Hello CHL,
While delta-sigma or sigma-delta is in essence an averaging process the oversampling is not the only thing responsible for the ENOB.
For example only OSR improves the number ob bits from n to ENOB=n+0.5*log2(OSR). Now this means that for a 5-bit gaing OSR of 1024 is needed.
Now if you look at delta-sigma it also produces a shaping function of the quantization noise, which enhanses the OSR. SNR of a 1-bit modulator with OSR = 1024 becomes:
SNR=6.02*n+1.78-5.17+9.03*log2(1024) this equals to aproximately 92 dB which translates to aprox ENOB of 15 bits. Hopefully my math was correct
.
Threre are number of resoureces regardin SDM. Something to get you started
link.