Evolution of the Raised Rims of Isolated Buildups in the Primorsk Region, Pricaspian Basin, Kazakhstan: A Quantitative Investigation of Compaction

Abstract

The Primorsk region in the North Caspian Sea of Kazakhstan contains several large isolated carbonate buildup reservoirs featuring raised rims (narrow, structurally elevated margins that partly or completely surround the buildups). Stratigraphic and facies data from the buildups show that the margin elevations (average 160 m) were generated by differential compaction during burial, that arose because of mechanical contrasts between rigid microbial boundstone and associated facies along the margin and more compressible grainstone and packstone facies in the buildup interior. Due to poor seismic resolution and a lack of deep well control, the distribution and thickness of microbial boundstone facies is poorly constrained in the raised rim areas. A series of 1-D and 2-D compaction and decompaction experiments were run using Basin2 software to determine sensitivities of the compaction process to facies thickness, depositional geometries, and burial history. While the thickness and depositional orientation of microbial boundstone facies along the margin resulted in corresponding measurable changes in rim morphology after compaction, the most important compaction sensitivity was burial history. The Primorsk buildups are overpressured reservoirs that experienced a past pressure release due to top-seal failure at near-lithostatic pressures and depths. The compaction models show that the overpressure was sufficient to delay compaction until the depressurization event, which was then followed by a period of rapid differential compaction. The rapid compaction episode has implications for genesis of vertical faults and fault-associated fractures in the raised rim, a region of Primorsk buildups dependent on non-matrix permeability for reservoir quality.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015