STRATIGRAPHIC DISTRIBUTION AND DEPOSITIONAL ENVIRONMENTS OF MICROBIAL FACIES IN AN ISOLATED CARBONIFEROUS CARBONATE BUILDUP CONSTRAINED BY BIOSTRATIGRAPHY, KASHAGAN FIELD, NORTH CASPIAN BASIN, OFFSHORE KAZAKHSTAN

Kashagan Field is an isolated Carboniferous carbonate buildup containing four major sequences, in ascending order: a Tournaisian–early Visean sequence (Sequence 1) that forms a backstepped platform above a Devonian (late Famennian) foundation; a late Visean aggradational platform (Sequence 2); a late Visean–Serpukhovian basinward-dipping progradational wedge (Sequence 3) that filled in accommodation space created after deposition of Sequences 1 and 2; and a terminal Bashkirian aggradational sequence (Sequence 4). The stratigraphic architecture of the Kashagan buildup is constrained by biostratigraphy (calcareous microfossils) in cored wells across the field, with correlations in terms of a modified Russian Platform substage scheme. The traditional foraminiferal zonation used for substages within the late Visean (post-Tulian) to early Serpukhovian interval is replaced by five local biozones defined by first appearances of the foraminifer Endothyranopsis crassa and the algae Fascifolium pantherinum, Asteroaoujgalia gibshmanae, Calcifolium okense, and Frustulata asiatica.

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.