STRATIGRAPHIC DISTRIBUTION AND DEPOSITIONAL ENVIRONMENTS OF MICROBIAL FACIES IN AN ISOLATED CARBONIFEROUS CARBONATE BUILDUP CONSTRAINED BY BIOSTRATIGRAPHY, KASHAGAN FIELD, NORTH CASPIAN BASIN, OFFSHORE KAZAKHSTAN
The Sequence 3 progradational margin underlies a present-day structural elevation (rim) around the margin of the buildup between 100 and 250 m higher than the interior region, and contains microbial facies not present in the buildup interior. Massive microbial boundstones occur in deeper portions of Sequence 3, followed by skeletal-rich microbial facies interbedded with non-microbial facies toward the top of the sequence. The thickness occupied by the A.gibshmanae, F.asiatica and late Serpukhovian zones in Sequence 3 is significantly greater in rim wells penetrating the microbial boundstone facies compared to the thickness represented by the same intervals in wells penetrating the buildup interior. The Sequence 3 margin is overlain by horizontal strata in Sequence 4 containing shallow non-microbial facies, indicating that the underlying microbial facies accumulated on progradational slopes. Early lithification of the microbial facies in the rim area resulted in low porosity relative to the facies in the buildup interior. These observations collectively imply that the elevated rim formed by differential compaction during burial that may be controlled by the distribution of microbial facies.