Seismic Expression of Karst-Related Features in the Persian Gulf and Implications for Characterization of Carbonate Reservoirs

Abstract

Karstification can have a positive and negative effect on carbonate reservoirs. For example, dissolution during karstification can enhance inter- and intra-granular porosity and permeability, whereas cave collapse can generate mega-pores and completely alter reservoir geometry and continuity. Karst may also pose challenges to drilling due to the unpredictable and highly variable porosity and permeability structure of the rock, and corresponding difficulty in predicting drilling mud-weight. Some of the largest karst-related features are imaged by seismic reflection data, thus they can be mapped directly, improving carbonate reservoir characterization and allowing development of safer drilling programs. In this study we use time-migrated 2D seismic reflection data to determine the distribution, scale and genesis of karst in a 3 km thick, Jurassic-Miocene carbonate-dominated succession in the Persian Gulf. We map 34 near-circular karst features on the top-Turonian regional unconformity that marks the top of the Upper Cretaceous Sarvak Formation. These sinkhole-like features are 0.8–10.2 km in diameter and 15–80 m deep, and are onlapped by overlying Coniacian strata, thus constraining their age. Additional subsidence, driven by differential compaction above in the stratigraphic succession overlying the sinkholes, occurred until the Early Miocene. We interpret that a 1100 m thick poorly imaged interval developed immediately below the sinkholes is related to subterranean collapse or poor seismic imaging below the highly geologically and geophysically heterogeneous karstified surface. There is no relationship between sinkhole diameter and depth, suggesting that the sinkholes did not widen as they deepened. Instead, the distribution of sinkholes along pan-African fault trends or around salt domes suggest a formation mechanism of cave collapse associated with fluid movement along pre-existing fault or fracture networks. These data indicate that seismic-scale karst features may be deep, wide and areally widespread at specific stratigraphic levels, suggesting that sub-seismic-scale karstic features may be even more widespread. Our study indicates that seismic reflection data can and should be used to determine the extent and scale of karstification with the specific aim of improving the characterization of carbonate reservoirs.

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