interpreted values will be able to justify the estimation of saturation-height relationships and relative permeability curves?
7. In a shaly-sand formation, while elucidating the distribution of shale volume and its associated influence on the porosity changes; do we need to depend on any other variable(s) than gamma-ray; bulk-density; compensated neutron; and induction/later-log/micro-resistivity – towards estimating shale volume (Clavier equation/neutron-density); shale resistivity; total and effective porosities; sand fractions; sand porosity (Thomas-Stieber method) & water saturation?
8. Which one of following two approaches remains to be closer in replicating the volume fraction of shale in a given zone of interest – in a shaly-sand formation? (and should be applied in Poupon and Leveaux equation – towards estimating water saturation – as against from using Archie’s equation?) (a) Clavier equation; or (b) Neutron-density equation.
This is quite an extensive set of questions, which includes several aspects of petrophysical analysis in shaly-sand formations. Let me provide a high-level response to each of your queries. For more detailed information, please consult a petrophysics textbook or a professional petrophysicist.
Finally, note that shaly-sand interpretation is a complex topic, and may require specialized knowledge and experience to deal with all the complexities and uncertainties. It's always recommended to work closely with an experienced petrophysicist and to integrate as much data as possible to make the most accurate interpretation.
A bit more detail on a few key points:
Each shaly-sand reservoir is unique, and these general guidelines may not apply in all cases. Therefore, it's always recommended to work with an experienced petrophysicist and to gather as much data as possible to understand the reservoir.
Greetings Suresh. Given that "shaly sands" can mean either thick beds of sandstone with high clay content (argillaceous sandstone) or interbedded thin shales and sandstones that have low clay content (heterolithics), one must decide which rock type is prevalent. My bias is that shaly-sand petrophysical workflows are appropriate for argillaceous sandstones and not for heterolithics. Resistivity tools do not have the vertical resolution to accurately measure heterolithics so an approach like the Thomas-Stieber method. It is critical to know the lithology and bed thickness before choosing a method. I found this paper that might help: Conference Paper Formation Evaluation for Thin Bed Shaly Sand Reservoirs
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