Reading Gamma Ray and SP Together

Two Curves, One Question

The two curves of this chapter answer different questions, and read together they answer a third. The gamma ray says how clean the rock is; the SP says whether it is permeable. Neither alone settles whether a bed is worth chasing, but the pair does. Lay them side by side and the well sorts itself into four kinds of rock.

The Four Cases

Read clean-or-shaly from the gamma ray and kicks-or-flat from the SP, and every bed falls into one of four boxes. Clean and kicking is the prize: a clean, permeable sand, low IGR and a full SP deflection, the rock that holds and gives up fluid. Clean but flat is the trap for the unwary: a low gamma ray promises a clean bed, but a flat SP says it will not flow, the signature of a tight, cemented sand or a tight carbonate. Hot but kicking is a shaly but permeable sand, a poorer reservoir that still takes mud filtrate. Hot and flat is plain shale. The lesson is that neither curve is a permeability log or a reservoir flag on its own; it is the agreement of the two that makes the call.

What Chapter 3 Has Given Us

Step back and count what these two curves delivered. The gamma ray gave a shale volume $V_{sh}$ for every foot of the well. The SP flagged the permeable beds and, inverted in a clean wet sand, handed us $R_w$, the water resistivity every saturation needs. Shale volume and $R_w$ are the first two links of the interpretation chain, and we now have both before reading a single porosity or resistivity log. The chapters that follow add porosity from the density, neutron, and sonic logs, then true resistivity from the deep tools, and finally close the chain with Archie. The gamma ray and the SP have set the table.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Rider, M. and Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French.